Nucleophilic Addition–Elimination (Oxford AQA International A Level Chemistry)

Nucleophilic Addition–Elimination Mechanism

• Amines undergo nucleophilic addition-elimination reactions with acyl chlorides and acid anhydrides

• The delta positive carbon atom is the site of the nucleophilic attack by the lone pair on the nitrogen in ammonia or the amine

• Chlorine is more electronegative than oxygen, so creates a stronger dipole along the C-Cl bond making the carbon more susceptible to attack from nucleophiles

  • This is why acyl chlorides are more reactive than acid anhydrides

• The reaction produces HCl as an elimination product (hence the name addition-elimination), but since both ammonia and amines are basic the HCl reacts to form ammonium chloride or the amine salt respectively

• Hence, two moles of ammonia /amine are needed in the overall equation:

RCOCl    + 2NH₃   →    RCONH₂    +   NH₄Cl

and

RCOCl    + 2R’NH₂   →    RCONHR’    +   R’NH₃Cl

Formation of amides: reaction mechanism

• Step 1 - addition

  • The lone pair on the nitrogen atoms carry out an initial attack on the carbonyl carbon

  • A pair of electrons from the C=O bond moves to the O atom forming a negatively charged oxygen

• Step 2 - elimination

  • A lone pair of electrons on negatively charged O reforms the C=O

  • The Cl group leaves

  • The H atom leaves and restores the electron density on the N atom

• Both reactions of acyl chlorides with ammonia and amines are vigorous however there are also differences

  • With ammonia - The product is a non-substituted amide and white fumes of NH₄Cl are formed

  • With amines - The product is a substituted amide and the HCl formed reacts with the unreacted amine to form a white organic ammonium salt

Nucleophilic addition-elimination mechanism

Primary amines and acyl chloride reaction mechansim