How do organohalides participate in substitution reactions?

Organohalides participate in substitution reactions by acting as the leaving group, being replaced by a nucleophile.

In more detail, organohalides, also known as alkyl halides, are organic compounds containing a halogen atom (such as fluorine, chlorine, bromine, or iodine) bonded to a carbon atom. They are known for their reactivity and are commonly used in organic synthesis. One of the most common reactions involving organohalides is the nucleophilic substitution reaction.

In a nucleophilic substitution reaction, a nucleophile, which is a species with a lone pair of electrons, attacks a carbon atom bonded to a halogen in the organohalide. The halogen then leaves, taking with it the bonding pair of electrons, hence the term 'leaving group'. This results in the substitution of the halogen with the nucleophile.

There are two main types of nucleophilic substitution reactions that organohalides can undergo: SN1 and SN2 reactions. In an SN1 reaction, the leaving group departs before the nucleophile arrives. This results in a carbocation intermediate, which is then attacked by the nucleophile. SN1 reactions are common with tertiary organohalides, where the carbocation is stabilised by neighbouring carbon atoms.

On the other hand, in an SN2 reaction, the nucleophile attacks the carbon atom at the same time as the leaving group departs. This results in a transition state where the carbon atom is partially bonded to both the leaving group and the nucleophile. SN2 reactions are common with primary organohalides, where steric hindrance is minimal.

The ability of organohalides to participate in these substitution reactions makes them extremely useful in the synthesis of a wide range of organic compounds. The type of nucleophile, the structure of the organohalide, and the reaction conditions can all be varied to achieve different outcomes, providing a high degree of control over the reaction.