Questions: Relative Reactivity of Carboxylic Acid Derivatives

Question 1

You want to synthesize an ester from a carboxylic acid derivative and an alcohol under mild conditions. Which derivative would react most readily?

• A. An amide — amines are excellent nucleophiles, suggesting high reactivity toward alcohol
• B. A carboxylic acid — it is the parent compound and reacts directly
• C. An acyl chloride — it has the best leaving group and the least resonance stabilization, making the carbonyl most electrophilic
• D. An ester — the reaction is an exchange of one ester for another and proceeds readily

Answer: (C)

Acyl chlorides sit at the top of the reactivity ladder: Cl⁻ is an excellent leaving group (weak base, stable anion), and chlorine's 3p orbital overlaps poorly with carbon's 2p, so there is minimal resonance donation into the carbonyl. This leaves the carbonyl carbon highly electrophilic, making acyl chlorides react readily with alcohols to give esters. Option A confuses the nucleophile (amine) with the leaving group — in amide hydrolysis, the amine would have to leave as NH₂⁻, an extremely strong base and terrible leaving group.

Question 2

A chemist attempts to convert an amide to an ester by stirring it with excess ethanol at room temperature. Why does this reaction fail?

• A. Ethanol is not nucleophilic enough to attack the carbonyl carbon
• B. The reaction fails because esters are thermodynamically less stable than amides and the equilibrium disfavors product formation — the reaction is just slow
• C. Nitrogen's strong lone-pair donation into the carbonyl makes the carbon less electrophilic, and the amide ion (NH₂⁻) is too strong a base to serve as a leaving group — both factors oppose the reaction
• D. Esters and amides cannot interconvert because they have different functional groups

Answer: (C)

Two reinforcing factors make amides the least reactive derivatives. First, nitrogen donates its lone pair strongly into the carbonyl π* orbital, giving the C–N bond roughly 40% double-bond character and making the carbonyl carbon far less electrophilic. Second, the leaving group would be NH₂⁻ (or NR₂⁻), an extremely strong base — strong bases resist departure. Converting an amide to an ester requires both overcoming this stabilization and forcing a terrible leaving group out, which demands harsh conditions or activating reagents. This reaction is strictly 'uphill' in the reactivity series.

Question 3

In nucleophilic acyl substitution, reactions spontaneously convert more reactive derivatives to less reactive ones, but not the reverse.

• T. True
• F. False

Answer: True

The reactivity hierarchy (acyl chlorides > anhydrides > esters > amides) reflects thermodynamic stability as well as kinetic reactivity. Converting an acyl chloride to an ester or amide releases the strain of the unstable starting material and produces a more stable, lower-energy product — thermodynamics and kinetics both favor the 'downhill' direction. Going 'uphill' (e.g., ester to acyl chloride) requires external activation (thionyl chloride, PCl₅) because the products are higher in energy than the starting materials.

Question 4

Acyl chlorides are highly reactive primarily because chlorine's lone pairs donate strongly into the carbonyl, making it very electrophilic.

• T. True
• F. False

Answer: False

This has the mechanism backwards. Chlorine's lone pairs are in 3p orbitals that overlap poorly with carbon's 2p orbital, so chlorine barely donates into the carbonyl. This is actually WHY acyl chlorides are reactive — the carbonyl remains highly electrophilic precisely because chlorine provides almost no resonance stabilization. By contrast, nitrogen in amides donates strongly into the carbonyl (both in 2p orbitals), which reduces carbonyl electrophilicity and makes amides unreactive.

Question 5

Both nitrogen (in amides) and oxygen (in esters) have lone pairs adjacent to the carbonyl. Why does nitrogen's donation so dramatically reduce reactivity compared to oxygen's?

Model answer: Nitrogen is a better electron donor than oxygen for two reasons: (1) nitrogen's lone pair is in a 2p orbital, matching carbon's 2p orbital for optimal overlap, giving amide C–N bonds substantial double-bond character (~40%); oxygen donates less effectively, partly due to its greater electronegativity withdrawing electron density. (2) If the reaction proceeds, nitrogen leaves as NH₂⁻ (pKa ~35), an extremely strong base and terrible leaving group, while oxygen leaves as RO⁻ (pKa ~16), a moderately strong base. Both factors — reduced electrophilicity AND worse leaving group — stack against amide reactivity.

The dual-factor analysis is the key insight: reactivity in nucleophilic acyl substitution depends on electrophilicity of the carbonyl carbon (controlled by heteroatom donation) AND leaving group quality (controlled by basicity of the departing anion). Amides score worst on both. This is not a coincidence — the same property (nitrogen's electron-donating ability) simultaneously reduces carbonyl electrophilicity and increases leaving-group basicity.