Chemistry Asked by Khushi Ladha on November 5, 2021
A question taken from a test series (Vibrant, India):
Amongst butan-2-ol and butane-2,3-diol, which has higher rate of dehydration?
The carbocation formed in butane-2,3-diol is more stable as it will rearrange to get +M stability from alcohol group. However, the product will be aldehyde (due to tautomerism) for butane-2,3-diol and alkene for butan-2-ol.
As rate is determined by stability of carbocation, butane-2,3-diol should have faster rate. Also, the products don’t give any extra hint. Right? But the answer given for higher rate is butan-2-ol. Is there any point that I have missed out?
For this question, given the lack of initial information, I'm making the following assumptions:
Butane-2,3-diol dehydration is nicely described in this paper from Emerson1:
Aqueous 2,3-butanediol was shown to react in a pseudo-first-order reaction in the presence of sulfuric acid to form methyl ethyl ketone. The reaction could be described by a pinacol rearrangement mechanism and had an activation energy of 36 kcal/mol.
Finding the data for dehydration of 2-butanol proved more elusive. One study2 conducted dehydration rate experiments for secondary alcohols at 250 °C and 40 bar pressure and obtained a rate constant for 3-hexanol to be $k = pu{(0.067 pm 0.021) h^{-1}}$. Assuming this rate would be similar to 2-butanol, we can find the activation energy via the arrhenius equation, which comes out to be around 48 kJ/mol, which is around 11.52 kcal/mol. Another source3 conducted the same experiment over a copper-chromite catalyst, and obtained an activation energy of 13 kcal/mol. This means that 2-butanol will dehydrate faster than butane-2,3-diol.
From a conceptual standpoint, this makes sense: The pinacol rearrangement mechanism would require a hydride shift, which would be the RDS and would take more time and energy than a simple carbocation formation. However, if you're comparing absolute reaction rates, a large number of factors come into play, and given the right conditions, butane-2,3-diol can dehydrate faster than butan-2-ol.
References:
Answered by Aniruddha Deb on November 5, 2021
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