how could be known that whether the sn or elimination rxn is being to be caried out in a rxn?

Dear Gopal,

The role of the hydroxide ion in a substitution reaction

In the substitution reaction between a halogenoalkane and OH^- ions, the hydroxide ions are acting as nucleophiles. For example, one of the lone pairs on the oxygen can attack the slightly positive carbon. This leads on to the loss of the bromine as a bromide ion, and the -OH group becoming attached in its place.

The role of the hydroxide ion in an elimination reaction

Hydroxide ions have a very strong tendency to combine with hydrogen ions to make water - in other words, the OH^- ion is a very strong base. In an elimination reaction, the hydroxide ion hits one of the hydrogen atoms in the CH₃group and pulls it off. This leads to a cascade of electron pair movements resulting in the formation of a carbon-carbon double bond, and the loss of the bromine as Br^-.

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