Alkyl halides are mono-halogen substituted alkanes.

They have an alkyl group bonded to a halogen atom.

Examples: Bromoethane, 1-Chloropropane, 2-Chloropropane,

Synthesis: by addition of a hydrogen halide to an alkene or by substitution of the –OH group in an alcohol using PCl₅, SOCl₂ or PBr₃

Properties: Alkyl halides are saturated molecules and will therefore undergo only substitution reactions. The C-halogen bond is polarised due to the higher electronegativity of halogen, the halogen atom acquires a little excess electron density while the carbon becomes electron deficient and constitutes an electrophilic centre. Any reagent that is nucleophilic in nature will seek this electron deficient carbon and form a bond resulting in substitution of halogen. Thus many reactions of alkyl halides are nucleophilic substitution reactions.

Use of alcoholic KOH or NaOEt/EtOH, will result in elimination(dehydrohalogenation).

Elimination converts an alkyl halide into an alkene.

Elimination reaction is opposite of an addition reaction.

In this reaction a halogen atom is lost from one carbon and hydrogen at the next carbon.

Reaction mechanism is the sequence of events (bond breaking and formation) that take place at the molecular level in a reaction and the energetics involved in the process.

In the above example only two events have happened, a carbon halogen bond has broken and a carbon oxygen bond has formed.

This could have happened in two possible ways.

Bond breaking first and bond formation next.

Bond formation and breaking simultaneously

The third possibility is bond formation first which would result in carbon having five bonds and that is not possible

Bond breaking first will result in a carbocation intermediate which in the second step will react with the nucleophile(base) forming the product.

The first step is slow and reversible; the overall rate depends on this step.

Since only the alkyl halide is involved in this step it is unimolecular. Hence it is termed Substitution Nucleophilic unimolecular ( S_N1).

The rate of the reaction depends on only the alkyl halide and not on the base.