0

There are two possible case π electrons shifting to C2 In this case, nucleophile attacks C3 It is stabilized by +I effect of two ethyl groups π electrons shifting to C3 In this case, nucleophile attacks C2 It is stabilized by +I effect of a methyl group and a propyl group.

How do I decide which one is more stable, and ultimately, which one is favoured?

Mathew Mahindaratne
  • 39,943
  • 27
  • 55
  • 107
DatBoi
  • 264
  • 3
  • 12

1 Answers1

1

Here what you should consider first is hyperconjugation, rather than inductive effect (a brief explanation by me).

You're right that $\ce{Nu-}$ will either attack on $\ce{C-3}$ or $\ce{C-2}$, and hence two carbocations would be formed. Now, we'll analyse the stability of these carbocations on the basis of hyperconjugation which is as follows,

  • carbocation at $\ce{C-2}$ will have a total of 5 HC structures

  • carbocation at $\ce{C-3}$ will have a total of 4 HC structures

Based on the above difference, it is easy to conclude that $\ce{Nu-}$ will attack on $\ce{C-2}$ carbon. Whereas based on inductive effect, those carbocations are nearly equally stable.

Note: HC is short for "hyperconjugative structures".

Rahul Verma
  • 3,071
  • 2
  • 21
  • 39
  • Oh thanks, but in my book it says that +I effect of c2h5>ch3 ,therefore electrons shifting to C2 is favoured more.any idea of what it's trying to say? – DatBoi Apr 15 '20 at 02:40
  • @DatBoi: your book had stated right order. It is bcoz of the fact that, ethyl group has two carbons, whereas methyl has only one. However, as you know that inductive effect isn't effective till next to neighbour C, hence the above diff. is near negligible. Also, in my this answer, I gave a brief explanation about the extent of stabilizing effect. So, any considerable diff. in hyperconjugation will overrule small diff. in inductive effect. – Rahul Verma Apr 15 '20 at 02:49
  • @YusufHasan: I think you've already noticed that I'd gave an explanation (short) on this in a previous answer. So, should I link that one in my post? it would be very effective, IMO. – Rahul Verma Apr 15 '20 at 03:00
  • Yusuf?! Who's that? – DatBoi Apr 15 '20 at 03:02
  • @DatBoi: he is Yusuf Hasan. Actually, he added some suggestion just after your comment, and then deleted it ahead of my comment. – Rahul Verma Apr 15 '20 at 03:06