Haloalkanes relative reactivities

The rates of 1 reactions decrease in the order tertiary > secondary > primary methyl. This trend is exactly the reverse of the trend observed in Sj 2 reactions. The relative reactivity of haloalkanes in Sj 2 reactions corresponds to the relative stability of carbocation intermediates that form during the reaction. We recall from Chapter 3 that the order of stability of carbocations is tertiary > secondary > primary methyl. A relatively stable tertiary carbocation forms faster than a less stable secondary carbocation, which in turn forms very much faster than a highly unstable primary carbocation. However, Sj l mechanisms are possible at primary centers that are resonance stabilized, such as allyl and benzyl. 

When enantiomerically pure (/ )- and (S)-ll were reacted with racemic 1-bromo-l-phenylethane (13), d.r. s of 97 3 were obtained. The somewhat lower d.r. obtained on reaction of the resolved complexes was explained as a mass action effect during the course of the reaction the reactive enantiomer of the haloalkane is depleted relative to the less reactive enantiomer. 

The reactivity order also appears to correlate with the C-X bond energy, inasmuch as the tertiary alkyl halides both are more reactive and have weaker carbon-halogen bonds than either primary or secondary halides (see Table 4-6). In fact, elimination of HX from haloalkenes or haloarenes with relatively strong C-X bonds, such as chloroethene or chlorobenzene, is much less facile than for haloalkanes. Nonetheless, elimination does occur under the right conditions and constitutes one of the most useful general methods for the synthesis of alkynes. For example,... 

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