Chirality center in 2-butanol

Noting the presence of one (but not more than one) chirality center is a simple, rapid way to determine if a molecule is chiral. For example, C-2 is a chirality center in 2-butanol it bears H, OH, CH3, and CH3CH2 as its four different groups. By way of... 

Chirality centers are often designated with an asterisk ( ). The chirality center in 2-butanol is C2 (Figure 5.4). The four different groups attached to C2 are a hydroxyl group, a hydrogen atom, a methyl group, and an ethyl group. (It is important to note that... 

In order of decreasing precedence the four substitu ents attached to the chirality center of 2 butanol are... 

There is another point of nomenclature that must be discussed, namely where a chiral center is involved. Taking the simple case of 2-butanol, CH3CH(OH)CH2CH3, we can explain the point as follows (Scheme 1). Two configurations are possible at the chiral center. In both the R and the S series, three conformations are possible. The +sc form in the R series and the -sc form in the S series are enantiomers and their free energies must be the same under achiral conditions. However, the - sc form in the R and that in the S series differ in free energies. Therefore, it is not sufficient to call a conformation -sc if a chiral center is involved. In this case we may have to call such conformations - sc(R) and — sc(S) to distinguish them. 

Because hydrogenation of the double bond does not involve any of the bonds to the chirality center, the spatial arrangement of substituents in (+)-3-buten-2-ol must be the same as that of the substituents in (+)-2-butanol. The fact that these two compounds have the same sign of rotation when they have the same relative configuration is established by the hydrogenation experiment it could not have been predicted in advance of the experiment. 

The first atoms in two or more substituents often are identical, in which case it is necessary to explore further and compare the atomic numbers of the second attached atoms. Precedence is given to the substituent with a second atom of higher atomic number. For example, in 2-butanol, CH3CH (OH)CH2CH3, two of the groups at the chiral atom have carbon as the first atom. We therefore must compare the other atoms bonded to these two carbons. It is convenient to represent the arrangement at the chiral atom as shown in 7, where the first atoms are shown attached to the chiral center and the second atoms are listed in their priority order thus, (C,H,H) for ethyl and (H,H,H) for methyl ... 

When there is more titan one chiral center in a molecule, the number of possible stereoisomers increases. Since each chiral center can have either the R or S configuration, for a molecule of n chiral centers, there will be 2" possible stereoisomers. Thus 3-pheny 1-2-butanol has two stereogenic centers and four possible stereoisomers. These are shown below with the configuration of each chiral center designated. 

The two methylene hydrogens labeled H and at C3 in 2-butanol are diastereotopic. We can illustrate this by imagining replacement of H or with some imaginary group Q. The result is a pair of diastereomers. As diastereomers, they have different physical properties, including chemical shifts, especially for those protons near the chirality center. 

The diastereotopic nature of H and at C3 in 2-butanol can also be appreciated by viewing Newman projections. In the conformations shown below (Fig. 9.17), as is the case for every possible conformation of 2-butanol, H and experience different environments because of the asymmetry from the chirality center at C2. That is, the molecailar landscape of 2-butanol appears different to each of these diastereotopic hydrogens. H and... 

The GC separation of enantiomeric d- and L-amino acids with non-chiral phases needs their conversion into diastereomeric derivatives. The second chiral center in the molecule [asterisk ( ) in Fig. 8] arises after their (9-esterification by stereochemically pure alcohols [(/ )- or (5)-2-butanol, 2-pentanol, pinacolol, menthol, etc.] or acylation of NH2 groups by chiral reagents, e.g., a-methoxy-a-trifluoromethylphenylacetyl chloride [MTPAC (III)], A-trifluoroacetyl-L-prolyl chloride [A-TFA-L-Pro-Cl (IV)], or the corresponding anhydride V-trifluoroacetylthiazolidine-4-carbonyl chloride (V). 

Illustration of the concept of the stereogenic center in the context of carbon. Whether in a chiral molecule like 2-butanol or an achiral molecule like meso-tartaiic acid, interconversion of two ligands at a stereocenter produces a new stereoisomer. 

Compounds in which one or more carbon atoms have four nonidentical substituents are the largest class of chiral molecules. Carbon atoms with four nonidentical ligands are referred to as asymmetric carbon atoms because the molecular environment at such a carbon atom possesses no element of symmetry. Asymmetric carbons are a specific example of a stereogenic center. A stereogenic center is any structural feature that gives rise to chirality in a molecule. 2-Butanol is an example of a chiral molecule and exists as two nonsuperimposable mirror images. Carbon-2 is a stereogenic center. 

The addition of methylmagnesium iodide to 2-phenylpropanal is stereoselective in producing twice as much syn-3-phenyl-2-butanol as the anti isomer (entry 5). The stereoselective formation of a particular configuration at a new stereogenic center in a reaction of a chiral reactant is called asymmetric induction. This particular case is one in which the stereochemistry can be predicted on the basis of an empirical correlation called Cram s rule. The structural and mechanistic basis of Cramls rule will be discussed in Chapter 3. 

Most of the biochemical reactions that take place in the body, as well as many organic reactions in the laboratory, yield products with chirality centers. Fo example, acid-catalyzed addition of H2O to 1-butene in the laboratory yield 2-butanol, a chiral alcohol. What is the stereochemistry of this chiral product If a single enantiomer is formed, is it R or 5 If a mixture of enantiomers i formed, how much of each In fact, the 2-butanol produced is a racemic mix ture of R and S enantiomers. Let s see why. 

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