Synthesis of n-Butyl Bromide and t-Pentyl Chloride

The synthesis of two alkyl halides from alcohols is the basis for these experiments. In the first experiment, a primary alkyl halide n-butyl bromide is prepared as 

In the second experiment, a tertiary alkyl halide f-pentyl chloride (f-amyl chloride) 

These reactions provide an interesting contrast in mechanisms. The n-butyl bromide synthesis proceeds by an Sj 2 mechanism, whereas f-pentyl chloride is prepared by an S l reaction. 

The primary alkyl halide n-butyl bromide can be prepared easily by allowing n-butyl alcohol to react with sodium bromide and sulfuric acid by Equation 1. The sodium bromide reacts with sulfuric acid to produce hydrobromic acid. 

Excess sulfuric acid serves to shift the equilibrium and thus to speed the reaction by producing a higher concentration of hydrobromic acid. The sulfuric acid also protonates the hydroxyl group of n-butyl alcohol so that water is displaced. The acid also protonates the water as it is produced in the reaction and deactivates it as a nucleophile. Deactivation of water keeps the alkyl halide from being converted back to the alcohol by nucleophilic attack of water. The reaction of the primary substrate proceeds via an Si 2 mechanism. 

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EXPERIMENT 23 Synthesis of n-Butyl Bromide and t-Pentyl Chloride 201... 

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