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I could not find any rule in Graphic representations (chemical formulae) of macromolecules (IUPAC Recommendations 1994) that would state that the bonds at the ends of the constitutional units must intersect the enclosing parentheses/braces. Only that they must be drawn:

Rule 1.3:

To make the formulae more concise, dashes representing chemical bonds may be omitted. At the ends of constitutional repeating units and monomeric units, dashes must be attached.

So the following representations of poly(vinyl chloride) would be unacceptable: $\ce{(CH2-CHCl)_n}$   or   $\ce{(CH2CHCl)_n}$

For irregular (co)polymers, they bonds use to be drawn inside of the brackets (probably for aesthetic/consistency reasons?), e.g.:

3-E2:

poly(chloromethylene/methylene)
(chlorinated polyethene that does not contain dichloromethylene units):

$\ce{\bond{...}-CH2-CHCl-CH2-CH2-CH2-CHCl-CHCl-CH2-CH2-CH2-\bond{...}}$

with constitutional units:     $\ce{-CH2\bond{-}}$    ,    $\ce{-CHCl\bond{-}}$

$\ce{(-CH2\bond{-}/-CHCl-)_n}$

But for the “normal” polymers, the side bonds are almost always drawn over the parentheses/braces, so that they intersect them.

Such feature is not in mhchem, but we have several other options:

  1. Use the “copolymer style”, i.e. the side bonds inside the parentheses:

    $\ce{(-CH2-CHCl\bond{-})_{n}}$    or    $\ce{[-CH2-CHCl\bond{-}]_{n}}$

  2. “Simulate” the crossing by inside/outside pair of bonds:

    $\ce{-(-CH2-CHCl\bond{-})_{n}\bond{-}}$    or    $\ce{-[-CH2-CHCl\bond{-}]_{n}\bond{-}}$

    (This style also seems to be used officially, for copolymers, when the end groups are specified)

  3. Use a negative space “hack” (see also e.g. Eliminate huge-locant-marked (by \overset) atom side space), i.e. use several \!'s before the symbol (left bond and the right bracket) to make it left-shifted:

    $\ce{(\!\!\!\!-CH2-CHCl-\!\!\!\!)_{n}}$    or    $\ce{[\!\!\!\!-CH2-CHCl-\!\!\!\!]_{n}}$

    (It may not work in web browsers as widely as supported mathjax constructs. If it worked and if mathjax had a support for longer bonds, it would be yet nicer.)

Is there yet better solution for this?

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mhchem doesn't support this yet.

I'll put it on my the (mid-term) to do list. Thanks for the link to the IUPAC document.

Please, please, don't write spacing commands inside \ce, like $\ce{(\!\!\!\!-CH2-CHCl-\!\!\!\!)_{n}}$ (your 3rd option), because this is not future-proof.

You might use the use dollar signs to escape to math feature like $\ce{$(\!\!\!\!-$CH2-CHCl$-\!\!\!\!)$_{n}}$ to get $\ce{$(\!\!\!\!-$CH2-CHCl$-\!\!\!\!)$_{n}}$ as a work-around.

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