Depto. Química Orgánica

SUBSTITUTION OF ALCOHOLS

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15/07/2017
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The OH group is one of the worst leaving groups ever.
It is imprescindible to convert it in something else in order to undertake its substitution.
For instance, the alcohol can be treated with a strong mineral acid to induce the protonation of the OH.

Once the OH is protonated, the leaving group turns into a water molecule:
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Protonation of the OH involves a severe increase in the C-O bond polarization, making much easier the spontaneous heterolitic cleavage, leading to a carbocation. The production of a carbocation always bring rearrangement problems, i.e. the formation of UNEXPECTED products.
Look carefully!!!
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Don't you see the Br group ended up on an unexpected position?
The only way to explaining that experimental result is to invoke a mechanism where a hydrogen rearrangement took place in the following manner:
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Image How come the  rearrangement?

What is the driving force for the rearrangements?
 ANSWER:
The different stability of carbocations. Click here
There's a way to avoid going through carbocations:
Derivatize the alcohol as an ester by means of an inorganic acid!!!
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Thionyl chloride
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Phosphorous tribromide
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Sulfonic acids