I have a question concerning acid/base reactions. Is it true that an alcohol is a better base than water? My Professor listed the basicity (because we talked about E1/E2 and Sn1/Sn2 reactions) of the following compounds the following way: RO- > OH- > ROH > H2O (with stronger base being the RO- and the weakest here H2O). I would like to understand that with respect to pka/pkb values...

Now I came across the following problem: Bronstedt Bases/Acids are determined by their equilibrium constant in WATER (pka/pkb). Alcohols (like ethanol have a pka about 16-19), water has a pks of 14, which makes alcohol a base by definition.

  1. if that happens -> ROH + H2O -> ROH2(+) + OH(-), wouldn't you calculate the pkb for ROH and the pks for ROH2+? The alcohol is the base here? Unfortunately I haven't found any pkb value for ROH on the internet... (I don't mean the conjugated base of ROH!) - alcohols are even considered amphoteric? how is that possible? why do I only find pka (and no pkb) values then, like for ammonia?
  2. Ammonia is considered amphoteric and has a pka and a pkb value. But if these equilibrium constants are determined in water, how can it have a pka value, if it only acts as a base there? Shouldn't be water the only molecule that can be considered amphoteric in water?
  3. If we would say molecules like H2O and NH3 can "generally" donate a proton or take up a proton and because of that are considered amphoteric, aren't there many more molecules than can take up and donate a proton when a strong acid/base is present? wouldn't there be many more amphoteric molecules?

So I'm really trying to understand why if alcohols can act as base AND as acid, why there are not two values for that (like for water (pka and pkb=14) and ammonia (pka=23 and pkb= 4,75), even though that would require two different reactions in alcohols too: ROH -> ROH2(+) ROH -> RO(-)

I hope I haven't confused you too much and explained my problem at least somewhat clearly. :)

  • 3
    $\begingroup$ Some pKa tables will list values for $\ce{R-OH2+}$ compounds, typically with values around -2 to -4. The pKa of $\ce{H3O+}$ is usually given as around -1.75. You can convert these to pKb if you prefer to work with that scale. Also remember that the values for water and its derivatives are distorted by water being the bulk solvent, so the values are not calculated using the normal 1M of everything. There are a number of questions on this site about the "actual" pKa of H2O if you need a refresher. $\endgroup$ – Andrew Jan 18 at 17:09
  • $\begingroup$ Thanks so much!! I guess I was just overthinking too much. Getting the pkb of the conjugated base of R-OH2+ totally makes sense. :) $\endgroup$ – Felix H. Jan 18 at 17:12

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