Alcohols - concept, properties, application. Chemical properties of monohydric and polyhydric alcohols

Alcohols are compounds containing one or more hydroxyl groups directly bonded to a hydrocarbon radical.

Classification of alcohols

Alcohols are classified according to various structural characteristics.

1. Based on the number of hydroxyl groups, alcohols are divided into

o monatomic(one group -OH)

For example, CH 3 – OH methanol,CH 3 – CH 2 – OH ethanol

o polyatomic(two or more -OH groups).

The modern name for polyhydric alcohols is polyols(diols, triols, etc.). Examples:

dihydric alcohol -ethylene glycol(ethanediol)

HO–CH 2 –CH 2 –OH

trihydric alcohol -glycerol(propanetriol-1,2,3)

HO–CH 2 –CH(OH)–CH 2 –OH

Diatomic alcohols with two OH groups at the same carbon atom R–CH(OH) 2 are unstable and, eliminating water, immediately turn into aldehydes R–CH=O. Alcohols R–C(OH) 3 do not exist.

2. Depending on which carbon atom (primary, secondary or tertiary) the hydroxy group is connected to, alcohols are distinguished

o primary R–CH 2 –OH,

o secondary R 2 CH–OH,

o tertiary R 3 C–OH.

For example:

In polyhydric alcohols, primary, secondary and tertiary alcohol groups are distinguished. For example, a molecule of the trihydric alcohol glycerol contains two primary alcohols (HO–CH2 –) and one secondary alcohol (–CH(OH)–) group.

3. According to the structure of radicals associated with the oxygen atom, alcohols are divided into

o limit(for example, CH 3 – CH 2 –OH)

o unlimited(CH 2 =CH–CH 2 –OH)

o aromatic(C 6 H 5 CH 2 –OH)

Unsaturated alcohols with an OH group at a carbon atom connected to another atom by a double bond are very unstable and immediately isomerize into aldehydes or ketones.

For example,vinyl alcohol CH 2 =CH–OH turns into acetaldehydeCH 3 –CH=O

Saturated monohydric alcohols

1. Definition

LIMITED MONO-ACHOLOGICAL ALCOHOLS – oxygen-containing organic substances, derivatives of saturated hydrocarbons, in which one hydrogen atom is replaced by a functional group (- OH)

2. Homologous series

3. Nomenclature of alcohols

Systematic names are given by the name of the hydrocarbon with the addition of a suffix -ol and a number indicating the position of the hydroxy group (if necessary). For example:

Numbering is based on the end of the chain closest to the OH group.

The number reflecting the location of the OH group is usually placed after the suffix “ol” in Russian.

According to another method (radical-functional nomenclature), the names of alcohols are derived from the names of radicals with the addition of the word " alcohol". In accordance with this method, the above compounds are called: methyl alcohol, ethyl alcohol, n-propyl alcohol CH 3 -CH 2 -CH 2 -OH, isopropyl alcohol CH 3 -CH(OH)-CH 3.

4. Isomerism of alcohols

Characteristic of alcohols structural isomerism:

· isomerism of OH group position(starting from C 3);
For example:

· carbon skeleton(starting from C 4);
For example, carbon skeleton isomers forC4H9OH:

· interclass isomerism with ethers
For example,

ethanol CH 3 CH 2 –OH and dimethyl ether CH 3 –O–CH 3

It is also possible spatial isomerism– optical.

For example, butanol-2 CH 3 C H(OH)CH 2 CH 3, in the molecule of which the second carbon atom (highlighted) is bonded to four different substituents, exists in the form of two optical isomers.

5. Structure of alcohols

The structure of the simplest alcohol - methyl (methanol) - can be represented by the formulas:

From the electronic formula it is clear that the oxygen in the alcohol molecule has two lone electron pairs.

The properties of alcohols and phenols are determined by the structure of the hydroxyl group, the nature of its chemical bonds, the structure of hydrocarbon radicals and their mutual influence.

O–H and C–O bonds are polar covalent. This follows from the differences in electronegativity of oxygen (3.5), hydrogen (2.1) and carbon (2.4). The electron density of both bonds is shifted towards the more electronegative oxygen atom:

The mobility of the hydrogen atom in the hydroxyl group of alcohol is slightly less than in water. Methyl alcohol (methanol) will be more “acidic” in the series of monohydric saturated alcohols.
Radicals in the alcohol molecule also play a role in the manifestation of acidic properties. Typically, hydrocarbon radicals reduce acidic properties. But if they contain electron-withdrawing groups, then the acidity of alcohols increases noticeably. For example, alcohol (CF 3) 3 C-OH due to fluorine atoms becomes so acidic that it is able to displace carbonic acid from its salts.

The next class of substances that I would like to consider are alcohols. These are compounds that contain an -OH group bonded to a carbon atom. Such a group is monovalent and any alkane can be converted into an alcohol by replacing one of the hydrogens with OH. For example, methane corresponds to methyl alcohol, ethane - ethyl alcohol, and so on. They are also abbreviated with the ending “ol”: methanol, ethanol, propanol.

Methanol, ethanol, propanol

Four isomeric butyl alcohols: n-butanol, sec-butanol, tert-butanol, isobutanol.

Alcohols are somewhat similar in properties to water: water also contains hydroxyl (that’s what the -OH group is called), but bonded to a hydrogen atom (therefore it can be called hydrogen hydroxide, although no one does that). Thanks to hydroxyl groups, the molecules are more strongly bonded to each other (due to hydrogen bonds), so even the lowest alcohol - methanol - is a liquid, albeit one that evaporates quite easily. Almost all lower alcohols, up to octanol, are liquid. Again, complexity arises here due to the large number of isomers.
General formula of alcohols C n H 2n+1 OH.
The most famous of the alcohols is ethanol, also known as ethyl alcohol - the same one found in alcoholic beverages. It boils at 78 and can be isolated from solution by distillation, but the concentration cannot be raised above 96% in this way (which does not, however, prevent one from obtaining 100% ethanol in other ways, for example, by removing water from 96% ethanol using a desiccant) . Surely everyone has heard about methanol, which is indistinguishable from ethanol in appearance and smell, but is deadly poisonous. However, without tasting it, methanol is an excellent solvent, as well as a fuel and intermediate for many chemical processes.

Since methanol and ethanol are controlled by law, the next alcohol, propanol, is often used instead. Moreover, n-propanol is much less common than its isomer - isopropanol, which is often used as a solvent and degreaser (also suitable for alcohol lamps, if that happens). It has a different odor from methanol and ethanol, is more viscous (especially at low temperatures) and boils at a slightly higher temperature.

Butanol and impurities of heavier alcohols form the basis of fusel oils - they are formed in small quantities during fermentation and have an unpleasant heavy odor. Otherwise, such alcohols are used primarily as reagents for the preparation of other compounds.

There can be several functional groups in a molecule, in particular alcohol groups. All the compounds discussed above are called monohydric alcohols - based on the number of hydroxyl groups. There are also dihydric alcohol ethylene glycol and trihydric alcohol glycerin:

Ethylene glycol and glycerin

Alcohols are similar to alkanes, in which an oxygen atom is “built” into the molecule. And indeed, in many textbooks they write that methanol can be obtained by incomplete oxidation of methane. This, however, requires very special conditions that are realized only in industry: high pressures, temperature control, and the use of catalysts. It is also obtained from the so-called. “synthesis gas” is a mixture of carbon monoxide and hydrogen, and synthesis gas, in turn, is obtained from methane and water at high temperature.

Production of methanol from synthesis gas

To produce ethanol, fermentation is used: certain types of microorganisms can convert sugars present in plant material (such as wheat or sugar cane) into ethanol, thereby producing energy. Ethanol is then separated by rectification and used, for example, as an additive to automobile fuel (so-called biofuel). Thus, about 60 million are produced. tons of ethanol per year (mainly in the USA and Brazil). On such a scale, I don’t want to talk about getting it from petroleum products, but there is still a way to get it from ethylene: a hydrocarbon in which two carbon atoms are connected not by one, but by a double bond. Under certain conditions, this bond can open, attaching a water molecule. This produces ethanol; other alcohols can be prepared in the same way from the corresponding alkenes

Ethylene hydration reaction

How and under what conditions, as well as other reactions of alcohols will be described in the next article.

Bioethanol production statistics can be found here: http://ethanolrfa.org/resources/industry/statistics/

DEFINITION

Saturated monohydric alcohols can be considered as derivatives of hydrocarbons of the methane series, in the molecules of which one hydrogen atom is replaced by a hydroxyl group.

So, saturated monohydric alcohols consist of a hydrocarbon radical and the -OH functional group. In the names of alcohols, the hydroxyl group is designated by the suffix -ol.

The general formula of saturated monohydric alcohols is C n H 2 n +1 OH or R-OH or C n H 2 n +2 O. The molecular formula of an alcohol does not reflect the structure of the molecule, since two completely different substances can correspond to the same gross formula, for example, the molecular formula C 2 H 5 OH is common to both ethyl alcohol and acetone (dimethyl ketone):

CH 3 -CH 2 -OH (ethanol);

CH 3 -O-CH 3 (acetone).

Just like the hydrocarbons of the methane series, saturated monohydric alcohols form a homologous series of methanol.

Let's compose this series of homologues and consider the patterns of changes in the physical properties of compounds of this series depending on the increase in the hydrocarbon radical (Table 1).

Homologous series (incomplete) of saturated monohydric alcohols

Table 1. Homologous series (incomplete) of saturated monohydric alcohols.

Saturated monohydric alcohols are lighter than water because their density is less than unity. Lower alcohols are miscible with water in all respects; as the hydrocarbon radical increases, this ability decreases. Most alcohols are highly soluble in organic solvents. Alcohols have higher boiling and melting points than the corresponding hydrocarbons or halogen derivatives, which is due to the possibility of their formation of intermolecular bonds.

The most important representatives of saturated monohydric alcohols are methanol (CH 3 OH) and ethanol (C 2 H 5 OH).

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

Alcohols are a diverse and broad class of chemical compounds.

Alcohols are chemical compounds whose molecules contain hydroxyl OH groups connected to a hydrocarbon radical.

A hydrocarbon radical consists of carbon and hydrogen atoms. Examples of hydrocarbon radicals - CH 3 - methyl, C 2 H 5 - ethyl. Often a hydrocarbon radical is simply denoted by the letter R. But if different radicals are present in the formula, they are denoted by R." R ", R """, etc.

The names of alcohols are formed by adding the suffix –ol to the name of the corresponding hydrocarbon.

Classification of alcohols

Alcohols are monohydric and polyhydric. If there is only one hydroxyl group in an alcohol molecule, then such an alcohol is called monohydric. If the number of hydroxyl groups is 2, 3, 4, etc., then it is a polyhydric alcohol.

Examples of monohydric alcohols: CH 3 -OH - methanol or methyl alcohol, CH 3 CH 2 -OH - ethanol or ethyl alcohol.

Accordingly, a molecule of a dihydric alcohol contains two hydroxyl groups, a molecule of a trihydric alcohol contains three, etc.

Monohydric alcohols

The general formula of monohydric alcohols can be represented as R-OH.

Based on the type of free radical included in the molecule, monohydric alcohols are divided into saturated (saturated), unsaturated (unsaturated) and aromatic alcohols.

In saturated hydrocarbon radicals, carbon atoms are connected by simple C – C bonds. Unsaturated radicals contain one or more pairs of carbon atoms connected by double C = C or triple C ≡ C bonds.

Saturated alcohols contain saturated radicals.

CH 3 CH 2 CH 2 -OH – saturated alcohol propanol-1 or propylene alcohol.

Accordingly, unsaturated alcohols contain unsaturated radicals.

CH 2 = CH - CH 2 - OH – unsaturated alcohol propenol 2-1 (allylic alcohol)

And the molecule of aromatic alcohols includes a benzene ring C 6 H 5.

C 6 H 5 -CH 2 -OH – aromatic alcohol phenylmethanol (benzyl alcohol).

Depending on the type of carbon atom bonded to the hydroxyl group, alcohols are divided into primary ((R-CH 2 -OH), secondary (R-CHOH-R) and tertiary (RR"R""C-OH) alcohols.

Chemical properties of monohydric alcohols

1. Alcohols burn to form carbon dioxide and water. When burning, heat is released.

C 2 H 5 OH + 3O 2 → 2CO 2 + 3H 2 O

2. When alcohols react with alkali metals, sodium alkoxide is formed and hydrogen is released.

C 2 H 5 -OH + 2Na → 2C 2 H 5 ONa + H 2

3. Reaction with hydrogen halide. As a result of the reaction, a haloalkane (bromoethane and water) is formed.

C 2 H 5 OH + HBr → C 2 H 5 Br + H 2 O

4. Intramolecular dehydration occurs when heated and under the influence of concentrated sulfuric acid. The result is unsaturated hydrocarbon and water.

H 3 – CH 2 – OH → CH 2 = CH 2 + H 2 O

5. Oxidation of alcohols. At ordinary temperatures, alcohols do not oxidize. But with the help of catalysts and heating, oxidation occurs.

Polyhydric alcohols

As substances containing hydroxyl groups, polyhydric alcohols have chemical properties similar to those of monohydric alcohols, but their reaction occurs at several hydroxyl groups at once.

Polyhydric alcohols react with active metals, hydrohalic acids, and nitric acid.

Preparation of alcohols

Let's consider methods for producing alcohols using the example of ethanol, the formula of which is C 2 H 5 OH.

The oldest of them is the distillation of alcohol from wine, where it is formed as a result of the fermentation of sugary substances. The raw materials for the production of ethyl alcohol are also starch-containing products, which are converted into sugar through the fermentation process, which is then fermented into alcohol. But the production of ethyl alcohol in this way requires a large consumption of food raw materials.

A much more advanced synthetic method for producing ethyl alcohol. In this case, ethylene is hydrated with water vapor.

C 2 H 4 + H 2 O → C 2 H 5 OH

Among polyhydric alcohols, the most famous is glycerin, which is obtained by splitting fats or synthetically from propylene, which is formed during high-temperature oil refining.

These are derivatives of hydrocarbons in which one hydrogen atom is replaced by a hydroxy group. The general formula of alcohols is CnH 2 n +1 OH.

Classification of monohydric alcohols.

Depending on the position where it is located HE-group, distinguish:

Primary alcohols:

Secondary alcohols:

Tertiary alcohols:

.

Isomerism of monohydric alcohols.

For monohydric alcohols characterized by isomerism of the carbon skeleton and isomerism of the position of the hydroxy group.

Physical properties of monohydric alcohols.

The reaction follows Markovnikov’s rule, so only song alcohol can be obtained from primary alkenes.

2. Hydrolysis of alkyl halides under the influence of aqueous solutions of alkalis:

If the heating is weak, then intramolecular dehydration occurs, resulting in the formation of ethers:

B) Alcohols can react with hydrogen halides, with tertiary alcohols reacting very quickly, while primary and secondary alcohols react slowly:

The use of monohydric alcohols.

Alcohols used primarily in industrial organic synthesis, in the food industry, medicine and pharmacy.