Ace Your Next Organic Chemistry
Exam.

With these Downloadable PDF Study Guides

Our Study Guides

A Primer On Organic Reactions

By James Ashenhurst

3 Factors That Stabilize Carbocations

Last updated: March 21st, 2019 |

Three main factors increase the stability of carbocations:

  • Increasing the number of adjacent carbon atoms: methyl (least stable carbocation) < primary < secondary < tertiary  (most stable carbocation)
  • Adjacent pi bonds that allow the carbocation p-orbital to be part of a conjugated pi-system system (“delocalization through resonance”)
  • Adjacent atoms with lone pairs

More details below!


Three Factors That Stabilize Carbocations

If electrons were money, carbocations would be the beggars of organic chemistry. Packing a mere six valence electrons, these electron-deficient intermediates figure prominently in many reactions we meet in organic chemistry, such as

  • nucleophilic substitution  (SN1) and elimination  (E1) reactions
  • additions of electrophiles to double and triple bonds
  • electrophilic aromatic substitution
  • additions to carbonyl compounds and enolate chemistry (albeit in masked form)

That’s a huge chunk of sophomore O-chem, right there.

Being electron-deficient (and therefore unstable), formation of a carbocation is usually the rate-limiting step in these reactions.

Knowing that,  then think about this: what happens to the rate of the reaction when the carbocation intermediate is made more stable? Well, the energy of the transition state leading to the reaction will be lower. Therefore, the activation energy will be lowered. 

What’s that going to do to the rate of the reaction? Since the activation energy is lowered, the reaction is going to speed up. 

So what are some of the factors that stabilize carbocations?

If you look through all of your organic chemistry textbook, you’ll find 3 main structural factors that help to stabilize carbocations.

  1. Neighboring carbon atoms.
  2. Neighboring carbon-carbon multiple bonds.
  3. Neighboring atoms with lone pairs.

Why is this? It all goes back to the core governing force in chemistry: electrostatics. Since “opposite charges attract, like charges repel”, you would be right in thinking that carbocations are stabilized by nearby electron-donating groups.

Let’s look at each of these in turn.

1) Carbocations are stabilized by neighboring carbon atoms.

The stability of carbocations increases as we go from primary to secondary to tertiary carbons. There’s two answers as to why this is. The age-old answer that is still passed around in many introductory textbooks points to carbons (alkyl groups in particular) as being “electron-releasing” groups through inductive effects. That is, a carbon (electronegativity 2.5) connected to hydrogen (electronegativity 2.2) will be electron rich, and can donate some of those electrons to the neighboring carbocation.   In other words, the neighboring carbon pays the carbocation with electrons it steals from the hydrogens. The second, (and theoretically more satisfactory explanation) is hyperconjugation, which invokes stabilization through donation of the electrons in C-H sigma bonds to the empty p orbital of the carbocation.

Whatever the explanation, this factor governs many key reactions you meet in Org 1 – from Markovnikoff’s rule, to carbocation rearrangements, through understanding the SN1 and E1 reactions.

1-substitution

2) Carbocations are stabilized by neighboring carbon-carbon multiple bonds.

Carbocations adjacent to another carbon-carbon double or triple bond have special stability because overlap between the empty p orbital of the carbocation with the p orbitals of the π bond allows for charge to be shared between multiple atoms.  This effect,  called “delocalization” is illustrated by drawing resonance structures where the charge “moves” from atom to atom. This is such a stabilizing influence that even primary carbocations – normally very unstable – are remarkably easy to form when adjacent to a double bond, so much so that they will actually participate in SN1 reactions.

2-adjacent double bonds

3) Carbocations are stabilized by adjacent lone pairs.

The key stabilizing influence is a neighboring atom that donates a pair of electrons to the electron-poor carbocation. Note here that this invariably results in forming a double bond (π bond)  and the charge will move to the atom donating the electron pair.  Hence this often goes by the name of “π donation”.

The strength of this effect varies with basicity, so nitrogen and oxygen are the most powerful π donors. Strangely enough, even halogens can help to stabilize carbocations through donation of a lone pair.  The fact that atoms that we normally think of as electron-wthdrawing (nitrogen, oxygen, chlorine) can actually be electron-donor groups is probably one of the most difficult factors to wrap your head around in Org 2.

This effect is tremendously important in the reactions of aromatic rings and also in enolate chemistry, where double bonds attached to donating groups (nitrogen and oxygen in particular) can be millions (or billions) of times more nucleophilic than alkenes that lack these groups.

3-lone pairs

The bottom line of this post is that by understanding the factors which affect the stability of carbocations, you can gain tremendous insight into many different reactions, even though they may appear vastly different.

Why is this important? Many reactions pass through carbocation intermediates. What do you think the effect of stabilizing the carbocation will be on the reaction rates? Here’s some specific examples.

4-apply concept

Comments

Comment section

89 thoughts on “3 Factors That Stabilize Carbocations

  1. can you please explain that if I have a benzyl carbocation and a t-butyl carbocation
    which will be more stable
    1st has stability due to benzyl resonance
    and 2nd has 9 possible hyperconjugative structures
    please answer

    1. Dear Jeetesh! You should know that resonance is more pronounce than hyperconjugation and will stabilize the cation more as compare to hyperconjugation.

          1. I do not have an experimental reference, but in “Electron Flow In Organic Chemistry” Paul Scudder claims on page 65 in his “carbocation stability ranking” chart that “tertiary cation” is more stable than “benzyl cation” (the benzyl shown being primary, i.e. two H’s).

            I’d imagine that a tertiary benzyl cation, the sp2 carbon bearing a phenyl group and two alkyl groups, should certainly be more stable than a tertiary cation bearing three alkyl groups (both have two hyperconjugations, however the aromatic pi donor beats the third hyperconjugation).

            A secondary benzyl cation vs tertiary alkyl cation would be a little more ambiguous.

          2. Dear all, carbocation stability could be inferred from the hydride affinity in gas phase. Please see Advanced Organic Chemistry: Part A 5th edition by Carey and Sundberg, Table 3.10., page 303. The lower the hydride affinity, the more stable the carbocation. (239 kcal/mol for benzylic cation VS 237 kcal/mol for t-Butyl cation)

        1. This is because the phenyl groups are not in a single plane and for resonance to occur, the R groups should lie in the same plane. This distortion of shape to the compound happens due to steric hinderance of the phenyl groups in the compound

  2. Oh man it is totally awesome. You told every thought tat we have while studying! And my every doubt is gone now !!n!n!n!

  3. What about the “bent or umbrella bond”? Don’t you think that bent bond participate in the stability of carbocations?

  4. out of ch3ch2ch2+ and ch3ch2+ which is more stable carbocation
    both are primary but
    the former one has a bulkier alkyl group and hence more inductive effect
    and the latter one has more no of alpha hydrogen and hence more no of hyperconjugative structures..
    both the reasons are clashing……!!!!!
    we expect the first one out of intuition but how can we forget the fact that hyperconjugaion is more dominant tha inductive effect?

    1. Both are primary carbocations; they will have very similar stabilities. The propyl carbocation can rearrange through a hydride shift to give a secondary carbocation.

  5. actually my main ques was about pinnacol pinnacolone rearrangement.
    H+ attacks on that OH which yields a more stable carbocation
    so which O should it attack?
    OH OH
    I I
    CH3—C—C—-C2H5
    I I
    CH3 C2H5

    1. oops the server omitted the spaces in the compound which messed it all up..
      it is

      (CH3)2–C(OH)————-C(OH)(C2H5)2

  6. I have only a little problem . well, which counts more, the resonance stabilisation or if its primary or secondary Carbon? Due this fact, which is more stable, +CH2-CH=CH2 or CH3CH(+)CH3? Thank you in advance xD

  7. This was so much help! Organic chem is a pain, are there more explanations of other orgo subjects?
    like spectroscopy, I really need help on that and could use a good website like this one

    Thanks.

  8. First of all ,thanks for explaining this so well.
    But, can u please tell me , generally which effect counts more, Inductive or Hyperconjugation?
    Like for example, if you have ethyl carbocation and if you have 2 methyl propane carbocation (primary carbocation) which will be more stable?

    1. It’s far more powerful to look at stabilization from the perspective of hyperconjugation, but it’s far easier to explain it from the perspective of inductive effects. If you are a major in a chemistry program I would take the time to learn hyperconjugation and apply that to your studies. It’s a very powerful concept.

  9. Hi! Thanks for your help : )

    I have a question. Would a secondary carbocation be considered more stable than a primary carbocation bonded with a halogen? It’s on a practice test and I’m a little confused o_O

    1. Hard to say without seeing the exact example, but my guess is that the latter situation would be more stable, since the halogen can donate a lone pair and every atom on the molecule can have a full octet. This is a more stable situation than a free carbocation where there is an empty orbital.

    1. Try looking at the strength of tertiary, benzyl, and allyl C-H bonds. Since C-H bond strengths measure homolytic cleavage, then you will then get the stability of the radicals. The weaker the C-H bond the more stable the radical.

  10. hello…i have a question can we say that the resonanse factor is more effective factor except of when the aromaticity is endangered?

  11. Hi,
    I was wondering if you could post the answers to these sample problems please? I’m doing them, but I have no way of checking if it’s right.
    Thank-you,
    TA Nguyen

    1. Hi, in each case the first carbocation is more stable. This applies the 3 factors we learned that stabilize carbocations.

      tertiary carbocation more stable than secondary
      allylic (resonance stabilized) carbocation more stable than non resonance stabilized carbocation
      carbocation adjacent to atom with lone pairs (oxygen) more stable than carbocation not adjacent to atom with lone pairs.

      Hope this helps
      James

    1. Hi Monica

      There’s several factors that are not always easy to judge by just looking at them – we need to do experiments. For example we know that carbocations increase in stability going from primary to secondary to tertiary.

      We also know that carbocations increase in stability if they are resonance stabilized.

      However – what factor is more important? substitution pattern or resonance?

      There is no way to figure this out just by looking at it. We have to do experiments (and we do!) [by measuring “ionization rates”]

      Those experiments tell us that secondary allylic carbocations are slightly easier to form than ordinary (non resonance stabilized) tertiary carbocations.

      Why is that? Good question (and this is where it can get complicated). Probably the fact that there is more electron density being donated from an adjacent p orbital than there is from the [hyperconjugation] C-H bonds adjacent to the tertiary carbocation.

      These factors can be in delicate balance. If I can make an analogy, it’s a bit like sports teams. What’s more important in football, to have a good offence or a good defence? Well, they’re both important. How do we know the relative importance of each? By playing a LOT of games and trying to figure it out by looking at the data. Thankfully with chemistry one mole of material gives us the chance to play 6x 10^23 “games” so we can figure this out pretty reliably.

      hope this helps! James

  12. Organic chemistry has always been the most exiting and beautiful subject for a geek like me and all these articles inspire me to go more in depth of this subject.

  13. Hi James,
    Thanks for a neat explanation. I am not an organic chemist and I have a question about stabilization of carbocations. Do you think it is probable to stabilize a carbocation by putting it next to sth that can stabilize it? Like solvation effects, or some negatively charged species?Like electrostatic stabilization?
    In all these 3 examples, carbocation is stabilized via intramolecular effects, how about intermolecular stabilization?
    Thanks.

  14. Hi James,

    I would like to pick your brain a little bit. I have been banging my head against the wall with this one and I don’t seem to figure it out.
    Let’s say you have two secondary amines. In the first one, one substituent is a propargyl (prop-2-yn) group and the second one is methyl. In the second amine, one substituent is an (indol-2-yl)methyl group (amine is bonded through a methylene group to the position 2 of indole) and the second one is methyl. Now, in both cases a secondary carbocation is formed next to the amine nitrogen. Which one should be more stable?

    In both cases the nitrogen stabilizes the positive charge via lone electron donation, but what about the alkynyl and indolyl moiety? I suppose they should both have stabylizing effects via delocalization but which one would stabilize the charged species more? What about the inductive effects? Should they play some significant roles in this case? I am so confused…

    1. There are two types of intermediates with positively charged oxygen. The intermediate where oxygen has a full octet is OK (and generally speaking more stable than a carbocation). Intermediates where oxygen have less than a full octet are very unstable, because a very electronegative atom (oxygen) with less than a full octet will have tremendous potential energy (and thus instability) for pulling electrons toward the nucleus.

    2. By positively charged oxygen I mean an electron with 6 electrons. It’s a more electronegative element so there will be MUCH greater electron-affinity pulling electrons toward the nucleus. Very unstable situation.

  15. Hi!

    I was just wondering, which would be more stable then between a tertiatry carbocation and a carbocation stabilized by resonance? (like in #2?)

    Thank you! Great article!

  16. (CH3)2–C+ —COOH , can this resonate ? …i mean theres pi sigma and +ve charge conjugate system ….but yet i was doing a question and this wasn’t the answer.
    So why cant it resonate please answer

    1. The resonance form would end up with less than a full octet on oxygen, which is extremely unstable. It’s not a significant resonance form.

  17. Hello, can you please tell me which is more stable carbocation…(CH3)3C or (C2H5)3C ?
    If the no of carbon atoms increases in an alkyl group…Its +I effect will decrease or increase ?

  18. So if you have a secondary carbocation that has a little bit of resonance stabilization and a tertiary carbocation (with no resonance stability), which is more stable? (My textbook says the secondary with resonance…)

  19. Dear James:

    I have no problem with -NH2, -OH since we establish in EAS that they are electron donating in general. But i don’t understand why halides should stabilize the carbocation. It is said in all textbooks that the mistmatch between 3p and 2p atomic orbitals results in a diminished resonance delocalization, and winning of the inductive effect for halides, making them electron-withdrawing. So it removes electron density and creates a bigger positive charge in the intermediate of aromatic reaction. but we have the same halide and the same carbocation here (Except its not in a ring) but its now donating whats the difference? Thanks!

  20. Thank you so much, sir! You’re metaphor on money and electrons made the concept so much easier! More powers to you!

  21. Is it possible for a non-adjacent atom with a lone pair to stabilize a carbocation? There’s a question in Brown’s Organic Chemistry 8th edition that asks why CH2CHCOOH (https://image.ibb.co/cdThRJ/structure_1.png) + HCl forms CH2ClCH2COOH rather than CH3CHClCOOH which would be expected due to the ordinarily greater stability of the secondary carbocation. I’m wondering if you can get the carbon backbone curling around upon itself (https://image.ibb.co/d6eBLd/structure_is_this_what_s_happening.png) so the lone pair on the hydroxyl oxygen can help to stabilize the primary carbocation?

  22. Will a primary alkyl halide be able to undergo an SN1 reaction if it is stabilised by a neighbouring oxygen atom? Or will the carbocation still be too unstable to react in this way?

    1. Depends on what you mean by “neighboring”. If it’s directly attached to the same carbon (such as H3C-O-CH2Cl) then the oxygen will easily form a pi bond with C and the Cl- will be displaced easily. For an example see benzyloxymethyl chloride. (these things tend to be unstable)
      If by neighboring you mean an C=O on the carbon adjacent to the carbon bearing the carbocation, then this will be unstable.

    1. That is an EXCELLENT question and the data contradicts somewhat. Cyclopropylmethyl cations are generally considered to be more stable than benzyl. I’m looking for a better reference than just March 5th ed. p. 222, but the references therein are to good, but somewhat obscure, reviews. Hydrolysis rates suggest cyclopropylmethyl cations are more stable.

      However another way to answer that is to look at 13-C NMR to determine the chemical shift of the carbocations. The more negative the chemical shift, the more unstable it is. The 2-cyclopropyl carbocation has a chemical shift of -86.8 ppm and the 2-phenylpropyl cation has a chemical shift of -61.1 indicating that the phenyl group is better at stabilizing. See https://pubs.acs.org/doi/10.1021/ja00731a026

  23. From gas phase dissociation energies, the tert butyl carbocation is about 7 kcal/mol more stable (232 kcal/mol) than the benzyl carbocation (238 kcal/mol) but substituent effects can greatly change these numbers. Source: March’s Advanced Organic Chemistry 5th ed. Table 5.2 page 224.

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.