Question

In which of the following alkenes the addition of $HBr$ is easiest?

• A. $C{H}_2=CH-Cl$
• B. $F-CH=CH-Cl$
• C. $C{H}_2=CH-N{O}_2$
• D. $C{H}_3-CH=C{H}_2$

Answer 1

The correct option is D $C{H}_3-CH=C{H}_2$

Order of reactivity of alkene toward electrophilic addition is directly proportional to the stability of carbocation formed.
Electron releasing group and electron donating will influence the stability of carbocation. Thus, it will decide rate of reaction in electrophilic addition reaction.
Electron donating group will stabilise the carbocation formed.
Electron withdrawing group will destabilise the carbocation formed. More the number of electron withdrawing group, less will be the stability of carbocation and it undergo addition of HBr hardly.
In compound (a), (b) and (c) we have electron withdrawing groups like $-F,-Cl,-N{O}_2$ , thus it will reduce the stability of carbocation.
Compound (d) has no electron withdrawing group, so it forms a relatively stable carbocation and undergoes addition $HBr$ easily.