Chapter 18, Problem 18.32P

Interpretation Introduction

Interpretation:

The complete, detailed mechanism for a given reaction is to be drawn, and the major product is to be predicted.

Concept introduction:

Intramolecular reactions are the ones in which the electron-rich and electron-poor sites are present within the same molecule. When ketones or aldehydes are treated with a strong base, they yield $\text{β}$-hydroxycarbonyl compounds via an aldol addition. Under these basic conditions, an enolate anion acts as a nucleophile. An intramolecular reaction is favored when it forms a five or six-membered ring. Intramolecular reactions involving the attack of an aldehyde $\text{C=O}$ are favored over ones involving the attack of a ketone $\text{C=O}$. The first step is the formation of an enolate ion. The ${\text{HO}}^{-}$ reversibly deprotonates the alpha carbon to yield an enolate anion. In step two, the newly formed enolate anion, which serves as a nucleophile, attacks the carbonyl carbon present within the same molecule. The product of this step is an alkoxide ion, which is subsequently protonated in step 3. The nucleophilic addition reaction to a ketone is generally less favorable than the addition to an aldehyde because in a ketone, there are two alkyl groups attached to the carbonyl carbon, but in an aldehyde, only one alkyl group is attached. The electron-donating effects from a ketone’s alkyl groups diminish the concentration of positive charge on the carbonyl carbon, making it less susceptible to the nucleophilic attack.