Acid Chlorides and Amide Formation

Introduction

Carboxylic acids react only slowly with alcohols and hardly at all with amines to give amides, because amines simply deprotonate the acid to form an unreactive salt. The solution is to activate the acid by turning it into an acid chloride. Acid chlorides are among the most reactive carbonyl compounds and transfer their acyl group rapidly to water, alcohols, ammonia and amines. Their reaction with amines is a key route to amides, the linkage found in nylon, paracetamol and proteins.

Core explanation

Making acid chlorides. Carboxylic acids are converted into acid chlorides by chlorinating reagents such as thionyl chloride (SOCl₂), phosphorus(V) chloride (PCl₅) or phosphorus(III) chloride (PCl₃). With thionyl chloride:

CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl

The by-products are gases, which makes purification easier. These reagents and their products are corrosive and toxic, so such work is done only in a fume cupboard by trained chemists.

Why acid chlorides are so reactive. The carbonyl carbon carries two electron-withdrawing atoms, oxygen and chlorine, making it strongly δ+. Chlorine's lone pairs donate very little into the C=O, and chloride is an excellent leaving group because it is the conjugate base of a strong acid. Both factors favour fast nucleophilic acyl substitution.

Reactions with oxygen nucleophiles.

- Water: CH₃COCl + H₂O → CH₃COOH + HCl (violent; ethanoyl chloride fumes in moist air as HCl forms). - Alcohols: CH₃COCl + CH₃CH₂OH → CH₃COOCH₂CH₃ + HCl. Unlike Fischer esterification, this is rapid, needs no catalyst and is not reversible. It also works with phenols, which are too weakly nucleophilic to esterify with carboxylic acids.

Reactions with nitrogen nucleophiles.

- Ammonia gives a primary amide: CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl. - A primary amine gives an N-substituted amide: CH₃COCl + 2CH₃NH₂ → CH₃CONHCH₃ + CH₃NH₃⁺Cl⁻ (N-methylethanamide). - A secondary amine gives an N,N-disubstituted amide.

Two equivalents of ammonia or amine are used because the HCl produced would otherwise protonate half of the amine, turning it into a non-nucleophilic ammonium ion. Alternatively a cheaper base such as pyridine or aqueous hydroxide is added to mop up the acid.

Mechanism with an amine. The nitrogen lone pair attacks the carbonyl carbon, forming a tetrahedral intermediate with O⁻ and a positively charged nitrogen. The C=O re-forms and Cl⁻ is expelled. A second amine molecule removes the proton from nitrogen, giving the neutral amide.

Properties of amides. In an amide the nitrogen lone pair is delocalised into the carbonyl group, giving the C–N bond partial double-bond character. As a result amides are planar around nitrogen, show restricted rotation, are almost neutral (not basic like amines) and are very resistant to hydrolysis. Primary and secondary amides can hydrogen bond strongly, so ethanamide is a solid melting at about 80 °C.

Acid anhydrides as milder alternatives. Ethanoic anhydride reacts similarly but more gently, producing ethanoic acid instead of corrosive HCl. It is preferred industrially, for example in making aspirin.

Step-by-step reasoning

To predict an acylation product:

1. Identify the acyl group RCO– in the acid chloride. 2. Identify the nucleophile's reactive atom (O in water or alcohol, N in ammonia or amine). 3. Join RCO– to that atom, removing one H from the nucleophile. 4. Write HCl as the by-product, or its ammonium salt if excess amine is present. 5. Name the product: acid, ester, amide or N-substituted amide.

Visual explanation

Picture methylamine's nitrogen lone pair as an arrow pointing to the carbonyl carbon of ethanoyl chloride. A second arrow pushes the C=O electrons onto oxygen. Then an arrow from O⁻ reforms the double bond while an arrow from the C–Cl bond sends chloride away, and a final arrow shows a second amine removing N–H.

Real-world analogy

An acid chloride is like a courier holding a parcel very loosely: almost anyone who reaches for it will take it. An ordinary carboxylic acid grips its parcel firmly and, faced with an amine, simply swaps a proton instead of handing over the parcel.

Real-world example

Paracetamol is made by acylating 4-aminophenol at nitrogen, industrially using ethanoic anhydride. Nylon-6,6 can be made from hexanedioyl dichloride and hexane-1,6-diamine: each acid chloride end reacts with an amine end, releasing HCl and building a polyamide chain.

Why?

Why do amines react with acid chlorides to give amides but with carboxylic acids to give salts? An amine is a base as well as a nucleophile. With a carboxylic acid, fast proton transfer gives a carboxylate that is no longer electrophilic. An acid chloride has no acidic O–H, so the amine can only act as a nucleophile at the carbonyl carbon.

Common misconception

"Amides are basic because they contain nitrogen, like amines." The nitrogen lone pair in an amide is delocalised into the carbonyl group and is not available to accept a proton, so amides are essentially neutral.

Worked example

Question: Give the product and by-products when propanoyl chloride reacts with excess ethylamine.

Reasoning: The nitrogen of ethylamine attacks the acyl carbon, chloride leaves and a second ethylamine takes the proton, forming ethylammonium chloride.

Answer: N-ethylpropanamide, CH₃CH₂CONHCH₂CH₃, plus CH₃CH₂NH₃⁺Cl⁻.

Quick check

1. Why is ethanoyl chloride observed to fume when a bottle is opened in moist air? Answer: It reacts with water vapour to form ethanoic acid and hydrogen chloride gas, which forms misty fumes.

Exam focus

Write balanced equations showing HCl or the ammonium salt, and draw the addition–elimination mechanism with the tetrahedral intermediate. Explain why acid chlorides beat carboxylic acids for making esters of phenols and for making amides.

Advanced insight

Living cells cannot use acid chlorides in water, so they activate carboxylic acids as thioesters (acetyl coenzyme A) or acyl phosphates instead. Laboratory peptide synthesis uses coupling reagents such as carbodiimides that convert the OH into a good leaving group in situ, avoiding the harshness of acid chlorides.

Summary

Acid chlorides are made from acids with reagents like SOCl₂ or PCl₅ and are highly reactive because of a strongly δ+ carbon and an excellent leaving group. They react rapidly and irreversibly with water, alcohols, phenols, ammonia and amines by addition–elimination. Two equivalents of amine are needed. Amides are planar, non-basic and resistant to hydrolysis.

Practice questions

1. Write the equation for ethanoyl chloride reacting with phenol. Answer: CH₃COCl + C₆H₅OH → CH₃COOC₆H₅ + HCl, giving phenyl ethanoate. 2. Name the product of benzoyl chloride with dimethylamine. Answer: N,N-dimethylbenzamide, C₆H₅CON(CH₃)₂. 3. Why is ethanoic anhydride often preferred to ethanoyl chloride industrially? Answer: It is cheaper, less violently reactive and less corrosive, and its by-product ethanoic acid is less hazardous than hydrogen chloride. 4. Explain why the C–N bond in an amide is shorter than in an amine. Answer: The nitrogen lone pair is delocalised into the carbonyl, giving the C–N bond partial double-bond character.