Lapworth model

The distribution of formal charges in the carbon skeleton is determined by the functional groups (or heteroatoms) present on it. In this context is very useful to use the "Lapworth model" of alternating polarities. 

A special case are the reactions in which a double deprotonation of a nitrocompound is involved [20], because then the "Lapworth model" does not apply anymore and the rule of alternating polarities is clearly violated (reaction 3, Scheme 2.3 see also Heading 5.5.5) ... 

As we have seen in Chapter 2 (Scheme 2.3, equation 3), the doubly deprotonated nitroalkanes are nitroalkenes with reactivity inversion which violate the Lapworth model of alternating polarities and react with electrophiles at the ipso- and a-positions ... 

Molecules as ionic aggregates. The Lapworth-Evans model... 

When, in 1832, Wohler and Liebig first discovered the cyanide-catalyzed coupling of benzaldehyde that became known as the benzoin condensation , they laid the foundations for a wide field of growing organic chemistry [1]. In 1903, Lapworth proposed a mechanistical model with an intermediate carbanion formed in a hydrogen cyanide addition to the benzaldehyde substrate and subsequent deprotonation [2]. In the intermediate active aldehyde , the former carbonyl carbon atom exhibits an inverted, nucleophilic reactivity, which exemplifies the Umpo-lung concept of Seebach [3]. In 1943, Ukai et al. reported that thiazolium salts also surprisingly catalyze the benzoin condensation [4], an observation which attracted even more attention when Mizuhara et al. found, in 1954, that the thiazolium unit of the coenzyme thiamine (vitamin Bi) (1, Fig. 9.1) is essential for its activity in enzyme biocatalysis [5]. Subsequently, the biochemistry of thiamine-dependent enzymes has been extensively studied, and this has resulted in widespread applications of the enzymes as synthetic tools [6]. 

There have been several experimental indications of the two-stage mechanism since it was first speculatively postulated by Lapworth (1901), e.g., the isolation of an intermediate in the electrophilic bromina-tion of l,l-di-(p-dimethylaminophenyl)ethylene by Pfeiffer and Wizinger (1928), and the interpretation of solvent effects in aromatic nitrations by Hughes et al. (1950). One might argue, however, that ethylene is not a good model for aromatic hydrocarbons and that solvent effects alone do not provide unequivocal evidence for a postulated mechanism. 

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