# Standard LaTeX notation of SN, multiple acid dissociation constants, and electronic effects

I find these inconsistencies a lot in older posts and till now haven't found a suitable response, so I decided to post eventually.

1. SN notation: I have found three popular notations for this:

• $S_N2$ $S_N2$
• $\mathrm{S_N2}$ $\mathrm{S_N2}$
• SN2 S<sub>N</sub>2
2. Multiple acid dissociation constants: I have seen these two variations:

• $K_{\mathrm a1}$ $K_{\mathrm a1}$
• $K_{\mathrm {a_1}}$ $K_{\mathrm {a_1}}$
3. Inductive/Resonance effect: I've seen:
• +I/+R (plain text)
• $+I/+R$ ($+I/+R$)
• +I/+R (<i>+I/+R</i>)

I wish to know which of these is the correct notations. I didn't find relevant info on the formatting wiki.

• Avoid MathJax where it is possible, don't use it for something that isn't mathematics (except text equations; and even that is an over-generalisation). I hence prefer S<sub>N</sub>2. Avoid double subscripts for those who are not eagle-eyed. Don't use double subscripts in inline mode. Therefore I actually prefer either $K_\mathrm{a}(1)$ or $K_\mathrm{a,1}$. I don't think there are official recommendations. Feb 27 '18 at 7:26
• @Martin-マーチン I find your logic great! I just edited my post to add one more case (sorry! but I figured out another meta question for that case would't be too good so..) I guess you would again prefer "plain text" mode for that but do have a look. Thanks! Feb 27 '18 at 8:45
• @Martin-マーチン I think we agree that K has to be in italics (because it is a variable) and a should be upright (because it is a label). But what about SN? (What is this anyway? It's not in the Green Book.) Feb 27 '18 at 8:55
• @mhchem SN is short "nucleophilic substitution". SN1 and SN2 refer to first and second order "nucleophilic substitution" respectively. Another related term is E1 and E2, referring to first and second order elimination reactions. Feb 27 '18 at 8:58
• @mhchem Indeed that is just an abbreviation, also common are SN<sub>1</sub>, or E<sub>1</sub>, albeit less frequent. I don't think that should be supported in maths mode, and I usually remove it when I see it. || I do prefer plain text for inductive/ resonance effect, as these are also abbreviations, and I don't think (but don't know) they should be sloped. I tend to avoid these, as I find them not very descriptive, and using +/- always confuses me. Feb 27 '18 at 9:56
• Ah, with that name I found it in the Green Book. In the Abbreviations section (printed page 163), not the Symbols section. All letters are printed upright. Feb 27 '18 at 10:28
• @mhchem Great find! Though is it a plain-text upright or a \mathrm upright? Feb 27 '18 at 11:25
• @GaurangTandon In a properly typeset document, one shouldn't see any difference. Feb 27 '18 at 12:34
• @mhchem Testing: $\mathrm{SN}$ and SN <-- well those two do look different, on my pc at least. Feb 27 '18 at 13:00
• @GaurangTandon Yes, here at C.SE one sees a difference. But in a document with proper typography, one shoudn't. Therefore, I cannot tell you if the authors of the Green Book used text font or math font, because it is indistinguishable. And therefore, there is no typographical convention what to use. The problem just exists for C.SE (and similar), so we have to decide. And we cannot rely on IUPAC or other authoritative bodies, because they simply do not have the need to decide between (the equivalent of) $\mathrm{S_N}$ and S<sub>N</sub> for their books. Feb 27 '18 at 13:23
• @mhchem Indeed, that is correct. Feb 27 '18 at 13:25
• @Martin-マーチン, @mhchem $\mathrm{S_N2}$ produces a small space between $_\mathrm{N}$ and $2$ while S<sub>N</sub>2 does not produce one. I don't think that space is needed. Also, HTML loads faster than MathJax. Feb 27 '18 at 13:41
• @mhchem Haven't looked into the rest but I think the 'a' can be either upright or Italic; Italic when it refers to 'acid', upright when referring to 'activity' that has symbol $a$. Both can be applied for acid equilibria for usually equivalent meaning. Feb 28 '18 at 20:08
• @linear I think you got it reversed. Upright $\mathrm{a}$ when it is and abbreviation vor acid, slanted $a$ when it would be referring to activity (although I would assume that it is hardly ever used that way, since the standard equilibrium constant $K$ or $K^\circ$ already implies activities). Mar 1 '18 at 5:43
• @ApoorvPotnis Different browser different rules... no space where I am sitting now. It is of course already known that MathJax renders client-side, which I think is another reason to avoid it whenever not necessary. Mar 1 '18 at 5:45

So, finally, I'll post the answer myself, based on the wonderful comments discussion above. Please upvote if you agree with all these notations, or downvote (and comment!) if you disagree with any of the points mentioned.

1. For nucleophilic substitution reactions: S<sub>N</sub>1, S<sub>N</sub>2, and S<sub>N</sub>i are acceptable notations.
• $\mathrm{S_N2}$ - though it is correct and prints upright text, it is discouraged because of unnecessary reliance on mathjax.
• $S_N2$ is completely incorrect, as the symbol must be upright.1
• $SN_2$, $\mathrm{SN_2}$, and SN<sub>2</sub> are obviously incorrect as the 2 is not supposed to be subscripted.
2. For elimination reactions: E1 and E2 are acceptable notations.
• $\mathrm{E2}$ - though it is correct and prints upright text, it is discouraged because of unnecessary reliance on mathjax.
• $E2$ is completely incorrect, as the symbol must be upright.2
• $E_2$, $\mathrm{E_2}$, and E<sub>2</sub> are obviously incorrect as the 2 is not supposed to be subscripted.
3. For multiple acid dissociation constants, no conclusive answer has been reached (since there is no official document indicating the notation). Until then, as per human convenience, $K_\mathrm{a,1}$ is preferred.
• $K_\mathrm {a_1}$ is discouraged because the second subscript would be too small to properly read for most people.
• $K_\mathrm {a(1)}$ is discouraged because of an unnecessary parentheses, when a simple comma would suffice.
• $K_{a,1}$ is incorrect because the $a$ must be upright.
4. Inductive, resonance, and mesomeric effects: plain-text +I, +R, and +M is encouraged. Again, there is no conclusive evidence due to lack of official documentation.
• Any mathjax form is discouraged because of its unnecessary reliance on mathjax.
• Italics <i>I</i> is discouraged because "+I effect" must be written upright. [citation needed]

Ok, so why is reliance on MathJax discouraged? (just in case you were wondering anyway) Because:

1. MathJax slows down page render. It's best to avoid it where no maths is involved.
2. An equally good plain-text alternative exists.
3. MathJax renders client side, avoid it where not necessary, as different browsers may have different rules (read)

it is discouraged.

References:

1. the Green Book. In the Abbreviations section (printed page 163), not the Symbols section. All letters are printed upright (thanks mhchem!)
2. the Green Book. In the Abbreviations section (printed page 158), not the Symbols section. All letters are printed upright (thanks mhchem!)
• Just in case there was some confusion: these notations are supposed to act as guidelines for future posts. This meta post is not advocating mass-scale notation conversion of old posts. Mar 31 '18 at 1:23
• All the texts I remember from introductory times had resonance and inductive effects set in italics—like I was doing on here. Otherwise I completely agree.
– Jan
Dec 5 '18 at 16:24
• @Jan Thanks for the comment (, and welcome back :) ) I wasn't completely sure about the resonance effects thing myself and was looking for citations, but i couldn't find it at that time. Is the italics notation mentioned in the Green Book, or is it a general agreed-upon convention, or something else? Dec 6 '18 at 4:17
• It was in my school books, mainly ^^'
– Jan
Dec 6 '18 at 15:20
• @Jan Oh that :P Dec 6 '18 at 16:28
• As Martin said in a different comment, I prefer inductive/resonance effects to be spelled out fully. The ±I or ±R notation is not universal, I have honestly only seen it here, and IMO we should try to stick to standard notation as far as possible (which seems to be the point of this post, really). But, truthfully, I don't think anybody will read this, so... it is mainly for editors to take note of, and different editors have different philosophies.
– orthocresol Mod
Dec 24 '18 at 2:32