Protons homotopic

Homotopic protons may be magnetically nonequivalent if the two protons have different coupling constants to the same third proton. The most common example of this occurs in para-substituted benzenes ... 

When you open your eyes, you cannot determine whether the molecule was rotated or not. From your point of view, the molecule appears exactly as it did before rotation, and it therefore has an axis of symmetry. The two protons on the middle carbon of propane are interchangeable by rotational symmetry and are therefore said to be homotopic. Homotopic protons are chemically equivalent. Below are other examples of homotopic protons. 

Identify the indicated sets of protons as unrelated, homotopic, enantiotopic, or diastereotopic ... 

Label the pairs of protons shown in boldface in each of the following compounds as homotopic, enantiotopic, or diastereotopic, as required. Assume normal rotational barriers and observations at room temperature. 

Aliphatic protons which are interconvertible by a rotational axis are termed homotopic and are chemically and magnetically equivalent. For example, the methylene protons of diphenylmethane are homotopic, as are the methylene protons and the methyl protons of propane. 

For the following compounds, label the spin systems present, label the symmetry properties of protons where possible (homotopic, enantiotopic,... 

In favorable circumstances, active (or dl) and meso stereoisomers may be distinguished directly an acyclic (84) and a cyclic (85) example are shown in Fig. 34. In both cases the methylene protons Hc in the active forms are related by a C2 axis and therefore homotopic and isochronous whereas the corresponding protons HA and Hb in the meso forms are not related by either C2 or a and are therefore diastereotopic and anisochronous. The situation is not altered when the dl form rather than an active isomer is compared with the meso form the (internally homotopic) methylene protons of the two enantiomers are externally enantiotopic and so remain isochronous. 

The compound with the ratio of signal intensities 1 1 1 in its 1H NMR spectrum is in accord with structure A (point group C3V). The compound with the ratio of signal intensities 2 2 2 1 1 1 in its H NMR spectrum is in accord with structure B (point group Cs). The chemically equivalent protons are those with the same numbers in the formulae given below These are homotopic in A and enantiotopic in B. 

What we have said so far explains to you why homotopic and enantiotopic groups appear identical in the NMR spectrum, but diastereotopic protons may not. Now we will give a quick guide to determining what sort of pair you are dealing with in a given molecule. 

These two molecules are identical, because just turning one over gives the other the protons are homotopic. Now for the next example. 

In Chapter 32 we showed that homotopic and enantiotopic protons are identical by NMR. Similarly, homotopic faces or groups are always chemically identical. Enantiotopic faces are also chemically identical, provided that all the reagents in the reaction in question are achiral or racemic. In Chapter 45. we will consider what happens to enantiotopic faces when enantiomerica ly pure reagents are used. 

Finally, for the other molecules, mentally replace each of the two hydrogens in the indicated set with X, a different group. In (a), the resulting products are enantiomers, and the protons are enantiotopic. Replacement of the protons in (b) produces two chirality centers (the carbon bearing the hydroxyl group is now chiral) and the indicated protons are diastereotopic. Replacement of one of the methyl protons in each of the groups in (c) produces a pair of double-bond isomers that are diastereomers these protons are diastereotopic. The protons in (f) are homotopic. 

A second possibility is that the protons are chemically identical and thus electronically equivalent. If so, the same product would be formed regardless of which H is replaced by X. In butane, for instance, the six -CH3 hydrogens on Cl and C4 are identical, would give the identical structure on replacement by X, and would show the identical NMR absorption. Such protons are said to be homotopic. 

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