Dehydrogenation of primary alcohols. Preparation from alcohols. On the topic: Catalytic dehydrogenation

Dehydrogenation reactions of alcohols are necessary to produce aldehydes and ketones. Ketones are obtained from secondary alcohols, and aldehydes from primary alcohols. Catalysts in the processes are copper, silver, copper chromites, zinc oxide, etc. It is worth noting that, compared to copper catalysts, zinc oxide is more stable and does not lose activity during the process, but can provoke a dehydration reaction. In general, the dehydrogenation reactions of alcohols can be presented as follows:

In industry, the dehydrogenation of alcohols produces compounds such as acetaldehyde, acetone, methyl ethyl ketone and cyclohexanone. The processes take place in a stream of water vapor. The most common processes are:

1. carried out on a copper or silver catalyst at a temperature of 200 - 400 ° C and atmospheric pressure. The catalyst is any carrier of Al 2 O 3, SnO 2 or carbon fiber, on which silver or copper components are deposited. This reaction is one of the components of the Wacker process, which is an industrial method for producing acetaldehyde from ethanol by dehydrogenation or oxidation with oxygen.

2. can proceed in different ways, depending on the structural formula of its original substance. 2-propanol, which is a secondary alcohol, is dehydrogenated to acetone, and 1-propanol, being a primary alcohol, is dehydrogenated to propanal at atmospheric pressure and a process temperature of 250 - 450 ° C.

3. it also depends on the structure of the starting compound, which affects the final product (aldehyde or ketone).

4. Methanol dehydrogenation. This process is not fully studied, but most researchers highlight it as a promising process for the synthesis of water-free formaldehyde. Various process parameters are offered: temperature 600 - 900 °C, active catalyst component zinc or copper, silicon oxide carrier, possibility of initiating the reaction with hydrogen peroxide, etc. Currently, most of the world's formaldehyde is produced by the oxidation of methanol.

Depending on the type of hydrocarbon radical, as well as, in some cases, the characteristics of the attachment of the -OH group to this hydrocarbon radical, compounds with a hydroxyl functional group are divided into alcohols and phenols.

Alcohols are compounds in which the hydroxyl group is connected to a hydrocarbon radical, but is not attached directly to the aromatic ring, if there is one in the structure of the radical.

Examples of alcohols:

If the structure of a hydrocarbon radical contains an aromatic ring and a hydroxyl group, and is connected directly to the aromatic ring, such compounds are called phenols .

Examples of phenols:

Why are phenols classified as a separate class from alcohols? After all, for example, the formulas

are very similar and give the impression of substances of the same class organic compounds.

However, the direct connection of the hydroxyl group with the aromatic ring significantly affects the properties of the compound, since the conjugated system of π-bonds of the aromatic ring is also conjugated with one of the lone electron pairs of the oxygen atom. Because of this, in phenols the O-H bond is more polar compared to alcohols, which significantly increases the mobility of the hydrogen atom in the hydroxyl group. In other words, phenols are much more pronounced than alcohols. acid properties.

Chemical properties of alcohols

Monohydric alcohols

Substitution reactions

Substitution of a hydrogen atom in the hydroxyl group

1) Alcohols react with alkali, alkaline earth metals and aluminum (cleaned from the protective film of Al 2 O 3), and metal alcoholates are formed and hydrogen is released:

The formation of alcoholates is possible only when using alcohols that do not contain water dissolved in them, since in the presence of water alcoholates are easily hydrolyzed:

CH 3 OK + H 2 O = CH 3 OH + KOH

2) Esterification reaction

The esterification reaction is the interaction of alcohols with organic and oxygen-containing inorganic acids, leading to the formation of esters.

This type of reaction is reversible, therefore, to shift the equilibrium towards the formation of an ester, it is advisable to carry out the reaction with heating, as well as in the presence of concentrated sulfuric acid as a water-removing agent:

Substitution of hydroxyl group

1) When alcohols are exposed to hydrohalic acids, the hydroxyl group is replaced by a halogen atom. As a result of this reaction, haloalkanes and water are formed:

2) By passing a mixture of alcohol vapor and ammonia through heated oxides of some metals (most often Al 2 O 3), primary, secondary or tertiary amines can be obtained:

The type of amine (primary, secondary, tertiary) will depend to some extent on the ratio of the starting alcohol to ammonia.

Elimination reactions

Dehydration

Dehydration, which actually involves the elimination of water molecules, in the case of alcohols differs by intermolecular dehydration And intramolecular dehydration.

At intermolecular dehydration In alcohols, one molecule of water is formed as a result of the abstraction of a hydrogen atom from one molecule of alcohol and a hydroxyl group from another molecule.

As a result of this reaction, compounds belonging to the class of ethers (R-O-R) are formed:

Intramolecular dehydration alcohols process occurs in such a way that one molecule of water is split off from one molecule of alcohol. This type of dehydration requires somewhat more stringent conditions, consisting in the need to use noticeably stronger heating compared to intermolecular dehydration. In this case, from one molecule of alcohol one molecule of alkene and one molecule of water are formed:

Since the methanol molecule contains only one carbon atom, intramolecular dehydration is impossible for it. When methanol is dehydrated, only ether (CH 3 -O-CH 3) can be formed.

It is necessary to clearly understand the fact that in the case of dehydration of unsymmetrical alcohols, intramolecular elimination of water will proceed in accordance with Zaitsev’s rule, i.e. hydrogen will be removed from the least hydrogenated carbon atom:

Dehydrogenation of alcohols

a) Dehydrogenation of primary alcohols when heated in the presence of copper metal leads to the formation aldehydes:

b) In the case of secondary alcohols, similar conditions will lead to the formation ketones:

c) Tertiary alcohols do not enter into a similar reaction, i.e. are not subject to dehydrogenation.

Oxidation reactions

Combustion

Alcohols easily react in combustion. This generates a large amount of heat:

2CH 3 -OH + 3O 2 = 2CO 2 + 4H 2 O + Q

Incomplete oxidation

Incomplete oxidation of primary alcohols can lead to the formation of aldehydes and carboxylic acids.

In the case of incomplete oxidation of secondary alcohols, only ketones can be formed.

Incomplete oxidation of alcohols is possible under the influence of various oxidizing agents, for example, such as atmospheric oxygen in the presence of catalysts (metallic copper), potassium permanganate, potassium dichromate, etc.

In this case, aldehydes can be obtained from primary alcohols. As you can see, the oxidation of alcohols to aldehydes essentially leads to the same organic products as dehydrogenation:

It should be noted that when using oxidizing agents such as potassium permanganate and potassium dichromate in an acidic environment, deeper oxidation of alcohols is possible, namely to carboxylic acids. In particular, this manifests itself when using an excess of oxidizing agent during heating. Secondary alcohols can only be oxidized to ketones under these conditions.

LIMITED POLYATHICAL ALCOHOLS

Substitution of hydrogen atoms of hydroxyl groups

Polyhydric alcohols are the same as monohydric ones react with alkali, alkaline earth metals and aluminum (cleaned from filmAl 2 O 3 ); in this case, a different number of hydrogen atoms of hydroxyl groups in the alcohol molecule can be replaced:

2. Since the molecules of polyhydric alcohols contain several hydroxyl groups, they influence each other due to a negative inductive effect. In particular, this leads to a weakening O-N connections and increasing the acidic properties of hydroxyl groups.

B O The greater acidity of polyhydric alcohols is manifested in the fact that polyhydric alcohols, unlike monohydric alcohols, react with some hydroxides of heavy metals. For example, you need to remember the fact that freshly precipitated copper hydroxide reacts with polyhydric alcohols to form a bright blue solution complex compound.

Thus, the interaction of glycerol with freshly precipitated copper hydroxide leads to the formation of a bright blue solution of copper glycerate:

This reaction is quality for polyhydric alcohols. For passing the Unified State Exam It is enough to know the signs of this reaction, but it is not necessary to be able to write the interaction equation itself.

3. Just like monohydric alcohols, polyhydric alcohols can enter into an esterification reaction, i.e. react with organic and oxygen-containing inorganic acids with the formation of esters. This reaction is catalyzed by strong inorganic acids and is reversible. In this regard, when carrying out the esterification reaction, the resulting ester is distilled off from the reaction mixture in order to shift the equilibrium to the right according to Le Chatelier’s principle:

If carboxylic acids react with glycerol a large number carbon atoms in the hydrocarbon radical, the resulting esters are called fats.

In the case of esterification of alcohols with nitric acid, a so-called nitrating mixture is used, which is a mixture of concentrated nitric and sulfuric acids. The reaction is carried out under constant cooling:

Glycerol ester and nitric acid, called trinitroglycerin, is an explosive. In addition, a 1% solution of this substance in alcohol has a powerful vasodilator effect, which is used for medical indications to prevent a stroke or heart attack.

Substitution of hydroxyl groups

Reactions of this type proceed through the mechanism of nucleophilic substitution. Interactions of this kind include the reaction of glycols with hydrogen halides.

For example, the reaction of ethylene glycol with hydrogen bromide proceeds with the sequential replacement of hydroxyl groups with halogen atoms:

Chemical properties of phenols

As already mentioned at the very beginning of this chapter, the chemical properties of phenols differ markedly from chemical properties alcohols This is due to the fact that one of the lone electron pairs of the oxygen atom in the hydroxyl group is conjugated with the π-system of conjugated bonds of the aromatic ring.

Reactions involving the hydroxyl group

Acid properties

Phenols are more strong acids than alcohols, and are dissociated to a very small extent in aqueous solution:

B O The greater acidity of phenols compared to alcohols in terms of chemical properties is expressed in the fact that phenols, unlike alcohols, are able to react with alkalis:

However, the acidic properties of phenol are less pronounced than even one of the weakest inorganic acids - carbonic acid. So, in particular, carbon dioxide, when passing it through an aqueous solution of alkali metal phenolates, displaces free phenol from the latter as an even weaker acid than carbonic acid:

Obviously, any other stronger acid will also displace phenol from phenolates:

3) Phenols are stronger acids than alcohols, and alcohols react with alkali and alkaline earth metals. In this regard, it is obvious that phenols will react with these metals. The only thing is that, unlike alcohols, the reaction of phenols with active metals requires heating, since both phenols and metals are solids:

Substitution reactions in the aromatic ring

The hydroxyl group is a substituent of the first kind, which means that it facilitates the occurrence of substitution reactions in ortho- And pair- positions in relation to oneself. Reactions with phenol occur under much milder conditions compared to benzene.

Halogenation

The reaction with bromine does not require any special conditions. When mixed bromine water with a phenol solution, a white precipitate of 2,4,6-tribromophenol is instantly formed:

Nitration

When phenol is exposed to a mixture of concentrated nitric and sulfuric acids (nitrating mixture), 2,4,6-trinitrophenol is formed, a yellow crystalline explosive:

Addition reactions

Since phenols are unsaturated compounds, they can be hydrogenated in the presence of catalysts to the corresponding alcohols.

The generally accepted mechanism of dehydration of alcohols is as follows (for simplicity, ethyl alcohol is taken as an example):

The alcohol adds a hydrogen ion step (1) to form a protonated alcohol, which dissociates step (2), giving a water molecule and a carbonium ion; then the carbonium ion step (3) loses a hydrogen ion and an alkene is formed.

Thus, the double bond is formed in two stages: the loss of the hydroxyl group as [step (2)] and the loss of hydrogen (step (3)). This is the difference between this reaction and the dehydrohalogenation reaction, where the elimination of hydrogen and halogen occurs simultaneously.

The first stage represents the Bronsted-Lowry acid-base equilibrium (Section 1.19). When sulfuric acid is dissolved in water, for example, the following reaction occurs:

A hydrogen ion moves from a very weak base to a stronger base to form an oxonium ion. The basic properties of both compounds are due, of course, to the lone pair of electrons that can bond the hydrogen ion. Alcohol also contains an oxygen atom with a lone pair of electrons and its basicity is comparable to that of water. The first stage of the proposed mechanism can most likely be represented as follows:

The hydrogen ion moves from the bisulfate ion to the stronger base (ethyl alcohol) to form the substituted oxonium ion of the protonated alcohol.

Similarly, step (3) is not the expulsion of a free hydrogen ion, but its transition to the strongest base available, namely

For convenience, this process is often depicted as the addition or elimination of a hydrogen ion, but it should be understood that in all cases what actually occurs is the transfer of a proton from one base to another.

All three reactions are given as equilibrium reactions, since each stage is reversible; as will be shown below, the reverse reaction is the formation of alcohols from alkenes (Section 6.10). Equilibrium (1) is shifted very much to the right; It is known that sulfuric acid is almost completely ionized in an alcohol solution. Since the concentration of carbonium ions present at any moment is very small, equilibrium (2) is shifted greatly to the left. At some point, one of these few carbonium ions reacts according to equation (3) to form an alkene. During dehydration, the volatile alkene is usually distilled off from the reaction mixture, and thus equilibrium (3) shifts to the right. As a result, the entire reaction comes to an end.

The carbonium ion is formed by the dissociation of a protonated alcohol; in this case the charged particle is separated from

neutral particle Obviously, this process requires significantly less energy than the formation of a carbonium ion from the alcohol itself, since in this case it is necessary to separate the positive particle from the negative one. In the first case, a weak base (water) is cleaved from a carbonium ion (Lewis acid) much more easily than a very strong base, hydroxyl ion, i.e. water is a better leaving group than hydroxyl ion. It has been shown that the hydroxyl ion is almost never cleaved from the alcohol; bond cleavage reactions in alcohol in almost all cases require an acid catalyst, the role of which, as in the present case, is to protonate the alcohol.

Finally, it should be understood that the dissociation of the protonated alcohol is only possible due to the solvation of the carbonium ion (cf. Section 5.14). The energy to break the carbon-oxygen bond is taken from the formation large number ion-dipole bonds between a carbonium ion and a polar solvent.

The carbonium ion can undergo various reactions; which one occurs depends on the experimental conditions. All reactions of carbonium ions end in the same way: they acquire a pair of electrons to fill the octet of a positively charged carbon atom. IN in this case a hydrogen ion is split off from a carbon atom adjacent to a positively charged electron-depleted carbon atom; a pair of electrons that previously bonded with this hydrogen can now form an -bond

This mechanism explains acid catalysis during dehydration. Does this mechanism also explain the fact that the ease of dehydration of alcohols decreases in the tertiary-secondary-primary series? Before answering this question, it is necessary to find out how the stability of carbonium ions changes.