In simple terms
A friendly intro before the formal notes — no formulas yet.
Acyl Chlorides: The Reactive Acylators
Acyl chlorides are like super-charged carboxylic acids, ready to donate their acyl group (RCO-) in rapid reactions. Their extreme reactivity makes them incredibly useful building blocks in organic synthesis.
Imagine a spring-loaded gift box. The acyl group is the gift inside. The electron-withdrawing chlorine and oxygen atoms act like a tightly wound spring, making the box (the carbonyl carbon) very unstable and ready to pop open. When a nucleophile (like an alcohol or amine) 'pokes' the box, it bursts open, delivering the acyl gift and ejecting the chloride 'lid'.
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RCOCl — reactive carbonyl derivative; fumes in air (HCl).
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Reactions: with alcohols → esters; with amines → amides; with arenes → ketones.
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Mechanism: nucleophilic addition–elimination at C=O.
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More reactive than carboxylic acids or esters.
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Key formulas
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$RCOCl + 2NH_3 \rightarrow RCONH_2 + NH_4Cl \ \ Reaction with a primary amine: RCOCl + 2R'NH_2 \rightarrow RCONHR' + R'NH_3Cl$
Full topic notes
Formal explanation with the rigour you need for the exam.
Structure and Reactivity
The reactivity of an acyl chloride stems from its electronic structure. The carbonyl group () is polar, with the carbon atom being electron-deficient (). This effect is significantly amplified by the attached chlorine atom, which is also highly electronegative. Both atoms pull electron density away from the carbonyl carbon, making it extremely electrophilic and highly susceptible to attack by nucleophiles. Furthermore, upon nucleophilic attack, the resulting tetrahedral intermediate can readily eliminate a chloride ion (). The chloride ion is a very stable anion and therefore an excellent leaving group, which drives the reaction forward.
Synthesis of Acyl Chlorides
Acyl chlorides are synthesised from their parent carboxylic acids. This cannot be achieved with simple reagents like HCl; a more powerful chlorinating agent is required. The two common reagents you must know are thionyl chloride () and phosphorus(V) chloride (). The reactions are typically carried out under anhydrous conditions, often with gentle heating in a fume cupboard due to the toxic and corrosive nature of the reagents and products.
Thionyl chloride () is often preferred as the by-products ( and ) are gases and can be easily removed, simplifying the purification of the liquid acyl chloride.
Using results in a liquid by-product, phosphorus(III) oxychloride (), which has a similar boiling point to many acyl chlorides, making separation by distillation more difficult.
Reactions of Acyl Chlorides
Acyl chlorides undergo rapid, often violent, reactions with nucleophiles at room temperature. These are all examples of nucleophilic addition-elimination. In each case, the nucleophile replaces the chlorine atom, and steamy fumes of HCl are produced as a by-product.
Reaction with Ammonia and Amines
Acyl chlorides react readily with concentrated ammonia and primary amines to form amides. With ammonia, a primary amide is formed. With a primary amine, an N-substituted amide is formed. These reactions are very fast and produce HCl. The HCl immediately reacts with a second molecule of the basic ammonia or amine. Therefore, the overall stoichiometry requires two moles of the amine/ammonia for every one mole of the acyl chloride.
Reaction with ammonia: $RCOCl + 2NH_3 \rightarrow RCONH_2 + NH_4Cl \ \ Reaction with a primary amine: RCOCl + 2R'NH_2 \rightarrow RCONHR' + R'NH_3Cl$
Examiners frequently ask why two equivalents of amine or ammonia are needed. The first equivalent acts as the nucleophile to attack the carbonyl carbon. The second equivalent acts as a base to neutralise the HCl by-product, forming an ammonium salt. If you only use one equivalent, half of your expensive amine/ammonia will be protonated and become unreactive, leading to a maximum 50% yield.
Order of Reactivity of Carboxylic Acid Derivatives
The reactivity of carboxylic acid derivatives towards nucleophilic addition-elimination depends on the leaving group's ability and the electrophilicity of the carbonyl carbon.
Order of Reactivity: Acyl Chloride > Acid Anhydride > Ester ≈ Carboxylic Acid > Amide.
Reasoning: is the best leaving group (conjugate base of a strong acid, HCl). is a good leaving group. and are poor leaving groups. is an extremely poor leaving group.
This order of reactivity means you can easily make a less reactive derivative from a more reactive one (e.g., acyl chloride → ester), but not the other way around under normal conditions.
Worked examples
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Propanoyl chloride () is reacted with propan-2-ol. (i) Name and draw the structure of the organic product formed. (ii) Write a balanced chemical equation for the reaction.
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(i) Product Name: Isopropyl propanoate (or 1-methylethyl propanoate).
Calculate the mass of N-ethylethanamide () formed when 5.00 g of ethanoyl chloride () is added to an excess of ethylamine (). ( values: C=12.0, H=1.0, O=16.0, N=14.0, Cl=35.5)
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Step 1: Write the balanced equation.
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Glossary
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Revision flashcards
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What is the functional group of an acyl chloride?
The -COCl group, known as the acyl chloride or acid chloride group.
Key takeaways
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Thionyl chloride () is often preferred as the by-products ( and ) are gases and can be easily removed, simplifying the purification of the liquid acyl chloride.
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Using results in a liquid by-product, phosphorus(III) oxychloride (), which has a similar boiling point to many acyl chlorides, making separation by distillation more difficult.
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Test Your Knowledge on Acyl Chlorides
Test Your Knowledge on Acyl Chlorides
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