A tertiary alcohol has a $\mathrm{p}K_\mathrm{a}$ of 18 while a primary alcohol has a $\mathrm{p}K_\mathrm{a}$ of 16. Shouldn't the tertiary alcohol be more acidic since its conjugate base is more stabilized due to induction?
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1$\begingroup$ Which tertiary and primary alcohol are you referring to? Please give some references for your data. $\endgroup$– bonApr 30, 2015 at 21:03
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1$\begingroup$ I meant (CH3)3COH for tertiary and CH3CH2OH for primary. $\endgroup$– Jordan ChanApr 30, 2015 at 21:07
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2$\begingroup$ It's other way around with induction - tert-butyl group is electron donating stronger than ethyl. $\endgroup$– MithoronApr 30, 2015 at 21:55
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1$\begingroup$ Here is a nice explanation. Considering polarizability and solvation effects you can explain the solution pKa data, as well as why it reverses in the gas phase. $\endgroup$– ronMay 1, 2015 at 16:59
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$\begingroup$ Methanol is a weaker acid than t-BuOH in the gas phase. pubs.acs.org/doi/pdf/10.1021/ja00723a029 $\endgroup$– user55119Dec 6, 2017 at 19:41
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2 Answers
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The short answer is that the steric hindrance of the tert butyl alcohol makes it hard to add an protonate the hydroxy group.
- Based on this lecture from the vanderbilt university. The answer is due to steric hindrance:
The hydroxy group of the tert butyl alcohol is intramolecular stabilized by hydrogen bonds (2nd image), between the methyl hydrogens and the oxygen. In the other hand, i.e. the hydroxy group of methanol is intermolecular stabilized with another methanol molecules ("Behave more like a water") (3th image). Therefore, the equilibrium $\ce{ROH}\rightleftharpoons\ce{RO^-}+\ce{H^+}$ is more displaced to the right with the methanol than with the tert-butil alcohol.
The protonation of ter butyl alcohol is slower than methanol (Figure 4th and 5th), for the same reason: steric hindrance.
answered May 1, 2015 at 4:01
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The conjugate bases of tertiary alcohols are LESS stabilized due to induction!
Because carbon groups are donating groups they push their electrons towards the negatively charged oxygen making it even more negative and therefore making it more reactive. For the conjugate bases of less substituted alcohols there are less carbon groups pushing their electrons into the already negatively charged oxygen which makes it less reactive. Now when comparing something like methanol and water their pKa's are very similar! Water has a pKa of 15.7 while methanol has a pKa of 16. When you look at the conjugate base of methanol the one carbon group is pushing its electrons towards the oxygen making the oxygen slightly more negative and therefore more reactive. The conjugate base of water does not have any donating groups pushing their electrons into the oxygen to make it more negative and is less reactive.
