Explain the difference between E1 and E2 reactions using examples of haloalkanes.

E1 and E2 reactions are two types of elimination reactions that haloalkanes can undergo. They are distinguished by their mechanisms and the number of steps involved in the reaction.

1. E1 Reaction: E1 stands for unimolecular elimination. In an E1 reaction, the reaction rate is determined by the concentration of one reactant, hence the term unimolecular. The reaction occurs in two steps.

Example: Consider the reaction of 2-bromo-2-methylpropane with a weak base like water.

Step 1: The leaving group (bromine in this case) leaves, forming a carbocation intermediate. This is the slow, rate-determining step.

Step 2: The base (water) then deprotonates the carbocation, leading to the formation of an alkene. This is the fast step.

2. E2 Reaction: E2 stands for bimolecular elimination. In an E2 reaction, the reaction rate is determined by the concentration of two reactants, hence the term bimolecular. The reaction occurs in one concerted step.

Example: Consider the reaction of 2-bromo-2-methylpropane with a strong base like ethoxide (CH3CH2O-).

In this reaction, the base deprotonates a hydrogen atom on the carbon adjacent to the one attached to the bromine atom, while at the same time, the bromine atom leaves. This results in the formation of an alkene in one step.

In summary, the main differences between E1 and E2 reactions are the number of steps involved in the reaction, the type of base used, and the formation of a carbocation intermediate in E1 reactions.