Secondary alkyl groups Carbocations

Rearrangements of this type involving carbocation intermediates often occur in Friedel-Crafts alkylations with primary and secondary alkyl groups larger than C2 and C3. Related carbocation rearrangements are discussed in Sections 8-9B and 15-5E. 

The hydrolysis of an ester with a tertiary alkyl group forms the same products as the hydrolysis of an ester with a primary or secondary alkyl group—namely, a carboxylic acid and an alcohol— but does so by a completely different mechanism. The hydrolysis of an ester with a tertiary alkyl group is an SnI reaction rather than a nucleophilic addition-elimination reaction, because the carboxylic acid leaves behind a relatively stable tertiary carbocation. 

Secondary carbocation Tertiary carbocation (G is a migrating group it may be either a hydrogen or an alkyl group)... 

Figure 6.11 A comparison of inductive stabilization for methyl, primary, secondary, and tertiary carbocations. The more alkyl groups there are bonded to the positively charged carbon, the more electron density shifts toward the charge, making the charged carbon less electron-poor (blue in electrostatic potential maps).

Carbocation rearrangements can also occur by the shift of an alkyl group with its ejection pair. For example, reaction of 3,3-dimethyJ-l-butene with HCI Leads to an equal mixture of unrearranged 2-chloro-3,3-dimethyTbutane and rearranged 2-chloro-2,3-dimethyibutane. In this instance, a secondary carbocation rearranges to a more stable tertiary carbocation by the shift of a methyl group. 

Figure 8.2 The structure of a secondary vinylic carbocation. The cationic carbon atom is sp-hybridized and has a vacant p orbital perpendicular to the plane of the tt bond orbitals. Only one R group is attached to the positively charged carbon rather than two, as in a secondary alkyl carbocation. The electrostatic potential map shows that the most positive (blue) regions coincide with lobes of the vacant p orbital and are perpendicular to the most negative (red) regions associated with the ir bond.

Strategy A Friedel-Crafts reaction involves initial formation of a carbocation, which can rearrange by either a hydride shift or an alkyl shift to give a more stable carbocation. Draw the initial carbocation, assess its stability, and see if the shift of a hydride ion or an alkyl group from a neighboring carbon will result in increased stability. In the present instance, the initial carbocation is a secondary one that can rearrange to a more stable tertiary one by a hydride shift. 

Recall that alkyl groups are electron donating, so the carbocation on the bottom (called a tertiary carbocation because it has three alkyl groups) will be more stable than the carbocation on the top (called a secondary carbocation because it has only two alkyl groups). 

Finally we learned that if we analyze the first factor (substrate), we will find two effects at play electroiucs and sterics. We saw that Sn2 reactions require primary or secondary substrates because of sterics—it is too crowded for the nucleophile to attack a tertiary substrate. On the other hand, SnI reactions did not have a problem with sterics, but electronics was a bigger issue. Tertiary was the best, because the alkyl groups were needed to stabilize the carbocation. 

In the step above, Br attacked the alkene at the less substituted carbon, in order to form the more substituted carbon radical (C ). Tertiary radicals are more stable than secondary radicals, for the same reason that tertiary carbocations are more stable than secondary carbocations. Just as alkyl groups donate electron density to... 

If we consider protonation of 2-methylbut-2-ene, then two different carbocations might be formed. One of these is tertiary, and thus favourable, because three electron-donating alkyl groups help to stabilize the cation by dispersing the charge (see Section 6.2.1). The alternative carbocation intermediate is less favourable, in that it is secondary, with just two alkyl... 

Silver halide precipitates at a rate that depends upon the structure of the alkyl group, tertiary > secondary > primary. Tertiary halides usually react immediately at room temperature, whereas primary halides require heating. That complexes actually are formed between organic halides and silver ion is indicated by an increase in water solubility in the presence of silver ion for those halides that are slow in forming carbocations. 

HIAs is only O.lkcalmol-1. This reflects almost complete cancellation of contributions from the extra CH2 group in the butyl structure between the cation and hydrocarbon. It indicates that HIAs provide a good approximation to differences in stability between a carbocation center and the corresponding group contribution from a hydrocarbon, independently of structural variations at carbon atoms not attached to the carbocation center. Moreover, a comparison between two secondary carbocations leads to almost complete cancellation of the contributions from the parent hydrocarbons and from alkyl groups of the carbocations too far removed from the charge center to influence stability. One is very close therefore to a comparison of stabilities comparable to that between isomeric cations. It should be noted that such intrinsic stabilities are not expressed in heats of formation of carbocations because they include uncanceled contributions from more remote portions of the structure. 

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