2- Bromobutane chirality

In a second example addition of hydrogen bromide converts 2 butene which is achiral to 2 bromobutane which is chiral But as before the product is racemic because... 

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). 

With n dissimilar chiral atoms the number of stereoisomers is 2" and the number of racemic forms is 2" as illustrated below for 2-chloro-3-bromobutan.e (n = 2). The R,S configuration is shown next to... 

Bromination of alkanes follows the same mechanism as chlorination. The only difference is the reactivity of the radical i.e., the chlorine radical is much more reactive than the bromine radical. Thus, the chlorine radical is much less selective than the bromine radical, and it is a useful reaction when there is only one kind of hydrogen in the molecule. If a radical substitution reaction yields a product with a chiral centre, the major product is a racemic mixture. For example, radical chlorination of n-butane produces a 71% racemic mixture of 2-chlorobutane, and bromination of n-butane produces a 98% racemic mixture of 2-bromobutane. 

As stated above, intermolecular coupling reactions between carbon atoms are of limited use. In the classical Wurtz reaction two identical primary alkyl iodide molecules are reduced by sodium. n-Hectane (C100H202), for example, has been made by this method in 60% yield (G. Stallberg, 1956). The unsymmetrical coupling of two alkyl halides can be achieved via dialkylcuprates. The first halide, which may have a branched carbon chain, is lithiated and allowed to react with copper(I) salts. The resulting dialkylcuprate can then be coupled with alkyl or aryl iodides or bromides. Although the reaction probably involves radicals it is quite stereoselective and leads to inversion of chiral halides. For example, lithium diphenyl-cuprate reacts with (R)-2-bromobutane with 90% stereoselectivity to form (S)-2-phenylbutane (G.M. Whitesides, 1969). 

Therefore, there are five different types of hydrogens in 2-bromobutane. However, the chemical shifts of the diastereotopic hydrogens on C-3 will be very similar. In general, the two hydrogens of a CH2 group are diastereotopic when a chirality center is present. 

Bromobutane is chiral by virtue of an asymmetric carbon atom (chiral carbon atom), marked by an. ... 

Most of (Jie biochemical reactions that take place in the body and manj organic reactions in tbe labaratory yield products chirality centers. Fw example, addition of HBr ta l cule. What predictions can we make about the stereochemistry of this rtu-rai product If a single enantianrer is formed, is it A or 5 If a noixtute oS enantiomers id formed, how much -of each In fact, the 2-bromobutane pre duerd is a racemic mixture of R and S enancioiners. Let s sec why. 

Let s now examine chlorination of the chiral starting material (/ )-2-bromobutane at C2 and C3. 

To understand why a racemic product results from the reaction of HBr with 1-butene, think about how the reaction occurs. 1-Butene is first proto-nated to yield an intermediate secondary (2") carbocation. Since the trivalent carbon is sp -hybridized and planar, the cation has no chirality centers, has a plane of symmetry, and is achiral. As a result, it can react with Br ion equally well from either the top or the bottom. Attack firom the top leads to (S)-2-bro-mobutane, and attack from the bottom leads to (i )-2-bromobutane. Since both pathways occur with equal probability, a racemic product mixture results (Figure 9.15). 

As seen in Figure 7.8, the bonds to the positively charged carbon are coplanar and define a plane of symmetry in the carbocation, which is achiral. The rates at which bromide ion attacks the carbocation at its two mirror-image faces are equal, and the product, 2-bromobutane, although chiral, is optically inactive because it is formed as a racemic mixture. 

When a reactant is chiral but optically inactive because it is racemic, any products derived from its reactions with optically inactive reagents will be optically inactive. For example, 2-butanol is chiral and may be converted with hydrogen bromide to 2-bromobutane, which is also chiral. If racemic 2-butanol is used, each enantiomer will react at the same rate with the achiral reagent. Whatever happens to (7 )-( )-2-butanol is mirrored in a corresponding reaction of (5)-(-b)-2-butanol, and a racemic, optically inactive product results. 

Nonsuperimposable mirror-image molecules are called enantiomers (from the Greek enantion, which means opposite ). The two stereoisomers of 2-bromobutane are enantiomers. A molecule that has a nonsuperimposable mirror image, like an object that has a nonsuperimposable mirror image, is chiral. Each of the enantiomers is chiral. A molecule that has a superimposable mirror image, like an object that has a superimposable mirror image, is achiral. To see that the achiral moleule is superimposable on its mirror image (i.e., they are identical molecules), mentally rotate the achiral molecule clockwise. Notice that chirality is a property of the entire molecule. 

---