Homotopic hydrogens

Homotopic hydrogens have identical environments and will have the same chemical shift. They are said to be chemical shift equivalent. 

An exception is the halogenation of an alkane (or cycloalkane) whose hydrogen atoms are all equivalent (i.e., homotopic). [Homotopic hydrogen atoms are defined as those that on replacement by some other group (e.g., chlorine) yield the same compound (Section 9.8).]... 

For example, analyzing 4-bromonitrobenzene (6) by the substitution test leads to the conclusion that there are two sets of homotopic hydrogen atoms, and HjjHj,. The two protons in each set have ecjuivalent chemical shifts. However, the coupling constants between all members of the spin set are not equivalent. Namely,... 

FIGURE 15.24 Substituting either of the two hydrogens gives identical molecules therefore the hydrogens in methylene chloride are homotopic. Homotopic hydrogens are equivalent. 

Homotopic (Section 15.6) Homotopic hydrogens are identical, both chemically and spectroscopically, under all circumstances. 

Each number represents another signal, with the highest number indicating the total number of proton NMR signals for the molecule. Repeated use of the same number indicates positions bearing homotopic hydrogens. 

B The fourth possibility arises in chiral molecules, such as (R)-2-butanol. The two — CH2- hydrogens at C3 are neither homotopic nor enantiotopic. Since replacement of a hydrogen at C3 would form a second chirality center, different diastereomers (Section 9.6) would result depending on whether the pro-R or pro-S hydrogen were replaced. Such hydrogens, whose replacement by X leads to different diastereomers, are said to be diastereotopic. Diastereotopic hydrogens are neither chemically nor electronically equivalent. They are completely different and would likely show different NMR absorptions. 

Homotopic (Section 13.8) Hydrogens that give the identical structure on replacement by X and thus show identical NMR absorptions. 

The term equivalent is overly general and therefore bland and of equivocal meaning. Thus the methylene hydrogen atoms in propionic acid (Fig. 1) are equivalent when detached (i.e. they are homomorphic), but, as already explained, they are not equivalent in the CH3CH2C02H molecules because of their placement — i.e. they are heterotopic. Ligands that are equivalent by the criteria to be described in the sequel are called homotopic from Greek homos = same and topos = place 6>, those that are not are called heterotopic . 

The symmetry planes (a) in molecules 30, 32, 34, 36, 38, Fig. 13 should be readily evident. It is possible to have both homotopic and enantiotopic ligands in the same set, as exemplified by the case of cyclobutanone (34) HA and HD are homotopic as are HB and Hc, HA is enantiotopic with HB and Hc HD is similarly enantiotopic with Hc and HB. The sets HAjB and HC>D may be called equivalent (or homotopic) sets of enantiotopic hydrogen atoms. The unlabeled hydrogens at position 3, constitutionally distinct — see Section 3.4 — from those at C(2, 4), are homotopic with respect to each other. Enantiotopic ligands need not be attached to the same atom — viz. the case of mew-tartaric acid (32) and also the just-mentioned pair Ha, Hc [or Hb, Hd] in cyclobutanone. 

The correct description for this pair of hydrogen atoms is homotopic. They are the same (homo) topologically and cannot be distinguished by chemical reagents, enzymes, NMR machines, or human beings. The molecule is achiral—it has no asymmetry at all. 

This molecule, the most symmetrical of the three, is achiral. The central carbon atom is completely nonstereogenic. Both planes are planes of symmetry and the hydrogens are homotopic. They are chemically and magnetically equivalent. 

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