1

In hyperconjugation, it is known that form an $\alpha$ hydrogen, one $\sigma$ electron goes to the corresponding carbon carbon bond, and there is apparently or practically, no bond between the hydrogen and carbon. I read in a textbook,

although one $\ce{C-H}$ bond of the alkyl is shown to be broken in each hyperconjugative structure, $\ce{H+}$ is never free from the rest of the molecule.

But then, still there is no bond. Is there any specific reason why the $\ce{H+}$ ion remains with the molecule though there is no bond attaching the ion to the rest of molecule?

bon
  • 15,369
  • 13
  • 62
  • 91
Aneek
  • 830
  • 4
  • 11
  • 24
  • Maybe too much chat recently ;) I don't like viewing of hyperconjugation with mesomeric structures - it confused you - and hyperconjugation as a concept altogether - it's only tries to describe things as they are, not causes anything. – Mithoron Aug 29 '15 at 18:30
  • @Aneek Have you studied molecular orbital theory. As Mithoron is suggesting this would give you a better understanding of how hyperconjugation occurs. – bon Aug 30 '15 at 17:27
  • More to the point - if you have resonant structures where in one of them bond is broken it simply means it's weaker than it would be if this structure wasn't present. – Mithoron Aug 31 '15 at 00:23
  • I believe what you saw in your textbook was a not-so-good but clear way of explaining the idea of hyperconjugation. By simplifying the idea of hyperconjugation to resonance is not chemically correct but it effectively certainly conveys the idea of the side-to-side overlap of orbitals. – Tan Yong Boon Aug 12 '17 at 14:30
  • Refer to this post for a clearer understanding of hyperconjugation. Interestingly, the most upvoted answer also used your textbook explanation. May I suggest that you look at the second most upvoted answer as well. https://chemistry.stackexchange.com/questions/8412/what-is-hyperconjugation – Tan Yong Boon Aug 12 '17 at 14:33

2 Answers2

2

Resonance structures do not exist on their own! Resonance is not a rapid equilibrium between different structures. It is a description of a single delocalised structure using contributing forms which only involve 2-centre-2-electron bonds. It is therefore meaningless to think about what would happen to an atom in a resonance contributor if it were left around.

In your example the two structures are resonance contributors to the single delocalised structure and so the hydrogen ion cannot move away from the molecule because it never exists on its own as a hydrogen ion. It is always bonded to the rest of the molecule.

Hyperconjugation can be explained through resonance but it is much more clearly explained through molecular orbital theory. In this picture you have donation of electron density from the $\ce{C-H~\sigma}$ orbitals into the empty $p$ orbital of the positively charged carbon. This is possible because the two orbitals can be aligned in a plane due to rotation around the C-C bonds.

bon
  • 15,369
  • 13
  • 62
  • 91
0

(1) The hyperconugation extension of the Lewis-structure model simply suggests to take more than the traditional mesomeric partial formulae into account. (2) even if those hyperconjugated forms would be the only ones, that would mean that there would be still be the (purely) ionic interaction. (For Na$^+$Cl$^-$ you also would not assume that the sodium ion would leave the "molecule").

Raphael J.F. Berger
  • 708
  • 6
  • 19