Which of the following orders of rotation bar | vedclass

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Question

(A) The rotation barrier about a $C=C$ bond is directly proportional to the double bond character of that bond. Resonance reduces the double bond char

Vedclass · Accepted Answer

$I > II > III$

Vedclass · Answer

(A) The rotation barrier about a $C=C$ bond is directly proportional to the double bond character of that bond. Resonance reduces the double bond character by introducing single bond character. In $(I)$,the phenyl group is conjugated with the $C=C$ bond. In $(II)$,both phenyl groups are conjugated with the $C=C$ bond,leading to more resonance and thus more single bond character than in $(I)$. In $(III)$,the presence of the $-OCH_3$ group (a strong electron-donating group) on one of the phenyl rings further increases the resonance contribution,leading to the highest single bond character and the lowest rotation barrier. Therefore,the order of double bond character (and thus rotation barrier) is $(I) > (II) > (III)$.

Which of the following orders of rotation barrier about the $C=C$ bond,as indicated in the structures $(I)$,$(II)$,and $(III)$,is correct?

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