Which of the following resonating structures of `1-`methoxy-`1`,`3`-butadiene is least stable?

Options

A. `CH_(2)=CH-overset(-)CH-overset(+)CH-overset(..)underset(..)CH_(3)`
B. `overset(-):CH_(2)-CH=CH-CH=overset(+)underset(..)O-CH_(3)`
C. `CH_(2)=CH-overset(-)overset(..)CH-CH=overset(+)underset(..)O-CH_(3)`
D. `overset(-): CH_(2)=overset(+)CH-CH=CH-overset(..)underset(..)O-CH_(3)`

Answer

D

Explanation

In resonance structures (2) and (3), all of the atoms have completed their octets while in structure (1) and (4), all of the atoms do not have complete octets. However, in structure (1), the `+ve` charge on the `C` atom can be dispersed by the `+R` effect of `underset(..)overset(..)OCH_(3)` group. Therefore, structure (4) is the least stable.
Also note that structures (2) and (3) have more number of covalent bond than (1) and (4).