Reactivity in Chemistry

Electrophilic Rearrangement

 

ER2. Pinacol Rearrangement

The pinacol rearrangement is a reaction of 1,2-diols.  It takes place under the influence of strong acids, inluding mineral acids like sulfuric acid.  It can also be brought about via the use of Lewis acids.  The reaction overall involves the loss of one of the hydroxyl groups, the conversion of the other hydroxyl group into a carbonyl, and the shift of an alkyl group.

 

 

The heart of the rearraction is the 1,2-shift of an alkyl group.  This shift is assisted by π-donation from the oxygen, converting the C-O bond into a carbonyl at the same time.

The entire mechanism would look like this:

In many rearrangements, formation of the actual cation is not necessarily involved.  The 1,2-shift could occur as the water molecule leaves.  The protonated oxygen may draw enough electron density toward itself to induce the shift.

Frequently there is a fine line between alternative reaction pathways.  In many cases, the stability of an intermediate determines whether a reaction happens in stages or in one step.  If the intermediate is relatively stable, the reaction will happen in steps.  The less stable the intermediate, the more likely the reaction will proceed in a concerted manner.

Problem ER2.1.

Fill in the products of the following pinacol rearrangements.

Problem ER1.2.

The Tiffeneau Demjanov rearrangement is superficially similar to a pinacol rearrangement.  It occurs when a β-amino alcohol is treated with nitrous acid.  The reaction results in loss of the amino group with rearrangement of the molecule.  Predict the product of the reaction.

This site is written and maintained by Chris P. Schaller, Ph.D., College of Saint Benedict / Saint John's University (with contributions from other authors as noted).  It is freely available for educational use.

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Structure & Reactivity in Organic, Biological and Inorganic Chemistry by Chris Schaller is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License

Send corrections to cschaller@csbsju.edu

This material is based upon work supported by the National Science Foundation under Grant No. 1043566.

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

 

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