Reactivity in Chemistry

Electrophilic Addition to Alkenes

 

EA8. Epoxidation

Earlier, we saw that alkenes can donate their pi electrons to electrophiles such as "Br+".  In the bromonium ion that results, a lone pair on the bromine can donate back to the incipient carbocation, leading to a more stable intermediate.

We have also seen that addition to alkenes can sometimes be concerted, happening all at once, rather than one step at a time.  For example, in hydroboration, the boron and the hydrogen add to the double bond at the same time. 

The boron is adding just slightly ahead of the hydrogen.  The initial interaction is donation from the pi bond to the Lewis acidic boron.  However, as soon as positive charge starts to build up on carbon, and negative charge starts to build up on boron, the hydride is immediately donated.  Time is not allowed for the charged intermediate to fully form before proceeding.

Really, that's what is happening to the bromine, too.  As the alkene starts to donate its pi electrons to the bromine and begins to build up positive charge, the bromine's lone pair is drawn back to the alkene.  As a result, the intermediate that we imagine with a full positive charge on carbon and no charge on bromine exists too fleetingly to be considered an intermediate at all.  As soon as it begins to form, it is already turning into something else.

 

That sort of concerted addition happens with some other electrophiles, too.  If an atom is electrophilic, but also has a lone pair to donate, that cyclic transition state can lead to the product in one step.

Alkene epoxidation is another example of this kind of reaction.  An epoxidation is the transfer of an oxygen atom from a peroxy compound to an alkene.  Peroxides contain O-O bonds, which are relatively weak and reactive.

To simplify a little bit, just look at the reaction from the point of view of the alkene.

When the oxygen atom is transferred, it forms an epoxide (sometimes called an oxirane).  It is a three-membered ring containing two carbons and an oxygen.

Like in a bromination, the electrophile is deceptive.  It is an oxygen atom, which we more naturally think of as a nucleophile.  However, just as Br2 contains an atom attached to a good leaving group (Br -), so do the kinds of oxygen compounds used in epoxidation.  Most often, these are "peroxy acids", carboxylic acids containing an extra oxygen.

As in the bromination, as soon as the alkene begins donating to the electrophile, a lone pair can donate back, so that an unstable cation does not have to form.

The entire mechanism is believed to be concerted, based on a number of lines of experimental evidence.  A number of things need to be accomplished; in addition to the oxygen donation, the leaving group must leave, and a proton must be transferred.

The reaction mechanism can be cleaned up slightly because it is thought to be an example of a pericyclic reaction.  Pericylcic reactions frequently involving three pairs of electrons moving in a circle.  Like the three pairs of electrons in a benzene ring, this structure is thought to be unusually stable.

 

Apart from peroxy acids, many other peroxides can be involved in epoxidations, as well as some metal oxides.  In some cases, the reaction is extremely slow, but works better in the presence of a catalyst.

Problem EA8.1.

Predict the products of the following reactions.

 

Epoxidation reactions display an almost counter-intuitive selectivity.  Unlike hydrogenation reactions, which are generally easier with less-substituted alkenes, epoxidations are much faster with more-substituted alkenes.  In the case of hydrogenations, the selectivity can be understood as a combination of steric factors (the alkene must bind to a catalyst) as well as thermodynamicic factors (more substituted alkenes are more stable, so they are less likely to react).  However, in epoxidations, the more electron-rich the alkene, the more easily it can be induced to react with the peroxide.  More substituted alkenes are generally more electron-rich than those that are substituted only with electron-poor hydrogens.

 

Problem EA8.2.

Predict the products of the following reactions.

 

Problem EA8.3.

Fill in the boxes in the following synthesis.

Problem EA8.4.

Fill in the boxes in the following synthesis.

Problem EA8.5.

Fill in the boxes in the following synthesis.

 

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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