The nucleophilic substitution reaction between bromomethane CH3Br and ammonia NH3 follows an SN2 mechanism. Here is a step-by-step description of the reaction mechanism:1. Nucleophile approach: Ammonia, which acts as a nucleophile due to its lone pair of electrons on the nitrogen atom, approaches the electrophilic carbon atom in bromomethane. The bromine atom is the leaving group, as it is more electronegative and can stabilize the negative charge when it leaves.2. Transition state formation: As ammonia approaches the carbon atom, a transition state is formed where the nitrogen atom in ammonia is partially bonded to the carbon atom in bromomethane. At the same time, the carbon-bromine bond is partially broken. This transition state is a high-energy state, and the reaction proceeds through a single concerted step.3. Bond formation and leaving group departure: The nitrogen atom in ammonia forms a new bond with the carbon atom in bromomethane, while the carbon-bromine bond is completely broken. The bromide ion Br- is released as the leaving group.4. Product formation: The final product of this reaction is methylamine CH3NH2 and a bromide ion Br- . The nitrogen atom in ammonia has effectively replaced the bromine atom in bromomethane through a nucleophilic substitution reaction.Overall, the reaction can be represented as:CH3Br + NH3 CH3NH2 + Br-