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This is the molecule I know as epoxypropane:

In my neck of the woods (example), this molecule is used as one of the standard examples of small chiral molecules. The name 'epoxypropane' makes a heck of a lot of sense to me $-$ an epoxy bridge spanning a single bond of what would otherwise be propane.

It seems, however, that most everyone thinks that a better name for this is propylene oxide, from Wikipedia upwards. Now, I'm an atomic physicist, so bear with me:

  • I can see how the name 'propylene oxide' fits that structure. You've taken propylene and you've oxidized it.
  • However, I cannot see why the name 'propylene oxide' doesn't also apply to other ways to oxidize propylene, say, sticking that oxygen over at one end or in the middle,

    i.e. what Wikipedia calls propionaldehyde (also propanal) and acetone, or making that bridge over the (1,3) combination in what Wikipedia calls oxetane,

    none of which is chiral.

Now, I imagine that IUPAC has all sorts of rules to suss these things out, so:

  • why is 'propylene oxide' the preferred name for epoxypropane? and
  • why is epoxypropane the only isomer encompassed by the designation 'propylene oxide'?
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why is 'propylene oxide' the preferred name for epoxypropane?

Propylene oxide is not the "preferred" IUPAC name (PIN): Wikipedia lists the PIN as 2-methyloxirane, which is in line with the 2013 IUPAC recommendations.

why is epoxypropane the only isomer encompassed by the designation 'propylene oxide'?

It's more of a tradition than anything else. Note how the two ring carbons in propylene oxide are the same as the two doubly bonded carbons in propylene: this is not a coincidence. A very common way of synthesising an epoxide is to oxidise the corresponding alkene (you can think of it as simply adding an oxygen atom across the double bond), and so "alkene oxide" has become a rather conventional way of referring to an epoxide (see also https://en.wikipedia.org/wiki/Styrene_oxide for another example).

So, the meaning of "propylene oxide" is not really "an oxidation product of propylene". It should pretty much be read as "the epoxide derived from propylene", or "the epoxide formed when you add an oxygen atom across the double bond of propylene".

In the strict sense of oxidation/reduction being addition/loss of oxygen, oxetane is an oxidised form of propylene, but there is no simple chemical oxidant that will perform this transformation. So, calling oxetane "propylene oxide" would never have - and has never - caught on.

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  • $\begingroup$ For full clarity, can acetone and propanal be considered to be oxidised forms of propylene? Or have you singled out oxetane for a reason, and the double bonds change the extent to which one can speak of oxidation there? $\endgroup$ – Emilio Pisanty Dec 19 '19 at 0:45
  • $\begingroup$ The issue isn’t so much about what is an oxidised form of propylene or not; they all are, including oxetane, which is what I wrote above. But if you’re asking about whether there are chemical transformations to take propylene to acetone or propanal, then yes, there are the Wacker oxidation and hydroboration/oxidation respectively. $\endgroup$ – orthocresol Dec 19 '19 at 9:15
  • $\begingroup$ ...and just to ward off any potential objections: the most common hydroboration/oxidation protocol uses H2O2/NaOH at the end, and this produces an alcohol instead of an aldehyde, but you can use other oxidants (NMO/TPAP) to go straight to the aldehyde. $\endgroup$ – orthocresol Dec 19 '19 at 9:56
  • $\begingroup$ No, I was just wondering why you had singled oxetane out. Thanks for the detailed response! $\endgroup$ – Emilio Pisanty Dec 19 '19 at 12:15
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The name propylene oxide does not fit the structure. It fits the synthesis, because this substance is made by oxidation of propylene (old name of propene). The oxidant is usually a peracid, or hydrogen peroxide.

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