This page covers the factors that influence the stability of carbocations. The purpose of this page is to provide a review of the fundamental nature of carbocations, and to describe different means by which carbocations are stabilized by nearby atoms or groups of atoms. This is important because carbocations are formed as intermediates in may organic reactions; the specific involvement of carbocations in various reactions is covered in related pages.
|lone pair electrons||resonance effects||resonance hybrid|
Carbocations are species bearing a formal "+" charge on carbon. The have sp2 hybridization and trigonal planar geometry, with an empty p orbital on carbon, perpendicular to the plane containing the substituents (see diagrams shown to the right). Carbocations are "hypovalent" species, inasmuch as they have only three shared pairs of electrons around carbon, instead of the usual four. Of course, this incomplete octet around carbon makes carbocations very unstable and very reactive. Nevertheless, carbocations are known to be formed as intermediates in many types of organic reactions. The relatively high energy of a carbocation , however, means that it will usually be formed in the rate-determining step for a reaction, and so it is important to understand how the substituents -- shown as R1, R2, and R3 in the diagrams above -- can act to stabilize a carbocation, thus making it easier to form from a neutral molecule in the first place.
There are basically two types of carbocations: those that are not stabilized by resonance effects, and those that are stabilized by resonance (either through lone pair electrons on adjacent atoms or through conjugated pi bonding electrons).
Simple Alkyl Carbocations
There are four possible degrees of alkyl substitution on a carbocation: three attached alkyl groups, two attached groups, one attached group, or no attached alkyl groups. These would be referred to as tertiary (3°), secondary (2°), primary (1°), or methyl carbocations, respectively. These four possibilities have been depicted explicitly below, using methyl groups in every case as the alkyl substituents; bear in mind, however, that any alkyl group could have been chosen:
[* - Of course, the methyl carbocation itself (CH3+) is the only example of a simple alkyl carbocation which is neither 3°, 2°, nor 1°. This turns out to be a trivial point, however, because the methyl carbocation is so unstable that it is never invoked as an intermediate in an organic chemical reaction. You should view with great skepticism any mechanism that purports to involove the methyl carbocation as an intermediate.]
Self-test question #1
Identify the following species as 1°, 2°, or 3° carbocations. Rank them from most to least stable.
It was long believed that the reason for the order of alkyl carbocation stability was simply as inductive stabilization of the positively charged carbon by its attached electron-releasing alkyl substituents. More recently, however, this notion has been more or less discarded in favor of "hyperconjugation" as the means be which alkyl substituents can act to stabilize a positive center to which they are attached. In molecular orbital terms, hyperconjugation is the overlap of the filled sigma orbitals of the C-H bonds adjacent to the carbocation with the empty "p" orbital on the positively charged carbon atom (see diagram, above). This electronic "spillover" helps delocalize the positive charge onto more than one atom. The more alkyl substituents, the more sigma bonds for hyperconjugation.
Note that it is not the sigma bonds that are directly attached to the carbocation that is involved in hyperconjugation; these orbitals are perpendicular to the empty "p" orbitals, and as such, cannot overlap with it. Rather, it is the sigma bonds one atom removed from the positively charged carbon atom that help to stabilize it. These bonds can rotate into an "eclipsed" conformation with the empty "p" orbital, and can thus interact with it. Another way of viewing the effect of hyperconjugation is via Lewis formulas, using "no-bond resonance" (see diagram below).
Self-test question #2
Can you think of a reason why the hydrogen atoms on carbons adjacent to a carbocation center are much more acidic than ordinary alkane hydrogen atoms? (Hint: Think of the second step in an E1 elimination reaction mechanism.)
How carbocations are formed
Carbocations are most often formed in one of two ways:
(These two modes of reactivity are discussed more fully on the related Web pages in this series that covers "SN1 and E1 Reactions," and "Additions to Alkenes.")
It is relevant at this point to note that carbocations are positively charged species. They are much more likely to be formed in acidic that in basic media; in fact, carbocations are never seen under conditions that are strongly basic.
Self-test question #3
Which of the compounds shown above would ionize to form the most stable carbocations?
Self-test question #4
(Review) Can you give the trivial and the IUPAC names for each of the four compounds shown in "Self-test question #3"?
Quantitative rankings of carbocation stability
We can get a more quantitative feel for the relative stabilities of the alkyl carbocations by examining data for the enthalpy of ionization (gas phase) for various alkyl chlorides: Of course, each of these reactions is much more endothermic in the gas phase than it would be in solution, where solvent molecules of appropriate polarity characteristics could help to stabilize the electrically charged products of the ionization reaction. (This is why "ionizing solvents" are often used for reactions that involve charged intermediates.) Nevertheless, the data clearly reflects the order of carbocation stability that we've already established: tertiary carboctions are the easiest (least endothermic) to form, the secondary, then primary, and the methyl carbocation is the most difficult to form.
Self-test question #5
Which of the two reactions shown below would be faster? Why?
Carbocations stabilized by resonance effects
Besides the simple alkyl carbocations, another class of carbocations are those that are stabilized by resonance. This usually occurs in one of two ways:
Let's look at each of these sorts of systems in some more detail.
1)Carbocations conjugated to pi bonds
The prototypical members of this class are the allylic and the benzylic carbocations:
Despite what might seem to be a greater degree of delocalization in the "benzyl" carbocation than in the "allyl" system, each of these ions is about as stable as the other. In fact, they are each about as stable as an ordinary 2° alkyl carbocation. Further alkyl substitution on the allylic or benzylic carbon atoms will further increase the stability. For example, each of the cations shown below is about as stable as an ordinary 3° alkyl carbocation, despite the fact that each of the drawings makes it appear as though these ions are 2°; they are 2°, but the are also conjugated!
Self-test question #6
Draw a second resonance form for the allylic cation shown below that accounts for its enhanced stability relative to an ordinary secondary alkyl carbocation (shown to the right, for comparison). Which of these two forms probably contributes more to the overall structure of the resonance hybrid? Why?
2)Carbocations conjugated to adjacent lone pairs
Carbocations form very readily on carbon atoms that have an attached heteroatom ("X"), especially when X=oxygen or nitrogen:
Carbocations stabilized in this way, i.e., by the (partial) "conversion" of non-bonding electrons into bonding electrons, are remarkably stable. For example, the compound MeOCH2+SbF6- can be isolated as a stable solid!
Self-test question #7
When formaldehyde is protonated, the hydrogen ion becomes attached to the carbonyl oxygen.
Can you draw a carbocation-like resonance form for protonated formaldehyde that accounts for this behavior?
Self-test question #8
When 1-methoxypropene is protonated, which atom acts as the H+ acceptor, and why?
Self-test question #9
The Friedel-Crafts acylation reaction involves the Lewis acid catalyzed ionization of an acid chloride to produce a carbocation that acts as an electophile in an electrophilic aromatic substitution reaction. Although simple vinyl cations are very unstable and are rarely formed, the "acylium" ion produced in the Friedel-Crafts reaction is relatively stable. Draw a resonance form that accounts for the stability of an acylium ion. What do you suppose is the hybridization of the carbonyl carbon after the ionization has taken place? What is the geometry about this carbon?
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Version 1.4.7, 3/19/97