Methyl groups, active

A tertiary carbonium ion is more stable than a secondary carbonium ion, which is in turn more stable than a primary carbonium ion. Therefore, the alkylation of ben2ene with isobutylene is much easier than is alkylation with ethylene. The reactivity of substituted aromatics for electrophilic substitution is affected by the inductive and resonance effects of a substituent. An electron-donating group, such as the hydroxyl and methyl groups, activates the alkylation and an electron-withdrawing group, such as chloride, deactivates it. 

The close structural similarities between polychloroprene and the natural rubber molecule will be noted. However, whilst the methyl group activates the double bond in the polyisoprene molecule the chlorine atom has the opposite effect in polychloroprene. Thus the polymer is less liable to oxygen and ozone attack. At the same time the a-methylene groups are also deactivated so that accelerated sulphur vulcanisation is not a feasible proposition and alternative curing systems, often involving the pendant vinyl groups arising from 1,2-polymerisation modes, are necessary. 

The acid cleavage of the aryl— silicon bond (desilylation), which provides a measure of the reactivity of the aromatic carbon of the bond, has been applied to 2- and 3-thienyl trimethylsilane, It was found that the 2-isomer reacted only 43.5 times faster than the 3-isomer and 5000 times faster than the phenyl compound at 50,2°C in acetic acid containing aqueous sulfuric acid. The results so far are consistent with the relative reactivities of thiophene upon detritia-tion if a linear free-energy relationship between the substituent effect in detritiation and desilylation is assumed, as the p-methyl group activates about 240 (200-300) times in detritiation with aqueous sulfuric acid and about 18 times in desilylation. A direct experimental comparison of the difference between benzene and thiophene in detritiation has not been carried out, but it may be mentioned that even in 80.7% sulfuric acid, benzene is detritiated about 600 times slower than 2-tritiothiophene. The aforementioned consideration makes it probable that under similar conditions the ratio of the rates of detritiation of thiophene and benzene is larger than in the desilylation. A still larger difference in reactivity between the 2-position of thiophene and benzene has been found for acetoxymercuration which... 

Based on the polarity of the C—H bond being C II1, the resonance canonicals predict that the methyl group activates the ortho and para carbons in an aromatic ring to electrophilic substitution. For SiH3 or SiMe3 the polarities are Si+H and Si+C, which gives the resonance canonicals, shown in equation 3. 

In all the above cases, C-2 is activated toward attack by phenyllithium compared with the same position in pyridine, while C-6 is about six- to sevenfold deactivated by the 3-methyl or 3-ethyl group compared with the oc-position of pyridine. Also, the 2,3- 2,5-isomer ratios remained unchanged in all cases. A 3-methyl group activates C-2 more than does a 3-ethyl, the activation in the former case being sufficient to overcome the deactivation of C-6 and resulting in an over-all activation of the 3-picoline nucleus compared with that of pyridine and a value of the total rate ratio greater than unity. In the case of 3-ethylpyridine, the activation is insufficient to overcome the normal deactivation of the position para to the alkyl group. 

When methylbenzene is nitrated with the nitric acid and sulfuric add mixture, the methyl group determines the location of the entering nitro group. It allows it to enter the 2- or 4-positions and eventually the 6-position (Figure 6.3.12). Methyl groups activate the ring. 

Thus it was stated that the methyl group activates the aromatic ring, whereas chloride and bromine deactivate it. 

Methyl groups activated by being a or y to an annular nitrogen in an N-heteroaromatic system can be readily diformylated.1 When an amino substituent is ortho to the methyl group, cyclization can ensue giving a fused pyrrole (Scheme 8).18-20 o-Methylamino groups result in A-methylpyrrole (41%) analogues.19... 

In another synthesis an aromatic methyl group activated by an ortho or para nitro substituent is converted into an aldehyde group, as illustrated by the preparation of 2,4-dinitrobenzaldehyde. ... 

Toluene undergoes nitration some 20-25 times faster than benzene. Because toluene is more reactive than benzene, we say that a methyl group activates the ring toward electrophilic aromatic substitution. (Trifluoromethyl)benzene, on the other hand, undergoes nitration about 40,000 times more slowly than benzene. We say that a triflu-oromethyl group deactivates the ring toward electrophilic aromatic substitution. 

Direct bromination of toluene cannot give 3.5-dibromotoluene because the methyl group activates the ortho and para positions. 

Terminal alkynes also add to [Tp Rh(CNR)]. Irradiation of the carbodiimide complex 1 in neat 1-alkyne leads to the activation of the sp C-H bond. In cases where other activatable C-H bonds were presented, competitive C-H activation at these positions was observed. For example, t-butylacetylene and trifluoromethyl acetylene give exclusively alkynyl hydride products, whereas 1-octyne and trimethylsilylacetylene also give products resulting from methyl group activation. In both of the latter cases, the sp C-H activation products are unstable and convert to the terminal alkynyl products at room temperature after a few days (Scheme 1). Similarly, the activation of arylalkynes leads to mixtures of sp and sp C-H activation products. The unsaturated fragment [Tp Rh(CNR)] was prepared either... 

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