Rank the compounds in each group in order of increasing...

Rank the compounds in each group in order of increasing reactivity in nucleophilic acyl substitution. a. $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CO}_{2} \mathrm{CH}_{3}, \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCl}, \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CONH}_{2}$ b. $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{H},\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CO}\right)_{2} \mathrm{O}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CONHCH}_{3}$

Rank the compounds in each group in order of increasing reactivity in nucleophilic acyl substitution.
a. $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CO}_{2} \mathrm{CH}_{3}, \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCl}, \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CONH}_{2}$
b. $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{H},\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CO}\right)_{2} \mathrm{O}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CONHCH}_{3}$

Key Concepts

Nucleophilic Acyl Substitution

This reaction mechanism involves a nucleophile attacking an acyl compound to form a tetrahedral intermediate, which then collapses by ejecting a leaving group. The overall reactivity in such reactions depends on both how strongly the carbonyl carbon attracts nucleophiles (its electrophilicity) and on the ability of the substituent leaving group to depart after nucleophilic attack.

Carbonyl Electrophilicity

The electrophilicity of the carbonyl carbon is a key factor in determining the reactivity of acyl compounds. A more electron?deficient carbonyl carbon is more prone to attack by nucleophiles, which means that substituents that withdraw electron density enhance the reaction rate. This principle explains why acyl compounds with more electrophilic carbonyls are generally more reactive in nucleophilic acyl substitution.

Leaving Group Ability

In nucleophilic acyl substitution, the rate of the reaction is significantly influenced by the quality of the leaving group. Good leaving groups, which can stabilize the negative charge once they depart (such as chloride in acid chlorides), facilitate a faster reaction. Conversely, poor leaving groups, like hydroxide or amide anions that are stabilized by resonance and thus less willing to leave, slow down the reaction.

Resonance Stabilization

Resonance stabilization involves the delocalization of electrons between the carbonyl group and adjacent atoms or groups. When substituents such as amide or ester groups donate electrons through resonance, they stabilize the carbonyl, reducing its electrophilicity and making it less reactive toward nucleophilic attack. The extent of resonance stabilization directly correlates to the observed order of reactivity in nucleophilic acyl substitutions.

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