Elimination reactions distinguishing features

The difficulty of distinguishing mechanisms at the ElcB-E2 borderline has also been discussed for reactions of secondary halides (1-X and 2-X) which feature a f-hydrogen made acidic by incorporation of an a-indenyl substituent (Scheme 1). 1,2-Elimination reactions of (/f,5 )-l-(l-X-ethyl)indene (1-X, X = Cl, Br, I, OBs) and the corresponding R,R isomers (2-X) promoted by water containing 25 vol.% acetonitrile... 

This equation also describes the overall reaction of either an 5 2 or a nucleophilic aromatic substitution process. In some cases, the only way to distinguish an reaction from these processes is that an is inhibited by radical inhibitors. Another distinguishing feature is that the order of the relative leaving group abilities of halides are opposite that found for nucleophilic aromatic substitution by the addition-elimination mechanism (see Chapter 3). 

This example is from biochemistry. It is a feature of biochemical reactions that enzymes almost always catalyse reactions in a completely stereospecific manner. They are able to distinguish between enantiotopic hydrogens because of the three-dimensional nature of the binding site (see Section 13.3.2). There are also occasions where chemical reactions are stereospecific refer to the stereochemistry of E2 eliminations for typical examples (see Section 6.4.1). 

The mechanism of phosphonate anion (135) addition to carbonyl derivatives is similar to the phosphonium ylide addition however, there are several notable features to these anion additions that distinguish the reactions fix)m those of the classical Wittig. The addition of the anion gives a mixture of the erythro (136 and 137) and threo (139 and 140) isomeric p-hydroxyphosphonates (Scheme 24). In the case of phosphine oxides, the initial oxyanion intermediates may be trapped. The anion intermediates decompose by a syn elimination of phosphate or phosphinate to give the alkene. The elinunation is stereospecific, with tile erythro isomer producing the ci.r-alkene (138), and the threo addition adduct producing the... 

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