Question

Give the major product for the following reaction... CH3CH(OH)CH2CH3 reacts with H+/heat

Answer 1

This is dehydration in alcohol in presence of acidic medium. Here the H+ protonates the OH group and converts the bad leaving group to a good leaving group . The water molecule get eliminate from the molecule generating a carbocation.

The carbocation will not undergo any rearragement here as it is the most stable possible carbocation.

Later the carbon next to carbocation will lose a proton and a double bond will form.

Points to remember

a) This is alpha-beta or 1,2 elimination in alcohol

b) It is neither Unimolecular nor bimolecular

c) the steps are reversible

d) the hydrogen is eliminate from beta carbon, if there are more than one kind of beta hydrogen then there will be formation of more than one product

e) According to Saytzeff's rule, more substituted alkene is more stable so the product with more alkylated alkene will be major product [More number of hyperconjugative structures] .

f) in case of geometrical isomers, trans alkene is more stable and hence will be major product if we are considering stereochemistry also

The reactiom mechanism will be

Нас CH CH3 -H20 он он CH CH3 H Base H C CH CH CH CH Base Нас CH CH3 CH3 (2Z)-but-2-ene Minor product (2E)-but-2-ene (less substituted alkene) Major product