Topic 19 · page 5 of 13
Acyl chlorides and amides
An acyl chloride reacts with ammonia to give an amide, and with a primary amine to give an N-substituted amide. The reaction is fast at room temperature and gives off hydrogen chloride.
Read first: Making and extending amines
Why acyl chlorides 19.7
The carbon of an acyl chloride is bonded to two very electronegative atoms, O and Cl, so it is strongly δ+. A nucleophile such as the lone pair on ammonia or an amine attacks it easily and the chloride leaves. This is why acyl chlorides react quickly and at room temperature, whereas a carboxylic acid with ammonia only forms a salt.
Ethanoyl chloride and concentrated ammonia give the amide ethanamide:
The hydrogen chloride reacts with a second molecule of ammonia, so with excess ammonia the equation is:
MethodAmide from an acyl chloride
- Reagents: the acyl chloride and concentrated ammonia (excess).
- Conditions: room temperature. The reaction is vigorous.
- Observation: white smoke of ammonium chloride, and a white solid amide.
Amides from amines 19.2(iv)
A primary amine has the same lone pair as ammonia, so it reacts with ethanoyl chloride in the same way. The product is an N-substituted amide, because the amine's carbon group ends up on the nitrogen.
The product is N-butylethanamide. Phenylamine reacts in the same way, giving N-phenylethanamide:
The reaction is vigorous at room temperature and gives misty fumes of hydrogen chloride. With excess amine, the HCl is taken up by a second amine molecule:
The general pattern is the same for any acyl chloride and any primary amine:
The amine loses an H atom and the acyl chloride loses the Cl atom. The new bond is C–N. The group –CO–NH– in the product is the amide link, the same link that joins nylon (page 7) and proteins (page 12).
Worked exampleName and write the product
Write the equation for propanoyl chloride, CH₃CH₂COCl, with methylamine, CH₃NH₂, and name the product.
- Join the acyl group CH₃CH₂CO– to the nitrogen, replacing one H: CH₃CH₂CONHCH₃.
- The other product is HCl.
- Name: the acid part has three carbons, so propanamide, and the methyl group is on N: N-methylpropanamide.
Worked examplePercentage yield of N-phenylethanamide
4.65 g of phenylamine reacts with excess ethanoyl chloride and gives 5.40 g of N-phenylethanamide (illustrative data). Calculate the percentage yield.
- M(C₆H₅NH₂) = 6 × 12.0 + 7 × 1.0 + 14.0 = 93.0 g mol⁻¹, so n = 4.65 ÷ 93.0 = 0.0500 mol
- Ratio 1 : 1, so theoretical n(product) = 0.0500 mol
- M(CH₃CONHC₆H₅) = 8 × 12.0 + 9 × 1.0 + 14.0 + 16.0 = 135.0 g mol⁻¹, so theoretical mass = 0.0500 × 135.0 = 6.75 g
- Percentage yield = 5.40 ÷ 6.75 × 100 = 80.0%
Check yourself
- Write the equation for ethanoyl chloride with concentrated ammonia. What would you see?
- Why is ethanoyl chloride used and not ethanoic acid?
- Write the equation for ethanoyl chloride with butylamine and name the organic product.
- Write the equation for propanoyl chloride with excess ammonia and name the amide.
- Give the structural formula of the product of ethanoyl chloride with phenylamine, and the name of the compound.
- 2.19 g of butylamine reacts with excess ethanoyl chloride. Calculate the mass of N-butylethanamide formed at 100% yield (M = 115.0 g mol⁻¹).
Answers cover these until you have tried the questions
- (or with 1 NH₃ and HCl as the second product). A vigorous reaction at room temperature with white smoke of ammonium chloride and a white solid of ethanamide.
- The acyl chloride has a strongly δ+ carbon and a good leaving group, so it reacts quickly at room temperature. Ethanoic acid only forms a salt with ammonia.
- . The product is N-butylethanamide.
- . The amide is propanamide.
- CH₃CONHC₆H₅, N-phenylethanamide, with HCl as the other product.
- n(butylamine) = 2.19 ÷ 73.0 = 0.0300 mol. Ratio 1 : 1, so n(amide) = 0.0300 mol. Mass = 0.0300 × 115.0 = 3.45 g.