D. Substitution Reactions Involving Water

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This page gives you the facts and simple, uncluttered mechanisms for the nucleophilic substitution reactions between halogenoalkanes and water.

The reaction of primary halogenoalkanes with water

The facts

There is only a slow reaction between a primary halogenoalkane and water even if they are heated. The halogen atom is replaced by -OH.

For example, using 1-bromoethane as a typical primary halogenoalkane:

An alcohol is produced together with hydrobromic acid. Be careful not to call this hydrogen bromide. Hydrogen bromide is a gas. When it is dissolved it in water (as it will be here), it's called hydrobromic acid.

The mechanism

The mechanism involves two steps. The first is a simple nucleophilic substitution reaction:

Because the mechanism involves collision between two species in this slow step of the reaction, it is known as an S_N2 reaction.

The nucleophilic substitution is very slow because water isn't a very good nucleophile. It lacks the full negative charge of, say, a hydroxide ion.

The second step of the reaction simply tidies up the product. A water molecule removes one of the hydrogens attached to the oxygen to give an alcohol and a hydroxonium ion (also known as a hydronium ion or an oxonium ion).

The hydroxonium ion and the bromide ion (from the nucleophilic substitution stage of the reaction) make up the hydrobromic acid which is formed as well as the alcohol.

The reaction of tertiary halogenoalkanes with water

The facts

If the halogenoalkane is heated under reflux with water, the halogen is replaced by -OH to give an alcohol. Heating under reflux means heating with a condenser placed vertically in the flask to prevent loss of volatile substances from the mixture. The reaction happens much faster than the corresponding one involving a primary halogenoalkane.

For example:

The mechanism

This mechanism involves an initial ionisation of the halogenoalkane:

followed by a very rapid attack by the water on the carbocation (carbonium ion) formed:

This is again an example of nucleophilic substitution.

This time the slow step of the reaction only involves one species - the halogenoalkane. It is known as an S_N1 reaction.

Now there is a final stage in which the product is tidied up. A water molecule removes one of the hydrogens attached to the oxygen to give an alcohol and a hydroxonium ion - exactly as happens with primary halogenoalkanes.

The rate of the overall reaction is governed entirely by how fast the halogenoalkane ionises. The fact that water isn't as good a nucleophile as, say, OH^- doesn't make any difference. The water isn't involved in the slow step of the reaction.

The reaction of secondary halogenoalkanes with water

Secondary halogenoalkanes use both an S_N2 mechanism and an S_N1.

Make sure you understand what happens with primary and tertiary halogenoalkanes, and then adapt it for secondary ones should ever need to.