If we had a hexane ring with an $\ce{OH}$ substituent vs. a hexane chain with an $\ce{OH}$ substituent, which of the two, if any, would result in a weaker conjugate base?

My confusion: The ring appears to be a bulky shape, but the chain would have one more hydrogen in it...

What do you think and do you have a source reference? There isn't anything in the books I have on structures and their influence on acidity.

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    $\begingroup$ It's not a "forum" but Q&A site (yes there's a difference ;) As far as your question is concerned, you need to specify which hexanol isomer you want to consider. Another thing is does it matter much which will be more acidic? I think that difference should be negligible for most purposes. $\endgroup$ – Mithoron Sep 20 '18 at 21:33
  • $\begingroup$ It's just for something silly like ranking acidity. Generally, in the text books i'm reading, there is not a way to distinguish from two very similar alkane molecules. I was more curious if there was a way I wasn't exposed to. (Probably no practicality, i'm just being picky?) I guess structure is more important, but I was hoping to find a generic "rule of thumb" similar to the "ARIO" or "CARDIO" pneumonic. Since Charge, Resonance, Induction, and Orbitals are all similar, the only thing I could think that would affect acidity would be its geometry?... In a normal case I would just check pka. $\endgroup$ – Ron Wilson Sep 21 '18 at 0:42
  • $\begingroup$ Well, simple picking the isomer and editing your question would do more good. BTW pKa values of such weak acids are quite uncertain. $\endgroup$ – Mithoron Sep 21 '18 at 19:45

My guess was that the electron donating character of the neighbouring methyl groups will play a deciding role. The more electrondonating group are attached to the hydroxyl carbon, the less stable the resulting alkoxide anion (the oxygen would become more negatively charged), and the less acidic.

So then a secondary alcohol would be less acidic than a primary alcohol, and cyclohexanol would be less acidic than n-hexanol. In comparison with e.g. hexane-3-ol the difference will be very subtle. One explanation I found on quora (https://www.quora.com/What-is-the-order-of-acidity-among-primary-secondary-and-tertiary-alcohols) points out the importance of steric factors. Indeed, the negative charge will be further stabilized by surrounding water molecules. The potential steric hindrance caused by neighbouring alkylgroups would appear to be less for the cyclohexanol anion than for hexane-3-ol anion which should lead to greater stabilization of the cyclohexanol anion. The latter would probably be slightly more acidic then. Keep in mind that this is all handwaving to some extent, definitely since these differences are so subtle and small unforeseen effects can easily change the ranking of acidity.


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