One of the first types of chemical compounds studied in the school organic chemistry curriculum are alkanes. They belong to the group of saturated (otherwise known as aliphatic) hydrocarbons. Their molecules contain only single bonds. Carbon atoms are characterized by sp³ hybridization.
Homologues are chemical substances that have common properties and chemical structure, but differ in one or more CH2 groups.
In the case of methane CH4, a general formula for alkanes can be given: CnH (2n+2), where n is the number of carbon atoms in the compound.
Here is a table of alkanes in which n is in the range from 1 to 10.
Isomerism of alkanes
Isomers are those substances whose molecular formula is the same, but whose structure or structure is different.
The class of alkanes is characterized by 2 types of isomerism: carbon skeleton and optical isomerism.
Let us give an example of a structural isomer (i.e., a substance that differs only in the structure of the carbon skeleton) for butane C4H10.
Optical isomers are two substances whose molecules have a similar structure, but cannot be combined in space. The phenomenon of optical or mirror isomerism occurs in alkanes, starting with heptane C7H16.
To give the alkane the correct name, IUPAC nomenclature must be used. To do this, use the following sequence of actions:
Using the above plan, we will try to name the next alkane.
Under normal conditions, unbranched alkanes from CH4 to C4H10 are gaseous substances, from C5H12 to C13H28 are liquid and have a specific odor, all subsequent ones are solid. It turns out that As the length of the carbon chain increases, the boiling and melting points increase. The more branched the structure of an alkane, the lower the temperature at which it boils and melts.
Gaseous alkanes have no color. And also all representatives of this class cannot dissolve in water.
Alkanes that have a gas state can burn, and the flame will be either colorless or have a pale blue tint.
Chemical properties
Under normal conditions, alkanes are quite inactive. This is explained by the strength of the σ bonds between the C-C and C-H atoms. Therefore, it is necessary to provide special conditions (for example, fairly high temperature or light) to make the chemical reaction possible.
Substitution reactions
Reactions of this type include halogenation and nitration. Halogenation (reaction with Cl2 or Br2) occurs when heated or exposed to light. During the reaction, which proceeds sequentially, haloalkanes are formed.
For example, we can write down the reaction of chlorination of ethane.
Bromination will proceed in a similar manner.
Nitration is a reaction with a weak (10%) solution of HNO3 or with nitric oxide (IV) NO2. Conditions for carrying out reactions are temperature 140 °C and pressure.
C3H8 + HNO3 = C3H7NO2 + H2O.
As a result, two products are formed - water and amino acid.
Decomposition reactions
When carrying out decomposition reactions, high temperatures are always required. This is necessary to break the bonds between carbon and hydrogen atoms.
So, when cracking Temperatures between 700 and 1000 °C will be required. During the reaction, -C-C- bonds are destroyed, a new alkane and alkene are formed:
C8H18 = C4H10 + C4H8
The exception is the cracking of methane and ethane. As a result of these reactions, hydrogen is released and alkyne acetylene is formed. A prerequisite is heating to 1500 °C.
C2H4 = C2H2 + H2
If you exceed the temperature of 1000 °C, you can achieve pyrolysis with complete rupture of bonds in the compound:
During the pyrolysis of propyl, carbon C was produced and hydrogen H2 was also released.
Dehydrogenation reactions
Dehydrogenation (removal of hydrogen) occurs differently for different alkanes. The reaction conditions are a temperature ranging from 400 to 600 °C, as well as the presence of a catalyst, which can be nickel or platinum.
An alkene is formed from a compound with 2 or 3 C atoms in its carbon skeleton:
C2H6 = C2H4 + H2.
If there are 4-5 carbon atoms in the molecule chain, then after dehydrogenation you will get alkadiene and hydrogen.
C5H12 = C4H8 + 2H2.
Starting with hexane, the reaction produces benzene or its derivatives.
C6H14 = C6H6 + 4H2
Also worth mentioning is the conversion reaction carried out for methane at a temperature of 800 °C and in the presence of nickel:
CH4 + H2O = CO + 3H2
Conversion is not typical for other alkanes.
Oxidation and combustion
If an alkane, heated to a temperature of no more than 200 °C, reacts with oxygen in the presence of a catalyst, then, depending on other reaction conditions, the resulting products will differ: these can be representatives of the classes of aldehydes, carboxylic acids, alcohols or ketones.
In the case of complete oxidation, the alkane burns to the final products - water and CO2:
C9H20 + 14O2 = 9CO2 + 10H2O
If the amount of oxygen is insufficient during oxidation, the end product will be coal or CO instead of carbon dioxide.
Carrying out isomerization
If you provide a temperature of about 100-200 degrees, a rearrangement reaction becomes possible for unbranched alkanes. The second prerequisite for isomerization is the presence of an AlCl3 catalyst. In this case, the structure of the molecules of the substance changes and its isomer is formed.
Significant the share of alkanes is obtained by isolating them from natural raw materials. Most often, natural gas is processed, the main component of which is methane, or oil is subjected to cracking and rectification.
You should also remember the chemical properties of alkenes. In 10th grade, one of the first laboratory methods studied in chemistry lessons is the hydrogenation of unsaturated hydrocarbons.
C3H6 + H2 = C3H8
For example, as a result of the addition of hydrogen to propylene, a single product is obtained - propane.
Using the Wurtz reaction, alkanes are obtained from monohaloalkanes, in the structural chain of which the number of carbon atoms is doubled:
2CH4H9Br + 2Na = C8H18 + 2NaBr.
Another method of preparation is the interaction of a carboxylic acid salt with an alkali when heated:
C2H5COONa + NaOH = Na2CO3 + C2H6.
In addition, methane is sometimes produced in an electric arc (C + 2H2 = CH4) or by reacting aluminum carbide with water:
Al4C3 + 12H2O = 3CH4 + 4Al (OH)3.
Alkanes are widely used in industry as low-cost fuels. They are also used as raw materials for the synthesis of other organic substances. For this purpose, methane, which is necessary for synthesis gas, is usually used. Some other saturated hydrocarbons are used to produce synthetic fats, and also as a basis for lubricants.
For a better understanding of the topic “Alkanes”, more than one video lesson has been created, in which topics such as the structure of matter, isomers and nomenclature are discussed in detail, as well as the mechanisms of chemical reactions are shown.
The content of the article
ALKANES AND CYCLOALKANES– hydrocarbons in which all carbon atoms are connected to each other and to hydrogen atoms by simple (single) bonds. Alkanes (synonyms - saturated hydrocarbons, saturated hydrocarbons, paraffins) - hydrocarbons with the general formula C n H 2 n+2 , where n– number of carbon atoms. The familiar polyethylene has the same formula, only the size n it is very large and can reach tens of thousands. In addition, polyethylene contains molecules of different lengths. In cycloalkanes, the carbon atoms form a closed chain; if there is one cycle, the formula of the cycloalkane is C n H 2 n .
Depending on the order of connection of carbon atoms in a chain, alkanes are divided into linear and branched. Accordingly, for alkanes with nі 4 it is possible that there is more than one substance with the same formula. Such substances are called isomers (from the Greek. isis– equal, identical and meros – share, part.
Names of alkanes.
The word "alkane" is of the same origin as "alcohol" ( see below). The obsolete term "paraffin" comes from the Latin parum - little, insignificantly and affinis - related; paraffins have low reactivity with respect to most chemical reagents. Many paraffins are homologues; in the homologous series of alkanes, each subsequent member differs from the previous one by one methylene group CH 2. The term comes from the Greek homologos - corresponding, similar.
Nomenclature (from lat. nomenclature– painting of names) the names of alkanes are constructed according to certain rules, which are not always unambiguous. So, if there are various substituents in an alkane molecule, then in the name of the alkane they are listed in alphabetical order. However, this order may vary in different languages. For example, the hydrocarbon CH 3 –CH(CH 3)–CH(C 2 H 5)–CH 2 –CH 2 –CH 3 in accordance with this rule will be called 2-methyl-3-ethylhexane in Russian, and in English 3-ethyl-2-methylhexane…
In accordance with the name of the hydrocarbon, alkyl radicals are also called: methyl (CH 3 -), ethyl (C 2 H 5 -), isopropyl (CH 3) 2 CH-, tues-butyl C 2 H 5 –CH (CH 3)-, rubs-butyl (CH 3) 3 C-, etc. Alkyl radicals are included as a whole in the composition of many organic compounds; in the free state, these particles with an unpaired electron are extremely active.
Some isomers of alkanes also have trivial names ( cm. TRIVIAL SUBSTANCE NAMES, e.g. isobutane (2-methylpropane), isooctane (2,2,4-trimethylpentane), neopentane (2,3-dimethylpropane), squalane (2,6,10,15,19,23-hexamethyltetracosane) , whose name comes from the Latin squalus– shark (an unsaturated derivative of squalane – squalene, a compound important for metabolism, was first discovered in the liver of a shark). The trivial name for the pentyl radical (C 5 H 11) is amyl. It comes from the Greek. amylon– starch: once upon a time, isoamyl alcohol C 5 H 11 OH (3-methylbutanol-1) was called “amyl alcohol of fermentation”, since it forms the basis of fusel oil, and it is formed as a result of the fermentation of sugary substances - products of starch hydrolysis.
The simplest member of the cycloalkane series C n H 2 n– cyclopropane ( n= 3). Its homologues are called the same as alkanes with the addition of the prefix “cyclo” (cyclobutane, cyclopentane, etc.). In cycloalkanes, isomerism is possible due to the presence of side alkyl groups and their location in the ring. For example, cyclohexane, methylcyclopentane, 1,1-, 1,2- and 1,3-dimethylcyclobutanes, 1,1,2- and 1,2,3-trimethylcyclopropanes are isomeric.
The number of alkane isomers increases sharply with increasing number of carbon atoms. The names of some alkanes, as well as the theoretical number of their possible isomers, are given in the table.
| Formula | Name | Number of isomers | Formula | Name | Number of isomers |
| CH 4 | Methane | 1 | C 11 N 24 | Undekan | 159 |
| C 2 H 6 | Ethane | 1 | C 12 N 26 | Dodecan | 355 |
| C 3 H 8 | Propane | 1 | C 13 N 28 | Tridecan | 802 |
| C 4 H 10 | Butane | 2 | C 14 N 30 | Tetradecane | 1858 |
| C 5 H 12 | Pentane | 3 | C 15 N 32 | Pentadecane | 4347 |
| C 6 H 14 | Hexane | 5 | C 20 N 42 | Eikosan | 366319 |
| C 7 H 16 | Heptane | 9 | C 25 N 52 | Pentacosan | 36797588 |
| C 8 H 18 | Octane | 18 | C 30 N 62 | Triacontan | 4111846763 |
| C 9 H 20 | Nonan | 35 | C 40 N 82 | Tetracontan | 62481801147341 |
| C 10 H 22 | Dean | 75 | C 100 N 202 | Hectane | about 5.921 10 39 |
Understanding most of the nomenclature names of saturated hydrocarbons is not very difficult even for those who did not study Greek in a classical gymnasium. These names come from Greek numerals with the addition of the suffix -an. It is more difficult with the first members of the series: they do not use numerals, but other Greek roots associated with the names of the corresponding alcohols or acids. These alcohols and acids were known long before the discovery of the corresponding alkanes; an example is ethyl alcohol and ethane (obtained only in 1848).
Methane (as well as methanol, methyl, methylene, etc.) have a common root “met”, which in chemistry denotes a group containing one carbon atom: methyl CH 3, methylene (methylidene) CH 2, methine (methylidine) CH. Historically, the first such substance was methyl (also known as wood) alcohol, methanol, which was previously obtained by dry distillation of wood. Its name comes from the Greek words methy - to intoxicate wine and hile - forest (so to speak, “wood wine”). The most amazing thing here is that methane, amethyst and honey have a common root! In ancient times, precious stones were endowed with magical properties (and many still believe in this). Thus, it was believed that beautiful purple stones protected against intoxication, especially if a drinking cup was made from this stone. Together with the negative prefix it turned out amethystos - counteracting intoxication. The word honey, it turns out, is present in almost all European languages: English. mead - honey (as a drink), German Met (in Old German metu), Dutch mede, Swedish mjöd, Danish mjød, Lithuanian and Latvian medus, not to mention the Slavic languages. All these words, including the Greek, come from the Indo-European medhu, meaning sweet drink. The Greek brandy Metaxa is not far behind them, although it is not at all sweet.
Ethane (as well as ether, ethanol, alcohol, alkane) have a common origin. Ancient Greek philosophers used the word aither to describe a certain substance that permeates the cosmos. When alchemists in the 8th century. They obtained an easily evaporating liquid from wine alcohol and sulfuric acid, it was called sulfuric ether. In the 19th century found out that sulfuric ether (in English ether) belongs to the so-called ethers and contains a group of two carbon atoms - the same as ethyl alcohol (ethanol); this group was called ethyl. Thus, the name of the substance “ethyl ether” (C 2 H 5 –O–C 2 H 5) is essentially “oil oil”.
The name ethane comes from “ethyl”. One of the names for ethanol, alcohol, is of the same origin as the word alkane (also alkene, alkyne, alkyl). In Arabic, al-kohl means powder, powder, dust. At the slightest breath they rise into the air, just like wine vapors - the “alcohol of wine”, which over time simply turned into alcohol.
Why is there a “t” in “ethane” and “ethanol”, and “f” in “ether”? After all, in English, unlike Russian, the words “ether” and “ethyl” have similar spellings and sounds. The combination th goes back to the Greek letter q (theta); in the Russian language until 1918, the letter “fita” had the same style, which, however, was pronounced as “f” and was used for the sole purpose of distinguishing words in which this letter comes from the Greek q and 247 (“fi”). In Western European languages, Greek. j went to ph, and q to th. In the Russian language, many words contain “fita” back in the 18th century. was replaced by the letter “f”: theater instead of “qeaftr”, mathematics instead of “maqematics”, theory instead of “qeory”... In this regard, it is interesting that in Dahl’s dictionary, published in 1882, it is written eqir, and in the encyclopedic dictionary of Brockhaus and Efron (1904) – “ether”.
By the way, esters in Western languages are ester, not ether. But the word “ester” does not exist in the Russian language, so any chemist’s eyes are hurt by the illiterate translation of the English polyester on the labels of textile products as “polyester” instead of “polyester”, “polyester fiber” (polyesters include, for example, lavsan, terylene, dacron).
The names “propane” and “butane” come from the names of the corresponding acids – propionic and butanoic (butyric). Propionic acid is the “first” (i.e. shortest chain) found in fats ( cm. FATS AND OILS), and its name is derived from the Greek. protos- first and pion– fat. Butane and butanoic acid butyric acid) – from Greek. butyron- oil; In Russian, butyrates are salts and esters of butyric acid. This acid is released when oil goes rancid.
Further, starting with pentane, the names are derived from Greek numerals. A rare exception is cetane, one of the names for C16 hexadecane. This word comes from the name cetyl alcohol, which was obtained in 1823 by the French chemist Michel Eugene Chevreul. Chevreul isolated this substance from spermaceti, a waxy substance from the head of the sperm whale. The word spermaceti comes from the Greek sperma - seed and ketos - large sea animal (whale, dolphin). From the Latin spelling of the second word (cetus) comes cetyl alcohol C 16 H 33 OH (hexadecanol) and cetane.
In the Russian language there are many words with the same roots as alkanes: Pentagon, heptachord (sound scale of 7 steps), dodecaphony (method of musical composition), octave, decima and undecima (musical intervals), octet and nonet (ensembles of 8 and 9 musicians), pentode, hexode and heptode (radio tubes); hexameter (poetic meter), octahedron, decade, decan, hectare, October, December, etc. and so on.
The alkane with the longest molecules was synthesized by English chemists in 1985. This is nonacontatrictan C 390 H 782, containing a chain of 390 carbon atoms. The researchers were interested in how such long chains would pack during crystallization (flexible hydrocarbon chains can fold easily).
Number of isomers of alkanes.
The problem of the theoretically possible number of isomers of alkanes was first solved by the English mathematician Arthur Cayley (1821–1895), one of the founders of an important branch of mathematics - topology (in 1879 he published the first article on the famous “problem of four colors”: are there enough of them to color any geographical cards; this problem was solved only in 1976). It turned out that there is no formula by which one can use the number of carbon atoms in a C alkane n H 2 n+2 calculate the number of its isomers. There are only so-called recurrent formulas (from the Latin recurrences– returning), which allow you to calculate the number of isomers n th member of the series, if the number of isomers of the previous member is already known. Therefore, calculations for large n were obtained relatively recently using computers and reduced to hydrocarbon C 400 H 802; for it, taking into account spatial isomers, a value was obtained that is difficult to imagine: 4.776·10 199. And starting from the alkane C 167 H 336, the number of isomers exceeds the number of elementary particles in the visible part of the Universe, which is estimated as 10 80. The number of isomers indicated in the table for most alkanes will increase significantly if we also consider mirror-symmetrical molecules - stereoisomers ( cm. OPTICAL ISOMERISM): for heptane - from 9 to 11, for decane - from 75 to 136, for eicosane - from 366,319 to 3,396,844, for hectane - from 5.921 10 39 to 1.373 10 46, etc.
From the point of view of a chemist, the number of structural isomers of saturated hydrocarbons is of practical interest only for the first members of the series. Even for a relatively simple alkane containing only one and a half dozen carbon atoms, the overwhelming number of isomers have not been obtained and are unlikely to ever be synthesized. For example, the last of the theoretically possible 75 isomers of decane were synthesized only in 1968. And this was done for practical purposes - to have a more complete set of standard compounds by which various hydrocarbons, for example, those found in oil, can be identified. By the way, all 18 possible octane isomers have been found in various types of oil.
But the most interesting thing is that, starting with heptadecane C 17 H 36, at first only some of the theoretically possible number of isomers, then many, and finally almost all are a striking example of “paper chemistry”, i.e. cannot exist in reality. The fact is that as the number of carbon atoms in the molecules of branched isomers increases, serious problems of spatial packing arise. After all, mathematicians treated carbon and hydrogen atoms as points, when in fact they have a finite radius. Thus, a methane “ball” has 4 hydrogen atoms on its “surface”, which are freely placed on it. In neopentane C(CH 3) 4 there are already 12 hydrogen atoms on the “surface”, located much closer to each other; but there is still room for them to be placed. But for alkane 4 (C 17 H 36), there is not enough space on the surface to accommodate all 36 hydrogen atoms in 12 methyl groups; This is easy to check if you draw a flat image (or, even better, make a three-dimensional model from plasticine and matches) for similar isomers, maintaining the constancy of the lengths of the C–C and C–H bonds and all angles between them). With growth n placement problems also arise for carbon atoms. As a result, despite the fact that the number of possible isomers with increasing n increases very quickly, the share of “paper” isomers grows much faster. A computer-based assessment showed that as n the ratio of the number of truly possible isomers to the number of “paper” ones quickly approaches zero. That is why the calculation of the exact number of isomers of saturated hydrocarbons for large n, which once aroused considerable interest, now has only theoretical significance for chemists.
Structure and physical properties of alkanes.
Alkanes have four sp 3 hybrid orbitals of the carbon atom ( cm. ORBITALS) are directed towards the vertices of the tetrahedron with an angle between them of about 109°28" - it is in this case that the repulsion between electrons and the energy of the system are minimal. As a result of the overlap of these orbitals with each other, as well as with s-orbitals of hydrogen atoms form s-bonds C–C and C–H. These bonds in alkane molecules are covalent non-polar or low-polar.
Alkanes are divided into primary carbon atoms (they are bonded to only one neighboring C atom), secondary (bonded to two C atoms), tertiary (bonded to three C atoms) and quaternary (bonded to four C atoms). Thus, in 2,2-dimethyl-3-methylpentane CH 3 –C(CH 3) 2 –CH(CH 3) – CH 2 –CH 3 there is one quaternary, one tertiary, one secondary and five primary carbon atoms. The different environments of carbon atoms greatly affect the reactivity of the hydrogen atoms associated with them.
The spatial arrangement of sp 3 orbitals leads, starting from propane, to a zigzag configuration of carbon chains. In this case, rotation of molecular fragments around C–C bonds is possible (in an ethane molecule at 20 ° C - at a speed of millions of revolutions per second!), which makes the molecules of higher alkanes flexible. Straightening of such chains occurs, for example, when stretching polyethylene, which consists of a mixture of alkanes with long chains. Alkane molecules interact weakly with each other, therefore alkanes melt and boil at much lower temperatures than similar substances with polar molecules. The first 4 members of the homologous series of methane are gases under normal conditions; propane and butane are easily liquefied under low pressure (a liquid propane-butane mixture is contained in household gas cylinders). Higher homologues are liquids with the smell of gasoline or solids that are insoluble in water and float on its surface. The melting and boiling points of alkanes increase with increasing number of carbon atoms in the molecule, while the temperature increase gradually slows down, for example, C 100 H 202 melts at 115 ° C, C 150 H 302 - at 123 ° C. Melting and boiling points for the first 25 alkanes are given in the table - it is clear that starting from octadecane, alkanes are solids.
| Table. MELTING AND BOILING TEMPERATURES OF ALKANES | ||
| Alkane | T pl | T bale |
| Methane | –182,5 | –161,5 |
| Ethane | –183,3 | –88,6 |
| Propane | –187,7 | –42,1 |
| Butane | –138,4 | –0,5 |
| Pentane | –129,7 | 36,1 |
| Hexane | –95,3 | 68,7 |
| Heptane | –90,6 | 98,4 |
| Octane | –56,8 | 125,7 |
| Nonan | –51,0 | 150,8 |
| Dean | –29,7 | 174,1 |
| Undekan | –25,6 | 195,9 |
| Dodecan | –9,6 | 216,3 |
| Tridecan | –5,5 | 235,4 |
| Tetradecane | +5,9 | 253,7 |
| Pentadecane | +9,9 | 270,6 |
| Hexadecane | 18,2 | 286,8 |
| Heptadecane | 22,0 | 301,9 |
| Octadecan | 28,2 | 316,1 |
| Nonadecane | 32,1 | 329,7 |
| Eikosan | 36,8 | 342,7 |
| Geneikozan | 40,5 | 356,5 |
| Docozan | 44,4 | 368,6 |
| Tricozan | 47,6 | 378,3 |
| Tetracosane | 50,9 | 389,2 |
| Pentacosan | 53,7 | 399,7 |
The presence of a branch in the chain dramatically changes the physical properties, especially the melting point. So, if hexane has a normal structure ( n-hexane) melts at –95.3° C, then its isomeric 2-methylpentane melts at –153.7° C. This is due to the difficulty of packing branched molecules during their crystallization. As a result, alkanes with chain branches do not crystallize upon rapid cooling, but transform into the glassy state of a supercooled liquid ( cm. GLASS). For example, if a thin ampoule of pentane is immersed in liquid nitrogen (temperature -196 ° C), the substance will turn into a white snow-like mass, while isopentane (2-methylbutane) solidifies into a transparent “glass”.
An original method of separating them is based on the difference in geometric shape of linear and branched alkanes: urea crystals have channels in which straight-chain alkanes can fit, but branched ones cannot.
Cycloalkanes with n= 2, 3 – gases, higher – liquids or solids. The largest cycle that chemists have been able to synthesize is cyclooctaoctacontadictane C 288 H 576. The different shapes of cycloalkane molecules with even and odd numbers of carbon atoms in the molecule lead to a strong even-odd effect regarding the melting point, as can be seen from the table. This effect is explained by the difference in the “convenience” of packing molecules of different shapes in a crystal: the more compact the packing, the stronger the crystal and the higher its melting point. For example, cyclododecane melts almost 70° higher than its closest homologue, cycloundecane. Of course, the mass of the molecule also matters: light molecules melt at a lower temperature.
| C 3 H 6 | –127,5 |
| C 4 H 8 | –50 |
| C 5 H 10 | –93,9 |
| C 6 H 12 | +6,5 |
| C 7 H 14 | –12 |
| C 8 H 16 | 14,3 |
| S 9 H 18 | 9,7 |
| C 10 N 20 | 10,8 |
| C 11 H 22 | –7,2 |
| C 12 H 24 | 61,6 |
| C 13 N 26 | 23,5 |
| C 14 N 28 | 54 |
| C 15 N 30 | 62,1 |
The ease of rotation around the C–C bond leads to the fact that the molecules of cycloalkanes are not planar (with the exception of cyclopropane), in this way they avoid strong distortion of bond angles. Thus, in cyclohexane and its higher homologues the bond angles are relaxed and close to tetrahedral (109°), while in a hexagon the angles are 120°, in an octagon - 135°, etc. Individual carbon atoms in such cycloalkanes do not occupy a rigidly fixed position: the ring seems to be in constant wave-like motion. Thus, a cyclohexane molecule can be in the form of different geometric structures (conformers) that can transform into each other (cycle inversion). Due to their external similarity, they were called “bathtub” and “chair” (in English literature, a “bathtub” is called a “boat”):
The shape of the chair is more stable; At ordinary temperatures, 99.9% of cyclohexane exists in the more stable chair form. The transition between two forms occurs through an intermediate “twist conformation” (from the English. twist– twist).
In cyclopropane, the angle decreases from 108° to 60°, resulting in high tension and "bent" bonds that are intermediate between normal s- and p-bonds; Due to their shape, these bonds are called “banana” bonds. In this case, the sp 3 orbitals of the carbon atoms overlap only partially. The result is duality in the chemical properties of cyclopropane. On the one hand, substitution of hydrogen atoms is possible in it (a reaction typical of alkanes), on the other hand, addition with ring opening is possible (reaction typical of alkenes, for example: cyclo-C 3 H 6 + Br 2 ® BrCH 2 CH 2 CH 2 Br).
Cycloalkanes with two rings and one common carbon atom are called spiroalkanes. If there are more than two common carbon atoms, then bicycloalkanes, tricycloalkanes, etc. are formed. As a result of such “cross-linking” of several cycles at once, chemists managed to obtain hydrocarbons, the spatial structure of which corresponds to various polyhedra: tetrahedron, cube, prism, etc. Bicyclic derivatives cyclohexane are found in essential oils, coniferous resin, and turpentine. A cycle of six and five carbon atoms is found in camphor, cholesterol, saccharin, piperine (it gives the hot taste to black pepper), nitrogenous bases - nucleotides, and other compounds (some carbon atoms in the cycles can be connected by double bonds, and some are substituted other atoms, such as in saccharin). A cycle of 17 carbon atoms (two of them connected by a double bond) is contained in civeton, an odorous substance, a component of musk, which is used in perfumery. The beautiful adamantane molecule contains three six-membered rings and its structure corresponds to the crystal lattice of diamond. The adamantane structure is found in the antiviral drug rimantadine, in hexamethylenetetramine (in the latter compound, 4 carbon atoms are replaced by nitrogen atoms, which are connected to each other by methylene bridges – CH 2 –). Below are the structures of some cycloalkanes, the molecules of which have more than one differently connected ring.
Bicyclodecane (tetrahydronaphthalene, decalin)
Adamantane
Chemical properties of alkanes.
Alkanes are the least chemically active organic compounds. All C–C and C–H bonds in alkanes are single, so alkanes are incapable of addition reactions. Alkanes are characterized by reactions of replacement of hydrogen atoms with other atoms and groups of atoms. Thus, when methane is chlorinated, methyl chloride CH 3 Cl, methylene chloride CH 2 Cl 2, trichloromethane (chloroform) CHCl 3 and carbon tetrachloride (carbon tetrachloride) CCl 4 are formed. These reactions follow a chain mechanism with the intermediate formation of free radicals.
When chlorinating alkanes, starting with propane, the very first chlorine atom can replace various hydrogen atoms. The direction of substitution depends on the strength of the C–H bond: the weaker it is, the faster the substitution of this particular atom. Primary C–H bonds are usually stronger than secondary ones, and secondary ones are stronger than tertiary ones. As a result, chlorination at 25° C along the secondary bond (CH 3) 2 CH–H occurs 4.5 times faster than through the primary bond C 2 H 5 –H, and the tertiary bond (CH 3) 3 C–H – at 6.7 times faster. The different reactivity of primary, secondary and tertiary hydrogen atoms can result in only one of several possible chlorination products being predominant. For example, when 2,3-dimethylbutane is chlorinated in a solution of carbon disulfide (CS 2), 95% of the 2-chloro derivative and only 5% of the 1-chloro derivative are formed, i.e. 19 times less. If we take into account that in the original alkane there are 6 times more primary hydrogen atoms than tertiary ones, then the ratio of their reactivity will be even greater (19 ґ 6 = 114). Carbon disulfide as a solvent reduces the reactivity of chlorine atoms and accordingly increases its selectivity. Lowering the temperature works the same way.
Bromine atoms are less active; The noticeable activation energy of this reaction leads to the fact that the bromination of alkanes, although it occurs by a chain mechanism, is much slower than chlorination, and only at elevated temperatures or in the light. The lower activity of bromine atoms also leads to increased selectivity of bromination. Thus, if the relative rate of photochemical bromination of ethane at 40°C is taken equal to 1, then the rate of bromination of propane (at the secondary H atom) will be already 220 under the same conditions, and the rate of bromination of isobutane (at the tertiary H atom) will be 19,000
Iodine atoms are the least active, therefore the reaction of iodination of alkanes RH + I 2 ® RI + HI is endothermic, possible only at high temperatures and occurs with very short chains. Moreover, the reverse exothermic reaction RI + HI ® RH + I 2 occurs very easily. When alkanes are iodinated, unsaturated compounds are also formed. For example, at 685° C, ethane, reacting with iodine, forms 72% ethylene and 10% acetylene. The same results were obtained with propane, butane and pentane.
The fluorination reaction of alkanes proceeds at a very high, often explosive, rate with the formation of all possible polyfluorinated derivatives of the original alkane. The energy released during the fluorination of alkanes is so great that it can lead to the breakdown of product molecules into radicals that begin new chains. As a result, the reaction rate increases like an avalanche and this leads to an explosion even at low temperatures. The peculiarity of the fluorination of alkanes is the possibility of destruction of the carbon skeleton by fluorine atoms with the formation of CF 4 as the final product with other halogens, such a reaction does not occur.
Nitration of alkanes (Konovalov’s reaction) also follows a radical mechanism: RH + NO 2 ® R· + HNO 2, R· + NO 2 ® RNO 2. The source of NO 2 is nitric acid, which decomposes when heated. The reaction is carried out in solution at temperatures above 150° C or in vapor under pressure up to 10 atm and a temperature of 400 – 500° C. In the latter case, C–C bonds in alkanes are also broken and a mixture of nitroalkanes is formed.
All alkanes burn by releasing heat, for example: C 5 H 12 + 8O 2 ® 5CO 2 + 6H 2 O. This reaction occurs, in particular, in the cylinders of internal combustion engines. To prevent the remains of unburned alkanes from entering the atmosphere, their catalytic afterburning is used in exhaust pipes (at the same time, CO is burned and nitrogen oxides are converted into harmless nitrogen). The reaction of oxygen with higher alkanes (in paraffin) occurs when a candle burns. Gaseous alkanes, such as methane, form explosive mixtures with air. Such mixtures can form in mines, as well as in residential buildings due to a leak of household gas if its content in the air reaches 5%.
Significant efforts of chemists were aimed at a detailed study of the reaction of low-temperature oxidation of alkanes in order to stop it at the stage of formation of valuable intermediate products - aldehydes, ketones, alcohols, carboxylic acids. Thus, in the presence of Co(II) and Mn(II) salts, butane can be oxidized to acetic acid, and paraffin to C12–C18 fatty acids. The oxidation of cyclohexane produces caprolactam, a monomer for the production of caprone, and adipic acid.
An important industrial reaction is the photochemical sulfochlorination of alkanes: a joint radical chain reaction with Cl 2 and SO 2 with the formation of alkanesulfonic acid chlorides RSO 2 Cl. This reaction is widely used in the production of detergents. When replacing chlorine with oxygen, a chain radical reaction of sulfonic oxidation of alkanes occurs with the formation of alkanesulfonic acids R–SO 2 –OH. Sodium salts of these acids are used as detergents and emulsifying agents.
At high temperatures, decomposition (pyrolysis) of alkanes occurs, for example: CH 4 ® C + 2H 2 (1000° C), 2CH 4 ® C 2 H 2 + 3H 2 (1500° C), C 2 H 6 ® C 2 H 4 +H2. The last reaction occurs at 500° C in the presence of a catalyst (Ni). Similarly, 2-butene CH 3 CH = CHCH 3 can be obtained from butane, at the same time a mixture of ethylene and ethane is formed. In contrast to this radical reaction, catalytic cracking of alkanes proceeds through an ionic mechanism and serves to produce gasoline from heavier petroleum fractions. When heated in the presence of Lewis acids, for example, AlCl 3, isomerization occurs: unbranched (normal) alkanes are converted into branched ones with the same number of carbon atoms. This reaction is of great practical importance for obtaining high-quality motor fuel ( cm. OCTANE NUMBER). Dehydrogenation of alkanes can be accompanied by ring closure (dehydrocyclization). In the case of hexane dehydrocyclization, the main product is benzene.
Methane at high temperature in the presence of a catalyst reacts with water vapor and carbon monoxide (IV) to form synthesis gas: CH 4 + H 2 O ® CO + 3H 2, CH 4 + CO 2 ® 2CO + 2H 2. Synthesis gas is used to produce motor fuels and methyl alcohol.
In recent years, the efforts of chemists have been aimed at creating catalysts that activate C–H bonds in alkane molecules under mild conditions. Some microorganisms “can” carry out such reactions, the enzymes of which are able to “digest” even paraffin with the formation of protein compounds. The task of chemists is to understand how natural catalysts work and to model enzymatic reactions that can occur at ordinary temperatures. In this case, various organometallic compounds are used as catalysts. For example, in the presence of some platinum compounds, methanol CH 3 OH can be obtained directly from methane, and in the presence of a triphenylphosphine complex of rhodium Rh[(C 6 H 5) 3 P] associated with CO molecules; During the reaction, CO molecules are introduced into the C–H bonds of alkanes to form aldehydes.
Cycloalkanes have chemical properties similar to alkanes. Thus, they are flammable, can be halogenated by a radical mechanism, and at elevated temperatures in the presence of catalysts they are dehydrogenated - they split off hydrogen and turn into unsaturated hydrocarbons. As mentioned, cyclopropane has special properties. Unlike alkanes, cycloalkanes are hydrogenated, when the ring opens and alkanes are formed, for example: cyclo-C 3 H 6 + H 2 ® C 3 H 8 (the reaction occurs when heated in the presence of a platinum catalyst). As the size of the cycle increases, the reaction becomes more difficult - thus, cyclopentane is already hydrogenated (to pentane) with great difficulty and at high temperature (300 ° C).
Being in nature and receiving.
The main sources of alkanes are oil and natural gas. Methane makes up the bulk of natural gas; it also contains small amounts of ethane, propane and butane. Methane is found in emissions from swamps and coal seams. Along with light homologues, methane is present in associated petroleum gases. These gases are dissolved in oil under pressure and are also located above it. Alkanes make up a significant portion of petroleum products. Oil also contains cycloalkanes - they are called naphthenes (from the Greek. naphtha- oil). Gas hydrates of alkanes, mainly methane, are also widespread in nature; they occur in sedimentary rocks on continents and at the bottom of the oceans. Their reserves probably exceed the known reserves of natural gas and in the future may become a source of methane and its closest homologues.
Alkanes are also obtained by pyrolysis (coking) of coal and its hydrogenation (production of synthetic liquid fuel). Solid alkanes are found in nature in the form of deposits of mountain wax - ozokerite, in the waxy coatings of leaves, flowers and plant seeds, and are part of beeswax.
In industry, alkanes are produced by the catalytic hydrogenation of carbon oxides CO and CO 2 (Fischer–Tropsch method). In the laboratory, methane can be obtained by heating sodium acetate with a solid alkali: CH 3 COONa + NaOH ® CH 4 + Na 2 CO 3 , as well as by hydrolysis of some carbides: Al 4 C 3 + 12H 2 O ® 3CH 4 + 4Al(OH) 3 . Homologues of methane can be obtained by the Wurtz reaction, for example: 2CH 3 Br + 2Na ® CH 3 –CH 3 + 2NaBr. In the case of dihaloalkanes, cycloalkanes are obtained, for example: Br–CH 2 –(CH 2) 4 –CH 2 Br + 2Na ® cyclo-C 6 H 12 + 2NaBr. Alkanes are also formed during decarboxylation of carboxylic acids and during their electrolysis.
Application of alkanes.
Alkanes in gasoline, kerosene, diesel oil, and fuel oil are used as fuel. Higher alkanes are found in lubricating oils, petroleum jelly and paraffin. A mixture of isomeric pentanes and hexanes is called petroleum ether and is used as a solvent. Cyclohexane is also widely used as a solvent and for the synthesis of polymers (nylon, nylon). Cyclopropane is used for anesthesia. Squalane is a high-quality lubricating oil, a component of pharmaceutical and cosmetic preparations, and an adsorbent in gas-liquid chromatography.
Alkanes serve as raw materials for the production of many organic compounds, including alcohols, aldehydes, and acids. Chlorine derivatives of alkanes are used as solvents, for example, trichloromethane (chloroform) CHCl 3, carbon tetrachloride CCl 4. A mixture of higher alkanes - paraffin is non-toxic and is widely used in the food industry for impregnation of containers and packaging materials (for example, milk cartons), and in the production of chewing gum. Pencils and the upper (near the head) part of matches are impregnated with paraffin for better burning. Heated paraffin is used for medicinal purposes (paraffin therapy). Oxidation of paraffin under controlled conditions in the presence of catalysts (organic transition metal salts) leads to the production of oxygen-containing products, mainly organic acids.
Ilya Leenson
Literature:
Petrov A.A. Chemistry of alkanes. M., Nauka, 1974
Azerbaev I.N. and etc. Syntheses based on petroleum hydrocarbons. Alma-Ata, Science, 1974
Rudakov E.S. Reactions of alkanes with oxidizing agents, metal complexes and radicals in solutions. Kyiv, Naukova Dumka, 1985
Parauşanu V. Production and use of hydrocarbons. M., Chemistry, 1987
The table shows some representatives of a number of alkanes and their radicals.
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Formula |
Name |
Radical name |
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CH3 methyl |
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C3H7 cut |
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C4H9 butyl |
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isobutane |
isobutyl |
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isopentane |
isopentyl |
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neopentane |
neopentyl |
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The table shows that these hydrocarbons differ from each other in the number of groups - CH2 -. Such a series of similar structures, having similar chemical properties and differing from each other in the number of these groups is called a homologous series. And the substances that make it up are called homologues. Homologues - substances similar in structure and properties, but differing in composition by one or more homologous differences (- CH2 -)
Carbon chain - zigzag (if n ≥ 3) σ - bonds (free rotation around bonds) length (-C-C-) 0.154 nm binding energy (-C-C-) 348 kJ/mol All carbon atoms in alkane molecules are in a state of sp3 hybridization
the angle between the C-C bonds is 109°28", so the molecules of normal alkanes with a large number of carbon atoms have a zigzag structure (zigzag). The length of the C-C bond in saturated hydrocarbons is 0.154 nm (1 nm = 1*10-9 m). a) electronic and structural formulas; b) spatial structure
4. Isomerism- STRUCTURAL isomerism of the chain with C4 is characteristic One of these isomers ( n-butane) contains an unbranched carbon chain, and the other, isobutane, contains a branched one (isostructure). The carbon atoms in a branched chain differ in the type of connection with other carbon atoms. Thus, a carbon atom bonded to only one other carbon atom is called primary, with two other carbon atoms - secondary, with three - tertiary, with four - quaternary. With an increase in the number of carbon atoms in the molecules, the possibilities for chain branching increase, i.e. the number of isomers increases with the number of carbon atoms. Comparative characteristics of homologues and isomers
1. They have their own nomenclature radicals(hydrocarbon radicals)
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The use of alkanes is quite diverse - they are used as fuel, as well as in mechanics, medicine, etc. The role of these chemical compounds in the life of modern man can hardly be overestimated.
Alkanes: properties and brief description
Alkanes are non-cyclic carbon compounds in which the carbon atoms are linked by simple saturated bonds. These substances represent a whole range with certain properties and characteristics. as follows:
N here represents the number of carbon atoms. For example, CH3, C2H6.
The first four representatives of the alkanes series are gaseous substances: methane, ethane, propane and butane. The following compounds (C5 to C17) are liquids. The series continues with compounds that are solids under normal conditions.
As for chemical properties, alkanes are low-active - they practically do not interact with alkalis and acids. By the way, it is the chemical properties that determine the use of alkanes.
However, these compounds are characterized by certain reactions, including the replacement of hydrogen atoms, as well as molecular splitting processes.
- The most characteristic reaction is halogenation, in which hydrogen atoms are replaced by halogens. The chlorination and bromination reactions of these compounds are of great importance.
- Nitration is the replacement of a hydrogen atom with a nitro group during a reaction with a dilute (10% concentration) Under normal conditions, alkanes do not react with acids. In order to carry out such a reaction, a temperature of 140 °C is needed.
- Oxidation - under normal conditions, alkanes are not affected by oxygen. However, after ignition in air, these substances enter into the final products of which are water and
- Cracking - this reaction occurs only in the presence of the necessary catalysts. The process involves the cleavage of stable homologous bonds between carbon atoms. For example, when butane is cracked, the reaction can produce ethane and ethylene.
- Isomerization - as a result of the action of certain catalysts, some rearrangement of the carbon skeleton of the alkane is possible.
Applications of alkanes
The main natural source of these substances are such valuable products as natural gas and oil. The areas of application of alkanes today are very wide and varied.
For example, gaseous substances used as a valuable source of fuel. An example is methane, which natural gas is made of, as well as a propane-butane mixture.
Another source of alkanes is oil , the importance of which for modern humanity is difficult to overestimate. Petroleum products include:
- gasoline - used as fuel;
- kerosene;
- diesel fuel, or light gas oil;
- heavy gas oil, which is used as a lubricating oil;
- the remains are used to make asphalt.
Petroleum products are also used to produce plastics, synthetic fibers, rubbers and some detergents.
Vaseline and petroleum jelly are products that consist of a mixture of alkanes. They are used in medicine and cosmetology (mainly for the preparation of ointments and creams), as well as in perfumery.
Paraffin is another well-known product, which is a mixture of solid alkanes. This is a solid white mass, the heating temperature of which is 50 - 70 degrees. In modern production, paraffin is used to make candles. Matches are impregnated with the same substance. In medicine, various thermal procedures are performed using paraffin.
Physical properties. Under normal conditions, the first four members of the homologous series of alkanes (C 1 - C 4) are gases. Normal alkanes from pentane to heptadecane ( C 5 - C 17 ) - liquids, starting from C 18 and above - solids. As the number of carbon atoms in the chain increases, i.e. As the relative molecular weight increases, the boiling and melting points of alkanes increase. With the same number of carbon atoms in the molecule, branched alkanes have lower boiling points than normal alkanes.
Alkanespractically insoluble in water, since their molecules are slightly polar and do not interact with water molecules, they dissolve well in non-polar organic solvents such as benzene, carbon tetrachloride, etc. Liquid alkanes are easily mixed with each other.
The main natural sources of alkanes are oil and natural gas. Various petroleum fractions contain alkanes from C5H12 to C 30 H 62. Natural gas consists of methane (95%) with an admixture of ethane and propane.
From synthetic methods for obtaining alkanes The following can be distinguished:/>
1 . Obtained from unsaturated hydrocarbons. The interaction of alkenes or alkynes with hydrogen (“hydrogenation”) occurs in the presence of metal catalysts (/>Ni, Pd ) at
heating:
CH z - C ≡CH+ 2H 2 → CH 3 -CH 2 -CH 3.
2. Receiving from halogen-conducted. When monohalogenated alkanes are heated with sodium metal, alkanes with double the number of carbon atoms are obtained (Wurtz reaction):
C 2 H 5 Br + 2 Na + Br - C 2 H 5 → C 2 H 5 - C 2 H 5 + 2 NaBr.
A similar reaction is not carried out with two different halogenated alkanes, since this produces a mixture of three different alkanes
3. Preparation from salts of carboxylic acids. When anhydrous salts of carboxylic acids are fused with alkalis, alkanes are obtained containing one less carbon atom compared to the carbon chain of the original carboxylic acids:
4.Methane production. An electric arc burning in a hydrogen atmosphere produces a significant amount of methane:
C + 2H 2 → CH 4 .
The same reaction occurs when carbon is heated in a hydrogen atmosphere to 400-500 °C at elevated pressure in the presence of a catalyst.
In laboratory conditions, methane is often obtained from aluminum carbide:
A l 4 C 3 + 12H 2 O = ZSN 4 + 4A l (OH) 3 .
Chemical properties. Under normal conditions, alkanes are chemically inert. They are resistant to the action of many reagents: they do not interact with concentrated sulfuric and nitric acids, with concentrated and molten alkalis, and are not oxidized by strong oxidizing agents - potassium permanganateKMn O 4, etc.
The chemical stability of alkanes is explained by their high strengths—C-C and C-H bonds, as well as their non-polarity. Non-polar C-C and C-H bonds in alkanes are not prone to ionic cleavage, but are capable of homolytic cleavage under the influence of active free radicals. Therefore, alkanes are characterized by radical reactions, which result in compounds where hydrogen atoms are replaced by other atoms or groups of atoms. Consequently, alkanes enter into reactions that proceed through the mechanism of radical substitution, denoted by the symbol S R ( from English, substitution radicalic). According to this mechanism, hydrogen atoms are most easily replaced at tertiary, then at secondary and primary carbon atoms.
1. Halogenation. When alkanes interact with halogens (chlorine and bromine) under the influence of UV radiation or high temperature, a mixture of products from mono- to polyhalogen-substituted alkanes The general scheme of this reaction is shown using methane as an example:
b) Growth of the chain. The chlorine radical removes a hydrogen atom from the alkane molecule:
Cl· + CH 4 →HC/>l + CH 3 ·
In this case, an alkyl radical is formed, which removes a chlorine atom from the chlorine molecule:
CH 3 + C l 2 →CH 3 C l + C l·
These reactions are repeated until the chain breaks in one of the reactions:
Cl· + Cl· → C l/> 2, CH 3 + CH 3 · → C 2 H 6, CH 3 + Cl· → CH 3 С l ·
Overall reaction equation:
| hv | ||
| CH 4 + Cl 2 | → | CH 3 Cl + HCl. |
The resulting chloromethane can be further chlorinated, giving a mixture of products CH 2 Cl 2, CHCl 3, CC l 4 according to the scheme (*).
Development of chain theory free radical reactions is closely connected with the name of the outstanding Russian scientist, Nobel Prize laureate N.I. Semenov (1896-1986).
2. Nitration (Konovalov reaction). When dilute nitric acid acts on alkanes at 140°C and low pressure, a radical reaction occurs:
In radical reactions (halogenation, nitration), hydrogen atoms at tertiary carbon atoms are first mixed, then at secondary and primary carbon atoms.This is explained by the fact that the bond between the tertiary carbon atom and hydrogen is most easily broken homolytically (bond energy 376 kJ/mol), then the secondary one (390 kJ/mol), and only then the primary one (415 kJ/mol).
3. Isomerization. Normal alkanes can, under certain conditions, transform into branched-chain alkanes:
4. Cracking is a hemolytic cleavage of C-C bonds, which occurs when heated and under the influence of catalysts.
When higher alkanes are cracked, alkenes and lower alkanes are formed; when methane and ethane are cracked, acetylene is formed:/>
C/> 8 H 18 → C 4 H 10 + C 4 H 8 ,/>
2CH 4 → C 2 H 2 + ZN 2,
C 2 H 6 → C 2 H 2 + 2H 2.
These reactions are of great industrial importance. In this way, high-boiling oil fractions (fuel oil) are converted into gasoline, kerosene and other valuable products.
5. Oxidation. By mild oxidation of methane with atmospheric oxygen in the presence of various catalysts, methyl alcohol, formaldehyde, and formic acid can be obtained:
 |
Mild catalytic oxidation of butane with atmospheric oxygen is one of the industrial methods for producing acetic acid:
t°
2 C 4/>H/>10 + 5 O/>2 → 4 CH/>3 COOH/>+ 2H 2 O .
cat
Alkanes in air burn to CO 2 and H 2 O:/>
С n Н 2 n +2 + (З n+1)/2O 2 = n CO 2 + (n +1) H 2 O.
