Branched Carbon Chains & Acidity (ochem q)

[shahcoco]

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Molecular formula C3H8O

There are 3 consitutional isomers. Why is the branched isomer with -OH at the end less acidic than the isomer that has a straight carbon chain with -OH at the end? Is it typical of branched carbon chains to have lower acidity relative to straight carbon chains?

 

[inasensegone]

Remember that alkyl groups are weakly electron donating. An acid can be described as an electron acceptor (Lewis acid). If the alkyl groups are e-donating, then this reduces acidity. A stronger acid can be made by having an electron withdrawing substituent instead of the alkyl groups. Anyway, with regards to the branched isomer, this means that the electron donating alkyl groups are closer to the Carboxylic acid functional group and therefore are better electron donators

 

[Czarcasm]

Molecular formula C3H8O

There are 3 consitutional isomers. Why is the branched isomer with -OH at the end less acidic than the isomer that has a straight carbon chain with -OH at the end? Is it typical of branched carbon chains to have lower acidity relative to straight carbon chains?

Well, if you consider the conjugate bases of each, the most highly branched alcohol is also the weakest alcohol because the surrounding solvent is less able to 'solvate' it essentially (which is a stabilizing effect), especially considering that positive dipoles of polar groups can associate themselves around the negatively charged oxygen atom and lower the overall energy. This is basically what attributes to its decreased acidity (higher pKa).

 

[Czarcasm]

Remember that alkyl groups are weakly electron donating. An acid can be described as an electron acceptor (Lewis acid). If the alkyl groups are e-donating, then this reduces acidity. A stronger acid can be made by having an electron withdrawing substituent instead of the alkyl groups. Anyway, with regards to the branched isomer, this means that the electron donating alkyl groups are closer to the Carboxylic acid functional group and therefore are better electron donators

Perhaps someone can correct me, but generally the whole alkyl groups are donating thing applies to carbocations and carbnanions via something known as hyperconjugation. In this case however, the electronegative oxygen atom is pulling from its neighbors with equal magnitude and so I think, irregardless of solvent's effect on branching, the negative density on the oxygen atom itself would be the same. (Whether the oxygen pulls intensly on 1 carbon atom, or with 1/3 the magnitude on 3 carbon neighbors = same overall electron density being contributed).

It would be different however, if we included some highly electronegative atoms nearby, for example, chlorine atoms, which would withdraw electron density from the oxygen. In this case, the oxygen would be more likely to lose its hydrogen (more acidic), because doing so would yield a pair of electrons that help stabalize the electropositive charge resulting from that inductive pull of neighboring electronegative atoms. In this case, however, I believe its the solvent that's the major contributor to the acidity trend.

 

[Czarcasm]

Confirmed in Textbook.

 

[inasensegone]

It looks like we were both correct 🙂