Butane chiral molecules

Only reaction 1 provides a direct pathway to this chiral molecule the intermediate 2-methyl-butanal may be silylated and reacted with formaldehyde in the presence of the boronated tartaric ester described on page 61. The enantiomeric excess may, however, be low. 

M. Mons, E Piuzzi, I. Dimicoli, A. Zehnacker, and F. Lahmani, Binding energy of hydrogen bonded complexes of the chiral molecule 1 phenylethanol, as studied by 2C R2PI Comparison between diastereoisomeric complexes with butan 2 ol and the singly hydrated complex. Phys. Chem. Chem. Phys. 2, 5065 5070 (2000). 

A wide variety of substituted y-butyrolactones can be prepared directly from olefins and aliphatic carboxylic acids by treatment with manganic acetate. This procedure is illustrated in the preparation of 7-( -OCTYL)-y-BUTYROLACTONE. Methods for the synthesis of chiral molecules are presently the target of intensive investigation. One such general method developed recently is the employment of certain chiral solvents as auxiliary agents in asymmetric synthesis. The preparation of (S.SM+H, 4-BIS(DIMETHYLAMINO)-2,3-DIMETHOXY-BUTANE FROM TARTARIC ACID DIETHYL ESTER provides a detailed procedure for the production of this useful chiral media an example of its utility in the synthesis of (+)-(/ )-l-PHENYL-l-PEN-TANOL from benzaldehyde and butyllithium is provided. 

Even a simple molecule such as //-butane has many chiral conformations. When rotation about the C-C bond is fixed, enantiomers of n-butane exist (Figure 3.1a). Chiral n-butane, even if it could be isolated, readily racemizes through C-C bond rotation with a barrier of around 3 kcal/mol. The corresponding half-life of racemization is expected to be shorter than 10-9 s at —78°C. The molecule behaves as an achiral molecule beyond this time scale. 2,2/-Dimethylbiphenyl is another example of a chiral molecule in a limited time (Figure 3.1//). Enantiomers with axial chirality exist when rotation of the C(1)-C(T) bond is restricted. Racemization takes place through C-C bond... 

The parent alkane is butane. The lUPAC name of this thiol is 2-butanethiol. Its common name is sec-butyl mercaptan. It is a chiral molecule due to the stereocenter at C-2. However, the stereochemical configuration was not indicated here. 

Stereoisomers are either diastereomers or enantiomers therefore, we also can apply these terms to conformational isomers. The gauche and anti forms of butane are diastereomers because they are not mirror images. The two gauche forms of butane are enantiomers because they are mirror images and not superposable (see the reflection in the mirror given). Hence, these forms of butane are chiral. However, butane is not a chiral molecule because these three isomers interconvert very rapidly at room temperature and because they interconvert through the intermediacy of the anti isomer, which is achiral (refer back to Figure 2.9 to see the interconver-sion). The anti isomer is achiral because there is a plane of symmetry when all four carbons are planar. 

There are many chiral molecules for which enantiomeric forms can be interconverted by a rotation about a single bond. The enantiomeric conformations of gauche butane provide an example, where rapid rotation interconverts the two under most conditions. If the rotation that interconverts a pair of such enantiomers is slow at ambient temperature, however, the two enantiomers can be separated and used. Recall from our first introduction of isomer terminology (Section 6.1) that stereoisomers that can be interconverted by rotation about single bonds, and for which the barrier to rotation about the bond is so large that the stereoisomers do not interconvert readily at room temperature and can be separated, are called atropisomers. One example is the binaphthol derivative shown in the margin. It is a more sterically crowded derivative of the biphenyl compound discussed previously as an example of a chiral molecule with no "chiral center". A second example is frans-cyclooctene, where the hydrocarbon chain must loop over either face of the double bond (Eq. 6.4). This creates a chiral structure, and the enantiomers interconvert by moving the loop to the other side of the double bond. 

The fourth possibility arises in chiral molecules, such as i -butan-2-ol. The two -CH2- hydrogens at C3 are neither homotopic nor enantiotopic. Since replacement of a hydrogen at C3 would form a second chirality center, different diastereomers (Section 5.6) would result depending on whether the pro-R or pro-S hydrogen were replaced. Such hydrogens, whose replacement by X leads to different diastereomers, are said to be diastereotopic. Diastereotopic hydrogens are neither chemically nor electronically equivalent. They are different and would likely show different NMR absorptions. 

The radical bromination of butane at C2 creates a chiral molecule (Figure 5-3). This happens because one of the methylene hydrogens is replaced by a new group, furnishing a stereocenter—a carbon atom with four different substituents. 

Addition to double bonds is not the only kind of reaction that converts an achiral molecule to a chiral one Other possibilities include substitution reactions such as the formation of 2 chlorobutane by free radical chlorination of butane Here again the prod uct IS chiral but racemic... 

It may be observed that the gauche conformation of butane (L) or any other similar molecule is chiral. The lack of optical activity in such compounds arises from the fact that L and its mirror image are always present in equal amounts and interconvert too rapidly for separation. 

J. Paul, I. Hearn, and B. J. Howard, Chiral recognition in a single molecule A study of homo and heterochiral butan 2,3, diol by Fourier transform microwave spectroscopy. Mol. Phys. 105, 825 839 (2007). 

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. 

To differentiate between these alternative mechanisms, H. C. Brown, M. S. Kharasch, and T. H. Chao, working at the University of Chicago, carried out the photochemical halogenation of optically active S-(H-)-l-chloro-2-methylbutanc. A number of isomeric products were, of course, formed, corresponding to attack at various positions in the molecule. Problem What were these products ) They focused their attention on just one of these products i,2--dichloro-2-mcthyl-butane, resulting from substitution at the chiral center (C-2). 

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