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This question comes from a second semester undergraduate organic chemistry course. Please refer to the image below with regard to the question.

My understanding of the trend in section 4a is that "larger" halogens such as iodine by themselves are able to stabilize positive and negative charges better than "smaller" halogens like fluorine.

Moving into section 4b, my professor transitions with "HOWEVER", telling me that the trend in 4b is contradictory in some way to the trend in 4a.

The idea that my professor is attempting to convey still very much eludes my comprehension. So I ask:

  • (Main question) Why is iodine-containing molecule less stable than the fluorine-containing molecule?
  • What is he trying to show me with this trend comparison?
  • Is there a decent source that can elaborate on this section of my handout?

A section from the course handout made by my professor.


Thank you,

Brian

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  • $\begingroup$ Could someone explain why such structures were used in 4b? This is a legitimate inquiry and I do not see how this question is preposterous. $\endgroup$ – Brian M. Jan 28 at 2:02
  • $\begingroup$ If the bonds in 4b were sigma bonds and there was no charge, the order of stability would be reversed. I want to know why that is. $\endgroup$ – Brian M. Jan 28 at 2:09
  • $\begingroup$ Well, you know what mesomeric stabilisation of cation is? That's exactly the context you're somehow missing. $\endgroup$ – Mithoron Jan 28 at 2:19
  • $\begingroup$ To answer your question, I do not. Is it analogous to induction? $\endgroup$ – Brian M. Jan 28 at 2:37
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The main idea is that a cation can be stabilized by an adjacent atom mainly in two ways:

  • Distributing charge over pi bonds, easily visualized with resonance structures; the effect is called mesomeric stabilisation.
  • Distributing charge over sigma bonds; i.e. induction stabilisation.

As induction stabilization works over sigma bonds, it is stronger if the bond has the correct polarization; that is, if the other atom has low electronegativity, so it holds a positive charge easily. In your example iodine would be better as explained in a).

Mesomeric stabilization occurs through pi bonds, which are significantly weaker than sigma bonds. In this case the main issue is good "contact", or overlap, between atomic orbitals. As carbon is so small, it overlaps better with small elements, so in this case fluorine has the advantage, as explained in b).

In general, mesomeric stabilisation is the main mechanism for carbocations. This happens because not only is the charge effectively spread over more than one atom, but also the electrons compromised in the pi bond lower their energy compared to just staying in the valence shell of the donor atom.

However, note that this is not the case for all elements; take for example sulphonium ions. Sulphur is a bigger atom that forms double bonds poorly. Hence the main mechanism is induction.

A good book (albeit a bit old) for reference is Morrison's Organic Chemistry.

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  • $\begingroup$ That was exactly the type of clarity I was looking for. You hit the nail on the head F Bat. Thank you. "Ah ha" moment achieved. $\endgroup$ – Brian M. Jan 28 at 18:13

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