Polyhydric alcohols are organic compounds, in one molecule of which there are several hydroxyl groups. The simplest representative of this group of chemical compounds is diatomic, or -1.2.

Physical properties

These properties largely depend on the structure of the hydrocarbon radical of alcohol, the number of hydroxyl groups, and their position. Thus, the first representatives of the homologous series are liquids, and the higher ones are solids.

If monohydric alcohols are easily miscible with water, then in polyatomic alcohols this process occurs more slowly and with increasing molecular weight substances gradually disappear. Due to the stronger association of molecules in such substances, and hence the emergence of fairly strong hydrogen bonds, the boiling point of alcohols is high. Dissociation into ions proceeds to such a small extent that alcohols give a neutral reaction - the color or phenolphthalein does not change.

Chemical properties

The chemical properties of these alcohols are similar to those of monoatomic ones, that is, they enter into reactions of nucleophilic substitution, dehydration and oxidation to aldehydes or ketones. The latter is excluded for trihydric alcohols, the oxidation of which is accompanied by the destruction of the hydrocarbon skeleton.

A qualitative reaction to polyhydric alcohols is carried out with copper (II) hydroxide. When an indicator is added to alcohol, a bright blue chelate complex precipitates.

Methods for obtaining polyhydric alcohols

The synthesis of these substances is possible by the reduction of monosaccharides, as well as the condensation of aldehydes with in an alkaline medium. Often I get polyhydric alcohols from natural raw materials - rowan fruits.

The most common polyhydric alcohol - glycerin - is obtained by, and with the introduction of new technologies in the chemical industry - synthetically from propylene, which is formed during the cracking of petroleum products.

The use of polyhydric alcohols

The areas of application of polyhydric alcohols are different. Erythritol is used for the preparation of explosives, quick-drying paints. Xylitol is widely used in the food industry in the preparation of diabetic products, as well as in the production of resins, drying oils and surfactants. Plasticizers for PVC and synthetic oils are obtained from pentaerythritol. Manit is a part of some cosmetic products. And sorbitol has found application in medicine as a substitute for sucrose.

Polyhydric alcohols can be considered as derivatives of hydrocarbons in which several hydrogen atoms are replaced by OH groups.

Dihydric alcohols are called diols or glycols, trihydric alcohols are called triols or glycerols.

The names of polyhydric alcohols are formed according to the general rules of the IUPAC nomenclature. Representatives of polyhydric alcohols are:

ethanediol-1,2 propanetriol-1,2,3

Ethylene glycol glycerin

Physical properties of alcohols.

Polyhydric alcohols are viscous liquids with a sweet taste, readily soluble in water and ethanol, and poorly soluble in other organic solvents. Ethylene glycol is a strong poison.

Chemical properties of alcohols.

Polyhydric alcohols are characterized by the reactions of monohydric alcohols and they can proceed with the participation of one or more –OH groups.

Interaction with active metals:

Interaction with alkalis. The introduction of additional OH groups, which are electron acceptors, into the molecule enhances acid properties alcohols, as electron density delocalization occurs.

Interaction with hydroxides heavy metals(copper hydroxide) –qualitative reaction to polyhydric alcohols.

Interaction with hydrogen halides:

Interaction with acids to form esters:

a) with mineral acids

nitroglycerine

Nitroglycerin is a colorless oily liquid. In the form of dilute alcohol solutions (1%), it is used for angina pectoris, because. has a vasodilating effect.

When glycerol interacts with phosphoric acid, a mixture of α- and β-glycerophosphates is formed:

Glycerophosphates - structural elements of phospholipids, are used as a general tonic

b) with organic acids. When glycerol interacts with higher carboxylic acids fats are formed

Dehydration reactions

dioxane (cyclic diester)

When heated, glycerin decomposes with the formation of a tear substance - acrolein:

Acrolein

Oxidation:

When glycerol is oxidized, a number of products are formed. With mild oxidation - glyceraldehyde (1) and dihydroxyacetone (2):

When oxidized under severe conditions, 1,3-dioxoacetone (3) is formed:

Biologically significant are penta- and six-hydric alcohols.

The accumulation of –OH groups leads to the appearance of a sweet taste. Xylitol and sorbitol are sugar substitutes for diabetics

Inositols - hexahydric alcohols of the cyclohexane series. Due to the presence of asymmetric carbon atoms, inositol has several stereoisomers; most important mesoinositis (myoinositis)

inositol mesoinositis

Mesoinositol refers to vitamin-like compounds (group B vitamins) and is a structural component of complex lipids. Phytic acid, which is mesoinositol hexaphosphate, is widely distributed in plants. Its calcium salt, called phytin, stimulates hematopoiesis, improves nervous activity in diseases associated with a lack of phosphorus in the body.

Phenols

Phenols are derivatives of aromatic hydrocarbons in which one or more hydrogen atoms are replaced by hydroxyl groups.

Polyhydric alcohols (polyalcohols, polyols) are organic compounds of the class of alcohols containing in their composition more than one hydroxyl group -OH.

Glucose C 6 H 12 O 6 is a monosaccharide (monose) - a polyfunctional compound containing an aldehyde or keto group and several hydroxyl groups, i.e. polyhydroxy aldehydes and polyhydroxy ketones.

Interaction of polyhydric alcohols with copper (II) hydroxide

Qualitative reactions with copper (II) hydroxide for polyhydric alcohols are aimed at determining their weak acidic properties.

When freshly precipitated copper (II) hydroxide is added in a strongly alkaline medium to aqueous solution glycerol (HOCH 2-CH(OH)-CH 2 OH), and then to a solution of ethylene glycol (ethanediol) (HO CH 2-CH 2 OH), the copper hydroxide precipitate dissolves in both cases and a bright blue color of the solution appears (saturated color indigo). This indicates the acidic properties of glycerin and ethylene glycol.

СuSO 4 + 2NaOH \u003d Cu (OH) 2 ↓ + Na 2 SO 4

The reaction with Cu(OH) 2 is a qualitative reaction to polyhydric alcohols with neighboring OH - groups, which causes their weak acidic properties. Formalin and copper hydroxide give the same qualitative reaction - the aldehyde group reacts according to the acid type.

Qualitative reaction of glucose with copper (II) hydroxide

The reaction of glucose with copper (II) hydroxide when heated demonstrates restorative properties glucose. When heated, the reaction of glucose with copper (II) hydroxide proceeds with the reduction of bivalent copper Cu (II) to monovalent copper Cu (I). At the beginning, a precipitate of copper oxide CuO yellow color. In the process of further heating, CuO is reduced to copper (I) oxide - Cu 2 O, which precipitates in the form of a red precipitate. During this reaction, glucose is oxidized to gluconic acid.

2 HOCH 2 - (CHOH) 4) - CH \u003d O + Cu (OH) 2 \u003d 2HOCH 2 - (CHOH) 4) - COOH + Cu 2 O ↓ + 2 H 2 O

This is a qualitative reaction of glucose with copper hydroxide for an aldehyde group.

Alcohols are a large group of organic chemical substances. It includes subclasses of monohydric and polyhydric alcohols, as well as all substances of a combined structure: aldehyde alcohols, phenol derivatives, biological molecules. These substances enter into many types of reactions both at the hydroxyl group and at the carbon atom that carries it. These Chemical properties alcohols should be studied in detail.

Types of alcohols

Alcohols contain a hydroxyl group attached to a carrier carbon atom. Depending on the number of carbon atoms to which the carrier C is connected, alcohols are divided into:

primary (connected to the terminal carbon);
secondary (connected to one hydroxyl group, one hydrogen and two carbon atoms);
tertiary (connected to three carbon atoms and one hydroxyl group);
mixed (polyhydric alcohols in which there are hydroxyl groups at secondary, primary or tertiary carbon atoms).

Alcohols are also divided depending on the number of hydroxyl radicals into monohydric and polyhydric. The former contain only one hydroxyl group at the carrying carbon atom, for example, ethanol. Polyhydric alcohols contain two or more hydroxyl groups on different bearing carbon atoms.

Chemical properties of alcohols: table

It is most convenient to present the material of interest to us through a table that reflects the general principles of the reactivity of alcohols.

Reactive bond, type of reaction	Reagent	Product
O-H bond, substitution	Active metal, active metal hydride, alkali or amides active metals	alcoholates
C-O and O-H bond, intermolecular dehydration	Alcohol when heated in an acidic environment	Ether
C-O and O-H bond, intramolecular dehydration	Alcohol when heated over concentrated sulfuric acid	Unsaturated hydrocarbon
C-O bond, substitution	Hydrogen halide, thionyl chloride, quasi-phosphonium salt, phosphorus halides	haloalkanes
C-O bond - oxidation	Oxygen donors (potassium permanganate) with primary alcohol	Aldehyde
C-O bond - oxidation	Oxygen donors (potassium permanganate) with secondary alcohol
alcohol molecule	Oxygen (combustion)	carbon dioxide and water.

Reactivity of alcohols

Due to the presence in the molecule of monohydric alcohol of a hydrocarbon radical - C-O bonds and O-N connections- this class of compounds enters into numerous chemical reactions. They determine the chemical properties of alcohols and depend on the reactivity of the substance. The latter, in turn, depends on the length of the hydrocarbon radical attached to the carrier carbon atom. The larger it is, the lower the polarity of the O-H bond, due to which the reactions proceeding with the elimination of hydrogen from alcohol will proceed more slowly. This also reduces the dissociation constant of the mentioned substance.

The chemical properties of alcohols also depend on the number of hydroxyl groups. One shifts the electron density towards itself along the sigma bonds, which increases the reactivity along O-N groups e. Because it polarizes C-O connection, then reactions with its rupture are more active in alcohols that have two or more O-H groups. Therefore, polyhydric alcohols, whose chemical properties are more numerous, are more likely to react. They also contain several alcohol groups, which is why they can freely react with each of them.

Typical reactions of monohydric and polyhydric alcohols

Typical chemical properties of alcohols appear only in the reaction with active metals, their bases and hydrides, Lewis acids. Also typical are interactions with hydrogen halides, phosphorus halides and other components to produce haloalkanes. Alcohols are also weak grounds, therefore, they react with acids, forming hydrogen halides and esters of inorganic acids.

Ethers are formed from alcohols by intermolecular dehydration. The same substances enter into dehydrogenation reactions with the formation of aldehydes from the primary alcohol and ketones from the secondary. Tertiary alcohols do not enter into such reactions. Also, the chemical properties of ethyl alcohol (and other alcohols) leave the possibility of their complete oxidation with oxygen. it simple reaction combustion, accompanied by the release of water with carbon dioxide and some heat.

Reactions on the hydrogen atom of the О-Н bond

The chemical properties of monohydric alcohols allow the breaking of the O-H bond and the elimination of hydrogen. These reactions proceed when interacting with active metals and their bases (alkalis), with active metal hydrides, and also with Lewis acids.

Alcohols also actively react with standard organic and inorganic acids. In this case, the reaction products are an ester or a halocarbon.

Reactions for the synthesis of haloalkanes (via the C-O bond)

Halogenalkanes are typical compounds that can be obtained from alcohols by several types of reactions. chemical reactions. In particular, the chemical properties of monohydric alcohols make it possible to interact with hydrogen halides, tri- and pentavalent phosphorus halides, quasi-phosphonium salts, and thionyl chloride. Also, haloalkanes from alcohols can be obtained in an intermediate way, that is, by the synthesis of an alkylsulfonate, which will later enter into a substitution reaction.

An example of the first reaction with hydrogen halide is indicated in the graphical appendix above. Here, butyl alcohol reacts with hydrogen chloride to form chlorobutane. In general, the class of compounds containing chlorine and a hydrocarbon saturated radical is called an alkyl chloride. by-product chemical interaction is water.

Reactions with the production of alkyl chloride (iodide, bromide or fluoride) are quite numerous. A typical example is the interaction with phosphorus tribromide, phosphorus pentachloride and other compounds of this element and its halides, perchlorides and perfluorides. They proceed by the mechanism of nucleophilic substitution. Alcohols also react with thionyl chloride to form chloroalkane and release SO 2 .

Visually the chemical properties of monatomic saturated alcohols containing a saturated hydrocarbon radical are presented as reactions in the illustration below.

Alcohols readily react with the quasi-phosphonium salt. However, this reaction is most advantageous when proceeding with monohydric secondary and tertiary alcohols. They are regioselective and allow the "implantation" of a halogen group in a strictly defined place. The products of such reactions are obtained with a high mass fraction of the yield. And polyhydric alcohols, whose chemical properties are somewhat different from those of monohydric ones, can isomerize during the reaction. Therefore, obtaining the target product is difficult. An example of a reaction in the image.

Intramolecular and intermolecular dehydration of alcohols

The hydroxyl group located at the supporting carbon atom can be cleaved off using strong acceptors. This is how intermolecular dehydration reactions proceed. When one alcohol molecule interacts with another in a solution of concentrated sulfuric acid, a water molecule is split off from both hydroxyl groups, the radicals of which combine to form an ether molecule. With intermolecular dehydration of ethanol, dioxane can be obtained - a dehydration product of four hydroxyl groups.