Question

Propose mechanisms to show the interchange of protons between ethanol molecules under

1. Acid catalysis
2. Base catalysis.

Short answer

Ethanol in the presence of an acid, gets protonated in the first step, as hydroxyl group present in ethanol acts as a base and abstracts the proton and gets converted to good leaving group, that is water. In the next step, the anionic part of an acid acts as a good base and abstracts the proton from hydroxyl so as to neutralize the positive charge on oxygen which leads to formation of ethanol as the product.

Step-by-step solution

Step-1. Explanation of part (a):

Ethanol in the presence of an acid, gets protonated in the first step, as hydroxyl group present in ethanol acts as a base and abstracts the proton and gets converted to good leaving group, that is water. In the next step, the anionic part of an acid acts as a good base and abstracts the proton from hydroxyl so as to neutralize the positive charge on oxygen which leads to formation of ethanol as the product.

Step-2. Explanation of part (b):

Ethanol in presence of a base, undergoes acid-base reaction in the first step as acid-base reactions are the fastest, thus, base abstracts the proton from hydroxyl group of ethanol converting it into ethoxide ion which is a good nucleophile, and in next step, ethoxide ion attacks at the proton of an acid, and forms ethanol.