Labelling experiments reveal the fate of individual atoms

It often happens that the atoms in starting material and product cannot be correlated without at least one of them being labelled. The fact that many elements exist as different isotopes provides us with a perfect way of doing this: a neutron more or less in the nucleus affects the physics (and hence the spectroscopic features) of an atom, but not its chemistry. The isomerization of Z-1-phenylbutadiene to the E diene in acid looks like a simple reaction. Protonation of the Z alkene would give a stabilized secondary benzylic cation that should last long enough to rotate. Loss of the proton would then give the more stable E diene.

However, reaction with D+ in D2O reveals that this mechanism is incorrect. The product contains substantial amounts of deuterium at C4, not at C2 as predicted by the proposed mechanism. Protonation must occur at the end of the conjugated system to produce the more stable conjugated cation, which rotates about the same bond and loses H or D from C4 to give the product. More H than D will be lost, partly because there are two Hs and only one D, but also because of the kinetic isotope effect, of which more later..

The easiest labels to use for this job are D for H, 13C, and 18O. None of these is radioactive; all can be found by mass spectrometry, while D and 13C can be found by NMR. Older work on mechanisms used radioactive tracers such as T (tritium, 3H) for H and 14C. The fi rst evidence for benzyne as the intermediate in the reaction of chlorobenzene with NH2 − came from radioactive labelling. If benzyne is an intermediate, the product should have 50% label at C1 and 50% at the two identical ortho carbons, as the scheme below shows.

The labelled aniline was degraded by the reactions shown here, which you must agree was a lot of work for the chemists concerned. Each potentially labelled carbon atom had to be isolated from any other labelled atom and the radioactivity measured. We shall follow the fate of the two labelled atoms with black and green spots. Since the two ortho positions are identical, we must put a black spot on both of them.

dicarboxylic acid is not a standard procedure and is not recommended. All the label came out in the CO₂ and almost exactly half of it was from the black and half from the green labelled carbons. This was the original evidence that convinced organic chemists in 1953 that benzyne was involved in the reaction.

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متحف الكفيل: إنجاز المرحلة الرابعة من مشروع متحف العتبة العسكرية المقدسة

تضمن عرضًا مسرحيًّا.. شعبةُ الخطابة النسويّة تقيم مجلس عزاء بذكرى شهادة الإمام الحسن (عليه السّلام)

الأمين العام للعتبة العبّاسية المقدّسة يتفقد مجموعة مواكب أمّ البنين (عليها السلام) لخدمة الزائرين

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مواضيع ذات صلة

The Hammett relationship

Systematic structural variation

Crossover experiments

Be sure of the structure of the product

Variation in the structure of the aldehyde

formation of an ester intermediate

formation of an intermediate dimeric adduct

Be sure of the structure of the product

Variation in the structure of the aldehyde

Proposed mechanism C: formation of an ester intermediate

Proposed mechanism B: formation of an intermediate dimeric adduct

Cross metathesis