Organic Chemistry Terminology (CIE A Level Chemistry)

• Organic reactions are often associated with terminology students should be familiar with
• Two of these important terms include:
  • Electrophilic substitution
  • Addition-elimination

Electrophilic substitution

• Electrophiles are species that are electron deficient and can act as an electron pair acceptor
  • Electrophiles are ‘electron loving’ species
• Substitution reactions are reactions that involve the replacement of one atom or group of atoms by another
• Electrophilic substitution reactions are therefore reactions in which an atom or group of atoms are replaced by an electrophile after an initial attack by the electron-deficient species
• An example of an electrophilic substitution reaction is the reaction of benzene with bromine in the presence of anhydrous aluminium bromide catalysts
  • The bromine acts as an electrophile and attacks the electron-rich benzene ring
  • A hydrogen atom is substituted by a bromine atom to form bromobenzene and hydrogen bromide
• Benzene undergoes substitution reactions rather than addition reactions because of the stability of the benzene ring

Electrophilic substitution of benzene by bromine

A hydrogen atom in benzene is substituted by a bromine atom, which acts as an electrophile

Addition-elimination

• Acyl chlorides are reactive organic compounds that undergo many reactions such as nucleophilic addition-elimination reactions
• In nucleophilic addition-elimination reactions, the nucleophilic addition of a small molecule across the C=O bond takes place followed by elimination of a small molecule
• Examples of these nucleophilic addition-elimination reactions include:
  • Hydrolysis
  • Reaction with alcohols to form esters
  • Reaction with ammonia and primary amines to form amides

Hydrolysis

• The hydrolysis of acyl chlorides results in the formation of a carboxylic acid and HCl molecule
• This is a nucleophilic addition-elimination reaction
  • A water molecule adds across the C=O bond
  • A hydrochloric acid (HCl) molecule is eliminated

Hydrolysis of propanoyl chloride to form propanoic acid and HCl

Acyl chlorides are hydrolysed to carboxylic acids via nucleophilic addition-elimination

Formation of esters

• Acyl chlorides can react with alcohols to form esters
• The esterification of acyl chlorides is also a nucleophilic addition-elimination reaction
  • The alcohol adds across the C=O bond
  • A HCl molecule is eliminated

Esterification of propanoyl chloride to form ethyl propanoate and HCl

Acyl chlorides are esterified with alcohols to form esters via nucleophilic addition-elimination

Formation of amides

• Acyl chlorides can form amides with primary amines and concentrated ammonia
• The nitrogen atom in ammonia and primary amine has a lone pair of electrons which can be used to attack the carbonyl carbon atom in the acyl chlorides
• The product is an amide (when reacted with ammonia) or N-substituted amide (when reacted with primary amines)
• This is also an example of a nucleophilic addition-elimination reaction as
  • The amine or ammonia molecule adds across the C=O bond
  • A HCl molecule is eliminated

Forming amides from propanoyl chloride

Acyl chlorides undergo reactions with ammonia and primary amines to form amides via nucleophilic addition-elimination