JEE Main · 2026 · 04 Apr 2026 · Shift 2 · Chemical Bonding and Molecular Structure

A longer carbon--oxygen bond reflects greater single-bond (lower bond-order) character. In acetaldehyde $(y)$ the $\mathrm{C}=\mathrm{O}$ is essentially a pure double bond with little delocalisation, so it is the shortest. In the acetate ion $(z)$ the negative charge is shared equally over two oxygens, fixing each carbon--oxygen bond at an order of about $1.5$. In tropone $(x)$ donation of the carbonyl $\pi$ electrons generates an aromatic six-$\pi$-electron tropylium ring carrying $\mathrm{C}-{\mathrm{O}}^{-}$; this aromatic stabilisation makes the charge-separated form dominate, giving that bond the greatest single-bond character and hence the greatest length. The order of carbon--oxygen bond length is $x>z>y$.