Question

Correct statements from the following is:

• A. Cyclobutenone is more easily enolised than triketocyclobutane
• B. Enol content of butane-2,3-dione is slightly lower than that of butane-2-one and much higher than that of cyclohexane-1,2-dione
• C. Keto content prediminates over enolic content in acetyl acetone
• D. $HCN$ and $HNC$ are tautomers of each other

Answer 1

The correct option is D $HCN$ and $HNC$ are tautomers of each other

(a) Cyclobutenone does not enolise because its enol is antiaromatic while triketocyclobutane enolises because it stabilises due to an increase in aromaticity.


(b) In biacetyl (A), the adjacent $C=O$ group destabilises the molecule due to electrostatic repulsion so it stabilises in an anti-conformation. Some repulsion is further relieved in the enol form but the enol content is still quite low because this happens at the expense of a loss of the $C=O$ bond energy.
In butane-2-one (B), no other factor except the loss of $C=O$ energy occurs during enolisation.
In cyclohexane-1,2-dione (C), the ring is rigid so the electrostatic repulsion can only be minimised by the enolisation of one of the $C=O$ groups.
Thus, enol content of (A) is slightly higher than that of (B) but much lower than (C).

(c) Enolic content predominates the keto product because it has a conjugated $\pi$ system and H-bonding.


(d) Hydrocyanic acid is an example of diad system in which the H atom oscillates between $C$ and $N$ atoms.