Prochiral non-equivalence

Some confusion can arise over use of the term prochiral to describe various sites within molecules and is perhaps best avoided for this reason. The term means literally, one step removed from being chiral (i.e., swap one of the protons for Z and you have a full chiral centre). The methylene in ethanol for example, would be a good example. What we have in the di-ethoxy molecule above is one prochiral centre acting in combination with another to render a pair of protons non-equivalent. 

The second method of determining tacticities is essentially an extension of the first, the only difference being that the chiral centre is placed in the side chain(s) of a monomer that would otherwise be prochiral. The adjacent olefinic protons in the polymer may be equivalent and uncoupled (HH or TT), or non-equivalent and coupled (HT). The application of this method to the case of 5,6-dicarboxylic esters of norbomadienes is described in Section 13.4.1.4. It turns out that an all-/rans polymer is syndiotactic and an all-cw polymer is isotactic, which is the opposite way round to what was found for 5,5-dimethylnorbomene (see above). 

In the H-NMR spectrum of hydroxyl-terminated PIB (Structure HI) the eight peak pattern in the 3.22-3.48 ppm range is clearly due to the AB part of a typical ABX system. This splitting pattern is most likely due to prochirality the two protons (gj and g2) in the - CH2OH group are magnetically non-equivalent, as they are part of a prochiral - CH2- group ... 

Nuclei are chemically equivalent if they can be interchanged by a non-trivial symmetry operation. Nuclei interchangeable by a pure rotation (C , n=2, 3,4,...) are called homotopic. Nuclei related only by a centre (i) of inversion or by a plane ([Pg.7]

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