Methylhexanes 3-methylhexane, optical activity

Heating optically active (5)-3-bromo-3-methylhexane with aqueous acetone results in the formation of racemic 3-methyl-3-hexanol. 

A special case of isomerization is the racemization of optically active compounds catalyzed, for example, by sulfuric acid or promoted A1C13. Thus, treatment of (+)-(5)-3-methylhexane at 60°C with 96% sulfuric acid yields a mixture of racemic 2- and 3-methylhexane68 (Scheme 4.4). At lower temperature (0 or 30°C), racemization occurs, but shift of the methyl group does not take place. It can be concluded that at 60°C methyl migration is faster than hydride abstraction to yield isomeric alkanes. At 0 or 30°C, hydride transfer occurs before methyl... 

Optically active polyaldehydes possessing optically active side chains, such as poly-(R)(+)-citronellal, poly-(R)(+)-6-methoxy-4-methylhexanal, and poly-(S)(+)-2-methylbutanal, have been prepared by Goodman (1, 22). The optical activity of the polymers was enhanced as compared with their model compounds. It was concluded that the enhancements of the optical activity arose from a conformational rigidity around the asymmetric center in the side chain of the polymer. From degradation studies of the polymers it was concluded that the optical activity of the monomer was unchanged, and no racemization had occurred during polymerization and degradation. 

Ordinary straight heptane has no optical activity, but one of the central carbon atoms in 3-methylhexane is circled because it has four different groups attached to it. It is this asymmetric arrangement around that carbon atom which causes the molecule to form a pair of optically active isomers. The pair of isomers are mirror images of each other. 

What about the configuration at C2, the newly formed chirality center As illustrated in Figure 9.19, the stereochemistry at C2 is established by attack of Br ion on a carbocation intermediate in the usual manner. But this carbocation does not have a plane of symmetry it is chiral because of the chirality center at C4. Since the carbocation has no plane of symmetry, it is not attacked equally well from top and bottom faces. One of the two faces is likely, for steric reasons, to be a bit more accessible than the other face, leading to a mixture of R and S products in some ratio other than 50 50, Thus, two diastereomeric products, (2/ ,4fi)-2-bromo-4-methylhexane and (2S,4/i)-2-bromo-4-methylhexane, are formed in unequal amounts, and the mixture is optically active. 

It was foimd that 3-methylhexane was strongly adsorbed by charcoal in amounts up to about two molecular layers. If greater amounts than this were adsorbed, the excess could be pumped off quite easily, and this material was fmmd to be unaffected in optical activity some of the strongly adsorbed material could be removed by prolonged pumping at 200°, but it could be removed completely only by solvent extraction. 

There is one further interesting and useful feature shown by branched alkanes. 3-Methylhexane (see Table 6.5) has a centre of optical activity, and the (-1-) form is observed to racemise during exchange. - The process over... 

Examples of this type of reaction are Sjyfl reactions in which the leaving group departs from a chirality center. These reacdiions almost always result in extensive and sometimes complete racemization. For example, heating optically active (5)-3-bromo-3-methylhexane... 

There are times when a three-dimensional representation is important, hut the subject is a racemic mixture of the two enantiomers rather than one enantiomer. Unless both enantiomers are drawn, the figure appears to focus only on a single enantiomer. Our molecule, 3-methylhexane, illustrates this point in Figure 4.16. If we are talking about a racemic mixmre of 3-methylhexane but want to show the three-dimensional structure of this molecule, we should, stricdy speaking, draw both of the enantiomers. In practice this is rarely done, and you must be alert for the problem. Unless optical activity is specifically indicated, it is usually the racemate that is meant. 

FIGURE 4.20 Optically active molecules, such as (i )-3-methylhexane, will rotate the plane of plane-polarized light. There are no compensating mirror-image molecules for this kind of molecule. 

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