Methyl groups activating effects

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... 

If activation is run at 405° in helium (run 250) one obtains a catalyst which has almost completely lost the capacity to exchange the methyl group. The effect is visible in the isotopic distribution which very closely approaches a random type. Two other runs on different samples of catalyst, one of which was a different preparation, gave the same random isotopic distribution pattern and nearly the same rate. 

Certain heteroaromatic compounds that have acidic methyl groups are effective in the Knoevenagel condensation. 6-Methyl-1,3,5-triazine (156), 2-methylpyridine V-oxides (157), quinolines and pyrimidines all condense with aromatic aldehydes in the presence of the usual catalysts. An example of a doubly activated methylene compound in which one activation group is a heterocycle is the 2-oxopro-pylthiazole (158), which condenses with a number of aldehydes. " ... 

The amino group activates the thiazole ring toward electrophilic centers. This point is illustrated by the rate constants of the reaction between 2-dialkylaminothiazoles (32) and methyl iodide in nitromethane at 25 C (Scheme 23) (158). The steric effects of substituents on nitrogen are... 

When two positions are comparably activated by alkyl groups substitution usually occurs at the less hindered site Nitration of p tert butyltoluene takes place at positions ortho to the methyl group m preference to those ortho to the larger tert butyl group This IS an example of a stenc effect... 

A quantitative study has been made on the effect of a methyl group in the 2-position of five-membered heteroaromatic compounds on the reactivity of position 5 in the formylation and trifluoroacetylation reaction. The order of sensitivity to the activating effect of the substituent is furan > tellurophene >selenophene = thiophene (77AHC(2l)ll9). 

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