Methyl deviations

FIGURE 1. The relationships between the methyl deviation, 5(CH3— Z), and Pauling electronegativity of the bonded Z atom in various homologous functional group series, CH3—(CH2)m—Z. See References 17b and 18... 

We have shown19 that there are separate but nearly parallel linear relationships between the methyl deviations, 6(CH3—Z), of methyl substituted species CH3—Z and the electronegativity of the atom in Z bonded to carbon for functionality in the second row of the periodic table (Z = —OH, —NH2, —CH3, —B<) and in the third row of the periodic table (Z = —Cl, —SH, —Al<). If the methyl deviations for CII3B1 and (CH3)4Ge, both containing fourth-row atoms, are now calculated and the results plotted versus the Pauling electronegativity, the two new points fall on the line established by the third-row series (Figure 1). 

A plot of A7/y°(SnR4, g) against A///TRII, g), where R is alkyl, is presented in Figure 3 where the enthalpy-of-formation values used are both experimentally determined (Table 3) and calculated. The calculated values are for tetramethyl and tetraethyl tin and methane. As stated above, the enthalpy-of-formation values for tetraethyl tin may be incorrect. That the calculated value for tetraethyl tin results in a better linear fit with tetrapropyl and tetrabutyl tin is further confirmation of this supposition. And, as discussed for the alkyl germaniums, the methyl deviations of methane and tetramethyl tin are too different for their measured values to fit a linear relationship such as equation 7. 

The early observations consisted of a review of enthalpies of formation of hydrocarbons and their stabilities from F. D. Rossini, Chem. Rev., 27, 1 (1940). The exception we know of is the relative stabilities of alkyl lithiums where methyl lithium is stabilized relative to the -alkyl family (and exhibits a methyl deviation opposite to the methyl halides) and the secondary and tertiary lithium alkyls are destabilized. See J. F. Liebman, J. A. Martinho Simoes and S. W. Slayden, in Lithium Chemistry A Theoretical and Experimental Overview (Eds. P. v. R. Schleyer and A.- M. Sapse), Wiley, New York, in press. 

For the nine substituents m- andp-methyl, p-fluoro, m- and p-chloro, m- and p-bromo, and m- and p-iodo, using the results for nitration carried out at 25 °C in nitromethane or acetic anhydride - (see tables 9.1, 9.5), a plot of logjoA/ j against cr+ produced a substituent constant p = —6-53 with a standard deviation from the regression line i = 0-335, 2 correlation coefficient c = 0-975. Inclusion of... 

R = -CH2CH(CH3)2, there occurs a modest deviation from a strict 0°-120° alternation which characterizes the trans-gauche sequence. This produces a helical structure with seven repeat units occurring in two turns. Even bulkier substituents, for example, o-methyl phenyl, produce still more open helices... 

Precise description of the pyramidal structures would also require that the bond angles be specified. The EPR spectrum of the methyl radical leads to the conclusion that its structure could be either planar or a veiy shallow pyramid. The IR spectrum of the methyl radical has been recorded at very low tempertures in frozen argon. This IR study puts a maximum of 5° on the deviation from planarity. A microwave study has also indicated... 

Thus curvature in an Arrhenius plot is sometimes ascribed to a nonzero value of ACp, the heat capacity of activation. As can be imagined, the experimental problem is very difficult, requiring rate constant measurements of high accuracy and precision. Figure 6-2 shows a curved Arrhenius plot for the neutral hydrolysis of methyl trifluoroacetate in aqueous dimethysulfoxide. The rate constants were measured by conductometry, their relative standard deviations being 0.014 to 0.076%. The value of ACp was estimated to be about — 200 J mol K, with an uncertainty of less than 10 J moE K. ... 

The aforementioned exception and the rather limited experimental material available do not allow any conclusions about the general applicability of the Hammett equation, using the same a- and p-values as for benzenes, to be drawn with certainty. The present author has pointed out that large deviations should be expected with strong - -M-substituents, as is also indicated from the rates of alkaline hydrolysis of methyl 5-amino- and 5-acylamino-2-thenoates. From the chemical shifts in the NMR spectra of thiophenes and benzenes it appears that another set of cr-values should be used in the thiophenes series which seems plausible since the transmission of the sub-... 

Using these assumptions and conventions, Imoto and co-workers have correlated a number of series of reactions of thiophenes and furans. The reactions studied are the acid-base equilibria pK values) and the acid catalyzed methylations (thiophenes only) of thiophene-and furan-carboxylic acids and the alkaline hydrolyses of their ethyl esters the side-chain bromination of the a-acetylthiophenes, and the a-mercuration of thiophenes and the polarographic half-wave potentials of the methyl esters of thiophene- and furan-carboxylic acids and of nitrothiophenes. The pK values were determined and the ester hydrolyses studied for all three substitution orientations in the thiophene series. For the 4-R-2-Y and 5-R-2-Y series, the p-values do not appear significantly different and the data could probably be combined into a single series unfortunately, however, no limits of accuracy are reported for the p-values, and some of the raw data are not readily available so recalculation is not easily possible. For the 5-R-3-Y series the p-values deviate considerably from the other values however, whereas they are higher for the pK values, they are lower for the ester hydrolyses, and it is possible that the differences are neither systematic nor significant. 

Because of the frequent mutual interference of electronic, inductive, and steric effects, and because of the influence of ring strain, the carbonyl stretching frequency is naturally not an absolute criterion for the methylation course. The heterocyclic systems in question are too diverse for this to hold. Careful inspection of Table I discloses certain deviations from the relationships mentioned. These deviations will now be discussed. 

The group contributions are given in Table 6. The group values were used to predict the enthalpy of formation of every compound in Tables 1-5 (except for those containing nitrogen). The estimated values were then compared to the experimental values. The agreement was usually to within about 10 kJ mol f The worst cases were for butyl methyl sulfone, t-butyl methyl sulfone and l-methyl-4-(l-methylethenylsulfonyl)benzene, for which the deviations were 12.4, 14.0 and 12.3 kJ mol-1, respectively. The average deviation was 4.4kJ mol-1. 

A good correlation with ordinary Hammett a values was based on 16 well-behaved substituents, and p-SOMe conformed well to this. Various other substituents showed deviations which were attributed to enhanced + R effects. These included p-SPh and this was explained in terms of 7t(pd) bonding, which was thus taken to play no part in the effect of p-SOMe on the methyl hyperfine splitting. More recently several 4-substituted benzyl radicals of the type RSO C6H4CH2 (n — 0,1 or 2 R = Me, Ph, Tol, COMe or OMe) have been examined by ESR spectroscopy249. The ability to delocalize spin density onto the substituent decreases in general as n increases and the effect of R depends on the oxidation state of sulfur. These authors have devised a new scale of substituent effects (sigma dot... 

In the course of this study, the authors determined /Lvalues for dibenzyl, methyl phenyl, methyl p-nitrophenyl, di-p-tolyl, di-isopropyl and tetramethylene sulphoxides and for diethyl, dipropyl and dibutyl sulphites. The /Lscales are applied to the various reactions or the spectral measurements. The /Lscales have been divided into either family-dependent (FD) types, which means two or more compounds can share the same /Lscale, family-independent (FI) types. Consequently, a variety of /Lscales are now available for various families of the bases, including 29 aldehydes and ketones, 17 carboxylic amides and ureas, 14 carboxylic acids esters, 4 acyl halides, 5 nitriles, 10 ethers, 16 phosphine oxides, 12 sulphinyl compounds, 15 pyridines and pyrimidines, 16 sp3 hybridized amines and 10 alcohols. The enthalpies of formation of the hydrogen bond of 4-fluorophenol with both sulphoxides and phosphine oxides and related derivatives fit the empirical equation 18, where the standard deviation is y = 0.983. Several averaged scales are shown in Table 1588. 

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