Organic Chemistry - Carbocations

CARBOCATIONS

Carbocations: Factors affecting their Stability	Carbocations and factors affecting instability
Carbocation Rearrangements: 1,2-H and 1,2-alkyl Shifts	Carbocation Rearrangements and Change in Ring Size

 

A species containing a positively charged carbon atom is called a carbocation (Fig. I.1). For many years these species were called "carbonium ions". In the early seventies George Olah proposed that the name "carbonium ion" should be reserved for pentacoordinated positive ions and that carbocations should be named "carbenium ions". Carbocations and their reactivity play an important role in chemistry since these species are intermediates in several kinds of reactions.

The more stable ones have been prepared in solution and in some cases even as solid salts. The less stable ones (more reactive) react in solution to produce new products.

By definition, every carbocation is an electron deficient species since it possesses a positive charge at a carbon atom. Carbocations are by their very nature unstable species.

Anything which donates electron density to the electron-deficient center will help to stabilize them.

Factors that stabilize them are the following:

Neighboring carbon atoms (inductive effect)
Neighboring carbon-carbon multiple bonds (resonance effect)
Neighboring atoms with lone pairs (resonance effect)

These factors are presented in the above posts and their stabilizing effect is explained.

 

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References

1. R. Bruckner, “Advanced Organic Chemistry – Reaction Mechanisms”, 2nd Edition, Elsevier, 2002

2. M.B. Smith & J. March “March’s Advanced Organic Chemistry”, 6th Edition, Wiley-Interscience, 2007

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Carbocation Rearrangements and Change in Ring Size

Carbocation rearrangements may give products where there is change in ring size. An example is shown below:
 

Fig. 1: A carbocation rearrangement with ring change is shown. The rearrangement gives a more stable tertiary carbocation

Another example of carbocation rearrangement and ring change (ring expansion) is shown in Fig. 2.  The reaction is a pinacol rearrangement and in this case leads to ring expansion. 

Fig. 2: A carbocation rearrangement with ring change is shown. The rearrangement gives a more stable tertiary carbocation since it is stabilized by the lone pairs of the oxygen atom. This rearrangement is known as the pinacol – pinacolone rearrangement.

Carbocation Rearrangements

Carbocation Rearrangements: 1,2-H and 1,2-alkyl Shifts

In a rearrangement a group moves from one atom to another in the same molecule. Most are migrations between adjacent atoms and are called 1,2-shifts. Carbocation rearrangements occur more frequently on secondary carbocations to form tertiary which are more stable and energetically more favorable. In general, the bonding electrons of a carbocation may shift between adjacent atoms to form a more stable carbocation.

What types of carbocation rearrangements are possible? When does a carbocation rearrangement occur?

Two types of carbocation rearrangements are possible: 1,2-H shift and 1,2-alkyl shift.
1,2-H shift (called 1,2 hydride shift, hydride ion = H:^-): If a carbocation is vicinal to a tertiary carbon bearing a H, a 1,2-H shift should occur (Fig. 1).

Fig. 1: Two possible types of rearrangement. The carbocation desires electron pair to complete the octet at the C⁺ atom. An 1,2-H shift provides these electrons and gives a more stable tertiary carbocation. In principle, a 1,2-methyl shift can occur but is less favorable since it gives a secondary carbocation.

1,2-alkyl shift: If a carbocation is vicinal to a tertiary carbon, a 1,2-alkyl shift should occur (Fig. 1).
Of these two examples shown in Fig. 1, hydride shift leads to a tertiary carbocation whereas alkyl (methyl) shift leads to a secondary carbocation. Because a tertiary carbocation is more stable than a secondary the hydride shift would occur more readily than the alkyl shift.

The following “rules” hold for carbocation rearrangements:

Carbocation rearrangements are equilibrium processes
Usually lead to more stable carbocations 
Sometimes lead to carbocations of equal stability (not so common)
Sometimes lead to less stable carbocations (very unusual but does happen)
Hydride shift is more common, favorable, than alkyl shift
The least bulky alkyl substituent shifts (usually CH₃-)
Only groups adjacent to C⁺ can migrate
Only carbon groups and H atoms can shift (1,2-OH shift is forbidden)

Carbocations are intermediates in S_N1 reactions and quite often yield rearranged products. A few examples are shown below:

Fig. 2: The S_N1 reaction above yields a rearranged reaction product. The intermediate secondary carbocation formed rearranges to a tertiary carbocation – a 1,2 hydride shift occurs – which is more stable. The carbocation reacts with CH₃OH and an S_N1 reaction occurs.

An example of 1,2-methyl shift is shown below leading to rearranged racemic products:

Fig. 3: A 1,2-alkyl shift gives a more stable tertiary carbocation. The carbocation reacts with CH₃NH₂ and an S_N1 reaction occurs.

Another example of carbocation rearrangement is shown in Fig. 4.  The reaction is known as the pinacol rearrangement and the carbocation formed is already stable since it is a tertiary carbocation. However it rearranges – a CH₃ shift occurs – because it is stabilized even more by the lone pairs of the oxygen atom.

Fig. 4: The 1,2-diol pinacol is treated with acid and a rearranged product pinacolone is produced. The reaction intermediate carbocation rearranges – a CH₃ shift occurs – even though it is a tertiary carbocation. The carbocation is stabilized by the electron lone pairs of the oxygen atom.

Carbocation rearrangements often give products where there is change in ring size. In the post entitled “Carbocation rearrangements and change in ring size” examples of such rearrangements are shown.

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Relevant Posts

Nucleophilic Substitution: S_N2 and S_N1 reactions and Stereochemistry

Carbocation Rearrangements and Change in Ring Size

S_N1 reactions and solvent effects

how does carbocation rearrangement occur?

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References

1. R. Bruckner, “Advanced Organic Chemistry – Reaction Mechanisms”, 2nd Edition, Elsevier, 2002
2. M.B. Smith & J. March “March’s Advanced Organic Chemistry”, 6th Edition, Wiley-Interscience, 2007
3. P. Muller, J. Mareda in G.A. Olah “Cage Hydrocarbons”, Wiley, 1990

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Key Terms

rearrangement, rearrangement of carbocation, secondary carbocation rearrangement, 1,2 shifts, carbocation rearrangements, secondary carbocations, 1,2 H-shift, 1,2 alkyl-shift, rearranged products, pinacol rearrangement

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Carbocations and factors affecting instability

Let’s consider the following carbocations and rank them in terms of their stability:

 

Fig. 1: Carbocation 1 is stabilized by the –OCH₃  substituent  since it is electron donating. The + charge is delocalized and the carbocation is stabilized. Carbocation 2 is destabilized by the electron withdrawing substituent NO₂. By drawing the resonance structure we see that there are two adjacent positive charges. This resonance structure is highly unfavorable!

The substituent (-OCH₃) stabilizes carbocation 1 by resonance and the resulting charge delocalization. The + charge is delocalized to the O atom.

The substituent (-NO₂⁺) in carbocation 2 is electron-withdrawing and the carbocation is destabilized. Notice the two adjacent + charges.

In general, carbocations are destabilized by neighboring electron-withdrawing groups if:

• Contain an atom more electronegative than C

• There is no atom with an electron pair that is directly attached to the carbocation (-CF₃, -CCl₃, -C(O)R, -⁺NR₃, -CN, -NO₂, -C(O)-OH, -S(O)₂OH)

For example let us consider the following carbocations and rank them in terms of stability:

 

Fig. 2: The carbocation with the CH₃ substituent is more stable than the carbocation with the electron-withdrawing CF₃ substituent.

The carbocation with the CH₃ group in Figure 1 is more stable than the carbocation with the -CF₃ group. Notice the partial + charge (d+) on the carbon atom due to the electron withdrawing effect of the fluorine atoms. The carbocation is destabilized due to the adjacent positive charge and partial positive charge.

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