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JEE Main levelSingle correct optionDifficulty 2 of 3g1085

The C-Cl bond length in chloroethene ( $CH X_{2} = CH - Cl$ ), compared with that in chloroethane, is

Alonger, because of the +M effect of chlorine
Blonger, because of the -I effect of the vinyl group
Cthe same, because the same two atoms are bonded
Dshorter, because the chlorine lone pair conjugates with the C=C bond (partial double-bond character)

Answer: D

In chloroethene a lone pair of chlorine delocalises into the pi bond, giving

X_{+} X_{2}^{2 +} Cl = CH - CH X_{2} X^{-}

character. This adds partial double-bond character to C-Cl, and the carbon is sp2, making the bond shorter than in chloroethane.

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