318 CHAPTER EIGHT Nucleophilic Substitution H CH3 Bimolecular transition state for hydrolysis of ethyl bromid 8.10 STEREOCHEMISTRY OF SN1 REACTIONS Although SN2 reactions are stereospecific and proceed with inversion of configuration at carbon, the situation is not as clear-cut for SNl reactions. When the leaving group is attached to the stereogenic center of an optically active halide ionization gives a carbo- cation intermediate that is achiral. It is achiral because the three bonds to the positively charged carbon lie in the same plane, and this plane is a plane of symmetry for the car- bocation. As shown in Figure 8.7, such a carbocation should react with a nucleophile at the same rate at either of its two faces. We expect the product of substitution by the SNl mechanism to be racemic and optically inactive. This outcome is rarely observed in prac- tice, however. Normally, the product is formed with predominant, but not complete, on of config For example, the hydrolysis of optically active 2-bromooctane in the absence of added base follows a first-order rate law, but the resulting 2-octanol is formed with 66% (R)-(-)-2-Bromooctane (S)-(+)-2-Octanol (R)-(-)-2-0ctanol to83%S,17%R FIGURE 8.7 Forma- 8 tion of a racemic product by nucleophilic substitution a carbocation intermediate 50% Back Forward Main MenuToc Study Guide ToC Student o MHHE Website8.10 STEREOCHEMISTRY OF SN1 REACTIONS Although SN2 reactions are stereospecific and proceed with inversion of configuration at carbon, the situation is not as clear-cut for SN1 reactions. When the leaving group is attached to the stereogenic center of an optically active halide, ionization gives a carbo￾cation intermediate that is achiral. It is achiral because the three bonds to the positively charged carbon lie in the same plane, and this plane is a plane of symmetry for the car￾bocation. As shown in Figure 8.7, such a carbocation should react with a nucleophile at the same rate at either of its two faces. We expect the product of substitution by the SN1 mechanism to be racemic and optically inactive. This outcome is rarely observed in prac￾tice, however. Normally, the product is formed with predominant, but not complete, inversion of configuration. For example, the hydrolysis of optically active 2-bromooctane in the absence of added base follows a first-order rate law, but the resulting 2-octanol is formed with 66% inversion of configuration. (R)-()-2-Bromooctane C H CH3 CH3(CH2)5 Br (R)-()-2-Octanol C H CH3 CH3(CH2)5 OH (S)-()-2-Octanol H CH3 (CH2)5CH3 HO C 66% net inversion corresponds to 83% S, 17% R H2O ethanol Bimolecular transition state for hydrolysis of ethyl bromide C Br CH3 O H H   H H 318 CHAPTER EIGHT Nucleophilic Substitution 50% 50%

FIGURE 8.7 Forma￾tion of a racemic product by nucleophilic substitution via a carbocation intermediate. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website