Chapter 18, Problem 18.6P

Interpretation Introduction

(a)

Interpretation:

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

Concept introduction:

There are two locations in an $\text{α,β-unsaturated}$ carbonyl that are subject to a nucleophilic attack. One is the carbonyl carbon, leading to a $\text{1, 2-}$ or direct addition product. The second is the $\text{β}$-carbon, leading to a $\text{1, 4-}$ or conjugate addition product. Uncharged nucleophiles heavily favor conjugate addition over direct addition. An uncharged nucleophile favors conjugate addition because the first step of the mechanism, nucleophilic addition, is reversible and occurs under thermodynamic control. Conjugate addition involves a resonance-stabilized enolate anion. The addition of the uncharged nucleophile to $\text{α,β-unsaturated}$ carbonyl compound does not require acidic condition. Although $\text{HCN}$ (${\text{CN}}^{-}$) is a charged nucleophile, it adds in a reversible manner.

Answer to Problem 18.6P

The product of given reaction is

The mechanism is drawn as

Explanation of Solution

Even though ${\text{NC}}^{\text{-}}$ is a charged nucleophile, it adds to an $\text{α,β-unsaturated}$ carbonyl reversibly. Catalytic amount of KOH increases the concentration of ${\text{NC}}^{\text{-}}$ as a result of the acid-base reaction with HCN. ${\text{NC}}^{\text{-}}$ then adds to the $\text{β}$-carbon, shifting the $\text{π}$ electron pair toward the carbonyl carbon and the carbonyl $\text{π}$ electron pair to the oxygen atom.

The negatively charged oxygen extracts a proton from the solvent molecule (${\text{H}}_{\text{2}}\text{O}$) to form a neutral enol form of the product. This proton transfer step involves a solvent molecule rather than HCN because of the basic conditions.

The enol form being relatively unstable rapidly tautomerizes to the more stable keto form of the product.

Thus, the final product of the reaction is

And the complete mechanism can be drawn as

Conclusion

The mechanism and the product of the given reaction were determined on the basis of reversible addition of cyanide ion to the $\text{α,β-unsaturated}$ carbonyl.

Interpretation Introduction

(b)

Interpretation:

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

Concept introduction:

There are two locations in an $\text{α,β-unsaturated}$ carbonyl that are subject to a nucleophilic attack. One is the carbonyl carbon, leading to a $\text{1, 2-}$ or direct addition product. The second is the $\text{β}$-carbon, leading to a $\text{1, 4-}$ or conjugate addition product. Uncharged nucleophiles heavily favor conjugate addition over direct addition. An uncharged nucleophile favors conjugate addition because the first step of the mechanism, nucleophilic addition, is reversible and occurs under thermodynamic control. Conjugate addition involves a resonance-stabilized enolate anion. The addition of the uncharged nucleophile to $\text{α,β-unsaturated}$ carbonyl compound does not require acidic condition.

Answer to Problem 18.6P

The product of the given reaction is

The mechanism is drawn as

Explanation of Solution

${\text{PhCH}}_{\text{2}}\text{SH}$ is an uncharged nucleophile that favors conjugate addition to an $\text{α,β-unsaturated}$ carbonyl compound. The uncharged nucleophile adds reversibly at the $\text{β-}$ carbon.

Two proton transfer steps follow and produce an uncharged enol. The enol form is relatively unstable and rapidly tautomerizes to the more stable keto form.

Thus, the final product of the reaction is

And the complete mechanism can be drawn as

Conclusion

The mechanism and the product of the given reaction were determined on the basis of reversible addition of the uncharged nucleophile to the $\text{α,β-unsaturated}$ carbonyl.

Interpretation Introduction

(c)

Interpretation:

The complete and detailed mechanism of the given reaction is to be drawn and the product is to be predicted.

Concept introduction:

There are two locations in an $\text{α,β-unsaturated}$ carbonyl that are subject to nucleophilic attack. One is the carbonyl carbon, leading to a $\text{1, 2-}$ or direct addition product. The second is the $\text{β}$-carbon, leading to a $\text{1, 4-}$ or conjugate addition product. Uncharged nucleophiles heavily favor conjugate addition over direct addition. An uncharged nucleophile favors conjugate addition because the first step of the mechanism, nucleophilic addition, is reversible and occurs under thermodynamic control. Conjugate addition involves a resonance-stabilized enolate anion. The addition of uncharged nucleophile to $\text{α,β-unsaturated}$ carbonyl compound does not require acidic condition.

Answer to Problem 18.6P

The product of given reaction is

The mechanism of the reaction is

Explanation of Solution

Pyrrolidine is uncharged nucleophile and favors conjugate addition to an $\text{α,β-unsaturated}$ carbonyl compound. The uncharged nucleophile adds reversibly to the $\text{β-}$ carbon.

After the nucleophilic addition, two proton transfer steps occur to produce an uncharged enol. The enol form is relatively unstable and rapidly tautomerizes to the more stable keto form.

Thus, the final product of the reaction is

And the complete mechanism of the reaction can be drawn as

Conclusion

The mechanism and the product of the given reaction were determined on the basis of reversible addition of the uncharged nucleophile to the $\text{α,β-unsaturated}$ carbonyl.