JEE Main · 2021 · 26 Feb 2021 · Shift 2 · Chemical Bonding and Molecular Structure

The molecule is buta-1,2-diene, an allene. The terminal $\mathrm{C}{\mathrm{H}}_2$ carbon has two hydrogens and one double bond, that is three sigma bonds and one pi bond, so it is $s{p}^2$. The second carbon lies between two double bonds; it forms only two sigma bonds and two pi bonds in mutually perpendicular planes, so it is $sp$. The third carbon carries one hydrogen, the methyl group and the double bond, three sigma bonds in all, so it is $s{p}^2$. The methyl carbon forms four sigma bonds and is $s{p}^3$. The hybridisation of carbons $1$, $2$, $3$ and $4$ is $s{p}^2$, $sp$, $s{p}^2$, $s{p}^3$.