Methyl groups electronic effects

For the methyl-substituted compounds (322) the increase in AG and AHf values relative to the unsubstituted thiazole is interpreted as being mainly due to polar effects. Electron-donating methyl groups are expected to stabilize the thiazolium ion, that is to decrease its acid strength. From Table 1-51 it may be seen that there is an increase in AG and AH by about 1 kcal mole for each methyl group. Similar effects have been observed for picolines and lutidines (325). 

The chemical shift effects of nitro and amino substituents are the largest normally encountered. The mildly electron donating methyl group induces far more modest shifts, as seen with 2- and 3-methylthiophene (113 and 114). 

Shima, Smid and Szwarc (56) studied the effect of the methyl substitution in the polymerization of butadiene, isoprene and dimethyl-butadiene. They showed that the electron-donating methyl group decreased the rate of polymerization catalysed by polystyrylsodium. This same electron releasing effect of the methyl is seen, since the 3.4-structure, not 1.2-structure, is produced predominantly from isoprene. This results from the anionic propagation mechanism of the alkali metal alkyl catalysed polymerization of dienes which produced 1.2 and 3.4-structures. 

M.O. calculations have predicted that all of the heterobenzenes have the same direction of polarization with negative end of the dipole towards the heteroatom67-70). This has been confirmed by substituent effects from pyridine, phos-phabenzene and arsabenzene 27). In each case the 4-methyl derivatives have dipole moments which exceed the parent compound since the electron donating methyl group reinforces the ring dipole. 

Recently the 2-naphthol system has been re-examined. From lifetime measurements at various pH values, Loken et al. (1972) and Ofran and Feitelson (1973) determined the rate constants (see Table 10). The effects of further ring substituents on the rate constants has been studied by Rosenberg and Brinn (1972) (see Table 10). Dissociation in the Si state is retarded by substituents which increase the electron density on the phenolic oxygen the electron-donating methyl group reduces k 1 while chloro- and bromo-substitution, as expected, increase it. 

Protophilic exchange of deuterium proceeds 50,000 times faster at an a than at a /3 position of selenophene. An electron-donating methyl group hinders the exchange, and this effect is most pronounced in 2-deuterio-3 - me thylselenophene. 

By releasing electrons, the methyl group (II) tends to neutralize the positive charge of the ring and so become more positive itself this dispersal of the charge stabilizes the carbonium ion. In the same way the inductive effect stabilizes the developing positive charge in the transition state and thus leads to a faster reaction. 

It is interesting to note that the above effect outweighs an undesirable rise in pKa. By adding an electron donating methyl group, there has been a rise in the pKa of the imidazole ring to 6.80 compared to 6.25 for thiaburimamide. (Coincidentally, this is... 

This relationship was then used to calculate the BDE for a-Toc, giving a value of 77.2 kcalmor, in excellent agreement with the experimental value of 77.3 kcalmoP. This empirical method does depend on the electronic effects of groups (methyls and para-ether) around the phenyl ring and provides some confirmation of the role these play in weakening the O—H bond and thus raising the antioxidant activity. In a similar manner, Jovanovic and coworkers obtained a correlation between the measured reduction potentials and the a+ constants for twenty-one substituted phenols at pH 7 (equation 59) and pH 0 (equation 60) ". ... 

The increasing number of electron-releasing methyl groups attached to the carbocation centre helps to spread the positive charge more effectively. 

Electronic and steric factors exert their impacts on this rearrangement. Thus, the rate of isomerization increased with electron depletion and decreased with electron enrichment of the pyrimidine ring. It was less facile when the triazole ring was substituted (89JHC687). Methyl groups with a positive mesomeric effect at C5 and/or C8 (16, R1 and or R3=Me) diminished the rate of acid-induced isomerization (pH 4) by retarding the approach of the nucleophile to C5 by electronic and (for C5) steric hinderance effects. Methyl groups at C3 and/or C7 (16, R and or R2=Me) exerted little electronic and no steric effects on C5, yet promoted the approach of the... 

The flash photolysis of benzothiazolino-spirochromenes (207 R = Me, CH Ph, Ph, OMe, SMe, etc.-, R = Me, Et, or Pr R = H, SMe, NO2, or OMe R = OMe, H, CHiOMe, CHjOCHaCHaOCHaCHjMe, or CHiSEt) was studied. The electron-donating substituents decrease the extent of photolysis. Hammett correlations involving inductive, resonance, and steric parameters were estabhshed for three compounds the steric effect is the most important polar solvents decrease the extent of photolysis. The kinetics of A -methylation of substituted 2-(2-furanyl)benzothiazoles have been determined spectrophotometrically at 80°C. The electron-donating methyl group accelerates the reaction, whereas the halogens have the opposite effect. 

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