(ORGANIC CHEMISTRY)
Splitting water from alcohols (dehydration):
Acidic reagents are used as catalysts for dehydration: sulfuric and phosphoric acids, aluminum oxide, etc. The order of elimination is most often determined by the Zaitsev rule (1875): when water is formed, hydrogen is most easily eliminated from the neighboring least hydrogenated carbon atom ...(ORGANIC CHEMISTRY)
Oxidation of alcohols
Alcohols are oxidized more readily than hydrocarbons, with the carbon containing the hydroxyl group being oxidized first. The most suitable oxidizing agent under laboratory conditions is a chromium mixture. In industry - atmospheric oxygen in the presence of catalysts. Primary...(ORGANIC CHEMISTRY)
Oxidation of ethyl alcohol to acetic acid.
Ethyl alcohol is oxidized to acetic acid under the influence of acetic acid bacteria of the genera Gluconobacter and Acetobacter. These are gram-negative chemoorganoheterotrophic, non-spore-forming, rod-shaped organisms, mobile or immobile. Acetic acid bacteria of these genera differ in ...(BASICS OF MICROBIOLOGY)
Catalytic paraffin dehydrogenation
An important industrial process is also catalytic dehydrogenation paraffins over chromium oxide: Most laboratory methods for the production of olefins are based on the reactions of elimination (elimination) of various reagents: water, halogens or hydrogen halides from the corresponding limiting derivatives ...(ORGANIC CHEMISTRY)
Depending on the type of hydrocarbon radical, as well as, in some cases, the peculiarities of attachment of the -OH group to this hydrocarbon radical, compounds with a hydroxyl functional group are divided into alcohols and phenols.
Alcohols refers to compounds in which the hydroxyl group is attached to a hydrocarbon radical, but not directly attached to the aromatic ring, if any in the structure of the radical.
Examples of alcohols:
If the structure of a hydrocarbon radical contains an aromatic nucleus and a hydroxyl group, and is connected directly to the aromatic nucleus, such compounds are called phenols .
Examples of phenols:
Why are phenols isolated in a class separate from alcohols? After all, for example, the formulas
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 nucleus significantly affects the properties of the compound, since the conjugated system of π-bonds of the aromatic nucleus is also conjugated with one of the lone electron pairs of the oxygen atom. Because of this, the O-H bond in phenols is more polar than in alcohols, which significantly increases the mobility of the hydrogen atom in the hydroxyl group. In other words, phenols have significantly more pronounced acidic properties than alcohols.
Chemical properties of alcohols
Monohydric alcohols
Substitution reactions
Substitution of a hydrogen atom in a hydroxyl group
1) Alcohols react with alkali, alkaline earth metals and aluminum (cleaned from the protective film Al 2 O 3), while 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 \u003d CH 3 OH + KOH
2) The reaction of esterification
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 desirable to carry out the reaction under heating, as well as in the presence of concentrated sulfuric acid as a dehydrating agent:
Substitution of a hydroxyl group
1) Under the action of hydrohalic acids on alcohols, 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 vapors with 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 (cleavage) reactions
Dehydration
Dehydration, which actually means the elimination of water molecules, in the case of alcohols differs by intermolecular dehydration and intramolecular dehydration.
When intermolecular dehydration alcohols, one water molecule is formed as a result of the elimination of a hydrogen atom from one alcohol molecule 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 of alcohols proceeds in such a way that one water molecule is split off from one alcohol molecule. This type of dehydration requires somewhat more stringent conditions of carrying out, consisting in the need to use a noticeably stronger heating in comparison with intermolecular dehydration. In this case, one alkene molecule and one water molecule are formed from one alcohol molecule:
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 asymmetric alcohols, the intramolecular elimination of water will proceed in accordance with the Zaitsev rule, i.e. hydrogen will be split off from the least hydrogenated carbon atom:
Dehydrogenation of alcohols
a) Dehydrogenation of primary alcohols when heated in the presence of metallic copper 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 subjected to dehydrogenation.
Oxidation reactions
Combustion
Alcohols are easy to burn. This produces a large amount of heat:
2СН 3 -ОН + 3O 2 \u003d 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 when various oxidizing agents act on them, 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, in fact, leads to the same organic products as dehydrogenation:
It should be noted that when using such oxidizing agents as potassium permanganate and potassium dichromate in an acidic medium, a deeper oxidation of alcohols is possible, namely, to carboxylic acids. In particular, this is manifested when using an excess of the oxidizing agent when heated. Secondary alcohols can be oxidized under these conditions only to ketones.
ULTIMATE MULTI-ATOMIC ALCOHOLS
Substitution of hydrogen atoms for hydroxyl groups
Polyhydric alcohols as well as monohydric react with alkali, alkaline earth metals and aluminum (stripped of the 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 through a negative inductive effect. In particular, this leads to a weakening communication O-N and increasing the acidic properties of hydroxyl groups.
B aboutthe higher acidity of polyhydric alcohols is manifested in the fact that polyhydric alcohols, in contrast to monohydric alcohols, react with some heavy metal hydroxides. For example, remember the fact that freshly precipitated copper hydroxide reacts with polyhydric alcohols to form a bright blue complex solution.
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 high quality for polyhydric alcohols.For passing the exam it is enough to know the signs of this reaction, and it is not necessary to be able to write down the interaction equation itself.
3. As well as monohydric alcohols, polyhydric alcohols can enter into an esterification reaction, i.e. react with organic and oxygen-containing inorganic acidswith the formation of esters. This reaction is catalyzed by strong inorganic acids and is reversible. In this regard, during 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 with a large number of carbon atoms in the hydrocarbon radical, resulting from such a reaction, enter into a reaction with glycerol, esters are called fats.
In the case of etherification 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 with constant cooling:
An ester of glycerin 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 an attack of a stroke or heart attack.
Substitution of hydroxyl groups
Reactions of this type proceed by the mechanism of nucleophilic substitution. Interactions of this kind include the reaction of glycols with hydrogen halides.
So, for example, the reaction of ethylene glycol with hydrogen bromide proceeds with the sequential replacement of hydroxyl groups by halogen atoms:
Chemical properties of phenols
As mentioned at the very beginning of this chapter, chemical properties phenols differ markedly from the chemical properties of 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 a hydroxyl group
Acidic properties
Phenols are stronger acids than alcohols, and are dissociated to a very small extent in aqueous solution:
B aboutthe higher acidity of phenols in comparison with alcohols in terms of chemical properties is expressed in the fact that phenols, in contrast to 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. So, in particular, carbon dioxide, when passing it through water solution phenolates of alkali metals, displaces free phenol from the latter as an even weaker acid than carbonic acid:
Obviously, phenol will also be displaced from phenolates by any other stronger acid:
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 nucleus
The hydroxyl group is a substituent of the first kind, which means that it facilitates the occurrence of substitution reactions in ortho- and couple-positions in relation to yourself. Reactions with phenol take place under much milder conditions than benzene.
Halogenation
The reaction with bromine does not require any special conditions... When bromine water is mixed 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 crystalline explosive yellow color:
Addition reactions
Since phenols are unsaturated compounds, their hydrogenation in the presence of catalysts to the corresponding alcohols is possible.
The generally accepted mechanism for the dehydration of alcohols is as follows (for simplicity, we took ethanol):
The alcohol attaches a hydrogen ion in step (1) to form a protonated alcohol, which dissociates in step (2) to give a water molecule and a carbonium ion; then the carbonium ion step (3) loses the hydrogen ion and an alkene is formed.
Thus, the double bond is formed in two stages: the loss of the hydroxyl group in the form [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 is the Bronsted-Lowry acid-base equilibrium (Section 1.19). When sulfuric acid is dissolved in water, for example, the following reaction occurs:
The hydrogen ion passed from a very weak base to a stronger base to form an oxonium ion. The basic properties of both compounds are, of course, due to the lone pair of electrons that can bind 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 passed from the bisulfate ion to a stronger base (ethyl alcohol) to form the substituted oxonium ion of the protonated alcohol.
Similarly, stage (3) is not the expulsion of the free hydrogen ion, but its transition to the strongest of the available bases, namely to
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, in fact, a proton is transferred from one base to another.
All three reactions are shown as equilibrium, 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 sulphuric acid almost completely ionized in alcoholic solution. Since the concentration of carbonium ions present at each moment is very low, equilibrium (2) is shifted strongly to the left. At some point, one of these few carbonium ions reacts according to equation (3) to form an alkene. Upon dehydration, the volatile alkene is usually distilled off from the reaction mixture, and thus equilibrium (3) is shifted to the right. As a result, the whole reaction comes to an end.
The carbonium ion is formed by the dissociation of the protonated alcohol; in this case, the charged particle is separated from
a neutral particle Obviously, this process requires much 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, the weak base (water) is cleaved from the carbonium ion (Lewis acid) much more easily than the very strong base, the hydroxyl ion, i.e., water is a better leaving group than the hydroxyl ion. It has been shown that the hydroxyl ion is almost never cleaved from alcohol; the reaction of cleavage of the bond in alcohol in almost all cases requires an acidic 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 becomes possible only due to the solvation of the carbonium ion (cf. Section 5.14). The energy for breaking the carbon-oxygen bond is taken due to the formation of a large number of ion-dipole bonds between the carbonium ion and the polar solvent.
Carbonium ion can enter into different reactions; which one occurs depends on the experimental conditions. All reactions of carbonium ions are completed in the same way: they acquire a pair of electrons to fill an octet of a positively charged carbon atom. 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 made a bond with this hydrogen can now form a -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 series tertiary secondary primary? Before answering this question, it is necessary to find out how the stability of carbonium ions changes.
Alkenes hydration Hydration of olefins is of the greatest industrial importance. The addition of water to olefins can be carried out in the presence of sulfuric acid - sulfuric acid hydration or by passing a mixture of olefin with steam over a phosphate catalyst Н3Р04 on aluminosilicate ...(ORGANIC CHEMISTRY)
Oxidation of alcohols
When alcohols burn, carbon dioxide and water are formed: Under the action of conventional oxidizing agents - chromium mixture, potassium permangate, the carbon atom at which the hydroxyl group is located is oxidized first. Primary alcohols give aldehydes during oxidation, which easily pass ...(ORGANIC CHEMISTRY)
Oxidation of ethyl alcohol to acetic acid.
Ethyl alcohol is oxidized to acetic acid under the influence of acetic acid bacteria of the genera Gluconobacter and Acetobacter. These are gram-negative chemoorganoheterotrophic, non-spore-forming, rod-shaped organisms, mobile or immobile. Acetic acid bacteria of these genera differ in ...(BASICS OF MICROBIOLOGY)
Catalytic dehydrogenation of alcohols
The transformation of alcohols into aldehydes and ketones can also be carried out by dehydrogenation - passing alcohol vapor over a heated catalyst - copper or silver at 300 ° C: The interaction of alcohols with organomagnesium compounds (Grignard reagents) leads to the formation of saturated hydrocarbons: This ...(ORGANIC CHEMISTRY)
Alcohol and alcohol-containing products
The excisable goods include only ethyl alcohol (raw and rectified alcohol), regardless of the type of raw material from which it is produced (food or non-food). Industrial alcohol (this is not ethyl alcohol) is not an excise product, it is obtained from wood or petroleum products. For the production of excise ...(Taxation of commercial activities)
Dehydrogenation reactions of alcohols are necessary to obtain aldehydes and ketones. Ketones are derived from secondary alcohols, and aldehydes from primary alcohols. The catalysts in the processes are copper, silver, copper chromites, zinc oxide, etc. It should be noted that, in comparison with copper catalysts, zinc oxide is more stable and does not lose activity during the process, but it can provoke a dehydration reaction. In general, the reaction of dehydrogenation of alcohols can be represented as follows:
In industry, the dehydrogenation of alcohols produces such compounds 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 supported. 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 parent substance. 2-propanol, which is a secondary alcohol, is dehydrated to acetone, and 1-propanol, as a primary alcohol, is dehydrated to propanal at atmospheric pressure and a process temperature of 250 - 450 ° C.
3. also depends on the structure of the starting compound, which affects the final product (aldehyde or ketone).
4. Dehydrogenation of methanol... This process is not fully understood, but most researchers identify it as a promising process for the synthesis of formaldehyde that does not contain water. Various process parameters are offered: temperature 600 - 900 ° C, active catalyst component zinc or copper, support silicon oxide, the possibility of initiating the reaction with hydrogen peroxide, etc. At the moment, most of the formaldehyde in the world is produced by the oxidation of methanol.