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Organic Chemistry · Carboxylic Acid Derivatives: Nucleophilic Acyl Substitution Reactions

Chemistry of Acid Anhydrides

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6 key termsSourcesReport an error
Reactivity rankings, mechanism steps, and pKa values (carboxylic acids ≈ 4.8, alcohols ≈ 16, amines ≈ 35) follow standard undergraduate organic chemistry conventions — verify against current sources before relying on them in assessments. Aspirin yield figures are stoichiometric theory; actual yields depend on procedure and are lab specific.
Want it in plain words first? Jump to Eli explains — the same idea, no jargon.
On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Check yourself
  8. Study tools
  9. Sources & references

In 30 seconds

An is a molecule in which two acyl groups share a single oxygen atom: RCO-O-COR'. When the two acyl groups are identical the anhydride is symmetrical (acetic anhydride, (CH3CO)2O, is the classic example); when they differ it is a . Anhydrides sit between acid chlorides and esters on the reactivity ladder: less reactive than acid chlorides (carboxylate is a worse leaving group than chloride) but far more reactive than esters or amides. They are prepared by dehydrating carboxylic acids or by combining an acid chloride with a carboxylate salt, and they react with water, alcohols, and amines to give acids, esters, and amides. Their big practical advantage over acid chlorides: no HCl gas is released, which makes them gentler, cheaper, and easier to handle — the reason acetic anhydride is the industrial reagent of choice for acetylations such as aspirin synthesis.

Why this matters

  • Aspirin synthesis: Salicylic acid + acetic anhydride (acid-catalyzed) → acetylsalicylic acid (aspirin) + acetic acid. One of the most famous reactions in organic chemistry and a guaranteed exam problem.
  • Safer acylation than acid chlorides: Because anhydrides release no HCl, they acetylate under milder conditions — cellulose to cellulose acetate (films, fibers, eyeglass frames), and p-aminophenol to acetaminophen (paracetamol).
  • Reactivity sweet spot: Anhydrides react fast enough to be useful but are stable enough to store and handle — a practical lesson in choosing the right derivative.
  • Biological relevance: Mixed anhydrides appear in metabolism — acyl phosphates (the topic's sibling file) are anhydride-like mixed derivatives that drive biosynthetic reactions.
  • Exams: Preparation routes, hydrolysis stoichiometry (one anhydride → two acids), and product prediction for alcoholysis and aminolysis are frequent questions.

The college version

Core Concepts

Structure and the two types

An anhydride is literally "without water": two carboxyl groups joined through an oxygen, formally the dehydration product of two acids:

RCOOH + R'COOH - H2O⟶ RCO-O-COR'

If R = R', the anhydride is symmetrical (acetic anhydride, benzoic anhydride). If R ≠ R', it is mixed — for example, acetic formic anhydride, CH3CO-O-CHO. Naming: replace "acid" with "anhydride" (acetic anhydride from acetic acid). Mixed anhydrides list both acyl groups alphabetically (acetic formic anhydride).

Reactivity: between acid chlorides and esters

Anhydrides undergo the same nucleophilic acyl substitution as acid chlorides, but more slowly. The leaving group is a carboxylate ion RCOO-, whose conjugate acid is weak (pKa ≈ 5) — a much worse leaving group than chloride. Two consequences:

  1. Anhydrides react with water, alcohols, and amines readily but controllably, often needing gentle heating or a catalyst.
  2. Because they carry two acyl groups, only one is consumed per substitution: the other half becomes a carboxylic acid. This stoichiometry (one anhydride gives one product molecule + one acid molecule) is essential for calculations.

Preparation

  • Dehydration of acids: Heating carboxylic acids with a dehydrating agent (e.g., P2O5) removes water. Industrially, acetic anhydride is used as the dehydrating agent for acetic acid — a process that makes more acetic anhydride.
  • Acid chloride + carboxylate salt: RCOCl + R'COO-Na+ ⟶ RCO-O-COR' + NaCl. Especially useful for mixed anhydrides.

Reactions: hydrolysis, alcoholysis, aminolysis

(RCO)2O + H2O ⟶ 2RCOOH

(RCO)2O + R'OH ⟶ RCOOR' + RCOOH

(RCO)2O + 2R'2NH ⟶ RCONR'2 + R'2NH2+RCOO-

The mechanism is the familiar addition–elimination: the nucleophile adds to an acyl carbon, the tetrahedral intermediate collapses, and the carboxylate leaves as the "second half" of the anhydride, then picks up a proton. In alcoholysis, one acyl group becomes the ester and the other becomes the carboxylic acid — a byproduct you must account for in yield calculations.

Acetic anhydride in synthesis: aspirin and beyond

Acetic anhydride transfers an acetyl group (CH3CO-) to nucleophiles. In aspirin synthesis, salicylic acid (an aromatic o-hydroxy acid) is acetylated at its phenolic –OH:

C6H4(OH)COOH + (CH3CO)2O H+⟶ C6H4(OCOCH3)COOH + CH3COOH

The product, acetylsalicylic acid, is an ester of the phenol — and still a carboxylic acid, which is why aspirin is an acid (and famously irritating to the stomach lining). The same acetyl-transfer chemistry acetylates amines (acetaminophen) and cellulose.

How It Works / Step-by-Step Process

Mechanism of the reaction of acetic anhydride with methanol (alcoholysis):

  1. Methanol's oxygen lone pair attacks an acyl carbon of (CH3CO)2O; the C=O π electrons move to oxygen, forming a tetrahedral intermediate.
  2. The intermediate collapses: an oxygen lone pair reforms the C=O bond while the C-O bond to the other acyl group breaks; the acetate ion CH3COO- departs.
  3. The acetate picks up the proton from the methoxy oxygen, giving acetic acid. Net: methyl acetate + acetic acid.

Common Confusions

Do Not ConfuseWithDifference
AnhydrideEsterAnhydride: RCO-O-COR (two acyl groups); ester: RCO-OR' (one acyl group + alkyl). Alcoholysis of an anhydride gives an ester AND an acid; hydrolysis of an ester gives one acid + alcohol
Symmetrical anhydrideMixed anhydrideSymmetrical: identical acyl groups; mixed: different groups, named by listing both
One anhydride → one productOne anhydride → two productsHydrolysis gives TWO acid molecules; alcoholysis/aminolysis give one acyl product + one acid (or salt) byproduct
Acetic anhydrideAcetic acidThe anhydride hydrolyzes to two acetic acids in water; the "vinegar smell" on the shelf comes from moisture, not the reagent itself
Anhydride reactivityAcid chloride reactivityAnhydrides are slower and release no HCl — gentler and often preferred for large-scale acetylation
Aspirin as anhydrideAspirin as esterAspirin is an ester of salicylic acid's phenolic –OH (and still a carboxylic acid); acetic anhydride is the reagent used to make it
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

An anhydride is like a double ice-cream cone with two scoops sharing one cone. When someone comes along with a spoon (a nucleophile), they take one scoop — the other scoop falls off and becomes a plain scoop on the ground (the carboxylic acid byproduct). The cone is still a cone, but now there's one scoop on it and one scoop of leftovers. One attack, two results: the new product plus an acid.

Worked example

Example 1: Stoichiometry of anhydride hydrolysis

How many grams of acetic acid (60.05 g/mol) are produced when 10.0 g of acetic anhydride ((CH3CO)2O, 102.09 g/mol) is completely hydrolyzed?

Balanced reaction first:

(CH3CO)2O + H2O ⟶ 2CH3COOH

Dimensional analysis — note the 1:2 mole ratio:

10.0 g (CH3CO)2O × 1 mol (CH3CO)2O102.09 g × 2 mol CH3COOH1 mol (CH3CO)2O × 60.05 g CH3COOH1 mol CH3COOH = 11.8 g

Answer: 11.8 g of acetic acid. The "×2" is the classic trap — both acyl groups become product.

Example 2: Aspirin synthesis yield

Salicylic acid (138.12 g/mol) reacts with excess acetic anhydride to give aspirin, acetylsalicylic acid (180.16 g/mol), and acetic acid. What is the theoretical yield of aspirin from 5.00 g of salicylic acid?

Balanced reaction first (1:1 mole ratio):

C6H4(OH)COOH + (CH3CO)2O ⟶ C6H4(OCOCH3)COOH + CH3COOH

Dimensional analysis:

5.00 g salicylic acid × 1 mol138.12 g × 1 mol aspirin1 mol salicylic acid × 180.16 g1 mol = 6.52 g

Answer: 6.52 g theoretical. With excess anhydride the acid is the limiting reagent; isolated yields in lab are typically 80–90% of this value.

Example 3: Predicting aminolysis products

Predict the products when acetic anhydride reacts with excess methylamine (CH3NH2).

Reason: Methylamine attacks an acyl carbon; acetate leaves and then deprotonates the methylammonium ion. Net: one acyl group becomes the amide; the other becomes acetate.

Answer: N-methylacetamide, CH3CONHCH3, plus methylammonium acetate, CH3NH3+CH3COO-. Compare with acid chlorides, where the byproduct is the amine hydrochloride — here it is the ammonium carboxylate salt.

Key takeaways

  • Anhydrides: RCO-O-COR'; symmetrical (same R) or mixed (different R).
  • Reactivity order: acid chloride > anhydride > thioester > ester > amide.
  • Leaving group in anhydride reactions is carboxylate RCOO- (pKa of conjugate acid ≈ 5).
  • Preparation: dehydration of acids (P2O5) or acid chloride + carboxylate salt.
  • Hydrolysis: (RCO)2O + H2O → 2RCOOH — one anhydride makes TWO acid molecules.
  • Alcoholysis: anhydride + alcohol → ester + carboxylic acid (1:1:1 stoichiometry).
  • Aminolysis: anhydride + 2 amine → amide + ammonium carboxylate salt.
  • Aspirin: salicylic acid + acetic anhydride → acetylsalicylic acid + acetic acid.
  • Advantage over acid chlorides: no HCl byproduct — milder, safer acylation.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

  1. Write the balanced hydrolysis of acetic anhydride. How many moles of acetic acid come from one mole of anhydride?

    Show answer

    (CH3CO)2O + H2O → 2CH3COOH. One mole of anhydride gives two moles of acetic acid.

  2. Why are anhydrides less reactive than acid chlorides but more reactive than esters?

    Show answer

    The leaving group is carboxylate (RCOO-), whose conjugate acid has pKa ≈ 5 — worse than chloride but far better than alkoxide (pKa ≈ 16) or amide nitrogen (pKa ≈ 35).

  3. Give two methods for preparing an acid anhydride.

    Show answer

    (1) Dehydration of the acid with P2O5 (or using acetic anhydride itself as the dehydrating agent for acetic acid); (2) acid chloride + carboxylate salt: RCOCl + R'COO- → RCO-O-COR' + Cl-.

  4. What are the products of the reaction of acetic anhydride with 1-butanol? With excess aniline?

    Show answer

    With 1-butanol: butyl acetate + acetic acid. With excess aniline: acetanilide (N-phenylacetamide) + anilinium acetate.

  5. In aspirin synthesis, which –OH of salicylic acid is acetylated, and why does the –COOH survive?

    Show answer

    The phenolic –OH (on the aromatic ring) is acetylated because it is the better nucleophile under the reaction conditions. The –COOH is not touched — it stays intact, which is why aspirin is still an acid.

  6. Why is acetic anhydride often preferred over acetyl chloride for industrial ?

    Show answer

    Acetyl chloride releases HCl, which is corrosive, must be neutralized, and can degrade sensitive substrates; acetic anhydride releases only acetic acid (or its salt), is cheaper, and gives milder, more selective acetylations.

Keep learning

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Acid anhydride
RCO-O-COR', two acyl groups on one oxygen
Symmetrical anhydride
Both acyl groups identical
Mixed anhydride
Two different acyl groups
Acetylation
Transfer of the CH3CO- group to a nucleophile
Carboxylate leaving group
RCOO-, the "second half" of the anhydride
Hydrolysis of an anhydride
Water splits the anhydride into two acids

Sources & references

  1. openstax.org — Organic Chemistry

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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