Amino acids (OCR A Level Chemistry)

• Amino acids are organic compounds that contain two functional groups:
  • A basic amino (-NH₂) group
  • An acidic carboxylic acid (-COOH) group
• Due to the presence of both a basic and acidic group in amino acids, they are said to be amphoteric
  • They can act as both acids and bases

Naturally occurring amino acids

• 2-aminocarboxylic acids are a type of amino acids in which the amine (-NH₂) group is bonded to the carbon atom next to the -COOH group
• These type of amino acids form the ‘building blocks’ that make up proteins
• There are 20 naturally occurring amino acids with the general structural formula of RCH(NH₂)COOH

General structural formula of amino acids

General structural formula of amino acids

• The R group varies in different amino acids and can be:
  • Acidic
  • Basic
  • Neutral

The R group varies in different amino acids

Acid / base properties of amino acids

• Amino acids will undergo most reactions of amines and carboxylic acids including acid-base reactions of:
  • Amines with acids
  • Carboxylic acids with bases

Reactions of the amine group

• The amine group is basic and reacts with acids to make salts
• For example, a general amino acid reacts with hydrochloric acid to form the ammonium salt:

Reactions of the carboxylic acid group

Reaction with aqueous alkalis

• An amino acid reacts with aqueous alkali such as sodium or potassium hydroxide to form a salt and water
• For example, a general amino acid reacts with sodium hydroxide to form a sodium salt:

Esterification with alcohols

• Amino acids, like carboxylic acids, can be esterified by heating with alcohol in the presence of concentrated sulfuric acid 
• The carboxylic acid group is esterified whilst the basic amine group is protonated due to the acidic conditions: