Reactions of Alcohols

Alcohols are organic molecule that contain one or more hydroxyl groups attached to the aliphatic or aromatic carbon group with the covalent Bond. The compounds obtained by replacing one hydrogen atom from aliphatic hydrocarbons by hydroxyl group are alcohols whereas those obtained by replacing the hydrogen atom of aromatic hydrocarbons are phenols. 

In this article, we have discussed the various alcohol reactions. 

Methods of Formation of Alcohol

The important methods of formation of alcohols are given below:

1. Preparation From Haloalkanes- Haloalkanes, when boiled with aqueous NaOH or KOH or moist silver oxide (AgOH), give alcohols. General reaction for the preparation of alcohols by this method is given below:

R-X + KOH (aq) → R-OH + KX

C2H5Br + KOH  → C2H5-OH + KBr

Primary haloalkanes give a good yield of alcohol. However, tertiary haloalkanes in this reaction give mainly alkenes due to dehydrohalogenation. Secondary haloalkanes give a mixture of alcohol and alkenes.

2. By Reduction of Aldehydes and Ketones- alcohols can be prepared by the reduction of aldehyde and ketones. The reduction is carried by common reducing agents such as hydrogen in the presence of a catalyst (platinum, palladium, and nickel), sodium in the presence of the alcohol, and lithium aluminium hydride.

Chemical Reaction of Alcohol 

In alcohols reactions, alcohol can act both as nucleophiles as well as electrophiles. Alcohols behave as nucleophiles in the reactions in which the bond between O-H is broken. Alcohols can behave as electrophiles in the reactions in which the bond between C-O is broken.

1. Oxidation of Alcohols- The oxidation of alcohols involves the formation of carbon-oxygen double bond (C=O) with cleavage of O-H and C-H bonds. This type of cleavage and formation of bonds occur in oxidation reactions. These reactions are also called dehydrogenation reactions because they involve the loss of hydrogen from alcohol. 

1. Oxidation of primary alcohols- A primary alcohol is easily oxidised to form first an aldehyde and then a carboxylic acid. Both the aldehyde and the acid formed to contain the same number of carbon atoms as the parent alcohol.

2. Oxidation of Secondary Alcohols- A secondary alcohol is easily oxidised to form a ketone with chromic anhydride. The ketone may be further oxidised under strong conditions to form a mixture of acids. While the ketone contains the same number of carbon atoms as the parent alcohol, the acids formed, contain a lesser number of carbon atoms.

3. Oxidation of Tertiary Alcohol- The oxidation of tertiary alcohol is very difficult because it does not have hydrogen in the carbon-bearing hydroxyl group (OH). However, the oxidation of tertiary alcohol can be possible when treated with acidic oxidising agents under very strong conditions at elevated temperatures, cleavage of various C-C bonds takes place. They form mixtures of ketones and carboxylic acids. Both the ketones and acids contain a lesser number4 of carbon atoms than the starting alcohols. 

2. Reduction of Alcohol- The Reduction of alcohol is not an easy step. The hydroxyl group (OH) is a poor leaving group. Therefore, the direct reduction of alcohol is not possible at normal room temperature and pressure. The alcohol can be first converted into any other compound by an oxidation reaction and then it can be reduced to the alkane form. Only indirect methods of reduction of alcohols are possible.

3. Acidic Reaction of Alcohol- Alcohols are weakly acidic in nature. Therefore, it reacts with active metals such as sodium, potassium, magnesium, aluminium, etc. to liberate hydrogen gas and form metal alkoxide.

Ethanol Reaction

Ethanol is lower alcohol that gives all the general acidic reactions. Therefore, it is called ethanol acid.

Reaction with Active Metal

C2H5OH + 2M → C2H5OM + H2

Reaction with Metal Hydrides

C2H5OH + NaH → C2H5O- Na + H2

Reaction with Carboxylic Acid

C2H5OH + ROH → RCOOC2H5

4. Dehydration Reaction of Alcohols- The most common question asked by the student in organic chemistry is primary alcohols undergo what reaction to form alkenes? The answer to this question is the dehydration reaction. When the alcohols are heated with a protonic acid such as conc.H2SO4 or H3PO4 at 443 K, they get dehydrated to form alkenes. This reaction mechanism involves three steps:

• Protonation of alcohol mechanism

• Elimination of Water molecule (dehydration)

• Elimination of hydrogen ion (deprotonation)

5. Hydrolysis of Alcohol

The hydrolysis reaction of alcohol is a kind of oxidation reaction. In this reaction, the water molecule acts as a catalyst. Aldehydes and ketones are formed as the main products in this hydrolysis reaction.

CH3CH2OH + H2O → CH3CHO + H2O + H2

Did You Know?

• Biological oxidation of methanol and ethanol occurs in the body.

• If an alcoholic person, by mistake, drinks denatured alcohol the methanol is oxidised in the body first to methanal and then to methanoic acid, which can cause blindness and death.