Question

When a mixture of methanol and ethanol is heated in the presence of concentrated sulfuric acid, the resulting organic product or products is/are :
A. CH₃OC₂H₅
​B. CH₃OCH₃, C₂H₅OC₂H₅
​C. CH₃OC₂H₅, CH₃OCH₃
​D. CH₃OC₂H₅, CH₃OCH₃, and C₂H₅OC₂H₅.

Answer 1

Hint: Alcohols undergo dehydration in the presence of protonic acids to give alkenes or ethers. The product formation depends on the reaction conditions. Protonic acids involve concentrated sulfuric acids or phosphoric acid.

Complete Step by Step Answer:
Symmetrical ethers may be formed by the dehydration of alcohols either in the presence of acids or heated alumina.
Dehydration means the loss of water.
Methanol and ethanol will undergo dehydration if heated in the presence of concentrated sulfuric acid at 413 K.
For example, if excess ethanol is taken at 413 K, ethoxyethane is the main product.
This reaction is the acid-catalysed dehydration of alcohol.
This is a bimolecular substitution reaction involving the attack of the alcohol molecule on a protonated alcohol molecule.

This reaction occurs in three steps:
1. Protonation of alcohol
The presence of two lone pairs of electrons on the oxygen atom of alcohol makes it a Lewis base.
So, they react with strong mineral acids like sulfuric acid to form oxonium salts.
Ethanol forms protonated ethanol and methanol forms protonated methanol.

Image: Formation of protonated alcohol

2. Due to the presence of a positive charge on the oxygen atom, the carbon next to the oxygen atom becomes electron deficient.
So, nucleophilic attack by the deprotonated alcohol molecule occurs on the protonated alcohol to form protonated ether molecules with the loss of water.

Image: Formation of protonated ether.

3. The alcohol molecule finally abstracts a proton from the protonated ether to form ethoxyethane, methoxymethane and methoxyethane.

Image: Formation of ether
So, ethoxyethane, methoxymethane and methoxyethane are formed when a mixture of methanol and ethanol is heated in the presence of concentrated sulfuric acid.
So, option D is correct.

Note: At 413 K, formation of ethers from primary alcohols is accompanied by formation of small amounts of alkenes. Therefore, it is the suitable way of preparation of ethers having unhindered primary alkyl groups. For secondary and tertiary alcohols due to presence of bulky alkyl groups, nucleophilic attack of lone pair of oxygen is not possible due to steric hindrance. So, alkenes are formed in large amounts.