Question

A student decides to synthesize 3-methylbutene by an acid catalyzed dehydration of isoamyl alcohol. On heating, instead of collecting a liquid with a boiling point of 20 degrees celcius as expected, the student instead collects a liquid which boils at 100 degrees celsius, followed by another which boils at 173 degrees celsius. When added to an aqueous solution of Br₂, neither liquid turns the solution clear. Explain in detail what went wrong, and identify the two products of the student's reaction.

Answer 1

isoamyl alcohol on acid catalysed dehydration is expected to give the corresponding alkene (2-methylbutene), but the temperature is the main issue.

There is an approximate border at around 413 K (very crudely speaking), below which the acid catalysed dehydration will result in the corresponding ether, or isoamyl ether(b.p. = 173 degree C) in this case.

Since elimination occurs at higher temperatures and substitution at lower temperatures, this reaction wasn't performed in sufficiently high temperatures and hence substitution took place.

1st step: Protonation of the alcohol oxygen took place.

2nd step: Since it's a primary alcohol, it is a S_N2 vs E2 scenario.

In this case S_N2 took place instead of E2 and the carbon adjacent to the protonated oxygen was attacked by and another isoamyl alcohol, resulting in water(b.p. = 100 degree C) and isoamyl ether(b.p. = 173 degree C).

Since none of the products contain unsaturation, hence they do not turn Br₂ solution clear.