Question

On treatment of the following compound with a strong acid, the most susceptible site for bond cleavage is :

• A. C1 - O2
• B. O2 - C3
• C. C4 - O5
• D. O5 - C6

Hint:

O2 is an enol-type (vinyl) ether oxygen whose lone pair is delocalised into C1=C, so the more basic ether oxygen O5 is protonated; compare the two bonds of O5: the primary ethyl carbon C6 versus the secondary C4.

Answer 1

The Correct Option is D

The molecule has two ether oxygens: O2 (ring oxygen, attached to the vinyl carbon C1 and to the allylic carbon C3) and O5 (attached to secondary C4 and primary ethyl C6).
The C1-O2 bond has partial double-bond character (O lone pair delocalised into C1=C), so it is not easily cleaved; the lone pair of O2 is also less available, making O2 less basic.
The more basic oxygen O5 is protonated by the strong acid, forming an oxonium ion.
Cleavage then goes by nucleophilic attack (SN2) on the less hindered alkyl carbon of O5: C6 (primary) is attacked in preference to C4 (secondary).
So the most susceptible bond is O5-C6.
Answer: (4) O5 - C6