Question

The choice of acid is important in the dehydration reaction. The following acids are not suitable for dehydration of 2-butanol: acetic acid and hydrobromic. Explain why each of the acids would not be a good choice to affect the dehydration in good yield.

Answer 1

In dehydration reaction, an organic compound (in this case, an alcohol) loses a water molecule to give a an alkene which contains a double bond as a result of the elimination of water molecule.

Usually we use strong acids such as sulfuric acid or phosphoric acid for carrying out dehydration reactions of organic alcohols.

Acetic acid is a very weak acid, and it is present in aqueous form, which means that even in its concentrated form it contains a lot of water. So using this to remove water molecules from another compound does not make sense because dehydration reaction can't be effectively carried out in an aqueous environment, Sulfuric acid, on the other hand, is 98% strong and has negligilbe water content.

Also, acetic acid is a very weak acid, and hence its conjugate base, the acetate ion, is a very strong nucleophile. So instead of carrying out elimination, it causes substitution reaction.

Similarly, with hydrobromic acid, we have the same problems. Firstly it is aqueous and contains a lot of water. Secondly, the Br- anion is a much better nucleophile as compared to the HSO4- anion produced by sulfuric acid. Thus it also supports the substitition reaction mechanism instead of elimination.

Thus sulfuric acid is preferred over thee two acids.