Carbocations in electrophilic addition

The orientation of addition of an unsymmetrical adduct, HY or XY, to an unsymmetrically substituted alkene will be defined by the preferential formation of the more stabilised carbanion, as seen above (cf. preferential formation of the more stabilised carbocation in electrophilic addition, p. 184). There is little evidence available about stereoselectivity in such nucleophilic additions to acyclic alkenes. Nucleophilic addition also occurs with suitable alkynes, generally more readily than with the corresponding alkenes. 

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. 

The formation of the sigma complex in electrophilic aromatic substitution has a higher activation energy than the formation of a carbocation in electrophilic addition to an alkene (Figure 13.1). Therefore, the rates of electrophilic aromatic substitution reactions are slower than the rates of electrophilic addition reactions to aUtenes for the same electrophile. For example, bromine reacts instantly with alkenes, but does not react at all with benzene except in the presence of a strong Lewis acid catalyst. 

Later in this chapter we ll see how allylic carbocations are involved in electrophilic addition to dienes and how the principles developed in this section apply there as well. 

Carbocations also feature as intermediates in electrophilic addition reactions (see Section 8.1) and in Friedel-Crafts alkylations (see Section 8.4.1). 

The most reactive alkenes in electrophilic addition are the most thermodynamically stable. This is because they also have the lowest activation energy when forming carbocations. Hydrogen halides add to alkynes in nearly the same way they add to alkenes. 

Carbocations, however formed, are very electrophilic. They react readily with nucleophiles, as shown in reaction (5.18). These reactions are important as steps in electrophilic addition to double bonds and unimolecular nucleophilic substitution (SnI) reactions. 

Why do the adduct carbocations 46 not react with a nucleophile to give an adduct of type 48, as happens in electrophilic addition to double bonds The main reason is probably thermodynamic. The hypothetical reaction (5.32) is very exothermic, based on experimental heats of formation. If we attribute this exothermicity to the stabilization of the benzene nucleus, we obtain a value of 150 kJ mol-1 for this stabilization. This stabilization would be lost in reaction (5.31) if the adduct 48 were formed, but is regained if the substituted product 47 is produced. This stabilization does not apply to reaction with alkenes, so addition can take place if the reaction is favourable energetically. 

The rate-determining step in electrophilic addition to an alkene is reaction of an electrophile with a carbon-carbon double bond to form a carbocation, an ion that contains a carbon with only six electrons in its valence shell and has a positive charge. 

Let us see how reaction rates are related to Markovnikov s Rule. In electrophilic addition reactions, more stable carbocations are formed more rapidly than less stable carbocations. This is because more stable carbocations are lower in energy than less stable carbocations, and it follows that the activation energy for the formation of more stable carbocations is also lower. For example, both isopropyl and propyl cations could be formed from propene and H (eq. 3.21), but the isopropyl cation is more stable (i.e., much lower in energy) than the propyl cation (Figure 3.12). Formation of the isopropyl cation, therefore, has a lower activation energy E and thus, the isopropyl carbocation is formed more rapidly than the propyl cation. Hence, the regioselectivity of electrophilic additions is the result of competing first steps, in which the more stable carbocation is formed at a faster rate. 

Other nucleophiles can also attack the caibocation produced as an intermediate in electrophilic addition. In bromine water, for example, the water molecule is present in a much higher concentration than the bromide ion, and so is the nudecphile more likely to attach to the carbocation (Figure 20.27). 

Both syn and anti addition occur in electrophilic addition reactions that form a carbocation intermediate. 

One of the most striking differences between conjugated dienes and typical alkenes is their behavior in electrophilic addition reactions. To review briefly, the addition of an electrophile to a carbon-carbon double bond is a general reaction of alkenes (Section 7.7). Markovnikov regiochemistry is found because the more stable carbocation is formed as an intermediate. Thus, addition of HCl to 2-methylpropene yields 2-chloro-2-methylpropane rather than l-chloro-2-methylpropane, and addition of 2 mol equiv of HCl to the nonconjugated diene 1,4-pentadiene yields 2,4-dichloropentane. 

In the absence of a good nucleophile, carbocation rearrangements may occur following addition of an electrophile to the alkene double bond (Section 9-3). Rearrangements are favored in electrophilic additions of acids whose conjugate bases are poor nucleophiles. An example is trifluoroacetic acid, CF3CO2H. Its trifluoroacetate counterion is much less nucleophilic than are halide ions. Thus, addition of trifluoroacetic acid to 3-methyl-1-butene gives only about 43% of the normal product of Markovnikov addition. The major product results from a hydride shift that converts the initial secondary cation into a more stable tertiary cation before the trifluoroacetate can attach. 

Finally, in electrophilic addition processes, many of the carbocation rearrangements that we previously met in El and S).jl processes are observed (Figure 11.6). 

Electrophile (Section 4 8) A species (ion or compound) that can act as a Lewis acid or electron pair acceptor an elec tron seeker Carbocations are one type of electrophile Electrophilic addition (Section 6 4) Mechanism of addition in which the species that first attacks the multiple bond is an electrophile ( electron seeker )... 

Electrophilic Addition. In the following example, an a-olefin reacts with a Lewis acid to form the most stable intermediate carbocation. This species, in turn, reacts with the conjugate base to produce the final product. Thus electrophilic addition follows Markovnikov s rule. 

Both steps in this general mechanism are based on precedent. It is called electrophilic addition because the reaction is triggered by the attack of an acid acting as an electrophile on the tt electrons of the double bond. Using the two tt electrons to fonn a bond to an electrophile generates a carbocation as a reactive intennediate normally this is the rate-detennining step. 

In general, alkyl substituents increase the reactivity of a double bond toward electrophilic addition. Alkyl groups are electron-releasing, and the more electron-rich a double bond, the better it can share its tt electrons with an electrophile. Along with the observed regioselectivity of addition, this supports the idea that carbocation fonrration, rather than carbocation capture, is rate-detenrrining. 

When formulating a mechanism for the reaction of alkynes with hydrogen halides, we could propose a process analogous to that of electrophilic addition to alkenes in which the first step is formation of a carbocation and is rate-determining. The second step according to such a mechanism would be nucleophilic capture of the car bocation by a halide ion. 

The regioselectivity of electrophilic addition is governed by the ability of an aromatic ring to stabilize an adjacent carbocation. This is clearly seen in the addition of hydrogen chloride to indene. Only a single chloride is formed. 

Electrophilic addition of HX to an alkene involves a two-step mechanism, the overall rate being given by the rate of the initial protonation step. Differences in protonation energies are usually explained by considering differences in carbocation stability, but the relief or buildup of strain can also be a factor. One of the following alkenes protonates much more easily than the other. 

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