Methylhexanes 3-methylhexane, stereochemistry

Formation of the 2-olefin was then conveniently measured by the percent racemization. Under conventional Oxo conditions the products were 4-methylhexanal (V), 3-ethylpentanal (VI), and 2,3-dimethyl-valeraldehyde (VII) in the ratio 93 4 3 and there was only 1.8% racemization of (V). At higher temperature the racemization of (V) increased to 32.2% at 145° and 93.9% at 180° C. The yield of (V) also dropped off to 61.3% at 180° C. The carbon monoxide pressures in these cases were in the range 71-95 atm. Decreasing the carbon monoxide pressure from 95 to 16 atm at 90° C increased the racemization from 1.8% to 8.7%. These results clearly show that under normal Oxo conditions (V) was mainly formed by a mechanism which did not change the stereochemistry at C3. The degree of racemization can be explained by the reversible formation of a 2-olefin intermediate. Pino et al. concluded that under conventional Oxo conditions, little isomerization occurred. 

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. 

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See also in sourсe #XX -- [ ]

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