What are the relative steric demands of carboxyl and methyl groups?

Abstract

The relative steric demands of carboxyl and methyl groups are compared by contrasting the difference quantity 5 -~ AH~'(g, ArCOOH) -AH~'(g, ARCH3) for a collection of alkylated benzoic acids and toluenes with the value for Ar = C6H5, the archetypical (i.e., unsubstituted) benzoic acid and toluene. We conclude that carboxyl and methyl groups are nearly the same size.KEY WORDS: Benzoic acids; enthalpy of formation; isomers; toluenes.This brief note discusses the relative steric demands of two of the most important groups in organic chemistry, --COOH and --CH 3. It seems almost obvious that --COOH is larger than --CH 3. After all, atomic oxygen is "bigger" than atomic hydrogen, and so a molecular group composed of a carbon with two oxygens and a hydrogen should be larger than a group composed of a carbon with three hydrogens. However, "size" is not the same as "steric demand." After all, the "lollipop stick" polyyne substituentshave increasing molar volume with increasing n, yet for all n > 0 they would be expected to "keep out of the way" of other groups to nearly identical amounts, and very probably, better than the t-butyl group that corresponds to the n = 0 case.How can we ascertain the steric demands of the COOH and CH 3 groups of interest? One way is to determine the steric demand of the two groups separately ~Instituto de Qufmica Ffsica "'Rocasolano," C.S.I.C., 141and take the difference of these quantities. It is important to specify the "environment" of these groups and to ensure that it is the same for both. We will take the environment to be aromatic rings for which there is an sp 2 carbon affixed to the COOH or the CH3. This suggests substituted benzoic acids, and in particular, one may define the steric demand of the COOH group by the difference of the (implicitly gas phase) enthalpies of formation of its o-and p-alkylated derivatives [1]. For R = CH3, C2H5, i-C3H 7, and t-C4H 9, the p-substituted benzoic acids (RC6H4COOH) are more stable than their o-isomers by 9.8 + 2.1, 15.4 + 2.6, 23.5 +__ 2.8, and 21.4 + 2.9 kJ mol -I, respectively. The corresponding p-alkyltoluenes (RC6H4CH3) are found to be more stable than their o-isomers by 1.1 + 1.5 (this, and all otherwise unreferenced enthalpy of formation data, is taken from Ref. ) kJ mol -I, respectively. In all cases, the p-disubstituted benzene is more stable than its o-isomer and in the cases of methyl, ethyl, and isopropyl substitution, the COOH group is considerably destabilized relative to CH 3. We would thus immediately conclude

Interplay of thermochemistry and structural chemistry, the journal (volume 24, 2013, issues 5–6) and the discipline

“…Roux, some of her Spanish coworkers (including her thesis advisor), and one of the current paper's authors showed [126] there was a nearly constant gas phase enthalpy of formation difference for alkylated benzoic acids and the corresponding toluenes in which COOH was replaced by CH 3 . We ask, then, does this near equality apply to the substituted benzoic acids in the paper by Vecchio? In the next paper, Miranda et al [127] reported a computational study using DFT to calculate the structure and energetics of 2-and 3-pyrrolidinone, tetrahydro-2-and -3-furanone, 2-, 3-, and 4-piperidinone, and tetrahydro-2-, -3-, and -4-pyranone.…”

Paradigms and paradoxes: a comparison of the enthalpies of formation of trinitromethyl and trimethylmethyl (t-butyl) containing species

“…In an earlier study, Colomina et al [2] compared the energetics of alkylated toluenes and benzoic acids from which they concluded that the methyl and carboxylic acid functionalities were very similar, at least from the vantage point of gas phase enthalpies of formation and size. In this study, it was shown that the difference of the enthalpies of formation of an alkylated toluene and corresponding alkylated benzoic acid was nearly a constant; and therefore, it was concluded that the steric demand of these two groups was nearly the same.…”