Structure, Bonding, and Stability of Carbocations

Carbocations are classified as primary, secondary, or tertiary according to the number of carbons that are directly attached to the positively charged carbon. They are named by appending cation as a separate word after the lUPAC name of the appropriate alkyl group. The chain is numbered beginning with the positively charged carbon (the positive charge is always at C-1). 

Common names that have been incorporated into lUPAC nomenclature such as isopropyl, vec-butyl, and so on, are permitted. Thus 1,1-dimethylethyl cation (CHsfsC may be called fert-butyl cation. 

Evidence from a variety of sources convinces us that carbocations can exist, but are relatively unstable. When carbocations are involved in chemical reactions, it is as reactive intermediates, formed in one step and consumed rapidly thereafter. 

Numerous studies have shown that the more stable a carbocation is, the faster it is formed. These studies also demonstrate that alkyl groups directly attached to the positively charged carbon stabilize a carbocation. Thus, the observed order of carbocation stability is 

FIGURE 4.8 Structure of methyl cation CHs . Carbon is sp -hybridized. Each hydrogen is attached to carbon by a (T bond formed by overlap of a hydrogen Is orbital with an sp hybrid orbital of carbon. All four atoms lie in the same plane. The unhybridized 2p orbital of carbon is unoccupied, and its axis is perpendicular to the plane of the atoms. 

Therefore, we would expect the same mixture of stereoisomeric forms of the product to result regardless of which reactant is used. This is, in fact, what is observed and is consistent with the SnI mechanism. 

On reaction with hydrogen chioride, one of the trimethyicyciohexanois shown yields a single product, the other gives a mixture of two stereoisomers. Expiain. 

As we have just seen, the rate-determining step in the reaction of ferf-butyl alcohol with hydrogen chloride is formation of the carbocation Convincing evidence from a 

Carbocations are classified according to the degree of substitution at the positively charged carbon. The positive charge is on a primary carbon in CHjCH2, a secondary carbon in (CH3)2CH, and a tertiary carbon in (CH3)3C+. Ethyl cation is a primary carbocation, isopropyl cation a secondary carbocation, and fert-butyl cation a tertiary carbocation. 

Methyl cation Ethyl cation Isopropyl cation tert-Butyl cation 

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Addition of the eiectrophile forms a new C- Br bond and generates a carbocation. This carbocation intermediate is resonance stabilized— three resonance structures can be drawn. 

The major carbon centered reaction intermediates in multistep reactions are carbocations (carbenium ions), carbanions, free radicals, and carbenes. Formation of most of these from common reactants is an endothermic process and is often rate determining. By the Hammond principle, the transition state for such a process should resemble the reactive intermediate. Thus, although it is usually difficult to assess the bonding in transition states, factors which affect the structure and stability of reactive intermediates will also be operative to a parallel extent in transition states. We examine the effect of substituents of the three kinds discussed above on the four different reactive intermediates, taking as our reference the parent systems [CH3], [CHi]", [CHi] , and [ CH2]. 

In Chapter 8, you will learn more about delocalized electrons, which were introduced in Chapter 2. We also will examine the structural features that cause a compoimd to be aromatic, as well as the features that cause a compoimd to be antiaiomatic. You will see how delocalized electrons influence some of the chemical properties with which you are already familiar, such as p/fg values, the stability of carbocations, and the products obtained from the reactions of certain alkenes. Then we will turn to the reactions of dienes, compounds that have two carbon-carbon double bonds. You will see that if the two double bonds in a diene are sufficiently separated, the reactions of a diene are identical to the reactions of an alkene. If, however, the double bonds are separated by only one carbon-carbon single bond, then electron delocalization will play a role in the products that are obtained. 

It follows from figures presented above that the influence of the structure on the stabilization of the carbocation is very high. For comparison, we can present, e.g., the difference in strengths of C—H bonds for R—H, which (relatively to methane) is equal to 26 for ethane, 38 for propane (in the CH2 group), and 50 kJ/mol for isobutane (in the tert-C—H group). 

A mechanism for the formation of the minor product is presented below. The alkyne attacks ICl, resulting in the formation of a new C-I bond, and a vinyl carbocation (see below for a justification for formation of a vinyl carbocation in this case). This carbocation then serves as an electrophile in an electrophihc aromatic substitution. The n electrons from the methoxyarene attack this carbocation, resulting in the formation of a new C-C bond, and a resonance-stabilized sigma complex. This intermediate is highly conjugated and has at least seven reasonable resonance structures in addition to the four shown. Deprotonation of the sigma complex restores aromaticity, thus yielding the product. 

The remarkable stability of the gold complexes is due to significant metal-metal bonding. However, their isolation and structural study are remarkable and greatly contributed to our knowledge of higher-coordinate carbocations. 

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