Ace Your Next Organic Chemistry
Exam.

With these Downloadable PDF Study Guides

Our Study Guides

A Primer On Organic Reactions

By James Ashenhurst

Three Factors that Destabilize Carbocations

Last updated: May 13th, 2019 |

Destabilizing Influences On Carbocation Stability 

In the recent post about 3 things that stabilize carbocations, I mentioned that carbocations are stabilized when their neighboring atoms donate electrons. This makes sense, right? Opposite charges attract, like charges repel.

Well, this post is about the “like charges repel” part. As you might be able to imagine, if  electron-donating neighbors are stabilizing for carbocations, electron-withdrawing neighbors must be destabilizing.

There’s at least three topics worth mentioning here – 1) replacing alkyl groups with H 2) electron-withdrawing neighbors, and 3) the effect of hybridization.

1. Decreasing The Number Of Carbon Substituents Reduces Carbocation Stability

If carbocations are stabilized as we increase the number of attached carbons, they are likewise destabilized as we replace these carbons by hydrogen. There’s two traditional ways to explain this.  The simple explanation invokes inductive effects as the major contributor to stabilizing carbocations. Hydrogens have a mere two electrons in their valence shell –  unlike carbon, they cannot withdraw electron density from neighboring atoms and donate it to the carbocation.

1-carbocations

The second, more theoretically robust explanation is hyperconjugation – the donation of electrons from a sigma bond (the C-H bond in this case) to the empty p-orbital. This is a more powerful concept, but many introductory courses don’t cover it.

2-hyperconjugation

2. Inductive Effects. Carbocations Are Destabilized By Neighboring Electron-Withdrawing Groups*

(*that lack lone pairs).

The best way to explain this is simply through pictures. Check out these functional groups.

If you look closely in each case you should notice the positive charge of the carbocation adjacent to either a full or partial positive charge on a neighboring substituents. This is true whether the neighbor is a group like CF3 or for aldehydes, nitro groups, sulfones, or many other groups containing atoms more electronegative than carbon.

The first to note with all of these electron withdrawing groups is  that none of the groups contain an atom with a lone pair directly bonded to the carbocation. This is important! Most people would consider halogens such as F, Cl, Br, and I to be electron withdrawing groups due to their electronegativity, but in fact all of these groups help to stabilize positive charges. Why? Because each of these atoms have lone pairs which can be donated to the carbocation, which is a stabilizing interaction.

3-ewgs

The second thing to note is that resonance, while possible, is not stabilizing in the case of groups such as aldehydes, ketones, and other groups with multiple bonds to electronegative atoms. The reason is that these resonance forms are really unstable – if you take the time to draw them out, you’ll see that you end up with a resonance form with the more electronegative atom lacking a full octet. The only first-row atoms you will ever observe lacking a full octet are boron and carbon.

3. The Greater The “s-character” Of The Carbocation, The Less Stable It Is

The trend here is pretty simple: the stability of a carbocation decreases as the s-character increases. That is, as we go from carbocations derived from sp3 to sp2 to sp- carbons.

You don’t see carbocations on double bonds very much, and here’s a good reason: compared to sp3, there is more s character in the orbitals, so the empty orbital is held more closely to the nucleus. This is destabilizing if we’re talking about positive charge.

4-hybrid

If this sounds somewhat familiar, it should – it’s the exact opposite of the argument for why negative charges are more stable on sp hybridized carbons as opposed to sp2 and sp3.

Remember the factors governing acidity? Well, all the factors there are pretty much a roadmap for things that stabilize negative charge. This whole post is talking about destabilizing positive charge. All of the trends that increase acidity will decrease the stability of cations. Cool how it can all connect, huh?

Comments

Comment section

10 thoughts on “Three Factors that Destabilize Carbocations

  1. pls i have a problem in tautomerism about comparing the rates of enolisation.for ex-a methylene group is surrounded by a carbonyl group and a phenyl group on both sides in first case,and is surrounded by carbonyl gp and alkyl gp in second case.how do we compare their rates

  2. But if resonance is not stabilizing in the case of carbonyls (you’ve shown that in the graphic on this page….because in the resonance form electronegative oxygen will have a sextet of electrons – and a positive charge)

    But then, why are alpha-hydrogens acidic in carbonyls (the reason why we see reactions like aldol condensation, Claisen condensation, etc.)

  3. Which is more stable among these pairs

    1). Benzyl carbocation & p-chloro benzyl carbocation

    2). Benzyl carbanion & p-chloro benzyl carbanion

  4. Hello,

    I am wondering why you said that a tertiary carbocation is sp3 hybridized? If a nucleophile attacked this, then the hybridization would be sp4 which does not exist (because carbon cannot be bound to five other groups). I learned that a carbocation is trigonal planar with sp2 hybridization and a vinylic carbocation is sp hybridized. Everywhere I have checked says that a carbocation is sp2 so I would appreciate the clarification. Thank you so much!

    1. Yes, you are correct. Flat alkyl carbocations are sp2 hybridized. We instructors need to come up with a better way of saying that “carbocation stability decreases as s-character of the carbocation increases” without invoking sp3 > sp2 > sp

  5. Hi James,

    you said here that “Most people would consider halogens such as F, Cl, Br, and I to be electron withdrawing groups due to their electronegativity…” but then cited CF3 as an electron withdrawing group. Is this a mistake? Did you maybe mean to cite the other halogens but leave fluor out?

    1. Not a mistake.

      If you look at F, Cl, Br, and I, they each have attached lone pairs. So while they do indeed “pull” electron density away via electronegativity, they also “push” electrons towards the carbocation through donation of a lone pair (forming a pi bond). This “donation” happens to outweigh the effect of electronegativity in the case of the halogens.

      CF3 is a pure EWG because there is no lone pair on the carbon to donate into the carbocation. By the same token, CCl3, CBr3, and even CI3 could be counted as electron withdrawing groups.

  6. Isn’t carbocation sp2 not sp3 as written (for example the methyl cation)?! This is what in most textbooks teach us.

    1. Yeah, I will adjust this image. On one hand, alkyl carbocations are sp2 hybridized. On the other, when talking about the stability trend, it’s easy to slip into saying, “sp3 is more stable than sp2 which is more stable than sp”. That sentence gets the main point across but is, as you point out, factually inaccurate.
      There are examples of sp3 hybridized carbocations (tetrahedral carbons) and they are very unstable e.g. bridgehead carbocations. https://pubs.acs.org/doi/abs/10.1021/jo990724x

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.