Provide an example of a substitution and an electrophilic substitution reaction.

A substitution reaction involves the replacement of an atom or group of atoms in a molecule.

In a substitution reaction, an atom or group of atoms in a molecule is replaced by another atom or group of atoms. This can occur through two different mechanisms: nucleophilic substitution and electrophilic substitution.

Nucleophilic substitution involves the attack of a nucleophile (an electron-rich species) on an electrophilic carbon atom. For example, in the reaction between ethyl bromide and sodium hydroxide, the hydroxide ion acts as a nucleophile and replaces the bromine atom in ethyl bromide to form ethanol and sodium bromide.

Electrophilic substitution, on the other hand, involves the attack of an electrophile (an electron-deficient species) on an aromatic ring. For example, in the reaction between benzene and nitric acid, the nitronium ion (NO2+) acts as an electrophile and replaces a hydrogen atom on the benzene ring to form nitrobenzene.

Both nucleophilic and electrophilic substitution reactions are important in organic chemistry and are used in the synthesis of a wide range of organic compounds. Understanding these reactions is essential for A-Level Biology students studying organic chemistry.