Posted by: Mark Foreman | August 30, 2013

Epichlorohydrin

Dear Reader,

I have just escaped from a Swedish cray fish party, and now it is time to think about organic chemistry again. Now I am sure that all of you will use some plastic during your daily lives, for example we wear plastic (nylon and polyester), we store data on plastic (polycarbonate CDs), we use plastic electrical leads (PVC), we use fluorinated polymers (PVDF) for fuel lines for diesel systems and we use plastics for paints and coatings for surfaces.

One important class of polymers for paints / coatings and glue are the epoxy resins. These are typically made using bisphenol A  and a bifunctional (two functional group) molecule known as epichlorohyrin.

Here is a molecule of epichlorohydrin in all its glory.

Epichlorohydrin, note one of the hydrogens in the molecule is hidden from sight.

Epichlorohydrin, note one of the hydrogens in the molecule is hidden from sight.

The bisphenol A is made normally by an electrophilic substitution reaction from phenol and acetone. The traditional route to phenol generates acetone as a side product, so the production of bisphenol A makes some sense. If phenol production occurs by the cumene route then where there is phenol there will never be a shortage of acetone. Here is bisphenol A.

Bisphenol A

Bisphenol A

We will explain this later but the epichlorohydrin is made from glycerol or from allyl chloride. But the more interesting part of the chemistry of epichlorohydrin is the reaction with the bisphenol A this reaction is normally done by heating the biphenol A with sodium hydroxide in water with epichlorohydrin present.

Here is the solvent accessable surface of epichlorohydrin, the more positive an atom is the more red it is and the more nagative an atom the more blue it is.

The solvent accessable surface of epichlorohydrin

The solvent accessable surface of epichlorohydrin

The epichlorohydrin could react with our phenol in three ways, the phenol as the phenolate anion could attack the alkyl chloride in an SN2 reaction but this is unlikely as the carbon bearing the chlorine only has a weak partial positive charge (calculated by Huckel theory to be 0.028). But the two epoxide carbons have much higher partial positive charges. The carbon bearing the CH2Cl group has a partial charge of 0.316 while the other one has a charge of 0.219. I think that the carbon with the lower partial positive charge will be the site of the reaction. Here is what I think the neutral form of the initial product will look like this.

Stage one product

Stage one product

While the alternative product would be

Alternative and wrong product

Alternative and wrong product

According to a molecular mechanics calculation the product of reacting a phenol molecule with the middle carbon of epichlorohydrin has a steric energy of 7.8423 kCal per mol while the reaction product in which the phenoxy group is attached at the end carbon is 3.9212 kCal per mol. I suspect that the transition states for the reactions forming these products would also be different.

The transition state for the reaction reacting at the end will be lower, so there is both a kinetic and thermodynamic reason why the less substituted end of the epoxide reacts. This sets up the molecule to then lose chloride to form a new epoxide group which can react with a second phenolate ion to form a link between two phenol groups. Here is the mechanism for the reaction.

The reaction of expichlorohydrin with one phenolate

The reaction of expichlorohydrin with one phenolate

I think that you should be able to work out how the second phenolate reacts with the epoxide which is generated by the loss of the chloride. If this is repeated again and again then it will generate polymers if we use a diphenol.

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