Question

Why are petroleum ether and sodium bicarbonate added to a mixture of HBr, tetrabutyl ammonium bromide, and 1-hexene, when trying to extract 2-bromohexane from the mixture?

Answer 1

1. It is an addition reaction of HBr to 1-hexene which follows Markonikov's rule which states that the H+ ion from HBr becomes bonded to the carbon atom that had the greatest number of Hydrogen atoms initially and Br- ion adds to carbon atom with more alkyl substituents.

2. In the process it forms a secondary carbocation which is stable.

3. After addition of HBr, tetrabutyl ammonium bromide and 1-hexene there is addition of petroleum ether and sodium bicarbonate.

-- HBr and 1-hexene are immiscible liquids.

-- After adding petroleum ether and sodium bicarbonate there is formation of two distinct layers- one is organic layer and other is aqeous layer.

--tetrabutyl ammonium which is a quarternary ammonium (R4N+) ion cross the interface and carry Br- ions with them into the organic phase where they react with 1-hexene to give 2-bromohexene.

--tetrabutyl ammonium ion being a phase transfer catalyst makes HBr more reactive to 1-hexene and increases the amount of addition possible due to formation of organic layer.