Question
Which of the following resonating structures of 1- methoxy - 1, 3 - butadiene is least stable?
A.
$$\mathop {\text{C}}\limits^\Theta {H_2} - CH = CH - CH = \mathop O\limits^ \oplus - C{H_3}$$
B.
$$C{H_2} = CH - \mathop {\text{C}}\limits^\Theta H - CH = \mathop O\limits^ \oplus - C{H_3}$$
C.
$$\mathop {\text{C}}\limits^\Theta {H_2} - C\mathop H\limits^ \oplus - CH = CH - O - C{H_3}$$
D.
$$C{H_2} = CH - \mathop {\text{C}}\limits^\Theta H - \mathop C\limits^ \oplus H - O - C{H_3}$$
Answer :
$$\mathop {\text{C}}\limits^\Theta {H_2} - C\mathop H\limits^ \oplus - CH = CH - O - C{H_3}$$
Solution :
Structures (A) and (B) are quite stable because here every atom has complete octet; in structures (C) and (D), every atom does not have complete octet; hence these are less stable than (A) and (B). However, structure (D) is stabilised by resonance, which is not possible in (C). Hence (C) is least stable.
Structures (A) and (B) are quite stable because here every atom has complete octet; in structures (C) and (D), every atom does not have complete octet; hence these are less stable than (A) and (B). However, structure (D) is stabilised by resonance, which is not possible in (C). Hence (C) is least stable.