Carbocations alkyl, stabilisation

The SN1 mechanism is specially favoured when the polar protic solvent is also a non-basic nucleophile. Therefore, it is most likely to take place when an alkyl halide is dissolved in water or alcohol. Protic solvents are bad for the SN2 mechanism because they solvate the nucleophile, but they are good for the SN1 mechanism. This is because polar protic solvents can solvate and stabilise the carbocation intermediate. If the carbocation is stabilised, the transition state leading to it will also be stabilised and this determines whether the SN1 reaction is favoured or not. Protic solvents will also solvate the nucleophile by hydrogen bonding, but unlike the SN2 reaction, this does not affect the reaction rate since the rate of reaction is independent of the nucleophile. 

Primary benzylic and allylic halides can undergo SN1 reactions because the carbocations are stabilised by resonance. These carbocations have stability similar to that of secondary alkyl carbocations. 

Tertiary carbocations are stabilised by three electron-donating (+1) alkyl groups (Section 4.3.1)... 

A tertiary carbocation is stabilised by three +I alkyl groups (Section 1.6.1)... 

An essential requirement for such stabilisation is that the carbocation should be planar, for it is only in this configuration that effective delocalisation can occur. Quantum mechanical calculations for simple alkyl cations do indeed suggest that the planar (sp2) configuration is more stable than the pyramidal (sp3) by = 84 kJ (20 kcal) mol-1. As planarity is departed from, or its attainment inhibited, instability of the cation and consequent difficulty in its formation increases very rapidly. This has already been seen in the extreme inertness of 1-bromotriptycene (p. 87) to SN1 attack, due to inability to assume the planar configuration preventing formation of the carbocation. The expected planar structure of even simple cations has been confirmed by analysis of the n.m.r. and i.r. spectra of species such as Me3C SbF6e they thus parallel the trialkyl borons, R3B, with which they are isoelectronic. 

Alkyl groups are said to have a positive inductive effect. This means they are electron-donating and can push electrons onto the positively charged carbon atom, thus stabilising the carbocation. It follows that tertiary carbocatlons, with their three alkyl groups, are the most stable species and that primary carbocatlons, with just one alkyl group, are the least stable species. This suggests that tertiary haloalkanes are most likely to react with a nucleophile via an Sj. 1 mechanism. 

The carbocations that are formed to generate these two products are shown on the left. You will recall that alkyl groups can exert a positive inductive effect (see p. 59), i.e. they can push electrons towards the positively charged carbon atom in the carbocation and so stabilise it. Therefore carbocation A will be more stable than carbocation B because it has two alkyl groups directly attached to the positively charged carbon atom, whereas there is only one alkyl group in carbocation B. 

Alcohols, phenols, ethers. The molecular ion of alcohols is weak or undetectable. Characteristic ions result from alpha-cleavage giving rise to resonance-stabilised carbocations ions the loss of the largest alkyl group is the preferred pathway although ions resulting from losses of the other groups may also be observed. 

Direct addition of a hydrogen halide to an alkene gives rise to an alkyl halide, the order of reactivity being HI>HBr>HCl. In the case of an unsymmetrical alkene, the regioselectivity of the reaction may be predicted from the mechanism of the reaction. Thus, the carbocation which is the most stabilised by charge dispersal will be the one which is formed preferentially. Classically the mode of addition is described as proceeding in the Markownikoff manner. 

The carbocation intermediate can be stabilised by neighbouring alkyl groups through inductive and hyperconjugation effects. However, it can also be stabilised by sharing the positive charge with the bromine atom and a second carbon atom. 

The mechanism of the Friedel-Crafts acylation is the same as the Friedel-Crafts alkylation. It involves an acylium ion instead of a carbocation. Like Friedel-Crafts alkylation, a Lewis acid is needed to generate the acylium ion (R-C = 0) but unlike a carbocation the acylium ion does not rearrange since there is resonance stabilisation from the oxygen ... 

Electronic factors also help in the formation of the carbocation because the positive charge can be stabilised by the inductive and hyperconjugative effects of the three alkyl groups ... 

Groups such as alkyl [16] and aryl [17], double bonds [18], oxygen [19] and sulphur [20] (that are known to stabilise carbocations), when attached to the carbon centres that are undergoing halogen exchange, activate the process (Figure 2.7). 

Vinylic carbocations are generally less stable than alkyl carbocations, as there are fewer +1 alkyl groups to stabilise the positive charge. As a consequence, alkynes (which give vinylic carbocations) generally react more slowly than alkenes (which give alkyl carbocations) in electrophilic addition reactions. 

Just as different substituents are able to stabilise carbocations by releasing electrons towards them, substituents can release electrons into double bonds or draw them out from it. Electron donating groups such as alkyl groups and ethers will increase the electron density of double bonds to which they are attached. Therefore, in hydrocarbons the more heavily substituted double bonds will be more electron rich. So, if there are two or more double bonds in a molecule, electron deficient reagents such as ozone or peracids will preferentially attack the more/most electron rich olefin. 

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