Some intermediates proposed in reaction mechanisms look so unlikely that it is comforting if they can be isolated and their structure determined. We feel more confident in proposing an intermediate if we are sure that it can really be made. Of course, this is not necessarily evidence that the intermediate is actually formed during reactions and it certainly does not follow that the failure to isolate a given intermediate disproves its involvement in a reaction. We shall use ketene as an example. Ketene looks pretty unlikely! It is CH₂=C=O with two π bonds (C=C and C=O) to the same carbon atom. The orbitals for these π bonds must be orthogonal because the central carbon atom is sp hybridized with two linear σ bonds and two p orbitals at right angles both to the σ bonds and to each other. Can such a molecule exist? When acetone vapour is heated to very high temperatures (700–750 °C) methane is given off and ketene is supposed to be the other product. What is isolated is a ketene dimer (C4H4O2) and even the structure of this is in doubt as two reasonable structures can be written.

The spectra fit the ester structure well, but not the more symmetrical diketone structure at all. There are three types of proton (cyclobuta-1,3-dione would have just one), with allylic coupling between one of the protons on the double bond and the CH₂ group in the ring. The carbonyl group has the shift (185 ppm) of an acid derivative (not that of a ketone, which would be about 200 ppm) and all four carbons are different.

Ozonolysis of ketene dimer gives a very unstable compound that can be observed only at low temperatures (–78 °C or below). It has two carbonyl bands in the IR and reacts with amines to give amides, so it looks like an anhydride (Chapter 10). Can it be the previously unknown cyclic anhydride of malonic acid? The two carbonyl bands are of high frequency, as would be expected for a four-membered ring—using the table on p. 413 we estimate 1715 + 50 cm⁻¹ (for the anhydride) + 65 cm⁻¹ (for the four-membered ring) = 1830 cm−1. Both the proton and the carbon NMR are very simple: just a 2H singlet at 4.12 ppm, shifted downfi eld by two carbonyls, a C=O group at 160 ppm, right for an acid derivative, and a saturated carbon shifted downfi eld but not as much as a CH₂O group.

All this is reasonably convincing, and is confirmed by allowing the anhydride to warm to –30 °C, at which temperature it loses CO₂ (detected by the 13C peak at 124.5 ppm) and gives another unstable compound with the strange IR frequency of 2140 cm⁻¹. Could this be mono meric ketene? It’s certainly not either of the possible ketene dimers as we know what their spectra are like, and this is quite different: just a 2H singlet at 2.24 ppm and 13C peaks at 194.0 and, remarkably, 2.5 ppm. It is indeed monomeric ketene.

مواضيع ذات صلة

Enolization is catalysed by acids and bases

Evidence for the equilibration of carbonyl compounds with enols

Tautomerism: formation of enols by proton transfer

Hydration of alkynes

Breaking a double bond completely: periodate cleavage and ozonolysis

Adding two hydroxyl groups: dihydroxylation

Electrophilic addition to alkenes can produce stereoisomers

Hydration of alkynes

Breaking a double bond completely: periodate cleavage and ozonolysis

Adding two hydroxyl groups: dihydroxylation

Electrophilic addition to alkenes can produce stereoisomers

Electrophilic additions to alkenes can be stereospecific