The first type of common electrophiles we will study is carbocations. A carbocation is a species that contains a positively charged trivalent carbon atom possessing a trigonal plannar structure. The simplest carbocation is, in principle, the methyl (CH3+) cation (Fig. 1.16). CH3+ has not been identified experimentally presumably due to its extremely high instability. It is, however, employed as a prototype from which all the carbocations are derived by replacing one or more hydrogen atoms with different groups. The central carbon atom in CH3+ is sp2 hybridized, with an empty p orbital holding a positive charge and perpendicular to the trigonal plane defined by the three sp2 orbitals. Each C─H bond is formed by the 1s‐sp2 overlap. When one hydrogen atom in CH3+ is replaced by an alkyl (R) group, a primary (1°) carbocation RCH2+ is formed. Replacement of the second and third hydrogen atoms with alkyl groups results in a secondary (2°) carbocation R2CH+ and a tertiary (3°) carbocation R3C+, respectively (Fig. 1.16). In general, all types of carbocations are energetic. Thus, they are usually very unstable and highly reactive. Most of them act as strong electrophiles primarily owing to the active empty p orbital in the central carbon atom which has highly strong tendency to accept a pair of electrons from a nucleophile (almost any types of nucleophiles).
The relative stability for different types of carbocations increases in the order of methyl cation CH3+, 1° carbocation RCH2+, 2° carbocation R2CH+, and a 3° carbocation R3C+. This is mainly attributed to the hyperconjugation effect of the C─H bonds in the alkyl groups, which can be well explained using the ethyl CH3CH2+ cation (a primary carbocation) (Fig. 1.17a). In CH3CH2+ one of the C─H bonds in the methyl group can overlap in sideway with one lobe of the unhybridized p orbital (hyperconjugation effect) in the primary CH2 carbon. It results in delocalization of the positive charge into the C─H bond domain. In addition, the positively charged CH2 carbon (sp2 hybridized) attracts electrons from the CH3 carbon (sp3 hybridized) through the C─C σ‐bond (inductive effect) also leading to delocalization of the positive charge. The charge delocalization lowers energy of the carbocation. The hyperconjugation effect is further demonstrated in Figure 1.17b using a MO model. The interactions (linear combinations) of a low‐energy C─H bond and a high‐energy p orbital lead to the formation of a bonding MO (with the energy level lower than that of the C─H bonding orbital) and an antibonding MO (with the energy level essentially the same as that of the p orbital). The difference in energy between the C─H bond and the bonding MO represents stabilization of the carbocation (decrease in energy) by a methyl group.
FIGURE 1.16 Structure of different types of carbocations.
FIGURE 1.17 (a) Overlap of a C─H bond of the methyl group in the ethyl cation (CH3CH2+) with one lobe of the empty p orbital in the carbocation (hyperconjugation) and (b) linear combination of the C─H bonding orbital with the empty p orbital giving rise to formation of bonding and antibonding molecular orbitals.
In (CH3)2CH+ (a secondary carbocation), two C─H bonds (each from one methyl group) can overlap simultaneously with one lobe of the unhybridized p orbital in the secondary CH carbon. In (CH3)3C+ (a tertiary carbocation), three C─H bonds (each from one methyl group) can overlap simultaneously with one lobe of the unhybridized p orbital in the tertiary carbon. As a result, the increase in number of the C─H bonds overlapping with the unhybridized p orbital (hyperconjugation effects) makes the positive charge delocalize to greater domains and further lowers the energies of the carbocations. In addition, the inductive effects through the methyl–C+ σ bonds are getting more appreciable as the number of methyl groups on the positive carbon increases. This also makes the positive charge delocalize to greater domains and further lowers the energies of the carbocations.
When unsaturated groups such as vinyl and phenyl are attached to a positively charged carbon, the carbocations are greatly stabilized. As a result, the stability of allylic cation (CH3=CHCH2+) and benzylic cation (PhCH2+) is even higher than a regular tertiary carbocation such as (CH3)3C+. The stabilization is due to large conjugation effects of the unsaturated groups. In each of the allylic and benzylic cations, the positively charged empty p orbital overlaps in sideways with the π bond of the unsaturated group (conjugation effect), which delocalizes the positive charge to the vinyl or phenyl group and lowers energy of the cation.
Usually, a carbocation is produced by dissociation of a tertiary or secondary haloalkane, which in turn is attacked by a nucleophile resulting in an SN1 reaction (Reaction 1.63) [1, 6].
Dissociation of the haloalkane to a carbocation can also be facilitated by a Friedel–Crafts catalyst, such as AlCl3 [1, 6]:
Another common route for generation of a carbocation is the electrophilic addition of a hydrogen halide to an alkene (Reaction 1.64) [1, 6].
Once formed, the intermediate carbocation reacts fast with the halide (a nucleophile) to give a haloalkane addition product.
The concentrations of the carbocations in Reactions 1.63 and 1.64 are extremely low, and their existence cannot be detected by common spectroscopic methods. However, in certain conditions, a secondary or a tertiary carbocation can be stabilized and identified experimentally. For example, 2‐chloromethane dissociates in the medium of antimony pentafluoride (SbF5, a very strong Lewis acid) to give isopropyl cation (CH3)2CH+ (Reaction 1.65) [1, 7].
Due
