HCl reacting with a carbonyl compound (say formaldehyde) sounds pretty simple. But often the simpler a thing looks, the more subtle it is under the skin. And this little reaction is actually my prelude to the next post.
The mechanism is studied using ωB97XD/6-311G(d,p) with a simulated solvent (acetic acid) included (but not explicit solvent setting up any hydrogen bonds).
The transition state itself does not convey what is happening, largely because the transition state normal mode is mass-weighted. This leads to the heavier Cl not moving much, and the formaldehyde conducting a bee-dance like wag. For more detail, indeed insight, we need the intrinsic reaction coordinate (IRC):
Is this model a realistic one? Well, the missing component is hydrogen bonds. Between a solvent (this is being done by the way in acetic acid as simulated solvent) and the chlorine, which must assume a large measure of being actually a chloride anion, countered by the oxenium cation. It is possible that the reaction may actually therefore not be concerted, but it might stop at the half-way stage of an ion-pair before continuing its journey. The calculated barrier (~20 kcal/mol) is actually quite reasonable for a thermal reaction, but hydrogen-bond stabilisation might be expected to reduce this to what in effect would correspond to a very fast room-temperature reaction.
Well, HCl + H2C=O does not sound complicated. But you can trust this blog to take something simple and make it less so!
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Does it means that the real reaction could run in non-polar solvents? The reagents are gases and a little bit could be absorbed in hexane...