Geminal couphng

Diastereotopic groups are not equivalent and are not isochronous they have different chemical shifts in the NMR spectrum. When the diastereotopic groups are hydrogens, they frequently split each other with a geminal couphng constant J. Figure 5.58c shows the substitution procedure for a molecule with two diastereotopic hydrogens. In this molecule, the replacement of first Ha. and then Hb yields a pair of diastereomers. Diastereomers are produced when substituent Y already has an adjacent stereocenter. Section 5.15 will cover this situation in detail. 

Geminal couphng Vicinal couphng V and V negligible (i.e., almost always zero)... 

B is more difficult. The alkene has obviously gone (no signals beyond 4.48) and there is one extra H. It looks as though HBr has added. The 17.7 coupling cannot be a tram alkene as the chemical shifts are too small, so it must be geminal couphng. This means that the molecule must be chiral so... 

Spin-spin splitting of NMR signals results from couphng of the nuclear spins that are separated by two bonds (geminal coupling) or three bonds (vicinal coupling). 

The mechanism of cis and trans couphng in alkenes is no different from that of any other three-bond vicinal coupling, and that of the terminal methylene protons is just a case of two-bond geminal coupling. All three types have been discussed already and are illustrated in Figure 5.19. 

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