Chapter 17, Problem 17.14YT

Interpretation Introduction

(a)

Interpretation:

It is to be determined whether the given product is of direct or conjugate addition to an α,β-unsaturated carbonyl.

Concept introduction:

In the case of $\text{α,β-}$ unsaturated carbonyl compounds, the weak resonance contributor with separated charges generates a small partial positive charge on the $\text{β}$ carbon due to the resonance hybrid. Thus, a nucleophile can attack at either the carbonyl carbon or the $\text{β}$ carbon. The polar $\text{π}$ bonds of $\text{α,β-}$ unsaturated carbonyl compounds are susceptible to nucleophilic attack at both the electron-poor atom of the polar $\text{π}$ bond and the $\text{β}$ carbon. Attack at the carbonyl carbon yields the direct addition (or $\text{1, 2-addition}$) product, whereas attack at the beta carbon yields the conjugate addition (or $\text{1, 4-addition}$). Direct addition to an $\text{α,β-}$ unsaturated polar $\text{π}$ bond dominates if the nucleophile adds irreversibly to the polar carbonyl bond, and when $\text{1, 4-addition}$ takes place, the nucleophile adds to the $\text{β}$ carbon and reversibly to the carbonyl group. So the product of the direct addition is alcohol on acidic workup while in conjugate addition, the carbonyl group is reverted and the nucleophile adds to the $\text{β}$ carbon.

Interpretation Introduction

(b)

Interpretation:

It is to be determined if the given product is of direct or conjugate addition to an α,β-unsaturated carbonyl.

Concept introduction:

In the case of $\text{α,β-}$ unsaturated carbonyl compounds, the weak resonance contributor with separated charges generates a small partial positive charge on the $\text{β}$ carbon due to the resonance hybrid. Thus, a nucleophile can attack at either the carbonyl carbon or the $\text{β}$ carbon. The polar $\text{π}$ bonds of $\text{α,β-}$ unsaturated carbonyl compounds are susceptible to nucleophilic attack at both the electron-poor atom of the polar $\text{π}$ bond and the $\text{β}$ carbon. Attack at the carbonyl carbon yields the direct addition (or $\text{1, 2-addition}$) product whereas attack at the beta carbon yields the conjugate addition (or $\text{1, 4-addition}$). Direct addition to an $\text{α,β-}$ unsaturated polar $\text{π}$ bond dominates if the nucleophile adds irreversibly to the polar carbonyl bond, and when $\text{1, 4-addition}$ takes place, the nucleophile adds to the $\text{β}$ carbon and reversibly to the carbonyl group. So the product of the direct addition is alcohol on acidic workup while in conjugate addition, the carbonyl group is reverted and the nucleophile adds to the $\text{β}$ carbon.