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I think sometimes it is necessary to show, that a reaction does not take place, therefore a broken arrow would be a nice thing to have. I played a little in the sandbox and found a somewhat working solution.

The fist solution requires a package, that is in the repository of MathJax, but not yet activated. therefore it has to be included via \require{cancel}. My first request is to activate it permanently on the site. Sometimes I would like to show that units cancel, etc.: $$\require{cancel} \begin{align} n&=c\cdot V\\ n&=1~\mathrm{\frac{mol}{\cancel{L}}}\cdot 1~\cancel{\mathrm{L}}\\ \end{align} $$

As for the broken arrows, this looks somewhat ugly, but it works:

  • \ce{\bcancel{->}\cancel{->}\xcancel{->}}: $\ce{\bcancel{->}\cancel{->}\xcancel{->}}$
  • \ce{\xcancel{<=>}\xcancel{->}\xcancel{<-}}: $\ce{\xcancel{<=>}\xcancel{->}\xcancel{<-}}$

A different possibility is to just use nrightarrow, but the arrow looks rather odd (too small) compared to the others in chem mode and harpoons do not even exist

  • \nrightarrow\ce{->}: $\nrightarrow\ce{->}$
  • \nleftarrow\ce{<-}: $\nleftarrow\ce{<-}$
  • \nleftrightarrow\ce{<->}: $\nleftrightarrow\ce{<->}$
  • \nrightleftharpoons\rightleftharpoons\ce{<=>}: $\nrightleftharpoons\rightleftharpoons\ce{<=>}$

Basically my idea is to define an addendum like \not which can be used like \not= or \not\in in $\not=$ or $\not\in$.

The following snipped makes use of of rlap, which produces a box of zero size and typesets the following element right of it.

  • \ce{\rlap{~~/\!/}->}: $\ce{\rlap{~~/\!/}->}$

Now it is much more convenient to have a command handy, that is why we can define it via \def\nnot{\rlap{~~/\!/}}, we can even add colour via \def\nnot{\color{red}{\rlap{~~/\!/}}} $\def\nnot{\color{red}{\rlap{~~/\!/}}}$ and now we can use it in an equation:

  • \ce{c \nnot-> d}: $\ce{c \nnot-> d}$
  • \ce{c \nnot<-> d}: $\ce{c \nnot<-> d}$
  • \ce{c \nnot<=> d}: $\ce{c \nnot<=> d}$

I admit, this is nowhere near an ideal solution, but since mhchem does not currently provide a feature for this I guess this is the best way to use it.

Lastly, I would like to second the demand on physics.se: https://physics.meta.stackexchange.com/questions/4220/siunitx-latex-package

But there seem to be not much efforts about that.
There is an official feature request by our own Manishearth. And it seems to be moving forward.

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    $\begingroup$ I absolutely second the demand for siunitx! $\endgroup$ – Klaus-Dieter Warzecha Mar 24 '15 at 18:48
  • $\begingroup$ @Klaus Unfortunately there is not much we can do to get that rolling, unless you know how to reimplement it into mathjax :/ $\endgroup$ – Martin - マーチン Mar 25 '15 at 4:12
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    $\begingroup$ Then we have to live without if for the time being. But it seems that Joseph Wright, the author of siunitx is aware of the demand and indicated that things might get easier with siunitx v3. $\endgroup$ – Klaus-Dieter Warzecha Mar 25 '15 at 5:27
  • $\begingroup$ Another way is to simply use the unicode letter for the cancel effect: \cancel{→} $\endgroup$ – Gaurang Tandon Apr 10 '18 at 6:57

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