Questions: Nucleophilic Acyl Substitution

Question 1

In nucleophilic acyl substitution, what distinguishes the outcome from nucleophilic addition to an aldehyde or ketone?

• A. Only acyl substitution proceeds through a tetrahedral intermediate; addition reactions do not
• B. In acyl substitution the product retains a carbonyl group because the leaving group departs; in addition the carbonyl is consumed and the nucleophile is retained
• C. Acyl substitution requires a basic catalyst while addition requires an acidic catalyst
• D. Addition reactions are faster because they do not require a leaving group

Answer: (B)

Both reactions begin with nucleophilic attack on the carbonyl carbon to form a tetrahedral intermediate — so the first step is identical. The divergence is in the second step: in nucleophilic addition (to aldehydes/ketones), the intermediate is protonated and the product is an alcohol with no carbonyl; there is no leaving group to expel. In nucleophilic acyl substitution, the tetrahedral intermediate collapses by ejecting the leaving group (e.g., Cl⁻, OR⁻), regenerating a new carbonyl in the product.

Question 2

Saponification (base-catalyzed ester hydrolysis) is reversible because the carboxylate product can react with the alcohol under basic conditions to re-form the ester.

• T. True
• F. False

Answer: False

Saponification is irreversible. The carboxylate anion produced under basic conditions is much less electrophilic than the ester — the negative charge on oxygen donates electron density into the carbonyl, greatly reducing the electrophilicity of the carbonyl carbon. Additionally, the alcohol product is deprotonated to an alkoxide under strongly basic conditions, and alkoxide is not a good leaving group even if any backward reaction were attempted. The thermodynamic sink of the stable carboxylate makes the reaction effectively irreversible.

Question 3

A student proposes that the tetrahedral intermediate in nucleophilic acyl substitution is simply a transition state, like the transition state in an SN2 reaction. Why is this wrong?

Model answer: A transition state is a saddle point on the energy surface — it has no finite lifetime and cannot be isolated or detected as a species. The tetrahedral intermediate in nucleophilic acyl substitution is a local energy minimum: it is a real molecule with four bonds to the carbonyl carbon, a finite (though short) lifetime, and in principle can be trapped or observed spectroscopically. The reaction coordinate has two transition states (one for formation, one for collapse of the intermediate) with the intermediate as a valley between them.

The distinction between transition state and intermediate is fundamental in mechanism. Transition states (like the SN2 trigonal bipyramidal TS) are peaks on the energy diagram — momentary and not isolable. Intermediates are valleys — they persist long enough to be called a 'species,' even if only transiently. The tetrahedral intermediate in NAS has been trapped in some systems and has distinct spectroscopic signatures. Recognizing this difference helps predict reactivity and design mechanisms correctly.