Consider the following molecules/species:
(x) tropone, (y) acetone, (z) acetate ion
The correct order of carbon - oxygen double bond length is :
← JEE Main 2026 · 4 Apr · Shift 2
ChemistrySingle correct+4 / -1Question 61 of 75
JEE Main 2026 · 4 Apr · Shift 2, Chemistry question 61
Answer and worked solution
Official answer: D
C-O bond length tracks how much single-bond character the bond has.
- Tropone (x): the dipolar form, an aromatic tropylium ring (6π electrons) carrying , is a major contributor because it is aromatic. The C-O bond therefore has the most single-bond character, so it is the longest.
- Acetate (z): the two equivalent resonance forms give each C-O a bond order of 1.5.
- Acetone (y): an essentially pure C=O, so it is the shortest.
Order x > z > y, as in NTA's key. (Measured lengths for tropone and acetate are close, about 1.26 Å; the key ranks them by the aromatic stabilisation of tropone's dipolar form.)
(D)