Methyl group classification

In this simplified example of phenylalanine, in the first iteration the methyl groups arc given a value of I in the first classification step because they contain a primaiy C-atom, The methylene group obtains a value of 2, and the methine carbon atom a value of 3. In the second step, the carbon atom of the methyl group on the left-hand side obtains an extended connectivity (EC) value of 2 because its neighboring atom had a value of 2 in the first classification step. 

However, one difficulty of the proposed classification is the fact that reactions of Type III probably overlap with those of Type II. One good example of this is redistribution reactions, presumably a reaction of the organic group. However, Maher and Evans 19) find that intramolecular exchange of methyl groups of monomeric trimethylthallium follows second-order kinetics in deuterobenzene and suggest a dimeric transition state. 

The water-soluble organometallic 2 is the only natural product able to transfer the methyl group as a carbanion (cf. Eq. 7). As a soft Lewis acid (in the classification of Pearson), the methylmercuronium ion [CH3Hg]+ is soluble in the presence of hard bases such as [N03]- or [S04]2. In turn, soft bases such as (organic) sulfides thus make it lipophilic and enable its uptake by living organisms. 

A methyl group is considered to "donate" two electrons and to be an "anionic, two-electron" ligand by the first classification system and is considered to "donate" one electron and to be an X-type ligand by tine second classification system. 

Studies of electrophilic substitutions on arenes are reported in which the experimental conditions allow a direct comparison of the relative reaction rates. For example, the relative reactivities of benzene and toluene toward halogenation, acetylation, sulfonation, nitration, and methylation have been determined. In all cases, electrophilic aromatic substitution was more rapid with toluene. For example, bromina-tion of toluene is some 600 times faster than that of benzene. Such studies have led to the classification of substituents as ring activators or deactivators, depending on whether the substituted arene reacts faster or slower than benzene itself. Thus, the methyl group of toluene is a ring activator. 

Figure 7.42 Steroids are biologically derived from cholesterol, and the ring-numbering scheme used to describe these compounds is shown. Androstane is an example of a steroid molecule with a methyl group at the Cl9 position, an important position for classification. 

Andno-esters, e.g. methyl anthranilate. Classification tests for both the amino- and ester functions are positive. These compounds are most conveniently characterized through the amino-group. 

An important group of polysiloxanes that is used in the medical, pharmaceutical and cosmetic industries are polydimethylsiloxanes (PDMS). Polydimethylsiloxane molecules consist of two functional groups [R Si-Oj, where R represents a methyl group. Polydimethylsiloxanes used in those fields are either linear (methyl silicon oils) or cyclic molecules (Figure 16.1) [9-12]. The cyclic PDMS include hexamethylcyclotrisiloxane (D3), octamethyl-cyclotetrasiloxane (OMCTS, D4), decamethylcyclopen-tasiloxane (D5), and dodecamethylcyclohexasiloxane (D6). A scheme for the classification of linear and cyclic... 

Infrared spectra of the most common cationic surfactant, distearyldimethylammo-nium chloride are very simple, little different from hydrocarbon spectra. The two methyl groups and the N-C group cannot be distinguished from the overpowering -CH2- bands, at least not by ordinary dispersive IR (23). Cross published information on the infrared spectra of the tetraphenylborate salts of common cationics (24). He finds IR spectroscopy suitable for the classification (he reserved the term identification for a technique which would give the exact length of the alkyl chain) of surfactants isolated by precipitation from complex samples. 

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