Why are α-haloaldehydes unstable?

I'm currently reading through Clayden's *Organic Chemistry*, and in chapter 30 an assertion is made that



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> Halo aldehydes are unstable and should be avoided.
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I should be clear that the context of this situation is under treatment with an enolate, so basic conditions are being employed. I believe the most likely hypothesis is that the instability has to do with the increased electrophilicity of the α-haloaldehyde's carbonyl group. The increased electrophilicity is supported by the equilibrium constant for the hydrate formation from acetaldehyde and chloroacetaldehyde, which is 1 and 37 respectively. This means that the rate of nucleophilic attack on the carbonyl group will be faster. Is it possible that the rate of nucleophilic attack is fast enough for O-attack on the carbonyl (hard-hard) as opposed to the expected C-C attack at the halogen center (soft-soft)? I have also proposed that α-haloaldehyde could eliminate H-Br to yield the ketene. I am less confident in my second proposal, as I have not seen any literature reporting this, and if so, I would expect equilibrium to lie on the side of the α-haloaldehyde.


