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    It is known that aldehydes without alpha hydrogens undergo cannizzaro reaction. However, Trichloracetaldehyde undergoes haloform reaction in strong base instead of cannizzaro. Any explanation....?
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    (Original post by adianadiadi)
    It is known that aldehydes without alpha hydrogens undergo cannizzaro reaction. However, Trichloracetaldehyde undergoes haloform reaction in strong base instead of cannizzaro. Any explanation....?
    Well, think about what's different about trichloracetaldehyde and another aldehyde without alpha hydrogens. In the final step of the cannizzaro mechanism what else could take place aside from a hydride shift?
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    (Original post by adianadiadi)
    It is known that aldehydes without alpha hydrogens undergo cannizzaro reaction. However, Trichloracetaldehyde undergoes haloform reaction in strong base instead of cannizzaro. Any explanation....?
    It's all about the ability of the leaving group. Take for example a Cannizzaro with benzaldehyde and a haloform reaction with Chloral. (Both using NaOH as the base/nucleophile)

    In both cases the first step (ie addition to the carbonyl to form a tetrahedral intermediate) is rapid and effectively under equilibrium.

    What then happens to this tetrahedral intermediate is all down to the ability of the three possible leaving groups.

    In both cases, the most likely result is that in OH- that just added will leave again, but this doesn't give any new products so that's a rather boring result.

    The benzaldehyde intermediate can therefore lose either phenyl- or H- to form products, of those hydride is favoured because it is able to directly transfer to another benzaldehyde and complete the disproportionation.

    For Chloral the choice is either loss of H- or -CCl3, of those, the latter is clearly more stable and hence the cannizzaro is far too slow to compete with the chloroform reaction.

    A rough scale for leaving group ability is provided by pKa values, a lower pKa means a compound is more acidic and therefore the compounds conjugate base is a better leaving group (in general, although the correlation isn't always perfect)

    In this case:

    C6H6 <---> C6H5- + H+ pKa = 43

    H2 <---> H+ + H- pKa = 36

    CHCl3 <---> CCl3- + H+ pKa = 15.5

    so in each of the two reactions the more stable leaving group (ie the conjugate base of the most acidic compound above) leaves.
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