Chemistry - What is the formally correct notation of t-Bu, i-Pr, p-NO2, n-Bu, a-HCl, etc.?

Solution 1:

In the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), single italic lower-case Latin letters are used in some stereodescriptors (such as ‘c’, ‘m’, ‘p’, ‘r’, ‘s’, and ‘t’), in descriptors for fusion sites (e.g. dibenzo[c,g]phenanthrene), in locants (‘o’, ‘m’, and ‘p’ in place of ortho, meta, and para, respectively; however, this usage is strongly discouraged), and in other structural descriptors (e.g. s-indacene).

However, single lower-case Latin letters that are used for numbering are not italicized (e.g. naphthalene-4a,8a-diol).

All these symbols are typically used in names (which are usually not typeset using MathJax/mhchem) and do not necessarily appear in formulas.

Furthermore, common abbreviations for organic structure fragments may be used in formulas, but not in text. According to the Graphical Representation Standards for Chemical Structure Diagrams (IUPAC Recommendations 2008), the permissible abbreviations include:

• isopropyl: iPr
• isobutyl: iBu
• sec-butyl: s-Bu
• tert-butyl: t-Bu

However, the ACS Style Guide recommends different abbreviations:

• isopropyl: i-Pr
• isobutyl: i-Bu
• sec-butyl: sec-Bu
• tert-butyl: t-Bu

Furthermore, authors may create their own abbreviations, but any abbreviations not included in the recommendations shall be defined clearly when they are used.

Also note (since the question mentions letters followed by an element) that several structural abbreviations in current usage are identical to an element symbol:

• Ac (acetyl, actinium)
• Ar (aryl, argon)
• Cm (carboxymethyl, curium)
• Nb (p-nitrobenzyl, niobium)
• Np (p-nitrophenyl, neptunium)
• Pr (propyl, praseodymium)

Solution 2:

tl;dr: That's probably a good approach.

So, there are at least three different things here. None of these abbreviations are standardised as far as I'm aware[1], so whatever is done is going to be just as non-standard as any other approach.

1. Specific shorthands for alkyl functional groups: t-Bu, i-Pr, n-Bu, c-Hex

   I've seen the method you've suggested used for these, and also the first three with superscripted initial characters, e.g. $^{\textrm{t}}\textrm{Bu}$ and $^{\textrm{i}}\textrm{Pr}$, and I'd never seen the last one until I read this question. I've also seen them written simply as tBu, iPr, and nBu. They're abbreviations for tert-butyl, isopropyl, n-butyl (straight-chain), and cyclo-hexyl respectively. (Here are links for IUPAC References for non-cyclic and cyclic alkanes.)

2. Substitution points on a benzene ring: (p-NO2Ph)_2CO

   This example denotes a para- substituted phenyl group (confusingly, benzyl refers to a $\ce{C6H5-CH2 -}$ group). The other options here are ortho- and meta-, which I would personally not find it confusing or uncommon to see abbreviated to o- and m- respectively, and $\ce{{\it p}-NO2Ph}$ seems a perfectly reasonable interpretation of what the person has written here.

3. Electronic configuration labels: 1s-H

   This seems like using an atomic orbital label (1s) as a hyphenated specifier for the hydrogen: I don't think it's a standard form of writing that at all, but the method you propose would presumably italicise the s, which would be fine -- the atomic orbital labels like this are often italicised.

4. Whatever the a-HCl is: I don't know.

   I couldn't find anything that looked appropriate for this one.

Most importantly, I can't think of any counter-examples where that wouldn't be a sensible thing to do.

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[1] Edit: I defer to @Loong's superior mastery of the ways of IUPAC in their answer.