11.5 Acid Derivatives: Esters, Amides & Anhydrides

Nomenclature and Physical Properties

The three acid derivatives covered here — anhydrides, amides, and esters — are all built by replacing a carboxylic acid's hydroxyl group with a different substituent, and every one retains the electrophilic carbonyl (C=O) that drives their chemistry.

Esters are named as two separate words: the alkyl group from the alcohol half first, then the acid-derived "-oate" name (ethyl acetate, methyl propanoate). A cyclic ester, introduced in the previous section, is a lactone.

Amides take the suffix -amide (acetamide, benzamide). Because the nitrogen is not part of the parent chain's numbering, substituents on nitrogen are labeled with an italicized N- locant rather than a number: N-methylacetamide has one methyl group on nitrogen, and N,N-dimethylacetamide has two. A cyclic amide is a lactam, and the four-membered lactam ring — the beta-lactam — reappears below as the structural centerpiece of an entire antibiotic class.

Anhydrides are named by citing the parent acid (or acids, for a mixed anhydride) followed by "anhydride": two molecules of acetic acid condensed together give acetic anhydride.

Physical properties split sharply along one axis: which derivatives can donate a hydrogen bond. Esters and anhydrides have no N-H or O-H, so they can only accept hydrogen bonds, never donate them — their boiling points sit below those of carboxylic acids of similar mass, and small esters are volatile enough to carry the fruity odors used in flavoring (isoamyl acetate smells like banana). Primary and secondary amides, by contrast, retain an N-H that both donates and accepts hydrogen bonds, and that N-H···O=C hydrogen-bonding pattern — repeated across a polypeptide backbone — is exactly the interaction that folds proteins into alpha helices and beta sheets (Chapter 10). Combined with an unusually large carbonyl dipole (explained below), this gives amides surprisingly high melting points; most are solids at room temperature, unlike comparably sized esters or anhydrides.

Important Reactions

Every reaction below proceeds through the same nucleophilic acyl substitution mechanism introduced in the previous section: a nucleophile adds to the electrophilic carbonyl carbon to form a tetrahedral intermediate, and a leaving group is expelled to restore the C=O. Here, the acid derivative itself is the electrophile, and a second nucleophile drives the substitution.

Anhydrides are useful acylating agents precisely because carboxylate is such a good leaving group: acetic anhydride reacts with water, alcohols, or amines to install an acetyl group while releasing acetic acid as byproduct. This is exactly how aspirin is manufactured — salicylic acid's phenolic OH attacks acetic anhydride, giving acetylsalicylic acid plus acetic acid.

Transesterification exchanges one ester's alkoxy group for a different one: an alcohol (or its conjugate base alkoxide) attacks the ester carbonyl, forms a tetrahedral intermediate, and collapses by ejecting the original alkoxy group as a new free alcohol. Because both the starting and product species are esters, the reaction is a genuine equilibrium, not a one-way path — pushing it to completion requires either a large excess of the new alcohol or continuous removal of the displaced alcohol as it forms, often by distilling it away since the displaced alcohol is frequently more volatile. Industrial biodiesel production runs exactly this reaction: a triglyceride (a triester of glycerol) is transesterified with excess methanol to yield fatty acid methyl esters (biodiesel) plus glycerol.

Hydrolysis of amides uses water as the nucleophile but needs far harsher conditions than ester hydrolysis — typically strong acid or strong base under reflux. Base-mediated hydrolysis is irreversible: hydroxide adds to the carbonyl, and the tetrahedral intermediate collapses to expel the amine (released as its deprotonated, neutral form), leaving a carboxylate that the departed amine can no longer re-attack. Acid-mediated hydrolysis is also effectively irreversible, because the liberated amine is immediately protonated to a non-nucleophilic ammonium salt under the strongly acidic conditions, preventing the reverse reaction from competing. This same electronic resistance to hydrolysis is why the peptide bond — a biological amide — is thermodynamically favorable to hydrolyze yet kinetically almost inert at physiological pH without a protease enzyme to lower the activation barrier; digestion would be immeasurably slow without one.

General Principles: What Controls Reactivity

The reactivity order toward nucleophilic acyl substitution is acid chloride > anhydride > ester > amide, and the underlying cause is how well each leaving-group heteroatom donates its lone pair into the carbonyl pi system by resonance. Strong lone-pair donation delocalizes the carbonyl's partial positive charge, making the carbon less electrophilic, and it simultaneously makes that same heteroatom a worse leaving group, since donating electron density into the system is the opposite of being ready to leave with it.

• Acid chlorides: chlorine's lone pairs sit in larger, poorly matched 3p orbitals that overlap weakly with the carbonyl carbon's 2p orbital, so resonance donation is minimal, and chloride is an excellent leaving group. Least stabilized, most reactive.
• Anhydrides: the bridging oxygen must split its lone-pair donation between two competing carbonyls at once, diluting the stabilization each one receives, and the leaving group — a resonance-stabilized carboxylate — is quite good. Second most reactive.
• Esters: a single alkoxy oxygen donates into one carbonyl with reasonably good orbital overlap, giving moderate resonance stabilization.
• Amides: nitrogen is less electronegative than oxygen and holds its lone pair more loosely, making it the best pi-donor of the group. This gives the amide C-N bond significant double-bond character — restricted rotation that is directly observable as distinct NMR signals for cis and trans rotamers — and turns nitrogen into an exceptionally poor leaving group. Most stabilized, least reactive.

Steric effects layer on top of this electronic picture: bulky substituents on the carbonyl carbon, or on the incoming nucleophile, physically hinder formation of the crowded tetrahedral intermediate and slow the reaction regardless of electronics — a pivaloyl (tert-butylcarbonyl) derivative reacts more slowly than an otherwise comparable acetyl derivative.

Strain can override the normal amide-resonance argument entirely. In a beta-lactam — the four-membered cyclic amide at the core of penicillin and cephalosporin antibiotics — ring geometry forces bond angles near 90° at both nitrogen and the carbonyl carbon, far from the roughly 120° that trigonal-planar sp2 centers need for good p-orbital alignment. This pyramidalizes the nitrogen and misaligns its lone pair with the carbonyl pi system, largely destroying amide resonance stabilization. The result is a carbonyl that behaves electronically like a reactive ester or ketone despite being formally an amide — exactly electrophilic enough for a nucleophilic serine in the active site of a bacterial transpeptidase to attack it, covalently and irreversibly acylating (and inactivating) the enzyme that cross-links peptidoglycan in the bacterial cell wall. Bacterial resistance genes exploit the very same strain: beta-lactamase enzymes hydrolyze the strained ring first, destroying the antibiotic before it ever reaches its target.

Worked Example: Ranking Reactivity

A passage presents four derivatives of acetic acid — acetamide, acetic anhydride, ethyl acetate, and acetyl chloride — and asks which reacts fastest with a given amine nucleophile. Working through the resonance-donation logic above: chlorine donates worst into the carbonyl and leaves most easily, so acetyl chloride reacts fastest. The anhydride's bridging oxygen splits its donation across two carbonyls, so it is next. Ethyl acetate's single ester oxygen donates more effectively than the anhydride's split oxygen but far less than nitrogen, placing it third. Acetamide's nitrogen is the strongest pi-donor of the four, most heavily stabilizing its carbonyl and most strongly resisting nucleophilic attack, so it reacts slowest. The full ranking is acetyl chloride > acetic anhydride > ethyl acetate > acetamide — the same general order that applies to any comparably substituted set of derivatives.