Substituent effect methyl group

Substitution by a methyl group increases AG° and AH°, and this increase is attributed to polar effects. As can be seen from Table III-15, there is an increase in AG and AH° of roughly 1 kcal/mole for each methyl group. Similar effects have been observed with picolines and lutidines (151). There is only a slight difference for the isomeric compounds, the substituent effect being weakest for the 5-derivative. 

In this paper, we have not concerned ourselves with the differing behavior of various alkyl groups, but have instead for the present considered the methyl substituent as roughly representative of the behavior of most alkyl group substituent effects. The present results make possible (through use of p/and pji values) analysis of reported substituent effects of various alkyl groups. Such an analysis is beyond the scope of this paper, however. 

The study of lactonization via an intermediate phenonium ion has been further pursued for several methyl 4-aryl-5-tosylhexanoates (55) as substrates. The intermediate phenonium ion (56) has two possibilities for ring closure, yielding products (57) or (58). In all the substrates, Ar contained one or two methoxy groups and sometimes also a methyl group. The effects of reaction medium, temperature, and time on the product ratios were examined. It was concluded that substrates (55) give y-lactone (57) selectively under thermodynamic conditions, but 5-lactone (58) under kinetic conditions. Substituents in Ar influence the selectivity through their electronic effects. 

Agman has examined the dynamic Stokes shift seen in coumarin fluorescence in considerable detail and resolution of the rotational effects of methyl group substituents as well as the... 

The greater stability of an equatorial methyl group compared with an axial one IS another example of a steric effect (Section 3 2) An axial substituent is said to be crowded because of 1,3 diaxial repulsions between itself and the other two axial sub stituents located on the same side of the ring... 

Our analysis of substituent effects has so far centered on two groups methyl and triflu oromethyl We have seen that a methyl substituent is activating and ortho para directing A trifluoromethyl group is strongly deactivating and meta directing What about other substituents ... 

Often the directing effects of substituents reinforce each other Brommation of p mtrotoluene for example takes place at the position that is ortho to the ortho para directing methyl group and meta to the meta directing nitro group... 

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 effect of substituents on the reactivity of heterocyclic nuclei is broadly similar to that on benzene. Thus mem-directing groups such as methoxycarbonyl and nitro are deactivating. The effects of strongly activating groups such as amino and hydroxy are difficult to assess since simple amino compounds are unstable and hydroxy compounds exist in an alternative tautomeric form. Comparison of the rates of formylation and trifiuoroacetylation of the parent heterocycle and its 2-methyl derivative indicate the following order of sensitivity to substituent effects furan > tellurophene > selenophene = thiophene... 

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