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Organic Chemistry · Carboxylic Acid Derivatives: Nucleophilic Acyl Substitution Reactions

Chemistry of Acid Halides

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Reactivity rankings, mechanism steps, and the pKa of HCl (≈ −7) follow standard undergraduate organic chemistry conventions — verify against current sources before relying on them in assessments. Lab-handling guidance here is general principle only; follow your institution's specific safety rules for acid chlorides and anhydrous reagents.
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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

Acid halides (acyl halides) have the structure RCO-X, where X is a halogen — almost always chlorine, giving an such as acetyl chloride (CH3COCl) or benzoyl chloride (C6H5COCl). They sit at the top of the reactivity ladder among carboxylic acid derivatives because the halogen is both electron-withdrawing (making the carbonyl carbon highly electrophilic) and an excellent leaving group. Acid chlorides are prepared from carboxylic acids with SOCl2, PCl3, PCl5, or oxalyl chloride, and they react with water, alcohols, amines, carboxylate salts, aromatic rings, hydrides, and Grignard reagents. Because they are the most reactive derivatives, they are the workhorses of acyl-transfer synthesis: almost any other derivative can be made from them.

Why this matters

  • Universal acylating agents: Acid chlorides convert acids into esters, amides, and anhydrides in high yield under mild conditions — the key bond-forming step in many drug and polymer syntheses.
  • Friedel–Crafts acylation: Acid chlorides plus AlCl3 attach acyl groups to aromatic rings — the route to aryl ketones such as acetophenone and a first step in countless pharmaceutical syntheses. Unlike alkylation, acylation does not rearrange.
  • Controlled reduction: The same starting material can be reduced to an aldehyde (with a special hydride) or all the way to a primary alcohol (with LiAlH4), giving chemists a choice of oxidation state.
  • Safety awareness: Acid chlorides fume in moist air and hydrolyze vigorously, releasing HCl gas. General lab practice is to handle them under anhydrous conditions in a fume hood and quench spills carefully; exact procedures vary by institution.
  • Exams: Reagent identification, product prediction, and mechanism drawing for hydrolysis// are classic questions.

The college version

Core Concepts

Why acid halides are so reactive

Two effects combine to make the acyl carbon of an acid chloride unusually electrophilic:

  1. : Chlorine is electronegative and pulls electron density away from the carbonyl carbon through the σ framework, increasing its partial positive charge.
  2. Weak resonance donation: A halogen's lone pairs can donate into the carbonyl π system, but the overlap is poor (the halogen is large and its lone pairs diffuse). Resonance stabilization of the ground state is small — much smaller than for esters or amides.

At the same time, chloride is an outstanding leaving group because HCl is a very strong acid (pKa ≈ −7), making Cl- an extremely weak base. Fast attack on an electron-poor carbon plus easy departure of the leaving group equals the fastest acyl substitution of the family.

Preparation from carboxylic acids

The standard route converts the –OH of the acid into –Cl:

RCOOH + SOCl2 ⟶ RCOCl + SO2 + HCl

is preferred because both byproducts are gases that escape, leaving the acid chloride essentially pure. Alternatives: PCl3, PCl5, and oxalyl chloride ((COCl)2, often with catalytic DMF). All versions must run under anhydrous conditions, since the product reacts with water.

Reactions with oxygen and nitrogen nucleophiles

  • Hydrolysis (water): RCOCl + H2O ⟶ RCOOH + HCl. Vigorous, often spontaneous — why acid chlorides cannot be stored in contact with moisture.
  • Alcoholysis (alcohol): RCOCl + R'OH ⟶ RCOOR' + HCl. A base (pyridine or extra alcohol) neutralizes the HCl before it can protonate the nucleophile.
  • Aminolysis (amine): RCOCl + 2R'2NH ⟶ RCONR'2 + R'2NH2+Cl-. Two equivalents of amine: one does the substitution, one absorbs the HCl. The classic amide synthesis.
  • Carboxylate salts (anhydride formation): RCOCl + R'COO- ⟶ RCO-O-COR' + Cl-. Useful for mixed anhydrides.

Friedel–Crafts acylation of aromatic rings

With a Lewis acid catalyst (AlCl3), an acid chloride acylates an aromatic ring:

C6H6 + CH3COCl AlCl3⟶ C6H5COCH3 + HCl

Acylation proceeds through a stable (RCO+) and does not rearrange. The ketone product is less reactive than benzene, so the reaction stops after one acylation.

Reduction and organometallic reactions

  • Reduction to a primary alcohol: RCOCl LiAlH4⟶ RCH2OH — hydride adds twice.
  • Reduction to an aldehyde: The bulky lithium tri-tert-butoxyaluminum hydride (LiAlH(O-t-Bu)3) delivers hydride once, stopping at RCHO.
  • Grignard reagents: Two equivalents of R''MgX give a tertiary alcohol (the first addition gives a ketone, which reacts again). Organocadmium or organocuprate reagents stop at the ketone.

How It Works / Step-by-Step Process

Mechanism of hydrolysis of an acid chloride (archetype for all acyl substitutions):

  1. Water's oxygen lone pair attacks the electrophilic acyl carbon; the C=O π electrons move onto oxygen, forming a tetrahedral intermediate.
  2. The intermediate collapses: an oxygen lone pair reforms the C=O bond while the C-Cl bond breaks; chloride leaves with the electron pair.
  3. A proton transfers (from the water-derived oxygen) to give the carboxylic acid and HCl.

For alcoholysis and aminolysis, replace water with the alcohol or amine; the leaving group and proton-balancing step are the same.

Common Confusions

Do Not ConfuseWithDifference
Acid chlorideAlkyl chlorideAcid chloride: RCOCl, acyl carbon sp², reacts with weak nucleophiles easily; alkyl chloride: RCH2Cl, sp³, needs strong nucleophiles or catalysts for SN2
SOCl2 product purityPCl5 byproductsSOCl2 gives only gaseous byproducts; PCl5 gives phosphorus oxychloride, which is harder to remove
LiAlH4 reduction of RCOClReduction to aldehydeLiAlH4 goes all the way to the primary alcohol; bulky LiAlH(O-t-Bu)3 stops at the aldehyde
Friedel–Crafts acylationFriedel–Crafts alkylationAcylation proceeds via acylium ion, does not rearrange, and the ketone product deactivates the ring (mono-acylation); alkylation rearranges and can poly-alkylate
"Reacts with water""Stable in water"Acid chlorides hydrolyze readily; they are stored and used under anhydrous conditions
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

An acid chloride is like a very eager coat hook: the chlorine atom pulls the hook's tip toward itself, making the hook extra grabby, and it lets go of its old coat easily. Because the hook grabs almost anything that comes near — water, alcohol, or amine — it reacts quickly with all of them. That's why chemists keep these hooks in dry boxes: even a little water in the air will snatch it first.

Worked example

Example 1: Choosing the reagent to make an acid chloride

Which reagent converts butanoic acid, CH3CH2CH2COOH, into butanoyl chloride, and what are the byproducts?

Approach: Replace –OH with –Cl using SOCl2:

CH3CH2CH2COOH + SOCl2 ⟶ CH3CH2CH2COCl + SO2 + HCl

Answer: SOCl2 (or PCl3, PCl5, oxalyl chloride). With SOCl2 the byproducts are SO2 and HCl, both gases, so the acid chloride is obtained by evaporation — no aqueous workup, which would destroy the product.

Example 2: Stoichiometry of acid chloride synthesis

How many grams of thionyl chloride (SOCl2, 118.97 g/mol) are required to convert 10.0 g of benzoic acid (C6H5COOH, 122.12 g/mol) to benzoyl chloride?

Balanced reaction first (1:1 mole ratio):

C6H5COOH + SOCl2 ⟶ C6H5COCl + SO2 + HCl

Dimensional analysis:

10.0 g C6H5COOH × 1 mol C6H5COOH122.12 g × 1 mol SOCl21 mol C6H5COOH × 118.97 g SOCl21 mol SOCl2 = 9.74 g

Answer: 9.74 g of SOCl2 (theoretical; a slight excess is normally used).

Example 3: Predicting the product of aminolysis

What is the product of benzoyl chloride, C6H5COCl, with excess aniline (C6H5NH2)?

Reason: The amine attacks the acyl carbon; chloride leaves; the second equivalent of aniline neutralizes the HCl. The nitrogen keeps one hydrogen and gains the benzoyl group.

Answer: N-phenylbenzamide, C6H5CONHC6H5, plus anilinium chloride. Two equivalents of amine are the signal that HCl must be neutralized.

Key takeaways

  • Acid chlorides: RCOCl; the most reactive carboxylic acid derivatives.
  • High reactivity = inductive withdrawal by Cl + weak resonance donation + excellent leaving group (Cl-, conjugate acid pKa ≈ −7).
  • Preparation: RCOOH + SOCl2 → RCOCl + SO2 + HCl (also PCl3, PCl5, oxalyl chloride); anhydrous conditions required.
  • Hydrolysis → acid; alcoholysis → ester; aminolysis → amide; carboxylate salts → anhydride; AlCl3 + arene → aryl ketone.
  • Aminolysis needs 2 equivalents of amine (one to neutralize HCl).
  • LiAlH4 → primary alcohol; LiAlH(O-t-Bu)3 → aldehyde; 2 equiv RMgX → tertiary alcohol.
  • Friedel–Crafts acylation: no rearrangement, stops after one acyl group.

Check yourself

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

  1. Why is an acid chloride more reactive toward nucleophiles than an ester?

    Show answer

    Chlorine withdraws electrons inductively, making the acyl carbon more electrophilic; chlorine's resonance donation is weak; and Cl- is an excellent leaving group because HCl is a very strong acid.

  2. Write the reaction of acetic acid with thionyl chloride and identify the byproducts.

    Show answer

    CH3COOH + SOCl2 → CH3COCl + SO2 + HCl. Both byproducts are gases and escape.

  3. How many equivalents of a primary amine are needed to convert RCOCl to RCONHR', and why?

    Show answer

    Two equivalents. The first is the nucleophile; the second neutralizes the HCl produced, preventing the amine from being protonated and the amide from reverting to a salt.

  4. What product forms when benzoyl chloride is treated with water? With methanol and pyridine? With excess CH3NH2?

    Show answer

    Water → benzoic acid + HCl. Methanol with pyridine → methyl benzoate (pyridine scavenges HCl). Excess methylamine → N-methylbenzamide + methylammonium chloride.

  5. Which reagent reduces an acid chloride to an aldehyde, and why does plain LiAlH4 not do this?

    Show answer

    Lithium tri-tert-butoxyaluminum hydride, LiAlH(O-t-Bu)3. Its three bulky alkoxide groups slow hydride delivery so only one addition occurs; LiAlH4 is small and fast enough to add twice, giving the alcohol.

  6. Why does Friedel–Crafts acylation not suffer from rearrangement, and why does it stop after one acylation?

    Show answer

    Acylation goes through the resonance-stabilized acylium ion RCO+, which does not rearrange. The product ketone is less reactive than benzene (the carbonyl withdraws electrons), so further acylation is slow — the reaction naturally stops at one acyl group.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Acid chloride
RCOCl, acyl group bonded to chlorine
Inductive withdrawal
Electron pull through σ bonds by an electronegative atom
Acylium ion
RCO+, the electrophilic species in Friedel–Crafts acylation
Thionyl chloride
SOCl2, converts RCOOH to RCOCl
Alcoholysis
Reaction of an acyl derivative with an alcohol
Aminolysis
Reaction with an amine

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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