Chlorination of higher alkanes

Chlorine does not discriminate among the different types of hydrogen atoms in a way that makes chlorination of higher alkanes a generally useful laboratory synthesis. 

The differences in the rates with which primary, secondary, and tertiary hydrogen atoms are replaced by chlorine are not large, however. Chlorine, as a result, does not discriminate among the different types of hydrogen atoms in a way that makes chlorination of higher alkanes a generally useful laboratory synthesis. (Alkane chlorinations do find use in some industrial processes, especially in those instances where mixtures of alkyl chlorides can be used.)... 

Chlorination of Higher Alkanes Relative Reactivity and Selectivity CHAPTER 3... 

CHLORINATION OF HIGHER ALKANES RELATIVE REACTIVITY AND SELECTIVITY... 

Chlorination of most of higher alkanes gives a mixture of isomeric monochloro products as well as more highly halogenated compounds. 

The chlorination of higher-molecular-weight alkanes yields a mixture of isomeric monochlorinated products. For example, the chlorination of butane or 2-methyl-propane, which have nonequivalent hydrogen atoms, yields significant amounts of isomeric monochlorinated derivatives. 

The transformation of l-methylthio-l-(methylsulfonyl)alkanes (254) to methyl esters can be efficiently carried out by oxidation or by a-chlorination followed by methanolysis (equation 152)145. The lithium or the sodium salt of (phenylsulfonyl)nitromethane (256) is a very useful reagent for the preparation of higher homologues of nitromethanes by alkylation since the salts are air insensitive, non-hygroscopic, and easily handled without decomposition. The oxidation of the resulting secondary a-nitro sulfone (257) gives... 

The much higher yields of 1-chloropropane than 2-chIoropropane reported by Gol dshleger et al. (34) do not arise necessarily from preferred attack at the terminal carbon of the alkane, as the internal isomers are themselves oxidized faster than the terminal isomer. If 1-chlorohexane or a mixture of 2- and 3-chlorohexanes was used as the reactant, then, when the 2- and 3-isomers had been consumed, 75% of the 1-isomer still remained (84). The ultimate oxidation product, carbon dioxide, was not formed, and it is thought that the major product from alkane oxidation are polychlorinated carboxylic acids formed by chlorination and reaction with the solvent. These acids are difficult to find in the reaction mixture and despite strenuous efforts have not been identified. 

The solvent must be nonmiscible with water and should have a boiling point as low as possible. Solvents such as dichloromethane (higher density than water) and diethyl ether, pentane, or the mixture of both (lower density than water) are commonly used. Freons, chlorinated solvents, or alkanes can be used. Toxic solvent should be avoided. 

In methane, all four hydrogen atoms are identical, and it does not matter which hydrogen is replaced. In the higher alkanes, replacement of different hydrogen atoms may lead to different products. In the chlorination of propane, for example, two monochlorinated (just one chlorine atom) products are possible. One has the chlorine atom on a primary carbon atom, and the other has the chlorine atom on the secondary carbon atom. 

Chlorination of alkanes, Photoinduced chlorination of alkanes with IBD is a well established reaction (1, 506) with the observed selectivity of tertiary> secondary primary positions. The reaction can also be induced with tri-alkylboranes. The reactivities of both reagents are similar, but yields are somewhat higher in the trialkylborane catalyzed reaction. 

The reported values for PCA-1, and in particular PCA-70, were higher than their respective true values. It is not clear why results for the PCA-70 mixture, whose GC profile and composition are similar to those of the PCA-60 standard, were less accurate then the results for the PCA-1 sample, whose GC profile and composition were quite different to the external standard. One possible explanation could be the amount of additives/stabilizers used by the manufactures, which are not measurable using ECNI or ECD detection. This makes the preparation of standard solutions from commercial products problematic for quantitation of PCAs and suggests that only pure PCA commercial formulations or synthetic mixtures prepared by free-radical chlorination of pure n-alkanes should be used for the preparation of external standards. 

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