Chloropropanes, dehydrochlorination

Deuterium kinetic isotope effects in the dehydrochlorination of 2-chloropropane [183]... 

Alternatively, 1,1,3,3- and 1,2,3,3-tetrachloropropene 21 and 22 or 1,1,2,3,3 penta-chloropropane 23 underwent dehydrochlorination with potassium t-butoxide to give, probably through 1,3,3-trichloropropyne 24, the dichlorovinylidenecarbene 25 which was trapped by olefins to lead to the dichloroethenylidenecyclopropanes 26. Then, the highly reactive allene 26 added electrophiles as well as nucleophiles such as methoxides to give, for instance, l-methoxy(2-chloroethynyl)cyclopropane 27, Eq. (9) 22). 

The optimal route to a particular compound is not always the shortest, as is illustrated by the simple problem of preparing 2-chloropropane on an industrial scale, shown in Scheme 21.1. Although 2-chloropropane is produced from propane in one step by direct chlorination, it must be separated from the 1-chloropropane formed concurrently this is not an easy separation. However, if the mixture of 1- and 2-chloropropane is treated with a base, both isomers undergo dehydrochlorination to give propane, and subsequent addition of hydrogen chloride produces 2-chloro-propane as the sole product. In this case, a three-step process is the preferred way to obtain pure 2-chloropropane. As this simple example demonstrates, an important... 

The unimolecular gas-phase elimination kinetics of 2-methoxy-l-chloroethane, 3-methoxy-l-chloropropane, and 4-methoxy-l-chlorobutane has been studied using density functional theory (DFT) methods. Results calculated for 2-methoxy-l-chloroethane and 3-methoxy-l-chloropropane suggest that the corresponding olefin forms by dehydrochlorination through a concerted nonsynchronous four-centered cyclic transition state. In the case of 4-methoxy-l-chlorobutane, in addition to the 1,2-elimination mechanism, anchimeric assistance by the methoxy group, through a polar five-centered cyclic transition state, provides 4-methoxybutene, tetrahydrofuran, and chloromethane. Polarization of the C-Cl bond is rate limiting in these elimination reactions. 

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