JEE Main · 2020 · 04 Sep 2020 · Shift 2 · General Organic Chemistry

The $\mathrm{C}-\mathrm{Cl}$ bond is shortest where the carbon holding chlorine is more $s$-rich and where the chlorine lone pair can delocalise into an adjacent $\pi$ system. In vinyl chloride, $\mathrm{C}{\mathrm{H}}_2=\mathrm{CH}-\mathrm{Cl}$, the chlorine lone pair conjugates with the double bond, giving the bond partial double-bond character, while the carbon is $s{p}^2$ hybridised; both effects draw the atoms closer. The other compounds, with $s{p}^3$ carbon and no such resonance, have longer $\mathrm{C}-\mathrm{Cl}$ bonds, so vinyl chloride has the shortest.