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(D) The reactivity of alkenes towards electrophilic addition of $HBr$ depends on the stability of the carbocation intermediate formed in the rate-dete

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$III > I > II > IV$

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(D) The reactivity of alkenes towards electrophilic addition of $HBr$ depends on the stability of the carbocation intermediate formed in the rate-determining step (r.d.s.).$1$. In $(III) \ MeOCH=CH_2$,the methoxy group $(-OCH_3)$ shows a strong $+M$ effect,which highly stabilizes the carbocation.$2$. In $(I) \ CH_3-CH=CH_2$,the methyl group shows $+I$ and hyperconjugation,providing moderate stability.$3$. In $(IV) \ CH_3-CO-CH=CH_2$,the carbonyl group $(-CO-)$ shows a $-M$ effect,which destabilizes the carbocation.$4$. In $(II) \ CF_3-CH=CH_2$,the trifluoromethyl group $(-CF_3)$ shows a very strong $-I$ effect,which strongly destabilizes the carbocation.Thus,the stability order of the carbocation (and hence reactivity) is: $III > I > IV > II$.

The decreasing order of reactivity of the following alkenes towards $HBr$ addition is:
$(I) \ CH_3-CH=CH_2$
$(II) \ CF_3-CH=CH_2$
$(III) \ MeOCH=CH_2$
$(IV) \ CH_3-CO-CH=CH_2$

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The decreasing order of reactivity of the following alkenes towards $HBr$ addition is:$(I) \ CH_3-CH=CH_2$$(II) \ CF_3-CH=CH_2$$(III) \ MeOCH=CH_2$$(IV) \ CH_3-CO-CH=CH_2$