Source: Organic Chemistry, The Ohio State University
Tags: nucleophilic acyl substitution, carboxylic acid derivatives, acid chloride, anhydride, ester, amide, leaving group, electrophilicity, reactivity order, Fischer esterification, Williamson ester synthesis, diazomethane, SOCl2, oxalyl chloride, P2O5
Difficulty: Intermediate Prerequisites: Nucleophilic addition to carbonyls (Ch. 19), leaving groups, acid-base chemistry, SN2 basics.
This chapter shifts from nucleophilic addition (aldehydes and ketones) to nucleophilic substitution at a carbonyl. The difference is one structural feature: carboxylic acid derivatives carry a leaving group (Y) bonded to the carbonyl carbon, so after a nucleophile attacks, Y departs rather than staying put. The derivatives you need to know are acid chlorides, anhydrides, esters, and amides, and the reactions in this chapter let you convert between them. If you are comfortable with tetrahedral intermediates and leaving-group ability from earlier chapters, this material builds directly on that foundation.
A nucleophile attacks the electrophilic carbonyl carbon of a carboxylic acid derivative, forming a tetrahedral intermediate. The leaving group (Cl, OCOR, OR, NR2) then departs, completing a substitution. Reactivity follows leaving-group ability: acid chlorides are most reactive, amides least. Carboxylic acids themselves can be converted into esters, acid chlorides, or anhydrides using specific reagents.
Nucleophilic acyl substitution
The replacement of a leaving group on a carbonyl carbon by a nucleophile, proceeding through a tetrahedral intermediate. Think of it as: the nucleophile kicks out the old group after briefly making the carbon sp3.
Carboxylic acid derivative
Any compound with the general structure RC(=O)–Y, where Y is a leaving group. The four main derivatives are acid chlorides (Y = Cl), anhydrides (Y = OCOR), esters (Y = OR'), and amides (Y = NR'2).
Tetrahedral intermediate
The sp3 carbon species formed when a nucleophile adds to the carbonyl carbon. It collapses by expelling the leaving group to regenerate the C=O.
Leaving group ability (in this context)
Determined by the stability of Y⁻ once it departs. Cl⁻ is very stable (good leaving group); R2N⁻ is very unstable (poor leaving group). This is why acid chlorides react fastest and amides react slowest.
Acyl bond vs. alkyl bond
In an ester RC(=O)–O–R', the bond between the carbonyl carbon and the oxygen is the acyl bond; the bond between oxygen and the alkyl group R' is the alkyl bond. Which one breaks matters for the mechanism and can be tested with isotope labelling.
Fischer esterification (acidic esterification)
Acid-catalysed conversion of a carboxylic acid and an alcohol into an ester and water. It is an equilibrium process, so conditions (excess alcohol or removal of water) drive the reaction forward.
Williamson ester synthesis
Deprotonation of a carboxylic acid with base to form the carboxylate, followed by SN2 alkylation with an alkyl halide (e.g. CH3–I).
Diazomethane (CH2N2)
A reagent that converts carboxylic acids into methyl esters by cleaving the O–H (alkyl) bond. It is selective and mild but toxic and explosive.
Nucleophilic acyl substitution requires Y⁻ to be a reasonable leaving group. In aldehydes, Y = H⁻; in ketones, Y = R⁻. Both are extremely poor leaving groups (very strong bases), so the tetrahedral intermediate simply collapses back to starting material rather than expelling them. That is why aldehydes and ketones undergo nucleophilic addition instead.
Reactivity depends on two factors working together.
Electronic factor (electrophilicity of the carbonyl carbon):
Cl is highly electronegative and a good leaving group, making the carbonyl carbon very electrophilic. Acid chlorides are the most reactive.
In anhydrides, the carboxylate anion (RCO2⁻) is a reasonable leaving group, so anhydrides are next.
OR' groups donate electrons into the carbonyl through lone-pair resonance, reducing electrophilicity. Esters are less reactive.
NR'2 donates even more strongly (nitrogen is less electronegative than oxygen), so amides are the least reactive.
The full order: acid chloride > anhydride > ester > amide.
Steric factor:
Bulky substituents on the carbon adjacent to the carbonyl slow the approach of the nucleophile. Reactivity increases as substituents shrink: R3C–C(=O)Y < R2CH–C(=O)Y < RCH2–C(=O)Y < H2CH–C(=O)Y. The sp2-to-sp3 geometry change at the carbonyl carbon during nucleophilic attack makes steric crowding worse in the tetrahedral intermediate.
When the nucleophile is weak and the carbonyl carbon is not electrophilic enough on its own, an electrophile (E⁺, typically H⁺ from acid catalyst) can protonate the carbonyl oxygen. This places a positive charge on or near the carbonyl carbon, making it far more attractive to the nucleophile. Acid-catalysed reactions of esters, acids, and amides all rely on this activation step.
Carboxylic acids can be converted into three other derivative classes.
1. Conversion into esters (RCO2R')
(a) Acidic esterification (Fischer esterification): Treating RCO2H with an alcohol (e.g. CH3OH) in the presence of an acid catalyst (HCl or H2SO4) gives the ester plus water. The mechanism proceeds through protonation of the carbonyl oxygen, nucleophilic attack by the alcohol, proton transfers, and loss of water.
Key points:
This is an equilibrium reaction. Excess alcohol or removal of water pushes the equilibrium toward ester.
Works with many different alcohols (methanol, ethanol, etc.).
¹⁸O-labelling experiments prove the acyl C–O bond breaks (the labelled oxygen from the alcohol ends up in the ester, not in the water). This confirms nucleophilic attack at the carbonyl carbon.
(b) Williamson ester synthesis: Deprotonate the acid with a base to form the carboxylate anion (RCO2⁻), then react with an alkyl halide (e.g. CH3–I) via SN2. This works well with primary and methyl halides. The usual SN2 limitations apply (no bulky or tertiary substrates).
(c) Diazomethane esterification: CH2N2 reacts with the O–H of the carboxylic acid to produce a methyl ester and N2 gas. It cleaves the alkyl bond (O–H), not the acyl bond. This is a clean, selective method for making methyl esters, though diazomethane itself is hazardous.
2. Conversion into acid chlorides (RC(=O)Cl)
This is one of the most useful transformations because acid chlorides are the most reactive derivatives, opening the door to further conversions.
Reagents that work:
SOCl2 (thionyl chloride) is the most common. The mechanism involves attack of the acid oxygen on the electrophilic sulfur of SOCl2, forming a good leaving group that is displaced by Cl⁻. SO2 and HCl are gaseous by-products, which drives the reaction forward.
Oxalyl chloride (ClC(=O)C(=O)Cl) with pyridine is a milder alternative that generates less acid, useful for sensitive substrates.
PCl3 also works, and PBr3 makes acid bromides (RC(=O)Br).
3. Conversion into anhydrides (RC(=O)–O–C(=O)R)
Treating a carboxylic acid with P2O5 (a powerful dehydrating agent) removes water and joins two acid molecules into an anhydride. This is not widely used in practice because it consumes two equivalents of the acid to make one equivalent of anhydride, which is wasteful.
Fischer esterification: RCO2H + R'OH, HCl catalyst → RCO2R' + H2O
Williamson ester synthesis: RCO2H + Base → RCO2⁻, then + R'–X → RCO2R' + X⁻
Diazomethane: RCO2H + CH2N2 → RCO2CH3 + N2
Acid chloride formation: RCO2H + SOCl2 → RC(=O)Cl + SO2 + HCl
Acid chloride (mild): RCO2H + (COCl)2 / pyridine → RC(=O)Cl
Anhydride formation: 2 RCO2H + P2O5 → (RCO)2O + H2O (absorbed by P2O5)
Fischer esterification is the basis for producing many fragrances and flavourings (fruit esters such as ethyl acetate and isoamyl acetate). Acid chloride formation is a routine first step in pharmaceutical synthesis whenever a more reactive intermediate is needed to couple with an amine or alcohol.
Students often confuse nucleophilic acyl substitution with nucleophilic addition. If there is a leaving group on the carbonyl, it is substitution. If there is only H or R (aldehyde/ketone), it is addition.
Students sometimes think any nucleophile can displace any leaving group. In practice, you can only move down the reactivity ladder (e.g. acid chloride → ester), not up (e.g. amide → acid chloride) without special reagents.
Fischer esterification is reversible. Students forget that simply mixing acid and alcohol does not guarantee high yield; you must shift the equilibrium.
Diazomethane cleaves the O–H bond (alkyl bond), not the C–O acyl bond. Students sometimes assume it works the same way as Fischer esterification mechanistically. It does not.
⚠️ The reactivity order (acid chloride > anhydride > ester > amide) appears in nearly every exam on this chapter. Know it cold, and be able to explain it using both leaving-group stability and resonance arguments.
⚠️ Be able to draw the full mechanism of Fischer esterification, including all proton transfers. Partial-credit mechanisms that skip steps lose marks.
⚠️ Know which reagent makes which product from a carboxylic acid: SOCl2 → acid chloride, alcohol + H⁺ → ester, P2O5 → anhydride, CH2N2 → methyl ester.
⚠️ ¹⁸O-labelling experiments are a favourite exam topic. Know that the label tracks where bond cleavage occurs (acyl vs. alkyl bond).
True or False: Ketones undergo nucleophilic acyl substitution readily.
Fill in the blank: The reactivity order from most to least reactive is acid chloride > ________ > ester > amide.
True or False: Fischer esterification is irreversible.
Fill in the blank: SOCl2 converts a carboxylic acid into a(n) ________.
True or False: Diazomethane cleaves the acyl bond of a carboxylic acid.
Answers: 1. False (H⁻ and R⁻ are poor leaving groups). 2. Anhydride. 3. False (it is an equilibrium). 4. Acid chloride. 5. False (it cleaves the O–H alkyl bond).
Q: Why can aldehydes and ketones not undergo nucleophilic acyl substitution?
A: Their substituents (H⁻ and R⁻) are very poor leaving groups because they are strong bases, so the tetrahedral intermediate collapses back to starting material rather than expelling them.
Q: Rank the four carboxylic acid derivatives in order of decreasing reactivity toward nucleophilic acyl substitution, and explain why acid chlorides are most reactive.
A: Acid chloride > anhydride > ester > amide. Acid chlorides are most reactive because Cl is highly electronegative (making the carbonyl carbon very electrophilic) and Cl⁻ is a stable, good leaving group.
Q: Draw the product of treating benzoic acid with SOCl2.
A: Benzoyl chloride (C6H5C(=O)Cl), with SO2 and HCl as by-products.
Q: A carboxylic acid is treated with CH3¹⁸OH under acidic conditions. Where does the ¹⁸O label appear in the product?
A: In the ester product, as RCO–¹⁸OCH3. The ¹⁸O from the alcohol ends up bonded to the carbonyl carbon (acyl bond cleavage), and the unlabelled oxygen leaves as water.
Q: Give two methods for converting a carboxylic acid into a methyl ester.
A: (1) Fischer esterification with CH3OH and acid catalyst. (2) Treatment with diazomethane (CH2N2).
This material connects directly to nucleophilic addition (Ch. 19) since the first step of the mechanism is the same. Understanding leaving-group ability here also reinforces SN1/SN2 concepts from earlier chapters. The interconversion of acid derivatives will become central in biological chemistry, especially when studying peptide bond formation and hydrolysis in biochemistry.
nucleophilic acyl substitution, carboxylic acid derivatives, acyl transfer, tetrahedral intermediate, acid chloride, acyl chloride, carboxylic acid anhydride, ester, carboxamide, amide, leaving group ability, electrophilicity, Fischer esterification, acid-catalysed esterification, Williamson ester synthesis, diazomethane CH2N2, methyl ester, SOCl2 thionyl chloride, oxalyl chloride, PCl3, PBr3, P2O5, acyl bond, alkyl bond, 18O labelling, isotope labelling, steric effects on reactivity, sp2 to sp3, CHEM 301 Chapter 20, organic chemistry OSU