Nucleophilic substitution reactions in halogenoalkanes involve the replacement of a halogen atom leaving group by a nucleophile. The mechanism of nucleophilic substitution can occur through two different pathways: SN1 Substitution Nucleophilic Unimolecular and SN2 Substitution Nucleophilic Bimolecular . The choice of the pathway depends on the structure of the halogenoalkane and the reaction conditions.In both mechanisms, the nucleophile plays a crucial role in the reaction, as it donates a pair of electrons to form a new bond with the electrophilic carbon atom, while the leaving group departs with the electron pair from the carbon-leaving group bond.SN1 Mechanism:1. Formation of a carbocation intermediate: The leaving group halogen atom departs from the carbon atom, resulting in the formation of a carbocation intermediate. This step is slow and rate-determining.2. Nucleophilic attack: The nucleophile attacks the carbocation, donating a pair of electrons and forming a new bond with the electrophilic carbon atom. This step is fast.SN2 Mechanism:1. Concerted nucleophilic attack and leaving group departure: The nucleophile attacks the electrophilic carbon atom from the opposite side of the leaving group, simultaneously donating a pair of electrons to form a new bond while the leaving group departs with the electron pair from the carbon-leaving group bond. This step is a single concerted process and is the rate-determining step.Example of a nucleophilic substitution reaction:Consider the reaction of bromoethane CH3CH2Br with hydroxide ions OH- .1. In the SN2 mechanism, the hydroxide ion nucleophile attacks the electrophilic carbon atom bearing the bromine atom from the opposite side of the leaving group Br- .2. The bromine atom departs with the electron pair from the carbon-bromine bond, while the hydroxide ion forms a new bond with the carbon atom.3. The product formed is ethanol CH3CH2OH .In summary, nucleophilic substitution reactions in halogenoalkanes involve the replacement of a halogen atom leaving group by a nucleophile through either an SN1 or SN2 mechanism. The nucleophile donates a pair of electrons to form a new bond with the electrophilic carbon atom, while the leaving group departs with the electron pair from the carbon-leaving group bond.