When one or more hydroxyl groups (-OH) are directly attached to the hydrocarbon chain then the compound formed is known as alcohol. Some examples of alcohol are phenol, methanol and ethanol. 

Alcohols can be classified into various types based on the number of hydroxyl groups in the structure of the compound. These types are:

• Monohydric Alcohol: Alcohols containing only one hydroxyl group are called monohydric alcohols. For example, ethanol and methanol
• Dihydric Alcohol: Alcohols containing two hydroxyl groups are called monohydric alcohols. Example, 1,2-Ethanol.
• Trihydric Alcohol: Alcohols containing three hydroxyl groups are called monohydric alcohols. Example 1,2,3-Propanol.
• Polyhydric Alcohol: Alcohols containing more than 3 hydroxyl groups are called monohydric alcohols. Example, 1,2,3,4-Pentanol.

Preparation of Alcohols

Alcohols can be synthetically prepared through various processes. Various processes used in the preparation of alcohol are,

• Hydrolysis of Halides
• From Alkenes
• From Carbonyl Compounds
• From Grignard Reagents
• Action of Nitrous Acid on Primary Amines
• Fermentation

These processes are discussed below in the article.

Hydrolysis of Halides

Both primary and secondary alcohols can be produced by boiling an aqueous solution of alkali hydroxides such as potassium hydroxide or sodium hydroxide with a suitable alkyl halide such as bromomethane, colour ethane, etc. The reaction is a nucleophilic substitution reaction. Below is an example of the preparation of alcohol using this method:

R-X + NaOH —-> R-OH + NaX

CH₄ + NaCl —-> CH₃OH + NaCl

From Alkenes

Alcohols are easily produced using the Alkenes. The process used in the production of alcohols from alkenes is given in detail below,

Hydration of Alkenes

Alkenes undergo additional reactions with water in the presence of a catalyst to produce alcohol. The addition reaction takes place according to Markovnikov’s rule. The basic reaction is as follows:

 

Hydroboration-Oxidation

Diborane (BH₃)₂ reacts with alkenes to form trialkyl boranes which are further converted to alcohol through oxidation by Hydrogen Peroxide in the presence of aqueous sodium hydroxide. The boron atom gets bonded to the sp2 hybridized carbon with the maximum number of Hydrogen. The result of the reaction appears to be opposite to the Markovnikov rule. The yield of alcohol through this reaction is very good in quantity. 

 

From Carbonyl Compounds

Alcohols are easily produced using Carbonyl Compounds. The process used in the production of alcohols from carbonyl compounds is given in detail below,

Reduction of Aldehydes and Ketones

The addition of Hydrogen to aldehydes and ketones in the presence of catalysts such as finely divided platinum, palladium or nickel leads to the reduction of aldehydes and ketones to alcohols. It can also be prepared to treat aldehydes and ketones with sodium borohydride NaBH₄ or lithium aluminium hydride LiAlH₄. Primary alcohols are formed using aldehydes whereas secondary alcohols are formed using ketones. The reactions for both of them are shown below:

 

Reduction of Carboxylic Acids and Esters

Carboxylic acids and Esters react with lithium aluminium hydride LiAlH to undergo a reduction reaction. This is an expensive process due to the cost associated with the catalyst i.e. lithium aluminium hydride. Thus it is not commercially feasible to produce alcohol using this method.

 

From Grignard Reagents

Alcohols are easily produced using Grignard Reagents. The process used in the production of alcohols from Grignard reagents is given in detail below,

Reaction between Grignard Reagent and Aldehydes and Ketones

Grignard reagents are Alkyl Magnesium halides (RMgX). These react with ketones and aldehydes. The primary alcohol is prepared with the reaction of the Grignard reagent with formaldehyde while reactions of other aldehydes and ketones with the Grignard reagent give secondary and tertiary alcohols respectively.

 

Hydroformylation of Alkenes

Alcohols can be manufactured using hydroformylation of alkenes. In this process, alkenes undergo a reaction in the presence of CO/H2 and a catalyst to form an aldehyde which is an intermediary and it finally reacts with hydrogen in presence of a catalyst to give alcohol. An example of this reaction is shown below:

 

Action of Nitrous Acid on Primary Amines

Nitrous Acid (HNO₂) reacts with primary amines to form alcohols with the evolution of Nitrogen gas. The primary amine first forms an intermediate which undergoes tautomerism and forms a diazonium cation by losing a water molecule. This diazonium cation further loses nitrogen gas and reacts with water to form alcohol. Example:

R-NH₂ + HNO₂ —-> R-OH + N₂

Fermentation

Alcohols are also produced commercially using the process of fermentation. Fermentation is the process of the breakdown of sugars into alcohol and other products. Fermentation is carried out by the action of different bacteria and depending on the type of bacteria, the products are different. For example, glucose is broken down to ethanol in the presence of yeast.

C₆H₁₂O₆ (fermentation)—-> 2C₂H₅OH + 2CO₂ + 2CH₃CHO

Also, Check

• Alcohols, Phenols and Ethers
• Physical properties of Alcohols, Phenols and Ethers
• Chemical reactions of Alcohols, Phenols and Ethers

FAQs on Preparation of Alcohols

Question 1: How can you prepare alcohol from aldehydes and ketones?

Answer:

Alcohol can be prepared from aldehydes and ketones using the following methods:

• Reduction of aldehydes and ketones
• Reaction of Grignard reagent with aldehydes and ketones

Question 2: What are Grignard reagents and how can you prepare secondary alcohol using Grignard reagents?

Answer:

Grignard reagents are Alkyl Magnesium halides (RMgX). These react with ketones and aldehydes. The primary alcohol is prepared with the reaction of Grignard reagent with formaldehyde while reactions of other aldehydes and ketones with Grignard reagent give secondary and tertiary alcohols respectively.

Question 3: Why is a reduction of carboxylic acids and esters not used to manufacture alcohol commercially?

Answer:

Reduction of carboxylic acids and esters to alcohols makes use of Lithium Aluminium Hydride as a catalyst which is very expensive and thus the process is not economically feasible to produce alcohol.

Question 4: What will be the result of the following reaction? Also, identify the type of reaction.

C₂H₅Cl + KOH

Answer:

Result of the above reaction is:

C₂H₅Cl + KOH—–>C₂H₅OH + KCl

This reaction is hydrolysis of alkyl halides and it is a nucleophilic substitution reaction.

Question 5: What is the Markovnikov rule?

Answer:

According to Markovnikov rule, the addition of hydrogen halide takes place such that the hydrogen is attached to the Carbon with maximum number of hydrogens while the halide ion gets attached to the carbon with minimum number of hydrogens.