Nucleophilic Addition–Elimination (AQA A Level Chemistry): Revision Note

Nucleophilic Addition–Elimination

• An important class of reactions that ammonia and amines undergo is nucleophilic addition-elimination with acyl chlorides and acid anhydrides

• Here is a reminder of the functional groups acyl chloride and acid anhydride:

• 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 it creates a stronger dipole along the C-Cl bond, making the carbon more susceptible to attack from nucleophiles, which 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

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

• This is followed by the elimination of an HCl molecule

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

  • With ammonia, the product is an amide, and white fumes of NH₄Cl are formed

• In excess NH₃, the HCl produced reacts with a second molecule of NH₃ to form ammonium chloride, NH₄Cl, which appears as white fumes:

HCl + NH₃ $\to$ NH₄Cl

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

Related topics

• Acylation

• Basic Properties