# Mhchem feature request: strikethrough and crossout reaction arrows

mhchem is a great $\mathrm{\LaTeX}$ and MathJax package and I don’t know what we would do without it. Especially its way to format reaction equations is great.

However, it is severely lacking the ability to easily access strikethrough and crossedout reaction arrows. These are used to indicate nonoccuring reactions (often depending on personal preference). Yes, you could work around by superimposing $//$ over the reaction arrow with the appropriate command (or $\mathrm{X}$) — but that’s a very high-level and low-accessability solution.

Much better would be one that uses the intuitive syntax of mhchem reaction formatting and also would include the added bonus of allowing for adding things above and below the reaction arrow. I propose the following syntax:

\ce{C6H6 + Br2 ->[FeBr3] C6H5Br + HBr} (already implemented)
\ce{C6H6 + Br2 -//->[$h \cdot \nu$] C6H5Br + HBr} (strikethrough)
\ce{C6H6 + Br2 -x->[$h \cdot \nu$] C6H5Br + HBr} (crossout)


Of course, this shouldn’t be restricted to simple arrows but should extend to mesomery (<-//->), equilibria harpoons (<=//=>) and hopefully also equilibrium double arrows (<--//-->).

I would love to see this implemented in a future version — not only of the MathJax mhchem but also for the $\mathrm{\LaTeX}$ package!

Note: I tagged this rather than since this is nothing for the SE devs to implement.

Thanks for your nice words about mhchem. I am happy to implement this feature. In fact, it already is listed on my to-do list.

There are four steps, that I need to decide on, for each mhchem feature request. You could help me with your input and suggestions.

1. Is this a common notation?

I think, we can agree on that. (Even though I am not a chemist, I've seen it several times.)

2. How should the output rendering look like? (Authoritative arguments)

To decide this, I would need a small collection of scans/photos that show how different publishers render this feature. These should be several well-known publishers (authorative arguments). Preferably, they don't use LaTeX. This is to see the bandwidth of possible typographic solutions. Further questions: What do IUPAC and ACS recommend? Did the recommendation or common practise change recently?

This is the point where I need the most help from you. (I am not a chemist and my chemical knowledge is shrinking every day ;-)

3. What should the LaTeX code (MathJax) be?

If 2 is decided, I need some LaTeX code to achieve it.

4. How should the input syntax look like?

Assuming, we find out in step 2 that the rendering should be a single strike, then the input syntax could look like this

normal  strike-through
->      -/->
<-      <-/-
<->     <-/->    ambiguous
<-->    <--/-->
<=>     <=/=>
<=>>    <=/=>>
<<=>    <<=/=>


or

normal  strike-through
->      /->    intuitive enough?
<-      /<-    maybe <-/ as well
<->     /<->
<-->    /<-->
<=>     /<=>
<=>>    /<=>>
<<=>    /<<=>


• This recent TeX.SE question talks about how to $\mathrm{\TeX}$ this. Well, something practically identical to double-crossed out.