3 Factors That Stabilize Carbocations

by James

in Drawing Reaction Mechanisms, Organic Chemistry 1, Where Electrons Are

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.

What’s that going to do to the rate of the reaction? It’s going to speed it 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.

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.

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.

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.


 

Related Posts:

{ 31 comments… read them below or add one }

vikash October 1, 2011 at 5:08 am

good tutorial.

Reply

dr klbajaj November 8, 2011 at 11:49 pm

To explain science in simplest way is an art.you have done it!

Reply

james November 9, 2011 at 12:13 am

thanks!

Reply

saleem khan November 18, 2011 at 9:25 am

information about organic chemistry

Reply

sanjay March 4, 2012 at 7:09 am

very simple way to understand chemistry

Reply

Jeetesh April 25, 2012 at 9:19 pm

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

Reply

Iqbal Safi March 19, 2013 at 1:24 am

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

Reply

Kushal Das November 16, 2013 at 4:01 pm

yes Iqbal,resonance is dominating mainly…..bt here it has been found that t-butyl is more stable…..

Reply

James Ashenhurst November 16, 2013 at 8:44 pm

Kushal, do you have a reference for that?

Reply

Manish Agnihotri April 26, 2013 at 3:38 am

Its an exceptional case, t-butyl carbocation is more stable …..

Reply

amna August 3, 2012 at 7:13 am

Good explanation one can eaisly understand by reading this article

Reply

james August 4, 2012 at 3:20 pm

Glad you found it helpful.

Reply

Sudhanshu August 10, 2012 at 7:03 am

Great article.

Reply

Sanathoi August 11, 2012 at 1:13 pm

Good article!

Reply

jasmine August 21, 2012 at 12:33 pm

this article rly helped me alot !! m glad u posted it :D

Reply

Ujjwalt January 1, 2013 at 1:25 pm

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!

Reply

Iqbal Safi March 19, 2013 at 1:27 am

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

Reply

sur March 29, 2013 at 2:46 am

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?

Reply

james March 31, 2013 at 4:08 pm

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

Reply

sur April 4, 2013 at 11:59 am

so the conclusion is that propyl carbocation is more stable….
bcz it can rearrange

Reply

james April 5, 2013 at 9:19 pm

No – once it’s rearranged, we’re discussing a different carbocation entirely.

Reply

sur April 4, 2013 at 12:27 pm

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

Reply

sur April 4, 2013 at 12:30 pm

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

(CH3)2–C(OH)————-C(OH)(C2H5)2

Reply

Rafi April 22, 2013 at 11:05 pm

Thank you very much, this has saved my life. Appreciate it.

Reply

Byrce July 2, 2013 at 6:39 am

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

Reply

james July 8, 2013 at 4:59 pm

Good question. In the examples you cited, the resonance counts more.

Reply

Rudd August 3, 2013 at 4:20 pm

Thanks. I’m now studying for my Organic Chem exam next week and this is really helpful for my studies

Reply

Michel Carroll October 14, 2013 at 10:23 pm

Had a hard time finding bare-bone explanation of cation stability. Thanks! Definitely bookmarking your website.

Reply

Nikel February 7, 2014 at 5:27 pm

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.

Reply

Mike June 14, 2014 at 10:36 am

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

Reply

James June 14, 2014 at 9:15 pm

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.

Reply

Leave a Comment