Question

When 2-methyl-2-butanol in HCl is converted to 2-chloro-2-methylbutanol via an SN1 mechanism, the product is not washed with water when excess acid is removed from the organic layer. Why is this?

Answer 1

Here is reaction of alcohol takes place with hydrogen halide (HCl) via SN1 mechanism, Let discuss it-

Alcohol reacts readily with hydrogen halides to produce alkyl halide and water, This reaction can be carried out by either passing hydrogen halides gas into alcohol or by heating alcohol with a concentrated aqueous acid.

Characteristic of reaction as follow:-

1) The reaction is catalysed by acids.

2) Rearrangement of alkyl group occurs, except with most primary alcohols.

3) The reaction is carried out through nucleophilic substitution, with protonated alcohol as substrate and halide ions as the nucleophile.

4)Primary alcohols do not undergo rearrangement simply because they do not react by SN1 mechanism instead they react by an alternative SN2 mechanism.while secondary and tertiary alcohol reacts through SN1 mechanism.

the reaction for above reactant as following

+ H₂O

So, you can see, there is a formation of product and water.

the mechanism for reaction-

The washing with water is to remove any excess HCl.

The washing with NaHCO₃  is to neutralize the acid that is still present, ( i.e. HCl + NaHCO₃ 6 NaCl + H₂CO₃, which immediately disintegrates to CO₂ and water.  Because CO₂8 gas is produced, venting is necessary.

Water simply washes away the HCl and NaHCO₃ that is still present.

As you told, when the excess acid (HCl) is removed from the organic layer, then there is no need to wash product with water.

Hope, you get it, what you want to know.Happy Learning.