Preparation of Alcohols: Definition and Methods

Preparation of Alcohol: Alcohol be it medicinal or recreational, the uses of alcohol in daily life are vast. Generally, alcohol is described as a compound that comprises one or more hydroxyl groups which are directly attached to a carbon chain. Alcohol is found in volatile oils which are extracted from various sources like leaves, stems, and flowers, of plants. It is a relatively common organic chemical with a lot of industrial applications.

Alcohol is of 4 types namely, monohydric alcohol, dihydric alcohol, trihydric alcohol, and a polyhydric alcohol. There are several ways in which alcohol is prepared in the industries and laboratories. In this article, we will learn about the preparation of alcohol and the uses of alcohol in human body. Scroll down to learn more!

Preparation of Alcohol: Definition

Alcohols are organic compounds with one or more hydroxyl groups \(( – {\rm{OH}})\) bonded to aliphatic carbon atoms. The general formula of Alcohol is \({\rm{ROH}}\)

Alcohols are compounds containing hydroxyl \(( – {\rm{OH}})\) attached to the alkyl group. Therefore, these are regarded as hydroxy derivatives of hydrocarbons and are also called aliphatic alcohols.

For example,

Alcohols in which the hydroxyl group \(( – {\rm{OH}})\) is present in the side chain of an aromatic hydrocarbon known as aromatic alcohols.

For example,

Classification of Alcohol

Alcohol can be classified as;

1. Monohydric alcohol: Alcohol with one hydroxyl group is known as monohydric alcohol. Examples- Methyl alcohol, ethyl alcohol, and propyl alcohol.
   Saturated monohydric alcohol is further classified into primary alcohol, secondary alcohol, and tertiary alcohol.
2. Dihydric alcohol: Alcohol with two hydroxyl groups is known as dihydric alcohol.
   Example, \({\rm{ – 1,2 – }}\)Ethanol.
3. Trihydric alcohol: Alcohol with three hydroxyl groups is known as dihydric alcohol.
   Example, \({\rm{ – 1,2,3 – }}\).
4. Polyhydric alcohol: Alcohol with four or more than four hydroxyl groups is known as polyhydric alcohol.
   Example- Hexaphenol.

What are the Methods of Preparation of Alcohol?

The methods of preparation of alcohol are discussed below:

Preparation of Alcohol from Alkenes

i. By acid-catalyzed Hydration: Alkenes can be hydrated to produce alcohols. Alkenes can be hydrated indirectly by adding sulphuric acid to produce alkyl hydrogen sulfate, which can then be hydrolyzed with hot water to produce alcohol. Acid-catalyzed hydration proceeds according to the Markovnikov rule.
For example,

ii. Hydration by Oxymercuration-Demercuration: The oxymercuration-demarcation reaction can also be used to convert alkenes to alcohol. In this reaction, an alkene is treated with mercuric acetate in a \({\rm{THF – }}\)water solution to produce a product that may then be reduced with \({\rm{NaB}}{{\rm{H}}_4}\) to produce alcohol. According to Markownikov’s rule, addition occurs in this reaction.

iii. By hydroboration – oxidation reaction: The hydroboration – oxidation reaction can also indirectly add water to olefins. Alcohol is generated when an alkene is treated with diborane \({{\rm{(B}}{{\rm{H}}_{\rm{3}}}{\rm{)}}_{\rm{2}}}\) and then treated with \({{\rm{H}}_{\rm{2}}}{{\rm{O}}_{\rm{2}}}\) in the presence of aqueous sodium hydroxide. Because diborane is electron-poor, it behaves as an electrophile in this process.

Preparation of Alcohol by Reduction of Aldehydes and Ketones

1. The reduction of aldehydes and ketones is used to prepare alcohols. The reduction process is carried out by common reducing agents as follows:

(i). Hydrogen in the presence of a catalyst such as finely divided platinum, palladium, nickel, and ruthenium.
(ii). Sodium in the presence of ethyl alcohol as a reducing agent.
(iii). Lithium aluminium hydride or sodium borohydride.

During reduction, aldehyde gives primary alcohols while ketone gives secondary alcohols.

For example,

For example,

Preparation of Alcohols from Carbonyl Compounds

Reduction of carbonyl compounds involves adding hydride ion reducing the carbonyl group to alkaloid ion, which on protonation gives alcohol.

The complex hydrides, \({\rm{LiAl}}{{\rm{H}}_4}\) and \({\rm{NaB}}{{\rm{H}}_4}\)

Preparation from Haloalkanes

When haloalkane is boiled with aqueous sodium hydroxide or potassium hydroxide, or silver oxide, it gives alcohol.

For example,

Preparation from Grignard’s Reagent

A. By reaction with aldehyde, ketones, and esters: Grignard’s reagents are alkyl or aryl magnesium halide. The \({\rm{C – Mg}}\) bond, Grignard’s reagents are a very versatile reagent in organic synthesis. Grignard reagent combines with aldehydes and ketones to generate an additional product, which decomposes in dilute hydrochloric or sulphuric acid to provide primary, secondary, and tertiary alcohols. Primary alcohols are produced by formaldehyde, while all other aldehydes produce secondary alcohols, and ketones produce tertiary alcohols.

This reaction obtains different types of alcohols:

i. Formaldehyde gives primary alcohol

ii. All aldehydes other than formaldehyde give secondary alcohol

iii. Ketone gives tertiary alcohol

iv. Ester gives secondary and tertiary alcohols

Esters can also be converted into alcohols by treating with suitable Grignard’s reagent. Secondary alcohols can be made by adding a suitable Grignard’s reagent to a formic acid ester and then acid hydrolyzing it.
For example,

Tertiary alcohols can be made by adding a Grignard reagent to an ester other than formic ester and then acid hydrolyzing it.

B. By reaction with ethylene oxide or oxirane:

In the presence of anhydrous ether, oxy alkanes such as oxiranes or ethylene oxide react with a Grignard reagent to form additional products, which are hydrolyzed by water in the presence of acid to form alcohols. It is an excellent method for converting an alkyl halide into primary alcohol containing two carbon atoms more than the alkyl halide.

For example, 

Preparation of Alcohol by Hydrolysis of Ester

Alcohols are generally prepared by hydrolysis of an ester with aqueous alkalies.

For example,

Preparation of Alcohol from Aliphatic Primary Amines

When primary amines react with nitrous acid, alcohol is formed along with the evolution of nitrogen gas.

For example,

Preparation of Alcohol by Reduction of Carboxylic Acids and Esters

a. In the presence of a strong reducing agent, lithium aluminium hydride, carboxylic acids are reduced to primary alcohols.

For example,

b. When esters are converted to alcohols, they produce a combination of two alcohols: one from the acyl group and one from the alkoxy group.

For example,

Industrial Preparation of Alcohol

The important methods for the preparation of alcohol on a large scale are given below;

1. By hydration of alkenes: The cracking of petroleum produces alkenes. In the presence of sulphuric acid, they are easily converted to alcohol by adding water.

In the case of unsymmetrical alkenes, the addition takes place according to Markownikov’s rule.

2. By Fermentation of Carbohydrates: One of the oldest methods of producing ethyl alcohol is through sugar fermentation. With the help of the enzyme invertase, sucrose is first converted to glucose and fructose. The fermentation process is carried out in anaerobic conditions, which means that there is no air present. The carbon dioxide produced during fermentation keeps the fermentation mixture from coming into contact with air. When the fermentation mixture is exposed to air, the oxygen in the air oxidizes ethanol to ethanoic acid, causing the mixture to sour.

Enzyme zymase then converts glucose and fructose to ethanol.

3. By Oxo Process: Aldehydes are formed when alkene reacts with carbon monoxide and hydrogen in the presence of octacarbonyl dicobalt as a catalyst at high temperature and pressure. Primary alcohols are produced by catalytic hydrogenation of aldehyde.

Solved Examples on Preparation of Alcohols

Q.1. Give the structures and IUPAC names of products expected from the following reactions:
a. The catalytic reaction of butanal.
b. Hydration of propene in the presence of dilute sulphuric acid.
c. The reaction of propanone with methylmagnesium bromide followed by hydrolysis.

Ans: a.

b.

c.

Q.2. Name two methods by which secondary alcohol can be prepared.
Ans: Secondary alcohols can be prepared by
i. Reduction of Ketone.
ii. The reaction of Grignard reagents on aldehydes followed by hydrolysis.

Q.3. How will you prepare pentan-1-ol from 1-bromopentane?
Ans: Pentan-1-ol can be prepared by treating 1-bromopentane with an aqueous solution of potassium hydroxide.

Q.4. Write the chemical reactions involved in the preparation of the following alcohols using Grignard’s reagent
a. \({\rm{2 – }}\)Pentylbutan\({\rm{ – 2 – Ol}}\)
b. \({\rm{3 – }}\)Methylpentan\({\rm{ – 3 – Ol}}\)
c. \({\rm{2 – }}\)Methylpentan\({\rm{ – 2 – Ol}}\)
Ans:

a.

b.

c.

Summary

From this article, we can conclude that alcohol is a relatively common organic chemical with a lot of industrial applications. The general formula of Alcohol is \({\rm{ROH}}\). Alcohol can be classified into monohydric alcohol, dihydric alcohol, trihydric alcohol, and polyhydric alcohol.

Alcohol is prepared when an alkene is treated with diborane \({{\rm{(B}}{{\rm{H}}_3})_2}\)and then treated with\({{\rm{H}}_{\rm{2}}}{{\rm{O}}_{\rm{2}}}\) in the presence of aqueous sodium hydroxide. According to Markownikov’s rule, addition occurs in this reaction. The oxymercuration-demarcation reaction can also be used to convert alkenes to alcohol.

FAQs on Preparation of Alcohols

Q.1. How are alcohols prepared?
Ans: Alcohols are prepared in the laboratory by the following methods;
1. Generally, through fermentation reaction.
2. Preparation of alcohol from alkene
3. By reduction of aldehydes and ketones
4. From Grignard reagent
5. From carbonyl compounds
6. By hydrolysis of ester

Q.2. What are alcohols? Give any two methods of preparation of alcohols?
Ans: Alcohols are organic compounds with one or more hydroxyl groups\(({\rm{ – OH}})\)bonded to aliphatic carbon atoms.
Two methods of preparation of alcohols are:
i. Preparation of Alcohol from Alkenes – By acid-catalyzed Hydration: Alkenes can be hydrated to produce alcohols. Alkenes can be hydrated indirectly by adding sulphuric acid to produce alkyl hydrogen sulfate, which can then be hydrolyzed with hot water to produce alcohol.
ii. Preparation of Alcohol by hydrolysis of ester: Alcohols are generally prepared by hydrolysis of the ester with aqueous alkalies.

Q.3. What is the industrial preparation of alcohol?
Ans: One of the oldest methods of producing ethyl alcohol is sugar fermentation. With the help of the enzyme invertase, sucrose is first converted to glucose and fructose. The fermentation process is carried out in anaerobic conditions, which means that there is no air present. The carbon dioxide produced during fermentation keeps the fermentation mixture from coming into contact with air. However, when the fermentation mixture is exposed to air, the oxygen in the air oxidizes ethanol to ethanoic acid, causing the mixture to sour. Enzyme zymase then converts glucose and fructose to ethanol.

Q.4. How do you make alcohol in chemistry?
Ans: Alcohols are formed by the reaction of Grignard reagents with aldehydes and ketones and the acid-catalyzed hydration of alkenes. 

Q.5. What are 4 types of alcohol?
Ans: The four types of alcohol are;
1. Monohydric alcohol
2. Dihydric alcohol
3. Trihydric alcohol
4. Polyhydric alcohol

Q.6. What is the name of a reaction to prepare alcohol?
Ans: Generally, fermentation of carbohydrate reaction is used to prepare alcohol.

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