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IUPAC recommends: [x+y] for the involved electrons and (x+y) for the involved atoms.
So: Diels Alder is [2+4] and (2+4) 1,3-dipolar cycloaddition is [2+4] and (2+3).
Actually, many people still use [2+3] for the 1,3-dipolar cycloaddition, but I think, the difference or the existence of both variants should be part of the text.
There's a page for this sort of stuff -- it's the pericyclic reactions page. No need to call attention to 2+2+2 (this is so far, exclusively metal-catalyzed) or ene (why is that special over, say, claisen)?
I clarified the two different notations. It all links to the IUPAC Gold Book but that should be sufficient. I also divided the photochemical and thermal reactions into different sections.
The Simple English article for this is actually quite good and may be worth porting to here.
Roches ( talk) 00:55, 26 February 2014 (UTC)
This page really incomprehensible. I admit to being terrible at chemistry but the use of "cyclic adduct" (undoubtedly a very basic concept), makes the definition appear circular to me. I realize that this is cited as "standard definition". As I understand it, it means a molecule/adduct is attached to a bond. Then it can either break that bond to replace it with two new sigma-bonds to the adduct or leaving the original sigma-bond intact but forming two new ones from the pi-bonds (forming something like a triangle). Would it be possible to simplify this by putting the formal definition later in the text, starting with an explanation in more layman's terms? -- Jaapkroe ( talk) 14:01, 2 July 2014 (UTC)