Chapter 9: Carboxylic Acid Derivatives

A carboxylic acid derivative is a compound with an acyl group (R-CO-) bonded to a leaving group instead of the OH of the parent carboxylic acid. The four common derivatives are:

1. Acyl halide (acid halide): R-CO-X (usually Cl).
2. Anhydride: R-CO-O-CO-R’.
3. Ester: R-CO-OR’.
4. Amide: R-CO-NR’R”.

What They Share

All four have:

• A carbonyl (C=O) with sp² carbon.
• A leaving group directly attached to the carbonyl carbon.
• The capacity to undergo nucleophilic acyl substitution (NAS): nucleophile attacks the carbonyl C, tetrahedral intermediate forms, leaving group departs, C=O reforms.

What Distinguishes Them

They differ in:

• The leaving group: chloride (acyl halide), carboxylate (anhydride), alkoxide (ester), amide nitrogen (amide).
• Reactivity: determined by how good the leaving group is AND how much the LG donates electron density into the carbonyl by resonance.
• Hydrolysis conditions: mild (acyl halides and anhydrides) to harsh (amides require strong acid/base + heat).
• Biological role: esters (fats, phospholipids, aspirin); amides (proteins, urea, nylon).

The Reactivity Order

Acyl halide > Anhydride > Ester > Amide

Acyl halides react fastest because chloride is an excellent leaving group AND does not donate much electron density into the carbonyl. Amides react slowest because amide nitrogen is a terrible leaving group AND donates its lone pair strongly into the carbonyl (resonance), reducing electrophilicity.

Naming the Derivatives

Why These Four Are Grouped Together

The unifying idea: all four can be interconverted via nucleophilic acyl substitution. You can convert an acyl halide to an anhydride, ester, or amide (going “down” the ladder). You can convert an amide to an ester with harsh conditions (going up). Understanding the reactivity order lets you predict which interconversions are favorable and which require activation.

Acyl halides (R-CO-X, usually X = Cl) are the most reactive of the carboxylic acid derivatives. They react with water, alcohols, amines, carboxylates, and even Friedel-Crafts electrophiles with minimal catalysis. Their high reactivity makes them the “workhorse” intermediates for installing acyl groups on any nucleophile.

Why Acyl Halides Are So Reactive

Two reasons:

1. Excellent leaving group. Chloride (Cl⁻) is the conjugate base of HCl (pKa −7). Very weak base, very stable anion, leaves effortlessly after nucleophile attack.
2. Minimal resonance donation. Chlorine’s lone pairs are in 3p orbitals, which have poor overlap with the carbonyl’s 2p system. So Cl donates only weakly into the C=O by resonance, keeping the carbonyl electrophilic.

Compare with esters: the OR’s oxygen lone pair is in a 2p orbital, well-matched to the carbonyl, so it donates strongly. This reduces the carbonyl’s electrophilicity. Acid chlorides have no such reduction.

Formation from Carboxylic Acids

Standard preparation (from Ch 8.9):

• SOCl₂ is the primary reagent. RCOOH + SOCl₂ → RCOCl + SO₂↑ + HCl↑. Both byproducts escape as gases.
• PCl₃ or PCl₅ also work but produce phosphorus byproducts.
• Oxalyl chloride (ClCOCOCl) is a milder alternative.

Typical Reactions

Acyl chlorides react with almost any nucleophile at room temperature, usually in the presence of a tertiary amine (like pyridine or triethylamine) to neutralize the HCl byproduct.

Friedel-Crafts Acylation (Briefly)

Acyl halides + Lewis acid (AlCl₃) + benzene → aryl ketone. The AlCl₃ coordinates to Cl, generating an acylium ion (RCO⁺) that acts as a super-electrophile in electrophilic aromatic substitution.

Net: C₆H₆ + RCOCl + AlCl₃ → C₆H₅COR + HCl + AlCl₃.

Friedel-Crafts acylation is useful for installing alkyl groups on aromatic rings (followed by Clemmensen or Wolff-Kishner reduction to remove the carbonyl, giving the alkyl-substituted benzene). This workaround avoids the rearrangement and over-alkylation problems of direct Friedel-Crafts alkylation.

Note: AAMC content outline does not cover Friedel-Crafts in detail, so this is Passage Peek territory.

An anhydride (literally “without water”) has the structure R-CO-O-CO-R’ - two acyl groups sharing a central oxygen. It is the product of removing water from two carboxylic acids, either explicitly or implicitly.

Anhydrides are second only to acyl halides in reactivity. Their leaving group (a carboxylate, RCOO⁻) is not as good as chloride but is much better than the alkoxide (ester) or amide nitrogen (amide).

Structure and Symmetry

• Symmetric anhydrides: both acyl groups identical. Example: acetic anhydride (CH₃CO-O-COCH₃).
• Mixed anhydrides: acyl groups different. Example: acetic formic anhydride (HCO-O-COCH₃).
• Cyclic anhydrides: both acyl groups on the same carbon chain, forming a ring. Example: succinic anhydride (5-membered ring), glutaric anhydride (6-membered ring), phthalic anhydride (aromatic).

Formation

From carboxylic acid + heat (especially for cyclic anhydrides):

Succinic acid → succinic anhydride + H₂O. Heat drives off water, forming the 5-membered ring. Easy for cyclic products; hard for acyclic because entropy favors two separate COOH molecules.

From acyl halide + carboxylate:

R-COCl + R’-COO⁻ → R-CO-O-CO-R’ + Cl⁻. Reacts at room temperature. This is how mixed anhydrides are prepared in the lab.

From acyl halide + carboxylic acid:

R-COCl + R’-COOH → R-CO-O-CO-R’ + HCl. Similar approach; HCl leaves as a gas.

Reactivity Relative to Acid Chlorides

Anhydrides are slower than acid chlorides but still fast. Carboxylate (the leaving group) is a better LG than alkoxide (ester) but not as good as chloride. Resonance donation from the second acyl group’s lone pairs is weaker than from a single OR, so the carbonyl remains highly electrophilic.

Typical Reactions

The same as acid chlorides, just slower:

• Water → two carboxylic acids (hydrolysis, reversible, essentially complete).
• Alcohol → ester + carboxylic acid (one of the two COOHs becomes the ester; the other becomes a free acid byproduct).
• Amine → amide + carboxylic acid.

The “half-the-acyl-wasted” pattern is why acid chlorides are usually preferred over anhydrides in synthesis: acid chlorides install 1 acyl per equivalent, while anhydrides install 1 acyl but waste the other acyl as a COOH byproduct.

Aspirin Synthesis (Classic Example)

Aspirin (acetylsalicylic acid) is made by treating salicylic acid with acetic anhydride:

• Salicylic acid (HO-C₆H₄-COOH) + acetic anhydride (CH₃CO-O-COCH₃) → aspirin (CH₃CO-O-C₆H₄-COOH) + acetic acid.

The aromatic -OH of salicylic acid attacks one of the two carbonyls of acetic anhydride, installing an acetyl ester group on the phenolic oxygen. The other acyl group leaves as acetic acid. Aspirin is an ester at the phenolic position; the carboxylic acid remains intact.

Cyclic Anhydrides in Biology

Succinic anhydride and related cyclic anhydrides appear in specialized biochemistry contexts but are less common than esters or amides. Maleic anhydride (cyclic diene-dione-like structure) is a reactive Diels-Alder dienophile in industrial chemistry.

Esters (R-CO-OR’) are the most common carboxylic acid derivatives in nature and industry. Triglycerides are triple esters of glycerol. Aspirin contains an ester. Biodiesel is a mix of fatty acid methyl esters. Every flavor and fragrance compound in fruit (banana, apple, pear) is an ester.

Structure

Esters have a carbonyl C=O and a C-OR single bond. The oxygen lone pair donates into the carbonyl via resonance, giving ester carbonyls less electrophilic character than ketones (and much less than acyl halides or anhydrides).

Formation

Four common routes:

1. Fischer esterification (Ch 8.8): carboxylic acid + alcohol + H⁺ cat. Reversible; drive forward with excess alcohol or water removal.
2. Acid chloride + alcohol + pyridine: fast and irreversible. Preferred for clean lab synthesis.
3. Anhydride + alcohol: acetic anhydride is a common acetylating agent for phenols and alcohols.
4. Carboxylate + alkyl halide (SN2): RCOO⁻ + R’X → RCOOR’ + X⁻. Works with primary halides in polar aprotic solvent. Slow with hindered substrates.

Hydrolysis

Acid-catalyzed hydrolysis: ester + water + H⁺ cat. → carboxylic acid + alcohol. This is the reverse of Fischer esterification. Reversible; pushed forward by excess water.

Base-promoted hydrolysis (saponification): ester + NaOH → sodium carboxylate + alcohol. Irreversible because the carboxylate is stable and cannot re-attack.

Saponification is the origin of the word “soap” (Latin “sapo”). Ancient Romans boiled animal fats with potash (potassium hydroxide from wood ashes) to make soap - a process that is still essentially the same today, though industrial scales have replaced the kettle.

Saponification Mechanism

1. OH⁻ attacks the ester carbonyl carbon.
2. Tetrahedral alkoxide intermediate forms.
3. Alkoxide (OR’⁻) leaves, reforming the carbonyl as a carboxylic acid.
4. Fast acid-base: the carboxylic acid is immediately deprotonated by hydroxide (or another OH⁻) to give the stable carboxylate anion.

Step 4 is irreversible because the carboxylate cannot re-attack the alcohol. This is why saponification is irreversible under basic conditions.

Transesterification

Swap one OR group for another:

R-CO-OR’ + R”-OH ⇌ R-CO-OR” + R’-OH (with acid or base catalyst)

Biodiesel production uses transesterification: triglycerides + methanol + base catalyst → fatty acid methyl esters (biodiesel) + glycerol. The glycerol is separated and used for other purposes.

Bioester Hydrolysis: Lipase

Pancreatic lipase and other esterases hydrolyze fats in the digestive tract. The enzyme active site uses a catalytic serine (nucleophile) and histidine/aspartate (general acid/base) to accelerate the hydrolysis billions of times.

Flavor and Fragrance Esters

Simple short-chain esters give fruit their characteristic smells:

Amides (R-CO-NR’₂) are the least reactive of the carboxylic acid derivatives. They are also the most biologically important - every peptide bond in every protein is an amide bond. Their stability and distinctive properties come from strong resonance donation of the nitrogen lone pair into the carbonyl.

Resonance and Planarity

The nitrogen lone pair donates heavily into the carbonyl via resonance. Two contributors:

1. Neutral amide (major): R-C(=O)-NR’₂. Lone pair on N; C=O double bond.
2. Dipolar contributor (~40%): R-C(-O⁻)=N⁺R’₂. Lone pair on N has moved into a double bond with C; charge is +1 on N, −1 on O.

The dipolar contributor is a significant (~40%) fraction of the true hybrid. This means:

• The C-N bond has substantial (partial) double-bond character.
• Rotation around the C-N bond is restricted (the barrier is ~75 kJ/mol, much higher than ordinary C-N single bonds).
• All six atoms of the amide plane (Cα, C=O, N, R’₂ and the carbonyl oxygen) lie in the same plane.

This planarity is the structural foundation of protein secondary structure (alpha helix, beta sheet). Peptide bonds cannot twist freely, restricting backbone conformations.

Why Amides Are So Unreactive

Two factors:

1. Nitrogen donates strongly into the carbonyl by resonance. The carbonyl carbon’s δ⁺ is much reduced. Nucleophilic attack is slow.
2. Amide nitrogen is a terrible leaving group. After nucleophile attack, the tetrahedral intermediate would have to eject NR’₂⁻, which is an extremely strong base (pKa of R₂NH is ~38-40). It essentially does not leave under ordinary conditions.

The result: amides resist hydrolysis and other NAS reactions. This stability is why proteins last long enough to fold and function.

Amide Hydrolysis

Amides eventually hydrolyze to carboxylic acid + amine, but only under harsh conditions:

• Acid hydrolysis: 6 M HCl, reflux for hours. Used in traditional protein sequencing (releases amino acids from proteins).
• Base hydrolysis: concentrated NaOH, high temperature. Saponifies the amide bond.
• Enzymatic hydrolysis: proteases (trypsin, chymotrypsin, pepsin) and peptidases catalyze amide hydrolysis at room temperature and physiological pH. Cells rely on these enzymes to turn over proteins.

Even enzyme-free, amide hydrolysis at neutral pH has a half-life of hundreds of years for ordinary peptide bonds. This is why abiotic synthesis of proteins was so difficult in early chemical-evolution experiments.

Peptide Bonds

A peptide bond is an amide bond between two amino acids: the -COOH of one amino acid + the -NH₂ of the next → peptide bond + H₂O.

Each peptide bond is planar (~6 atoms in plane), restricting rotation around the C-N bond. This is the origin of the Ramachandran plot and the alpha helix / beta sheet secondary structures in proteins.

In cells, peptide bonds are formed by the ribosome, which uses activated aminoacyl-tRNAs as reactive intermediates (the amino acid is esterified to the tRNA, making the carbonyl more electrophilic). Water is not released per se - the tRNA is ejected as the product.

Nylon and Synthetic Polyamides

Nylon is a polyamide - a chain of amide bonds repeating with alkyl groups. Nylon 6,6 (most common) is made from adipic acid (HOOC-(CH₂)₄-COOH) and hexamethylenediamine (H₂N-(CH₂)₆-NH₂). Each amide bond connects one monomer to the next, and the resulting polymer has the same planarity and stability as protein backbones (which is why nylon fibers are so strong).

The reactivity ladder acyl halide > anhydride > ester > amide is not arbitrary. Two factors combine to create the ranking:

1. Leaving group ability of the attached group after nucleophile attack.
2. Resonance donation of the attached group’s lone pair into the carbonyl in the starting material.

These two effects point in the same direction: groups that are good leaving groups (weak bases like Cl⁻) are also poor resonance donors; groups that are bad leaving groups (strong bases like NR₂⁻) are also strong resonance donors. This coincidence amplifies the reactivity differences across the ladder.

Factor 1: Leaving Group Ability

After nucleophile attack, the tetrahedral intermediate must eject its attached group for NAS to complete. The leaving group ability depends on the conjugate acid pKa:

Weaker bases (higher conjugate acid acidity, more stable anions) are better LGs. Cl⁻ leaves effortlessly; R₂N⁻ essentially never leaves.

Factor 2: Resonance Donation

The attached group donates lone pair electrons into the carbonyl via resonance. The more donation, the more electron density on the carbonyl C, and the LESS electrophilic it is:

• Cl in acyl halide: Cl’s 3p lone pair overlaps poorly with C’s 2p. Weak donation. Carbonyl retains full δ⁺ character.
• OR in ester: O’s 2p lone pair donates moderately. Some reduction of δ⁺.
• NR₂ in amide: N’s 2p lone pair donates strongly (N is less electronegative than O). Substantial reduction of δ⁺.

Combine both factors:

• Acyl halide: Cl is a good LG AND a weak donor → carbonyl stays electrophilic AND tetrahedral intermediate collapses quickly. Very reactive.
• Anhydride: carboxylate is a decent LG AND a weak donor (the second C=O pulls electrons away). Quite reactive.
• Ester: alkoxide is a poor LG AND a moderate donor. Moderate reactivity.
• Amide: amide N is a terrible LG AND a strong donor. Very unreactive.

Implications for Interconversion

Going DOWN the ladder (more reactive → less reactive) is thermodynamically favorable:

• Acyl halide + alcohol → ester (go from Cl to OR; Cl- is more stable than OR-).
• Ester + amine → amide (go from OR to NR2; OR- is more stable than NR2-).

Going UP the ladder requires activation:

• Amide → ester requires harsh hydrolysis first (to give COOH), then Fischer esterification.
• Ester → anhydride is possible with DCC or similar activating reagents.
• Ester or acid → acyl halide requires SOCl₂ or PCl₃.

Never try to run the reverse of the ladder directly - you need to activate the starting material (usually via the carboxylic acid + SOCl₂ route).

A Unifying Rule

The rule for predicting whether an NAS interconversion will work:

If the new leaving group (the LG displaced by the nucleophile) is a WEAKER base than the incoming nucleophile, the reaction will work.

• Acyl halide + alcohol: Cl⁻ (weak base, pKa of HCl −7) leaves; RO⁻ comes in. Since RO⁻ is a stronger base than Cl⁻, Cl⁻ is the better LG and the reaction proceeds.
• Ester + hydroxide: OR⁻ leaves; OH⁻ comes in. OH⁻ and OR⁻ have similar basicity; the reaction is thermodynamically neutral unless other factors drive it (like the carboxylate stabilization in saponification).
• Amide + alcohol: NR₂⁻ would have to leave; RO⁻ would come in. NR₂⁻ is much more basic than RO⁻, so NR₂⁻ is the WORSE LG. The reaction does not work spontaneously.

Nucleophilic acyl substitution (NAS) is the mechanism by which ALL carboxylic acid derivatives react with nucleophiles. Two phases: addition (nucleophile joins the carbonyl carbon, giving a tetrahedral intermediate) and elimination (the leaving group departs, reforming the carbonyl). Section 8.10 introduced this; here we walk through it in detail with both base and acid catalysis.

Base-Catalyzed (Anionic) NAS

When the nucleophile is already anionic (hydroxide, alkoxide, amide), no separate acid catalyst is needed:

Step 1: Addition. Nu⁻ attacks the carbonyl C. The pi electrons flow onto the oxygen. Product: tetrahedral alkoxide intermediate.

Arrows: Nu⁻ lone pair → C; C=O pi bond → O.

Step 2: Elimination. The oxygen’s lone pair reforms the C=O double bond. The electrons that were in the C-LG bond leave with LG.

Arrows: O’s lone pair → C; C-LG bond → LG.

Net: R-CO-LG + Nu⁻ → R-CO-Nu + LG⁻. One-step-ish mechanism (two arrows, two chemical events, through one stable tetrahedral intermediate).

Acid-Catalyzed (Protonated) NAS

When the nucleophile is neutral (water, alcohol, amine), acid catalysis makes the carbonyl more electrophilic:

Step 1a: Protonate carbonyl oxygen. H⁺ adds to the C=O oxygen.

Step 1b: Neutral nucleophile attacks. The protonated carbonyl is a much stronger electrophile; water, alcohol, or amine can attack its carbon.

Step 2a: Proton shuffle. Proton transfers happen: the new nucleophilic OH/NH gets deprotonated, and then the original leaving group (OH in a carboxylic acid; OR in an ester) gets protonated to become a good leaving group.

Step 2b: Elimination. The protonated LG (now a neutral water or alcohol) leaves with the bond electrons. The C=O reforms.

Step 3: Final deprotonation. The C=O⁺ gets deprotonated to give the neutral product.

This looks complicated but is just Fischer esterification / hydrolysis mechanism from Section 8.8. Each step is a simple proton transfer or attack.

The Tetrahedral Intermediate Is the Branching Point

The tetrahedral intermediate can:

1. Expel the LG (forward to product): standard NAS.
2. Expel the Nu back (reverse to starting material): equilibrium situation.
3. Be protonated and give the stable addition product (in a ketone/aldehyde, which has no LG): this is why aldehydes and ketones give addition not substitution.

Which path wins depends on:

• Relative basicity of Nu vs LG: the worse base (= better LG) is expelled preferentially.
• Stability of the tetrahedral intermediate.
• Solvent and conditions.

Why NAS Requires a Leaving Group

The essential difference between aldehydes/ketones (Ch 6) and carboxylic acid derivatives (Chs 8-9) is the leaving group. Aldehydes and ketones have H or R groups, neither of which can leave. Derivatives have Cl, OR, OCOR’, NR₂, or OH (with acid catalysis) - all of which can leave.

This single structural difference splits the carbonyl world into two:

• Aldehydes/ketones: nucleophilic addition only. New Nu stays attached; no group is lost.
• Carboxylic acids/derivatives: nucleophilic acyl substitution. Nu replaces LG; net one group in, one group out.

Carboxylic acid derivatives can be interconverted by nucleophilic acyl substitution. The general rule: going down the reactivity ladder is easy; going up requires activation.

The Ladder (Reactive to Unreactive)

Acyl halide → Anhydride → Ester → Amide

You can go FROM more reactive TO less reactive spontaneously or easily (the energy is downhill). You cannot go from less reactive to more reactive without putting in energy.

Easy Downward Steps

Acyl halide → anything: acyl halide + Nu (alcohol, amine, carboxylate, etc.) → derivative. Room temperature, fast, with mild base (pyridine) to absorb HCl.

• Acyl halide + alcohol + pyridine → ester + pyridinium chloride. Standard lab method for making esters.
• Acyl halide + amine + pyridine → amide + pyridinium chloride. Standard method for amide synthesis.
• Acyl halide + carboxylate → anhydride + chloride. Convenient for mixed anhydride synthesis.

Anhydride → ester, amide: anhydride + alcohol → ester + carboxylic acid. Anhydride + amine → amide + carboxylic acid. Slower than acyl halides but works without strong activation.

Ester → amide: ester + amine → amide + alcohol. Slow but possible; often needs heat. This is the last step in some antibiotic syntheses.

Hard Upward Steps

Amide → ester: requires amide hydrolysis first (strong acid or base, reflux, hours). Then re-esterify with Fischer or acid chloride.

Ester → acid chloride: not direct. Hydrolyze ester to carboxylic acid, then use SOCl₂ to make acid chloride.

Amide → acid chloride: similar; hydrolyze to acid, then SOCl₂.

Carboxylic acid → acid chloride: use SOCl₂, PCl₃, or oxalyl chloride. A one-step conversion.

The Carboxylic Acid As a Hub

In multi-step syntheses, the carboxylic acid (RCOOH) is often the central “hub” compound:

• From acid chloride: add water (hydrolysis). Fast.
• From anhydride: add water. Fast.
• From ester: add base and water (saponification). Then acidify to get the COOH.
• From amide: acid or base hydrolysis. Slow.

From the acid, you can go forward again:

• Acid + SOCl₂ → acid chloride.
• Acid + alcohol + H⁺ cat. → ester (Fischer).
• Acid + amine + DCC → amide.
• Acid + heat (if beta-keto or 1,3-diacid) → decarboxylation.

DCC Coupling (Peptide Chemistry)

DCC (dicyclohexylcarbodiimide) is the workhorse coupling reagent for making amide bonds from free carboxylic acids without going through acid chlorides:

R-COOH + DCC → O-acylisourea intermediate.
O-acylisourea + R’NH₂ → amide + DCU (dicyclohexylurea byproduct).

Mechanism: DCC’s central carbon is highly electrophilic; the carboxylic acid’s oxygen attacks it, installing the activating group. The amine then attacks the activated acyl carbon, ejecting DCU as a stable leaving group.

DCC is used in peptide synthesis because it works at room temperature, does not racemize chiral alpha-centers, and produces a non-toxic urea byproduct that is easy to filter off. EDC (a water-soluble variant) is used when the byproducts need to wash away in aqueous workup.

Practical Interconversion Example

From aspirin (an ester/COOH hybrid) to a methylated analog:

1. Hydrolyze the aspirin’s ester: aspirin + NaOH → salicylate + acetate.
2. Re-esterify with methanol + H⁺ (Fischer): salicylate + CH₃OH → methyl salicylate.
3. Done.

This two-step sequence swaps one ester for another via the free acid intermediate. Direct transesterification (aspirin + methanol + acid) would also work but is slower and less clean.

Hydrolysis is the cleavage of a carboxylic acid derivative by water, breaking the C-LG bond and giving the parent carboxylic acid + the leaving group. The rate depends on the derivative (reactivity ladder) and the conditions (acid vs. base catalysis).

Rate Order for Hydrolysis

Acyl halide >>> anhydride > ester >> amide

At physiological pH and room temperature:

• Acyl halides: hydrolyze in seconds to minutes (without catalysis).
• Anhydrides: hydrolyze in minutes to hours.
• Esters: hydrolyze in days to weeks without catalysis; minutes with acid/base catalysis at reflux.
• Amides: hydrolyze in hundreds of years without catalysis; hours with strong acid/base at reflux.

This rate hierarchy parallels the reactivity ladder exactly.

Base-Catalyzed Hydrolysis (Saponification for Esters)

Mechanism:

1. OH⁻ attacks the carbonyl C.
2. Tetrahedral alkoxide intermediate forms.
3. LG departs, reforming the C=O.
4. The product carboxylic acid is immediately deprotonated by another OH⁻, giving the stable carboxylate.

Step 4 is irreversible for ester hydrolysis (saponification). Once the carboxylate forms, it cannot re-attack the alcohol.

For amide hydrolysis under basic conditions, the rate is slow (amides resist), but eventually the same mechanism gives carboxylate + amine.

Acid-Catalyzed Hydrolysis

Mechanism:

1. Acid protonates carbonyl O, activating the electrophile.
2. Neutral water attacks the activated C.
3. Proton transfers shuffle: the water-derived OH gets deprotonated; the original LG (OR or NR₂) gets protonated to a better LG.
4. LG departs as neutral (ROH or R₂NH for ester/amide).
5. C=O reforms and gets deprotonated, giving carboxylic acid.

For Fischer ester hydrolysis, this mechanism is reversible (same mechanism, run in reverse, is Fischer esterification). Drive forward with excess water.

For amide hydrolysis under acid, water loss is very slow because amide N is a poor LG even after protonation.

Rate Differences Explained

Acyl halides hydrolyze fastest: excellent LG (Cl⁻), minimal resonance from Cl. Even uncatalyzed water attack is fast. This is why acyl halides must be kept dry.

Anhydrides hydrolyze moderately fast: good LG (RCOO⁻), weak resonance donation. Acetic anhydride reacts with moist air slowly but completely.

Esters hydrolyze slowly: poor LG (RO⁻), moderate resonance from OR. Needs acid or base catalysis to proceed at reasonable rates.

Amides hydrolyze extremely slowly: terrible LG (R₂N⁻ is a strong base), strong resonance from N. This is why proteins and peptides are stable.

Enzyme-Catalyzed Hydrolysis

Biological hydrolysis uses enzymes that can accelerate the reaction by factors of $10^{10}$ or more:

• Proteases (pepsin, trypsin, chymotrypsin) hydrolyze amide/peptide bonds. Usually at a serine active site (serine proteases) or a cysteine/aspartate active site (cysteine/aspartyl proteases). Neutral pH, 37°C.
• Esterases and lipases hydrolyze esters. Pancreatic lipase digests dietary fats at pH 7-8.
• Amidases hydrolyze amides. Slower, less common than proteases.

The enzymes’ power comes from: active site positioning that brings water and substrate together; general acid/base catalysis; stabilization of the tetrahedral intermediate via an “oxyanion hole”; and covalent catalysis (serine proteases form a covalent acyl-enzyme intermediate).

Amide bond formation is one of the most common reactions in organic synthesis. Carboxylic acid derivatives + amines give amides, with the specific conditions depending on which derivative you start with.

Acyl Halide + Amine (Fastest)

R-COCl + R’-NH₂ + pyridine → R-CO-NHR’ + pyridinium chloride.

Pyridine acts as a base to absorb the HCl that forms; without it, the amine would get protonated to its ammonium form and lose nucleophilicity.

This reaction is fast (minutes at room temperature), irreversible, and high-yielding. It is the standard lab method for amide bonds when the starting acid is easily converted to its chloride.

Anhydride + Amine

R-CO-O-CO-R + R’-NH₂ → R-CO-NHR’ + R-COOH.

Half of the anhydride becomes the amide; the other half becomes the carboxylic acid byproduct. Slower than acyl halide + amine but still fast at room temperature.

Ester + Amine (Slower)

R-COOR’ + R”-NH₂ → R-CO-NHR” + R’-OH.

This “aminolysis” of esters requires heat and time. Rate depends on the ester’s structure: methyl esters react faster than bulky esters. For peptide synthesis, ester intermediates are sometimes used as “active esters” - N-hydroxysuccinimide (NHS) esters or pentafluorophenyl esters are common examples. They are more reactive than methyl/ethyl esters because their LG (NHS anion or pentafluorophenolate) is a weak base.

Carboxylic Acid + Amine Directly (Slow)

R-COOH + R’-NH₂ ⇌ R-CO-NHR’ + H₂O.

This equilibrium is unfavorable at room temperature because:

1. The carboxylic acid protonates the amine, turning both into unreactive zwitterion forms.
2. Even if they do come together, the -OH is a poor LG.

To force this reaction, you need extreme heat (150°C+) with water removal. A cleaner alternative is DCC coupling (see below).

DCC Coupling (Standard for Peptide Synthesis)

DCC (dicyclohexylcarbodiimide, C₆H₁₁-N=C=N-C₆H₁₁) activates the carboxylic acid at room temperature:

Step 1: R-COOH + DCC → O-acylisourea (activated intermediate).
Step 2: O-acylisourea + R’-NH₂ → amide + DCU (dicyclohexylurea, insoluble byproduct).

Mechanism in detail:

1. DCC’s central carbon is highly electrophilic.
2. The COOH’s oxygen attacks it, forming a C-O bond and breaking one of the C=N bonds.
3. The resulting O-acylisourea has a greatly activated acyl carbonyl.
4. The amine attacks this activated acyl carbon.
5. The N=C-N portion of the DCC rearranges to give urea (DCU), which precipitates out.

DCC is the “gold standard” for peptide synthesis because it is done at room temperature, does not racemize chiral alpha-centers (a big concern in peptide chemistry), and DCU can be filtered off.

Related coupling reagents:

• EDC (ethyl-(dimethylaminopropyl)carbodiimide): water-soluble, easier workup.
• HATU, HBTU, PyBOP: more reactive, used for difficult couplings.
• BOP, TBTU: similar applications.

All follow the same acid-activation-then-amine-attack logic.

Peptide Bond Formation in Biology

Cells make peptide bonds via the ribosome. The mechanism is essentially:

1. Amino acid is esterified onto tRNA via ATP activation (forming aminoacyl-tRNA).
2. The activated amino acid’s carbonyl is more electrophilic than a free acid.
3. The next amino acid’s -NH₂ attacks the activated acyl carbonyl.
4. The tRNA is kicked out as the LG.
5. Result: new peptide bond + empty tRNA.

The ribosome’s peptidyl transferase center (PTC) is actually a ribozyme - the catalysis is done by RNA, not protein. The chemistry is the same NAS mechanism you just learned, just with tRNA as the leaving group (instead of chloride or alkoxide) and ribosome positioning for specificity.

Esters can be made from any of the four carboxylic acid derivatives + an alcohol. The conditions (and speed) depend on the starting material.

Acyl Halide + Alcohol (Best for Clean Ester Synthesis)

R-COCl + R’-OH + pyridine → R-COOR’ + pyridinium chloride.

Fast, room temperature, irreversible. Pyridine neutralizes the HCl byproduct.

Anhydride + Alcohol

R-CO-O-CO-R + R’-OH → R-COOR’ + R-COOH.

Commonly used for phenol acetylation (e.g., aspirin synthesis from salicylic acid + acetic anhydride). Half the anhydride becomes the ester; the other half is the carboxylic acid byproduct.

Ester + Alcohol (Transesterification)

R-COOR’ + R”-OH ⇌ R-COOR” + R’-OH.

Equilibrium, driven by the same techniques as Fischer esterification (excess alcohol or water removal). Used in biodiesel production: triglycerides + methanol + base catalyst → fatty acid methyl esters + glycerol.

Carboxylic Acid + Alcohol (Fischer Esterification, Review from Ch 8.8)

R-COOH + R’-OH + H⁺ cat. ⇌ R-COOR’ + H₂O.

Reversible. Excess alcohol or water removal drives it forward. The classic lab method for making esters from the most accessible starting material.

Amide + Alcohol → Ester (Essentially Does Not Work)

Amides cannot be converted directly to esters under mild conditions because the amide N is a terrible leaving group. To go from amide to ester, you must first hydrolyze the amide to the carboxylic acid (strong acid or base, reflux), then do Fischer esterification. Going up the ladder requires two steps and energy input.

Cyclic Esters: Lactones

When a carboxylic acid and an alcohol are on the same molecule (intramolecular), the internal Fischer esterification gives a lactone (cyclic ester). Five- and six-membered lactones form spontaneously and are often isolable.

• Gamma-butyrolactone (GBL): 4-carbon chain with COOH and OH on opposite ends gives a 5-ring lactone.
• Delta-valerolactone: 5-carbon chain → 6-ring lactone.
• Vitamin C (ascorbic acid): a complex 5-membered lactone with multiple hydroxyls.

Cyclic esters (lactones) are important in natural product chemistry and pharmaceuticals. Many antibiotics (erythromycin, rapamycin) are macrolide lactones.

Biology: Triglycerides and Phospholipids

Triglycerides are triple esters of glycerol (a triol) and three fatty acids. Formation in cells uses acyl-CoA (an activated thioester of fatty acid) + glycerol-3-phosphate + enzymes. Hydrolysis (lipase) runs the reverse: triglyceride + water → glycerol + 3 fatty acids.

Phospholipids have a similar structure: glycerol + 2 fatty acid esters (at C1 and C2) + 1 phosphate ester (at C3). The phosphate carries a polar head group (choline, serine, ethanolamine, inositol). This amphipathic structure is the basis of cell membranes.

Polyester Plastics

Polyethylene terephthalate (PET, used in soda bottles and polyester fabric) is a polymer of ethylene glycol (diol) + terephthalic acid (dicarboxylic acid) linked via ester bonds. Industrial synthesis uses transesterification of dimethyl terephthalate + ethylene glycol with a transesterification catalyst. Billions of tons are produced annually.

Carboxylic acid derivatives are the workhorse functional groups of biochemistry. Peptide bonds (amides) connect amino acids into proteins. Thioesters (like acetyl-CoA) activate carboxylic acids for transfer reactions. Esters link fatty acids into triglycerides and phospholipids. Understanding their organic chemistry unlocks their biological function.

Peptide Bonds: The Amide Backbone of Proteins

Every protein is a chain of amino acids connected by amide bonds (peptide bonds). The properties of amides explain protein behavior:

1. Planarity. Amide nitrogen’s lone pair delocalizes into the carbonyl, giving the C-N bond ~40% double-bond character. Six atoms (Cα, C=O, N, H, Cα) lie in a plane. This restricts rotation around C-N, which is why protein structure is dominated by the Ramachandran phi/psi plot showing allowed combinations of backbone torsion angles.

2. Stability. Amide hydrolysis has a half-life of hundreds of years at physiological conditions. Proteins last long enough to fold, function, and be recycled by cellular proteases.

3. Hydrogen bonding. The amide -NH can donate H-bonds; the amide C=O can accept them. This is the basis of alpha-helix and beta-sheet secondary structure.

Thioesters: Activated Biological Acyl Groups

Coenzyme A (CoA) has a terminal -SH group. When a carboxylic acid is attached to it, the result is a thioester: R-CO-S-CoA. Key example: acetyl-CoA (CH₃CO-S-CoA).

Thioesters are more reactive than regular esters because:

1. Sulfur’s 3p orbitals overlap poorly with C’s 2p orbitals, so S donates weakly into the carbonyl via resonance (compared to O in a regular ester).
2. Thioester’s -SCoA LG is a softer, more polarizable anion, making it a slightly better LG than alkoxide.

Net effect: thioesters are about $10^{3}$ to $10^{5}$ more reactive than regular esters. This makes them ideal for biological acyl transfer reactions - they are reactive enough to transfer the acyl group but stable enough to exist as discrete metabolic intermediates.

Acetyl-CoA is the “activated acetate” of metabolism. It:

• Enters the Krebs cycle by condensing with oxaloacetate → citrate (Claisen-like condensation).
• Starts fatty acid synthesis.
• Acetylates histones (gene regulation) and neurotransmitters.
• Is used for cholesterol and isoprenoid biosynthesis.

Aspirin: Pharmacological Use of an Ester Group

Aspirin (acetylsalicylic acid) is synthesized from salicylic acid + acetic anhydride, installing an acetyl ester on the phenolic oxygen. The original -COOH of salicylic acid is preserved.

Aspirin’s biological mechanism: the ester is a reactive acyl donor that transfers the acetyl group to a specific serine residue (Ser530) in cyclooxygenase (COX-1 and COX-2) enzymes. This acylation covalently inhibits the enzyme, preventing prostaglandin synthesis and giving aspirin its anti-inflammatory, analgesic, and antipyretic effects.

The mechanism: aspirin’s acetyl-OR group (where R is the phenolate of salicylic acid) transfers to the enzyme’s serine -OH. The leaving group is salicylate itself (the free acid).

This is a specific example of an organic chemistry acetylation happening inside the human body, exploiting the same principles you learned about ester reactivity.

Other Ester-Based Pharmaceuticals

• Procaine, lidocaine: local anesthetics with ester or amide functional groups. Hydrolysis rate of the linker determines duration of action.
• Cocaine: an ester alkaloid. Cleaved by plasma esterases, which is why cocaine has a short half-life.
• Statins (pravastatin, simvastatin, atorvastatin): HMG-CoA reductase inhibitors; some are administered as lactone prodrugs that are hydrolyzed in the liver to the active acid form.

Biological Amides Beyond Proteins

Amide bonds also appear in:

• Penicillin and related beta-lactam antibiotics (4-ring amide is the key pharmacophore).
• Acetaminophen (an amide linker): N-acetyl-para-aminophenol, a common analgesic.
• Urea: diamide of carbonic acid, the nitrogen-disposal product in mammals.
• DNA and RNA: no amides in the backbone (phosphate esters), but base-sugar linkages and some other structural features are amides.