Question

which of the following alkyl halide gives a mixture of alkene on the hydrohalogenation
​

Answer 1

Answer:

Ease of dehydrohalogenation of alkyl halides is: tert > sec > pr

The alkyl halides, in which halogen is attached to a terminal carbon, yield a single alkene but alkyl halides in which the halogen atom is attached to a non-terminal carbon atom and both adjacent position have hydrogen atoms yield a mixture of alkenes. For example: KOH (alc)

CH3CH2CH2Cl CH3CH=CH2chloropropane propene

Cl | KOH (alc) CH3CH2CHCH3CH3CH=

CHCH3+ CH3CH2CH=CH22-chlorobutane 2-butene 1-butene (major) (minor) In the first reaction, chloropropane can lose hydrogen only from C2; therefore, it gives only one product, i.e., propene. However, in the second reaction, 2-chlorobutane can lose hydrogen from any of the two-β-carbon atoms and, hence, it gives a mixture of 2-butene (80%) and 1-butene (20%). Now you may ask why 2-butene is the major product? Dehydrohalogenation follows Saytzeff rule which says that the more highly substituted alkene is the dominant product.