Chapter 16, Problem 98P

(a)

Interpretation Introduction

Interpretation:

Reaction mechanism has to be proposed for the given reaction.

Concept introduction: A cyclic acetal is formed on a combination of an aldehyde and a diol. An acetal is a protecting group for aldehyde and ketones.  A cyclic acetal on heating with aqueous acid gives back aldehyde and diol.  Heating in aqueous acidic medium causes the deprotection of aldehyde group.

(b)

Interpretation Introduction

Interpretation:

Reaction mechanism has to be proposed for the given reaction.

Concept introduction: Nucleophile ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}\text{SH}$ is a weak base.  The weak base forms a conjugate addition product.  The conjugate addition product undergoes tautomerism.  Keto-enol tautomerism results in a gain of aromaticity in the molecule.  Therefore in this particular case enol form is more stable than the keto form.