S-2-Bromobutane

Whatever happens at one enantiotopic face of the double bond of as or trans 2 butene hap pens at the same rate at the other resultrng mail mrxture of R) and (S) 2 bromobutane... 

Draw the reaction of (S)-2-bromobutane with US ion to yield 2-butanethiol, CH3CH2CH(SH)CH3. What is the stereochemistry of the product ... 

Would you expect two enantiomers such as (R)-2-bromobutane and (S)-2-bromobutane to have identical or different IR spectra Explain. 

In the first systematic study on nucleophilic substitutions of chiral halides by Group IV metal anions, Jensen and Davis showed that (S )-2-bromobutane is converted to the (R)-2-triphenylmetal product with predominant inversion at the carbon center (Table 5)37. Replacement of the phenyl substituents by alkyl groups was possible through sequential brominolysis and reaction of the derived stannyl bromides with a Grignard reagent (equation 16). Subsequently, Pereyre and coworkers employed the foregoing Grignard sequence to prepare several trialkyl(s-butyl)stannanes (equation 17)38. They also developed an alternative synthesis of more hindered trialkyl derivatives (equation 18). 

These are also reactions in which inversion of configuration occurs signifying the reaction to be of Sn2 type. For example racemisation of S-2-bromobutane on heating with bromide ions in acetone results in inversion. 

Stereochemistry of the SnI reactions The SnI reaction is not stereospecific. The carbocation produced is planar and 5p -hybridized. For example, the reaction of (S)-2-bromobutane and ethanol gives a racemic mixture, (S)-2-butanol and (R)-2-butanol. 

One-step addition of H-Br to the top face of the double bond gives (S)-2-bromobutane. Addition of H-Br to the bottom face gives (fi)-2-bromobutane. Since 1-butene is achiral, the probability of addition to either face of the double bond is equal, and the product will be racemic (an equal mixture of enantiomers). 

The essential feature of the 8 2 mechanism is that the reaction takes place in a single step without intermediates when the incoming nucleophile attacks the alkyl halide or tosylate (the substrate) from a direction directly opposite the group that leaves. As the nucleophile comes in on one side of the substrate and bonds to the carbon, the halide or tosylate departs from the other side, thereby inverting the stereochemical configuration. The process is shown in Figure 11.3 for the reaction of (S)-2-bromobutane with HO , leading to (/2)-2-butanol. 

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. 

In contrast to the Sn2 reaction, the SnI reaction of (S)-2-bromobutane forms two substitution products—one with the same relative configuration as the reactant and the other with the inverted configuration. In an SnI reaction, the leaving group leaves before the nucleophile attacks. This means that the nucleophile is free to attack either side of the planar carbocation. If it attacks the side from which the bromide ion left, the product will have the same relative configuration as the reactant. If it attacks the opposite side, the product will have the inverted configuration. 

Fig. 10. Stereoview, down the c axis, of the calculated positions for (R)- and (S)-2-bromobutane (heavy and thin line, respectively). Dashed line X-ray model (one equivalent position shown). Dotted line crystallographic twofold axis...

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