1.1.2- Trichloro-ethane elimination reactions

In this, as in many other cases in aqueous solution, OH" plays the role of the base. Note that for compounds such as 1,1,2,2-tetrachloroethane and pentachloroethane, the base catalyzed reaction is important at quite low pH values (/NB = 4.5, i.e., pH at which the neutral and base catalyzed reaction are equally important, see Table 13.7 and Section 13.3). In fact, for polyhalogenated alkanes a small7NB value (e.g., <7) is indicative of an E2 reaction, or, in special cases, of an E1CB reaction see below. Some other examples of compounds reacting by an E2-mechanism include 1,1,2-trichloro-ethane, 1,1,2-tribromoethane, and l,2-dibromo-3-chloroethane (see Table 13.7). A high /NB value (e.g., >10) does not, however, necessarily exclude ( elimination, because this reaction may also occur with water as base, or by an alternative to the SN1 mechanism (i.e., an El mechanism, see below). 

As indicated in Table 13.7,1,2-dibromoethane (BrCH2-CH2Br) and 1,1,1-trichloro-ethane (CH3-CC13) are examples in which both hydrolysis and elimination are important. If in such cases the reactions occur by SN2 and E2 mechanisms, respectively, the ratio of the hydrolysis versus elimination products should vary with varying pH and temperature, since the two competing reactions likely exhibit different pH and temperature dependencies. On the other hand, if the reaction mechanisms were more SN1- and El-like, a much less pronounced effect of temperature or pH on product formation would be expected, since the rate-determining step in aqueous solution may be considered to be identical for both reactions ... 

Iron(II) porphyrins react readily with haloalkanes in the presence of reducing agents, e.g., excess iron powder, to give chlorocarbene complexes 15,32). With the insecticide DDT [2,2-bis(4-chlorophenyl)-1,1,1 -trichloro-ethane, (C1C6H4)2CHCC13], the reaction proceeds one step further, by elimination of HC1 from the carbene complex, to give diarylvinylidene complexes 33) ... 

The three different second-order processes thus exhibit widely different kinetic behaviour towards the varying base concentration at constant buffer ratio. In theory this dependence should provide a means of assigning the mechanism. An advantage over the isotopic exchange approach is that it should be possible to detect carbanion intermediates that eliminate more rapidly than they protonate. Unfortunately, the kinetics are not always clear-cut. The E2 mechanism can, under certain conditions, follow specific base catalysis, especially if one base is of much greater catalytic efficiency than the other bases present (e.g. the E2 reaction of l,l,l-trichloro-2,2-di-p-chlorophenyl-ethane with sodium thiophenoxide in methanol) . Alternatively, the base may be sufficiently powerful to produce a kinetically significant concentration of lyate ions (e.g. the E2 reaction of alkyl bromides with phenoxides in ethanol) " . 

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