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So everywhere that I've read, there's the reasoning that the acidity of a compound is determined by how stable the conjugate base is, this should be the same for alcohols too, but on Chemistry LibreTexts there's an article which says otherwise, and the reason being that if the electron donating ie. inductive effect was responsible for this then we would have obsereved the same acidity order of alcohols in the gaseous phase too, where the relative acidity trend is quite opposite of what it is in solution.

The rationale behind this as is given in the article is:

Polarizibility almost completely accounts for the trend in gas-phase acidities. As the size of the substituent increases, the acid becomes stronger due to the ability for the charge to be distributed over a larger volume, thereby reducing the charge density and, consequently, the Coulombic repulsion.In solution, however, the ions can be stabilized by solvation, and this is what leads to the inversion of acidity ordering. Brauman and Blair showed that smaller ions are better stabilized by solvation, which is consistent with the Born equation.

Now there are these two things that are not clear to me

  1. If the substituents are almost similiar in size such that the charge almost gets a similiar size to spread, let's say they are amine and methyl groups on the beta carbon, will we then take the inductive effect into account to determine the acidity of the alcohol?

  2. Why is this then only applicable to alcohols? Why not the same reasoning for determing basicity and acidity of other functional groups?

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    $\begingroup$ That's just a thing to show whys and hows of "inductive effect" of alkyl groups vs hydrogen. See, carbon is actually more electronegative then hydrogen, and yet ethanol is weaker acid then water in aqueous solution. In gas phase, not so much. And, yeah, polarisability is probably more adequately correlated parameter for gas phase in such situation. $\endgroup$
    – Mithoron
    Aug 10, 2021 at 17:27
  • $\begingroup$ @Mithoron Could you expand on the whys and hows, I'm not sure how electronegativity and inductive effect correlate. $\endgroup$ Aug 11, 2021 at 6:16

1 Answer 1

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As mithoron has said in the comments, alkyl basically acts as an EWG in the gas phase, as shown e.g. by Boand et al. It's easily seen that it is very very weak, which could be easily explained, as there's not much of a difference between the electronegativity of C and H, and the existence of hyperconjugation "donates" some electrons.

But, another common fact is that alkyl substitutions on ammonium ions decrease their acidity, as is shown by Aue et al., which means that it should be regarded as an EDG for ammonium ions.

So, what is really happening here is that alkyls are highly polarizable compared to hydrogen, which means that whenever an alkyl is near a charge, no matter positive or negative, it will create a dipole that disperses charge, which would act as a stabilizing force. In this way, we could explain the stability of carbocations, carbanions, etc.

Boand, G.; Houriet, R.; Gaumann, T. Gas-Phase Acidity of Aliphatic Alcohols. J. Am. Chem. Soc. 1983, 105, 2203-2206. DOI 10.1021/ja00346a600

Aue, D. H.; Webb, H. M; Bowers, M. T. Quantitative relative gas-phase basicities of alkylamines. Correlation with solution basicity. J. Am. Chem. Soc. 1972, 94, 4726-4728. DOI 10.1021/ja00768a049

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