Carbocation achiral

Because an S jl reaction occurs through a carbocation intermediate, its stereochemical outcome is different from that of an S 2 reaction. Carbocations, as we ve seen, are planar, sp2-hybndized, and achiral. Thus, if we carry out an S jl reaction on one enantiomer of a chiral reactant and go through an achiral carbocation intermediate, the product must be optically inactive (Section 9.10). The symmetrical intermediate carbocation can react with a nucleophile equally well from either side, leading to a racemic, 50 50 mixture of enantiomers (Figure 11.10). 

Figure 5. Stereochemistry of the addition of HBr to 1-butene the intermediate achiral carbocation is attacked equally well from both top and bottom, leading to a racemic product mixture.

When the halide ion reacts with this achiral carbocation in the second step, reaction is equally likely at either face. 

Butene accepts a proton from HX to form an achiral carbocation. 

Stepwise solvolysis of chiral substrates through a planar achiral carbocation reaction intermediate normally results in the formation of racemic products. However, the solvolysis of chiral tertiary derivatives 6-Y proceeds with either... 

In contrast, when R—X in Scheme 4.4 is enantiomerically enriched endo-2-norbornyl brosylate (2 in Fig. 4.5) under the same reaction conditions, solvolysis is still accompanied by racemisation, but the rate of racemisation is identical with that of product formation [14]. The achiral carbocation in this reaction, therefore, is captured by solvent much faster than it can undergo internal return, i.e. k2 i in Scheme 4.4. In this case, therefore, the initial ionisation is the rate-determining step. 

Racemization. An asymmetric carbon atom undergoes racemization when it ionizes to a planar, achiral carbocation. A nucleophile can attack the carbocation from either face, giving either enantiomer of the product. 

Attack by acetate can occur on either side of the planar, achiral carbocation intermediate, resulting in a mixture of both the R and S enantiomeric acetates. The ratio of enantiomers is probably close to 50 50. 

The chiral tertiary alcohol (/ )-3-methyl-3-hexanol reacts with HBr by an SnI pathway. HBr protonates the hydroxyl group, which dissociates to yield a planar, achiral carbocation. Reaction with the nucleophilic bromide anion can occur from either side of the carbocation to produce ( )3-bromo-3-methylhexane. 

Stereochemistry develops in nucleophilic solvents where carbocations are shortlived intermediates. Any observation of stereospecificity excludes the intervention of achiral carbocations. Chirality may be conferred on otherwise planar carbocations by their environment. Ion pairing is the best known example of asymmetric solvation (Section 4). Incorporation of a carbocation in micelles has recently been recognized as another way of controlling its stereochemistry (Section 5). 

FIGURE 7.8 Electrophilic addition of hydrogen bromide to ( ) and (Z)-2-butene proceeds by way of an achiral carbocation, which leads to equal quantities of (/ )- and (S)-2-bromobutane. 

The product, therefore, can exist as a pair of enantiomers. The question now arises as to how these enantiomers are formed. Is one enantiomer formed in greater amount than the other The answer is no-, the carbocation that is formed in the first step of the addition (see the following scheme) is trigonal planar and is achiral (a model will show that it has a plane of symmetry). When the halide ion reacts with this achiral carbocation in the second step, reaction is equally likely at either face. The reactions leading to the two enantiomers occur at the same rate, and the enantiomers, therefore, are produced in equal amounts as a racemic mixture. 

Experiments in which nucleophilic substitution takes place at a chiral center provide us with information about the stereochemical course of the reaction. One of the compounds studied to determine the stereochemistry of an S l reaction utilized the following chloroalkane. When either enantiomer of this molecule undergoes nucleophilic substitution by an S l pathway, the product is racemic. The reason is that ionization of this secondary chloride forms an achiral carbocation. Attack of the nucleophile can occur from either side of the planar carbocation carbon, resulting... 

Because a planar and achiral carbocation intermediate is formed that can be attacked with roughly equal probability from either face, reaction at a chiral center results in racemization of stereochemistry. 

Stereochemical analysis can add detail to the mechanistic picture of the S l substitution reaction. The ionization mechanism results in formation of a carbocation intermediate which is planar because of its sp hybridization. If the carbocation is sufficiently long-lived under the reaction conditions to diffuse away from the leaving group, it becomes symmetrically solvated and gives racemic product. If this condition is not met, the solvation is dissymmetric, and product with net retention or inversion of configuration may be obtained, even though an achiral carbocation is formed. The extent of inversion or retention depends upon the details of the system. Examples of this effect will be discussed in later sections of the chapter. 

The mechanism of hydrohalogenation illustrates why two enantiomers are formed. Initial addition of the electrophile (from HCl) occurs from either side of the planar double bond to form a car-bocation. Both modes of addition (from above and below) generate the same achiral carbocation. Either representation of this carbocation can then be used to draw the second step of the mechanism. 

Figure 7-3 The mechanism of hydrolysis of (S)-(1-bromoethyl)benzene explains the stereochemistry of the reaction. Initial ionization furnishes a planar, achiral carbocation. This ion, when trapped with water, yields racemic alcohol.

In the preceding section, we saw that Sj 2 reactions at chiral centers occur with inversion of configuration. In contrast, an S l reaction at a chiral center reaction usually gives a mixture of enantiomers. For example, (5)-3-bromo-3-methylhexane reacts with water, a poor nucleophile, to give a racemic mixture of 3-methyl-3-hexanol. The reaction occurs by way of an achiral carbocation intermediate with a plane of symmetry (Figure 10.4). Because the carbocation intermediate has a plane of symmetry, the nucleophile can attack equally well from either side of the plane to give a racemic mixture. 

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