Question

Which will be the least stable resonating structure?
A. $ C{H_2} = CH - \mathop C\limits^ + H - \mathop C\limits^ - H - N{H_2} $
B. $ \mathop C\limits^ - {H_2} - \mathop C\limits^ + H - CH = CH - N{H_2} $
C. $ \mathop C\limits^ - {H_2} - CH = CH - CH = \mathop N\limits^ + {H_2} $
D. $ C{H_2} = CH - \mathop C\limits^ - H - CH = \mathop N\limits^ + {H_2} $

Answer 1

Hint: There are two major factors which affect the stability of the resonating structures in the given question. First, the octet of each atom must be complete and second, stabilized negative charge i.e., the negative charge will be more stabilized near the electron withdrawing group.

Complete Step By Step Answer:
Resonating structures are the set of Lewis structures which describe the delocalization of pi electrons in a polyatomic ion or molecule. The first factor which affects the stability of resonating structure is complete octet and among given options, it is clearly observed that the octet of each atom in option (C) and (D) are completely filled and thus, these structures will be comparatively more stable than the structures in option (A) and (B).
Both the structures in option (A) and (B) consist of carbocation whose octet is incomplete. So, to compare stability we need to look at the second factor i.e., stabilized negative charge. In structure (A), the carbanion is relatively less stable than in structure (B) because in structure (A), the carbanion is directly bonded to the $ N{H_2} $ group which is an electron donating group and destabilizes the carbanion.
Thus, we can conclude that the least stable resonating structures among given options is structure (A) i.e., $ C{H_2} = CH - \mathop C\limits^ + H - \mathop C\limits^ - H - N{H_2} $ . Hence, option (A) is the correct answer.

Note:
It is important to note that the stabilization of positive charge is also a major factor which affects the stability of the resonating structures. A carbocation is more stable on the most substituted carbon atom and its stability follows the order $ {3^o} > {2^o} > {1^o} $ . For the given question, the carbocation in both cases was secondary carbocation and thus, this factor was neglected.