Beta protons

Since the prepared PVC samples had a high degree of syndiotacticity, it is expected that the trans planar grouping should predominate for PVC in solution. On irradiation, the radical would reside most likely in one of the trans-trans sequences. If the proposed radical, -CH2-CH-CH2- is formed, it follows that the four beta protons must be equivalent from geometric considerations (Figure 4). There are only two dihedral angles that fulfill this requirement, 0 = 60° or 30°. 

Therefore, due to the four equivalent beta protons, a quintet is expected, which should be further split into ten lines by the alpha proton (Figure 5). This constructed spectrum can be applicable to either 3-chloropentane or PVC, the R groups being CH3 or polymer chains respectively. If this PVC radical could be observed in the liquid state, all ten lines would be visible. However, for the solid at LNT, the central eight lines will overlap, and the spectrum will appear as a symmetrical sextet with a theoretical line intensity ratio of 1 5 10 10 5 1. 

In principle, a planar carbonium ion should lose a beta proton from either side of the plane with equal facility and consequently there should be no stereoselectivity in El reactions. This simple interpretation can be distorted, however, in solvents which do not favour ionic dissociation, or if neighbouring group participation in the ionisation is afforded by a beta hydrogen or another group. 

A change to a less basic solvent can influence the stereochemistry of unimolecular elimination. In nitrobenzene or acetic acid, erythro-3-deutcro-2-butyl tosylate yields deuterium-free m-2-butene and monodeuterated /ra 5-2-butene, suggestive of a jy/i-elimination (104). In more basic solvents, such as water, acetamide and 80% aqueous ethanol, the labelling of olefins is reversed and a/i/i-elimination is followed. The change in stereochemistry is attributed to the relative basicities of the tosylate anion and solvent and hence the tendencies of these species to accept a beta proton from the carbonium ion (105). 

Additional to the polar factors, the steric requirements of the alkyl substituents may play a dominating role by hindering approach of the base to a beta proton or alternatively causing a divergence from the requisite coplanarity of reacting bonds in the transition state for elimination. Often, along a series of alkyl substituents, both steric and inductive effects vary in the same direction and the interpretation of the salient orientating features is a matter of personal preference. 

Of course, varying degrees of solvation other than illustrated in this scheme will be encountered and the desolvation of the carbonium ion centre and the loss of a beta proton may occur synchronously or in discrete steps. 

The two protons on the beta carbon (Cp) are labeled Hpj and Hp2, and the proton on the alpha carbon (CJ is labeled H ,. Although Hpi and Hp2 may at first appear to be identical—especially if we draw the molecule in two dimensions—the three Newman projections showing the rotamers resulting from rotation about the Cp—C , bond reveal that the beta protons do not experience the same chemical environment. 

The values above represent the effect of a few functional groups on the chemical shifts of alpha protons. The effect on beta protons is generally about one-fifth of the effect on the alpha protons. For example, in an alcohol, the presence of an oxygen atom adds -1-2.5 ppm to the chemical shift of the alpha protons, but adds only -bO.5 ppm to the beta protons. Similarly, a carbonyl group adds +1 ppm to the chemical shift of the alpha protons, but only -1-0.2 to the beta protons. 

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