3
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Problem

How many resonance structures can be drawn for the following molecule?

4‐aminobut‐3‐en‐2‐ylium

(a) 1
(b) 4
(c) 3
(d) 2

Answer

(d) 2

Question

I think that the molecule can be drawn in three ways, which contradicts the book's answer. Can anyone explain?

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  • $\begingroup$ Asking is how many (more) structures can be drawn. First one is already here. You can draw two more, thus answer is D. $\endgroup$ Dec 4, 2019 at 6:19
  • $\begingroup$ @MathewMahindaratne But that isn't what the question asks. The one shown can be drawn, too. They should specify "additional" if that is what they want. $\endgroup$ Dec 4, 2019 at 7:33
  • $\begingroup$ @ SendersReagent: Compare Q1 & 2, to see how different they were written. Anyway, you and I can't read writer's mind. That's the bottom line. $\endgroup$ Dec 4, 2019 at 17:16

1 Answer 1

-2
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Just shift the pi-bond from the double bond to the right side. The carbocation then shifts to the left. That's the second resonance structure. The first one is already shown, making a total of two different structures (D).

You might be wondering about the allylic lone pair on the Nitrogen atom. Well, that could have resulted in another resonance pair, but if you shift the lone pair to the adjacent C-C bond on the right, there will be a total of 3 charges on the resonance structure. This is a highly unstable form, so it's not a significant resonance structure. Thus, it can be neglected.

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