Question

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

• A. $\stackrel{\ominus }{C}{H}_2-CH=CH-CH=\stackrel{\oplus }{O}-C{H}_3$
• B. $C{H}_2=CH-\stackrel{\ominus }{C}H-CH=\stackrel{\oplus }{O}-C{H}_3$
• C. $\stackrel{\ominus }{C}{H}_2-\stackrel{\oplus }{C}H-CH=CH-O-C{H}_3$
• D. $C{H}_2=CH-\stackrel{\ominus }{C}H-\stackrel{\oplus }{C}H-O-C{H}_3$

Answer 1

The correct option is C $\stackrel{\ominus }{C}{H}_2-\stackrel{\oplus }{C}H-CH=CH-O-C{H}_3$

The octet of all atoms are complete in structures (a) and (b). The resonating structure in which all the atoms have complete octet is more stable than the structure having incomplete octet of atoms. Thus, structures (a) and (b) are stable.
In structrue (d), the electron deficiency of positive charged carbon is duly compensated by lone pair electrons of adjacent oxygen atom, while such neighbour group support is not available in structure (c). Hence, structure (c) is least stable in comparison to other structures.