A Primer On Organic Reactions
3 Factors That Stabilize Carbocations
Last updated: February 20th, 2020 |
Three Factors That Affect Carbocation Stability
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!
Table of Contents
- Three Factors That Stabilize Carbocations
- Carbocations Are Stabilized By Neighboring Carbon Atoms
- Carbocations Are Stabilized By Neighboring Carbon-Carbon Multiple Bonds
- Carbocations Are Stabilized By Adjacent Atoms Bearing Lone Pairs
- Formation Of A Carbocation Is Often The Rate-Limiting Step In A Reaction Mechanism. So By Understanding How Carbocations Are Stabilized, You Can understand The Effect Of Substituents On Reaction Rates
- (Advanced) References and Further Reading
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.
- Neighboring carbon atoms.
- Neighboring carbon-carbon multiple bonds.
- 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.
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.
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.
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.
5. Formation Of A Carbocation Is Often The Rate-Limiting Step In A Reaction Mechanism. So By Understanding How Carbocations Are Stabilized, You Can understand The Effect Of Substituents On Reaction Rates
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.
A large number of papers have been written on various experimental and theoretical methods for quantitatively determining the relative stability of carbocations, both in the gas phase and in solution (usually superacid).
- Quantitative preparation and enthalpy of rearrangement of the sec-butyl cation
W. Bittner, E. M. Arnett, and M. Saunders
Journal of the American Chemical Society 1976, 98 (12), 3734-3735
This paper describes a novel calorimetric method for measuring the heat of isomerization of the sec-butyl cation to the tert-butyl cation in superacid solution. Using this, the value obtained is 14.5 ± 0.5 kcal/mol.
- Stabilities of carbocations in solution. 14. An extended thermochemical scale of carbocation stabilities in a common superacid
Edward M. Arnett and Thomas C. Hofelich
Journal of the American Chemical Society 1983, 105 (9), 2889-2895
This paper provides a table of the stability of 39 carbocations, measured calorimetrically by heat of formation (ionization in superacid from the corresponding carbinol).
- Carbonium Ions. I. An Acidity Function (C0) Derived from Arylcarbonium Ion Equilibria
C. Deno, J. J. Jaruzelski, and Alan Schriesheim
Journal of the American Chemical Society 1955, 77 (11), 3044-3051
Carbocation stability can also be expressed in pKR+, which is defined in the paper. The carbocations studied in this paper are all relatively stable arylcarbonium ions.
- Hydride affinities of carbenium ions in acetonitrile and dimethyl sulfoxide solution
Jinpei Cheng, Kishan L. Handoo, and Vernon D. Parker
Journal of the American Chemical Society 1993, 115 (7), 2655-2660
Carbocation stabilities can also be expressed in terms of hydride affinity (R+ + H– -> RH). The carbocations studied in this paper are also fairly stable arylcarbonium ions, as these are measured electrochemically in either DMSO or acetonitrile.
- Photoelectron spectroscopy of methyl, ethyl, isopropyl, and tert-butyl radicals. Implications for the thermochemistry and structures of the radicals and their corresponding carbonium ions
A. Houle and J. L. Beauchamp
Journal of the American Chemical Society 1979, 101 (15), 4067-4074
Table III in this paper has heats of formation for the basic alkyl cations (methyl, ethyl, isopropyl, and t-butyl), but in the gas phase. The numbers agree with intuition from solution-phase experiments; stability increases from methyl -> ethyl -> isopropyl -> t-butyl.