Page 35 of my textbook, Mass Spectrometry by Jürgen H. Gross, says the following in a section on ionization energy (IE) and charge localization:

Molecules with π-bonds have lower IEs than those without, causing the IE of ethene to be lower than that of ethane. Again the IE is reduced further with increasing size of the alkene. Aromatic hydrocarbons can stabalize a single charge even better and expanding π-systems also help making ionization easier.

My questions are as follows:

  1. Why do molecules with π-bonds have lower IEs than those without?

  2. Why does expanding π-systems help making ionization easier?

I would greatly appreciate it if people could please take the time to clarify this.

  • 1
    $\begingroup$ $\pi$ MOs are higher in energy than $\sigma$ MOs, so they are somewhat 'easier' to remove. Furthermore $\pi$ MOs are more delocalized (spread out in space), thus helping to stabilize a charged species by lowering the overall charge density. $\endgroup$ – LoschmidtsSchnitzel Jun 24 '19 at 20:00
  • $\begingroup$ @LoschmidtsSchnitzel Thanks for the comment. Just to clarify, so the answer to number 2 is that expanding $\pi$-systems are further delocalized, thus lowering the overall charge density and making ionization easier? $\endgroup$ – The Pointer Jun 24 '19 at 20:19
  • $\begingroup$ The delocalization helps to increase the stability of the charged species and makes it thus more likely to actually remain in the ionized state. Think of an allyl carbenium ion vs a secondary carbenium ion $\endgroup$ – LoschmidtsSchnitzel Jun 25 '19 at 9:23
  • $\begingroup$ @LoschmidtsSchnitzel So molecules sometimes lose their ionization after becoming ionized? And so a challenge is ionizing them and keeping them ionized? Is this the phenomenon that number 2 is referring to? $\endgroup$ – The Pointer Jun 25 '19 at 9:27
  • $\begingroup$ Have you heard of flame colouring? If you dip a sodium solution into a flame, the flame will turn bright yellow. The highest electron of sodium is raised to a higher energy level by the thermal input, then relaxes back into the lower orbital - releasing a characteristic amount of energy which happens to be a photon in VIS. (this is the basis of a lot spectroscopy techniques). Its not a textbook style example of ionization, but quite intuitive. Try any introductory organic chemistry book and look at reaction mechanisms involving charged species (SN1, E1 vs SN2/E2) $\endgroup$ – LoschmidtsSchnitzel Jun 25 '19 at 9:36

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