Protons stereochemical equivalence

Whenever an organic acid contains two or more chemically identical (i.e., stereochemically equivalent) functional groups, statistical factors that originate in the entropy of formation of the acid and/or its conjugate base contribute to the variation of thermodynamic dissociation constants with the degree of dissociation of the acid. Such statistical effects are implicitly included in equations that are often used to describe acid-base equilibria in synthetic and natural polymers. Because those equations have frequently been applied to proton binding by humic substances, a brief discussion of statistical ef-... 

Chemically equivalent protons should also be stereochemically equivalent. Thus, a given set of protons is chemically equivalent only if all the protons exist in identical environment even when stereochemical formula of the molecule is written. This principle cem be grasped better with the help of an exaimple. The compound 1,2-dichloropropane is expected to give three n.m.r. signals in the following manner ... 

Since the inception of our work Jere, Miller and Jackson have published kinetic and stereochemical data on the hydrogenation of alanine (19). Important in their analysis is the observation that amino acids must be in their protonated form to undergo facile hydrogenation since reduction of carboxylate anions is significantly more endothermic than protonated acids (19). Control of pH is important for two reasons at neutral pH amino acids exist as zwitterions and the resultant hydrogenation products are basic. For these reasons a full equivalent of phosphoric acid (or similar acid) is required to obtain high yields. 

Five- and six-membered rings formed by coordination of diamines with a metal ion have the stereochemical characteristics of cyclopentane and cyclohexane. The ethylenediamine complexes have puckered rings and the trimethylenediamine complexes have chair conformations. The methylene protons are nonequivalent in these nonplanar conformations, taking on the character of equatorial and axial substituents. They are made equivalent as the result of rapid conformational inversion at room temperature, just as in the alicyclic compounds (Fig. 7.1). This has been observed in nmr studies of planar and octahedral complexes of ethylenediamine-type ligands with a number of metals. 

The nondegenerate geminal pairs are usually named according to their chemical shifts (e.g., downfield of ft ) rather than their stereochemical relationships (pro-R and pro-S). In structure calculations, this usually is dealt with by creating a pseudo-atom right between the pro-R and pro-S positions in 3D space. The NOE restraints are applied to the pseudoatom and not to the real atoms, and the distance limit is increased a bit to account for the ambiguity (we do not really know which restraint applies to which of the two positions in space). Similarly, a pseudoatom is created at the center of the three equivalent protons of a CH3 group, and the distance restraint is applied to the pseudoatom. 

Stereochemical differences often result in different chemical shifts for protons on the same carbon atom. For example, the two protons on Q of allyl bromide (3-bromopropene) are not equivalent. Ha is cis to the —CH2Br group, and Hb is trans. Ha absorbs at 8 5.3 Hb absorbs at 8 5.1. There are four different (by NMR) types of protons in allyl bromide, as shown in the structure at right in the margin. 

Olah and coworkers obtained the parent cyclopropylcarbinyl cation and characterized it by both and NMR spectroscopy. The NMR spectrum of the cation shows two overlapping quartets (/ = 8 and 6.5 Hz) for the methine protons, and two sets of doublets for the methylene protons, <5 H 4.64 and 4.21. Thus the methylene hydrogens are stereochemically non equivalent, which is unexpected for classical cyclopropylcarbinyl or cyclobutyl cations. The NMR spectrum shows only two signals 108.4 (CH), and... 

One can interpret the above data, admittedly limited by the small number of different temperatures and the signal-to-noise ratio present for exchanging sites with no attached protons to enhance these resonance signals, as indicative of two unique sites for the carbonyls in a ratio of 2 1, equivalent sites for both central dmtc carbons, and two sites for the four dmtc methyls in a 1 1 ratio. Although the CO-enriched NMR study contradicts each of these conclusions for the low temperature structure, vide infra, the above case offers a convenient point of departure for analyzing the stereochemical nonrigidity of these complexes. 

NMR is a useful alternative to proton NMR. It is a reasonably sensitive nucleus which does not suffer from the relaxation problems associated with NMR. Phosphorus trichloride can be used as a reagent for self-recognition by a chiral substrate. Two molecules of an enantiomerically enriched alcohol react with each PCI3 molecule to form a phosphonate. Four stereochemically distinct species are possible (R,R) and (5, S) (a pair of enantiomers), (R,S) and (5, R) which are meso compounds. There are consequently three resonances in the 3ip NMR spectrum, whose non-equivalence is typically 0.5 ppm and integration gives results within 2% of those obtained by chiral GC methods. Subsequently methyl phosphoryl chloride has been employed in the same way giving improved chemical shift non-equivalences, A5 = 1 ppm (Scheme 3.3). 

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