Hydrolysis of Haloalkanes

Haloalkanes react with aqueous NaOH to form alcohols. the mechanism for which, is nucleophilic substitution:

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In this mechanism, the hydroxide ion acts as a nucleophile because it is donating an electron pair the the partially positive carbon atom. The hydroxide ion attacks the carbon from the opposite side to which the halogen is bonded because the bromine is very large in comparison, making it nigh on impossible for the nucleophile to attack from the front. 

The carbon atom is then making 5 bonds (10 bonding electrons in total), but it can only accommodate 8 electrons in its valence shell, therefore the bromine leaves as the C - Br bond breaks by heterolytic fission as the bromine comes away with both of the electrons from the bonding pair. 

The resulting solution contains alcohol, excess hydroxide and halide (in this case bromide) ions. 

From the perspective of organic synthesis, this reaction is a useful transition from haloalkane to alcohol, but from an organic analysis perspective, this reaction can also be used to identify the type of haloalkane when combined with another subsequent step. However, this is a destructive analytical technique and you will not get your sample back!

To test for the type of halogen in your haloalkane, you must firstly add nitric acid (HNO₃) to your reaction mixture. This will remove any other species that could give a false positive result in the next step. You then ad silver nitrate (AgNO₃). This will produce precipitates of various different colours:

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From left to right:

• Iodide produces a yellow precipitate ( I^-_aq) + Ag⁺_(aq) --> AgI_(s) )
• Iodide produces a yellow precipitate ( Br^-_aq) + Ag⁺_(aq) --> AgBr_(s) )
• Iodide produces a yellow precipitate ( Cl^-_aq) + Ag⁺_(aq) --> AgCl_(s) )

So, finally, an example to summarise the above: a sample of a haloalkane can be hydrolysed to substitute the halogen for an OH group. The resulting solution contains halides which can be tested from with nitric acid and silver nitrate. The colour of the precipitate formed can be used to identify the halogen present in the haloalkane. 

In a subsequent post, I will cover explaining the rate of hydrolysis of haloalkanes.