How many individuals have purchased a 2-tube pack of epoxy at the local hardware, or even grocery store? It’s a routine item we use to repair an assortment of objects with. But what is it that makes epoxy glue so strong? Its name hints at its contents. Epoxy glues involve epoxy compounds—epoxides. What is an epoxide?
It’s Simple? Actually, Yes
An epoxide is a molecule that contains the tiny 3-member ring in its structure, one member of which is oxygen, and two of which are carbon. The epoxy link is illustrated in the image featured with this article, Figure 1. R₁, R₂, R, and R₄ are pendant groups. How is such a 3-member ring generated? Frequently through oxidation of an alkene. Doubtless the reader will want to be provided an instructive example.¹
Consider the simple oxidation of propene, CH₃CH=CH₂.² First, consider what would be a logical oxidizing agent for the reaction? It is not practical to attempt oxidation by means of oxygen gas. That gas, in order to be divided up into two oxygen atoms, O, would require a violence that would destroy the alkene.
Since we seek to add but one lone, single, atom of oxygen to the double bond in the mildest way possible, the logical choice is some kind of unstable peroxide. Why? A peroxide wants to break down, anyway. It doesn’t require “violent” conditions to give up an atom of oxygen! If a single molecule of hydrogen peroxide is decomposed, one O atom is released, giving water, H₂O.
The reaction, written in shorthand, is,
H₂O₂ → H₂O + [O]
Now, unfortunately, most organic compounds are not soluble in either water or hydrogen peroxide. So an organic peroxide or something similar is required. One possibility is to use a peroxycarboxylic acid, which acts in similar fashion to hydrogen peroxide,
RCO₂OH → RCOOH + [O]
The reaction we might use involves the combining of peroxyacetic acid and propene gas. See Figure 2. Note that the resultant products are de-oxidized, ordinary acetic acid plus the epoxide of propene—more commonly called propylene oxide. Yes, there are more practical synthetic methods for the preparation of propylene oxide. Our purpose, however, has been one of demonstrating how an epoxide can be prepared in general. Now, once prepared, of what value are epoxides (aka oxiranes)?
Commercial Epoxide Use
To what practical uses have epoxides been put? The oxirane ring is relatively unstable, being subject to opening in a host of reactions. Although the best known application for polymeric forms of oxiranes involves so-called epoxy resins, a host of other substances are readily derivable from epoxides.
Perhaps the most important epoxide is also the simplest one, ethylene oxide, (CH₂)₂O. It is one of the most diverse starting substances in the field of organic chemistry.
¹ Symbols for the elements discussed in this article are, for carbon, C, for hydrogen, H, and for oxygen, O.
² Propene is more commonly called propylene.