Basic Factors in Isomerization

The equilibrium distributions of butane, pentane, and hexane isomers have been experimentally determined (6,21) and are diagrammed in 

For butane and hexane, the experimental equilibria agree fairly well with equilibria calculated from thermodynamic data (22). However, the pentane isomer favored thermodynamically, neopentane (2,2-dimethyl-propane), is not obtained, possibly because of steric hindrance or the instability of ions intermediate in the reaction mechanism. 

Besides the limiting of isomerization by equilibria, the rate at which equilibrium is approached is also important. Butanes and pentanes 

The rates at which the above conversions take place vary considerably as shown in Table II. The interconversion of 2-methylpentane and 3-methylpentane is extremely rapid while the isomerization of 2,3-dimethylbutane to 2,2-dimethylbutane is very slow. Thus, the ratedetermining step in the formation of 2,2-dimethylbutane (neohexane) from n-hexane is the slow isomerization of 2,3-dimethylbutane to 2,2-dimethylbutane. In the isomerization of w-hexane to 2,3-dimethylbutane, the rate-determining step is the isomerization of n-hexane to the methylpentanes. 

The butanes show little tendency to crack or disproportionate (7) thus butane isomerization is fairly straightforward. However, the suppression of side reactions becomes more difficult as the molecular weight increases. With pentanes, disproportionation to isobutane and hexane is pronounced, amounting to as much as 63%. A typical composition of pentane disproportionation products is shown in Table III. Besides lowering the yield of isopentane, such side reactions shorten the life of the catalyst. Adding small amounts of cyclic hydrocarbons (7,15,18) 

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