Isomer distribution of tertiary

The stereospecificity of the hydrocarbon ozonation was investigated by measuring the isomer distribution of tertiary alcohols formed on ozonating cis- and fmri5-l,2-dimethylcyclohexane and cis- and trans-DecR-lin. Other oxidation products are formed in these ozonations, but the tertiary alcohols are easily separated from these by gas chromatography. Some results are shown in Table II. The observed isomer distributions of tertiary alcohols are independent of time for up to 3 hours ozonation... 

Table II. Isomer Distribution of Tertiary Alcohols from the Ozonation of cis- and mws-Hydrocarbons...

The effect of various additives on the isomer distribution of tertiary alcohols, obtained from ct5-l,2-dimethylcyclohexane, was investigated to determine the reason for the lack of complete stereospecificity in the ozonation. The results are summarized in Table III. None of the addi-... 

The ability of the Fe (DPAH)2/02/PhNHNHPh system (where PhNHNHPh is a mimic for flavin rednctases) to monooxygenate saturated hydrocarbons closely parallels the chemistry of the methane monooxygenase proteins. However, the enzyme oxygenates 2-Me-bntane with an isomer distribution of 82% primary alcohol, 10% secondary, and 8% tertiary. The present model gives a distribution of 21% primary, 29% secondary, and 50% tertiary. Clearly the protein affords a cavity that is selective for -CH3 groups. 

The distributions of products within a certain carbon number fraction are far from equilibrium. In the Cg-fraction, for example, the dimethylhexanes would be thermodynamically favored over the trimethylpentanes, but the latter are predominant. The distribution within the trimethylpentanes is also not equilibrated. 2,2,4-TMP would prevail at equilibrium over the other TMPs, constituting 60-70% of the product, depending on the temperature. Furthermore, 2,2,3-TMP as the primary product is found in less than equilibrium amounts. Qualitatively, the same statement is valid for the other carbon number distributions. Products with a tertiary carbon atom in the 2-position dominate over other isomers in all fractions. 

Whereas step 1 is stoichiometric, steps 2 and 3 form a catalytic cycle involving the continuous generation of carbenium ions via hydride transfer from a new hydrocarbon molecule (step 3) and isomerization of the corresponding carbenium ion (step 2). This catalytic cycle is controlled by two kinetic and two thermodynamic parameters that can help orient the isomer distribution, depending on the reaction conditions. Step 2 is kinetically controlled by the relative rates of hydrogen shifts, alkyl shifts, and protonated cyclopropane formation, and it is thermodynamically controlled by the relative stabilities of the secondary and tertiary ions. (This area is thoroughly studied see Chapter 3.) Step 3, however, is kinetically controlled by the hydride transfer from excess of the starting hydrocarbon and by the relative thermodynamic stability of the various hydrocarbon isomers. 

The fact that constant growth parameters will predict the isomer distribution data reasonably is remarkable. It is not necessary that the kinetic constants governing chain growth are independent of chain length and structure but that certain ratios of these parameters are constant. The fraction of tertiary carbons has been reported to decrease with carbon number beyond Cio (i7). The SCG scheme predicts a maximum and subsequent decrease, but the maxima occur at C12-C14 for products considered in this chapter. For the cobalt product, all schemes predict yields of dimethyl species that are often too large by factors of two to four. The simple schemes with constant growth parameters as described here are unable to predict the isomer distribution sensibly for products from fixed-bed iron (16) and from fixed-bed nickel... 

If one of several possible isomers predominates, a reaction is said to be regioselective. The data for the chlorination of butane and 2-methylpropane indicate that the chlorination of alkanes is not very regioselective. In fact, there doesn t appear to be a simple explanation for the product distribution in these chlorination reactions. For example, in the chlorination of butane, the major product, 2-chlorobutane, arises when a chlorine atom replaces a secondary hydrogen atom rather than a primary hydrogen atom. However, in the case of 2-methylpropane, the major product, l-chloro-2-methylpropane, arises when a chlorine atom replaces a primary hydrogen atom rather than a tertiary hydrogen atom. 

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