Question

Which of the following resonating structures of 1-methoxy- 1,3-butadiene is least stable?
(a) $\overset{\ominus}{C}H_2 - CH = CH - CH = \overset{\oplus}{O} - CH_3$
(b) $CH_2= CH - \overset{\ominus}{C}H - CH = \overset{\oplus}{O} - CH_3$
(c) $\overset{\ominus}{C}H_2 - C\overset{\oplus}{H} - CH = CH - O - CH_3$
(d) $CH_2 = CH - \overset{\ominus}{C}H - \overset{\oplus}{C}H - O - CH3$

Answer 1

OPTION c IS LEAST STABLE

option 1,2 are having 2 double bond which makes them more stable then other 2 option in c and d the octet of positive carbon is not complete but in 4th it is in resonance with oxygen which is not happening in c.

Answer 2

Answer: (c)

Explanation:

The octet of all atoms are complete in structures a and b. The molecule in which all the atoms have complete octet is more stable than atom which have incomplete octet. Larger the number of resonating structures, larger will be the stability, thus structures a and b are stable. In structure (d), the electron deficiency of positive charged carbon is duly compensated by one pair electrons of adjacent oxygen atoms while such neighbour group support is not available in structure (c). Hence, structure (c) is least stable in comparison to structure (d).