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What is the basic difference between aqueous and alcoholic $\ce{KOH}$? Why does alcoholic $\ce{KOH}$ prefer elimination whereas aqueous $\ce{KOH}$ prefers substitution?

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In alcoholic solution, the $\ce{KOH}$ is basic enough ($\mathrm{p}K_{\mathrm{a}} =15.74$) to deprotonate a small amount of the alcohol molecules ($\mathrm{p}K_{\mathrm{a}}= 16–17$), thus forming alkoxide salts ($\ce{ROK}$). The alkoxide anions $\ce{RO-}$ are not only more basic than pure $\ce{OH-}$ but they are also bulkier (how much bulkier depends on the alkyl group). The higher bulkiness makes $\ce{RO-}$ a worse nucleophile than $\ce{OH-}$ and the higher basicity makes it better at E2 eliminations.

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  • $\begingroup$ I don't get how the alcoholate salts are formed. Where is the 'R' group coming from? $\endgroup$ – Internet Guy Jul 30 '17 at 4:21
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    $\begingroup$ @InternetGuy $\ce{ROH}$ is the notation for a generic alcohol, where $\ce{R}$ is usually an alkyl chain. Thus, when $\ce{KOH}$ deprotonates an alcohol, you get an alcoholate salt $\ce{ROK}$, i.e. $\ce{KOH + ROH <<=> ROK + H2O}$. $\endgroup$ – Philipp Jul 30 '17 at 11:15
  • $\begingroup$ Hmm, isn't $\ce{RO^-}$ often considered a stronger nucleophile that $\ce{OH^-}$? (or did you factor in solvent interactions as well?) :-) $\endgroup$ – paracetamol Dec 31 '17 at 7:01
  • $\begingroup$ Does that mean that in ethanolic KOH, it is the ethoxide that is the primary basic species involved in proton abstraction from the substrate. Does that also mean that the concentration of the alkoxide ion would be factored into the rate law? $\endgroup$ – Tan Yong Boon Jun 12 at 3:45
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$\ce{OH-}$ acts as a nucleophile. Reactions carried out in alcohol tend to be elimination reactions, and reactions carried out in water (aqueous) tend to be substitution reactions.

If water were used as a solvent in an elimination reaction involving $\ce{KOH}$, the equilibrium would be shifted towards the reactants (water reacting with product), so substitution is favored.

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  • $\begingroup$ Can you explain your answer in detail please $\endgroup$ – ragvri Sep 1 '14 at 3:21
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The reason is quite straightforward-

  1. $\ce{OH^-}$ is a weak base and a stronger nucleophile specially under polar protic conditions. Hence substitution occurs.

  2. $\ce{RO^-}$ is a strong base owing to the inductive effect (+ I effect) of the R group. Hence under alcoholic conditions, $\ce{RO^-}$ extracts the $\beta$ hydrogen of the halides and gives an alkene.

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Aqueous KOH is more solvated by water and not able to abstract H+from substrate but act as nucleophile but alc KOH is less solvated and alkoxide ion can abstract acidic hydrogen

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