33.3 Acyl chlorides
- Syllabus
- 9701–2028–2029
- Topic
- 33.3
- Level
- A2
Learning objectives
33.3.1Preparation of acyl chlorides• Recall preparation of acyl chlorides: (a) reaction of carboxylic acids with PCl ₃ and heat, PCl ₅ or SOCl ₂33.3.2Reactions of acyl chlorides• Describe reactions of acyl chlorides:- (a) hydrolysis on addition of water at room temp. to produce the carboxylic acid and HCl- (b) reaction with an alcohol at room temp. to produce an ester and HCl- (c) reaction with phenol at room temp. to produce an ester and HCl- (d) reaction with ammonia at room temp. to produce an amide and HCl- (e) reaction with a primary or secondary amine at room temp. to produce an amide and HC l33.3.3Mechanism: acyl chlorides in reactions• Describe the addition–elimination mechanism of acyl chlorides in reactions in 33.3.2(a)–(e)33.3.4Hydrolysis reactivity: acyl vs alkyl chlorides• Explain the relative ease of hydrolysis of acyl chlorides, alkyl chlorides and halogenoarenes (aryl chlorides)
Prepare acyl chlorides from carboxylic acids with chlorinating agents
A carboxylic acid is converted to the corresponding acyl chloride when the -OH in RCOOH is replaced by Cl. The accepted chlorinating choices are PCl3 with heat, PCl5, or SOCl2.
| Reagent | Condition | Overall reaction |
|---|---|---|
| PCl3 | heat | 3RCOOH + PCl3 -> 3RCOCl + H3PO3 |
| PCl5 | no additional condition specified | RCOOH + PCl5 -> RCOCl + POCl3 + HCl |
| SOCl2 | no additional condition specified | RCOOH + SOCl2 -> RCOCl + SO2 + HCl |
Ethanoic acid, CH3COOH, gives ethanoyl chloride, CH3COCl, with any of the three reagent choices. The root name stays eth- and the ending changes from -oic acid to -oyl chloride.
Only PCl3 is explicitly paired with heat in this outcome. Acyl chlorides are RCOCl, not alkyl chlorides RCl; preserve the carbonyl group and use the reagent-specific by-products.
The nucleophile determines the product of an acyl-chloride reaction
At room temperature, an acyl chloride reacts at its carbonyl carbon. Replace Cl by the atom bearing the nucleophile's lone pair, then identify the new carboxylic-acid derivative; HCl is produced in every specified overall reaction.
| Reactant with RCOCl at room temperature | Main organic product | General overall equation |
|---|---|---|
| water, H2O | carboxylic acid | RCOCl + H2O -> RCOOH + HCl |
| alcohol, R'OH | ester | RCOCl + R'OH -> RCOOR' + HCl |
| phenol, C6H5OH | phenyl ester | RCOCl + C6H5OH -> RCOOC6H5 + HCl |
| ammonia, NH3 | primary amide | RCOCl + NH3 -> RCONH2 + HCl |
| primary amine, R'NH2 | N-substituted amide | RCOCl + R'NH2 -> RCONHR' + HCl |
| secondary amine, R'2NH | N,N-disubstituted amide | RCOCl + R'2NH -> RCONR'2 + HCl |
In practice, excess ammonia or amine can neutralise the HCl to form an ammonium or alkylammonium chloride salt. This acid capture does not change the identity of the amide formed from the first nucleophile molecule.
Do not omit phenol or secondary amines from the reaction set. Tertiary amines have no N-H bond and are not included in this amide-forming objective; the specified reactants are ammonia, primary amines and secondary amines.
All five acyl-chloride reaction families share addition-elimination
Water, alcohols, phenol, ammonia and primary/secondary amines all supply a lone pair to the electrophilic carbonyl carbon of RCOCl. The attacking atom is O for water/alcohol/phenol and N for ammonia/amines.
- Draw a curly arrow from the nucleophile lone pair to the carbonyl carbon, and a second arrow from the C=O pi bond to O. This addition gives a tetrahedral intermediate with O-. 2) Draw an arrow from the O- lone pair back to reform C=O, and an arrow from the C-Cl bond to Cl, eliminating Cl-. 3) Transfer the proton from the newly attached O-H or N-H group; HCl is formed or captured by excess base.
RCOCl+HNuRCONu+HCl(Nu=O- or N-centred group)
| Incoming nucleophile | Attacking atom | Group retained in product | Product family |
|---|---|---|---|
| H2O | O | -OH | carboxylic acid |
| R'OH or C6H5OH | O | -OR' or -OC6H5 | ester |
| NH3 | N | -NH2 | primary amide |
| R'NH2 or R'2NH | N | -NHR' or -NR'2 | substituted amide |
Addition is only the first step: the tetrahedral intermediate then eliminates chloride and the carbonyl is restored. Do not draw direct SN2 displacement at the carbonyl carbon or leave the final product with a permanently single C-O bond.
Hydrolysis becomes harder from acyl chloride to alkyl chloride to aryl chloride
ease of hydrolysis: acyl chloride>alkyl chloride>aryl chloride
| Chloride type | Local electronic structure | Hydrolysis pathway and conditions | Why it has this position |
|---|---|---|---|
| acyl chloride, RCOCl | carbonyl O and Cl withdraw electron density, making the carbonyl C strongly delta-positive | water attacks the carbonyl at room temperature; vigorous addition-elimination | highly electrophilic reaction centre and Cl- leaves as C=O reforms |
| alkyl chloride, RCl | C-Cl is a polar single bond at an sp3 carbon, but the carbon is less electrophilic | aqueous OH- and heating/reflux are normally required for nucleophilic substitution | water alone is a weaker nucleophile and attack is less favourable |
| aryl chloride, ArCl | a Cl lone pair overlaps with the aromatic pi system; C-Cl gains partial double-bond character at an sp2 carbon | does not readily undergo ordinary hydrolysis; much harsher conditions are needed | strengthened C-Cl bond is difficult to break |
Compare the actual reaction centre, not chlorine alone. Acyl-chloride hydrolysis attacks C=O and then expels chloride; alkyl-chloride hydrolysis substitutes at saturated carbon; aryl chloride is protected by pi-system overlap and a strengthened C-Cl bond.
Do not explain all three with one SN1 or SN2 mechanism. Acyl chlorides use addition-elimination, alkyl chlorides use nucleophilic substitution, and aryl chlorides resist ordinary substitution because their C-Cl bonding is different.