Question

Which halide on dehydrohalogination gives an alkene which will show cis-trans isomerization? 10 a) butylbromide c) 2-bromo-4,4-dimethylpentane b) cyclohexylbromide d) 2-bromo-3,3-dimethylbutane

Answer 1

Dehydrohalogenation leads to alkene formation by removal of HX, product depends on reactant :

(a) Butyl bromide will give Butene (or 1-Butene ) by HBr elimination from 1,2 positions, this product cannot show cis -trans isomerization, as it has only one substituent.

(b) Cyclohexyl bromide will give Cyclohexene by HBr elimination , this product cannot show cis -trans isomerization, as double bond is in 6-membered ring which is small ring to exhibit cis-trans isomerzation, product have double bond locked in cis fashion.

(c) 2-Bromo-4,4-dimethyl pentane can give 2 product by HBr elimination from 1,2 positions

4,4-dimethyl pent-1-ene: , and other from 2,3 positions :

4,4-dimethyl pent-2-ene this product can show cis -trans isomerization, as it has one substituent on each carbon of double bond. : Correct answer

(d) 2-Bromo-3,3-dimethyl butane will give one  product by HBr elimination from 1,2 positions

3,3-dimethyl but-1-ene: which cannot show cis -trans isomerization, as it has only one substituent.