What are SN1 and SN2 mechanisms? Discuss with examples of haloalkanes reacting with aqueous KOH.

SN1 and SN2 are two different types of mechanisms that are used in organic chemistry for substitution reactions.

1. SN1 Mechanism: SN1 stands for Substitution Nucleophilic Unimolecular. In this mechanism, the rate of the reaction depends on the concentration of one reactant, hence the term 'unimolecular'. The reaction occurs in two steps:

   • Step 1: The leaving group leaves, forming a carbocation intermediate.
   • Step 2: The nucleophile attacks the carbocation and forms a new bond.

   For example, if we take a haloalkane like 2-chloro-2-methylpropane and react it with aqueous KOH, the chlorine atom leaves first to form a carbocation. Then, the hydroxide ion (OH-) from KOH attacks the carbocation, resulting in 2-methylpropan-2-ol.

2. SN2 Mechanism: SN2 stands for Substitution Nucleophilic Bimolecular. In this mechanism, the rate of the reaction depends on the concentration of two reactants, hence the term 'bimolecular'. The reaction occurs in one step, where the nucleophile attacks the substrate and the leaving group leaves simultaneously.

   For example, if we take a haloalkane like bromoethane and react it with aqueous KOH, the hydroxide ion (OH-) from KOH attacks the carbon atom bonded to the bromine atom, while the bromine atom leaves at the same time. This results in ethanol.

In both mechanisms, the nucleophile (OH- from KOH) replaces the halogen atom in the haloalkane. However, the rate and the steps of the reaction are different.