# Functional group tagging - revisited

Back in 2015 it was decided that we should create and apply tags for a bunch of functional groups. See: Should we create tags for common functional groups?

I'll be upfront about what I think: after ~2 years of having this system, I've decided I don't actually like it. Reasons:

1. Reactions involving multiple functional groups will easily break the five-tag limit. This is trivial, but it provided the original motivation for me to post this.

2. Tags are intended to connect experts with questions they can answer (source: help centre). Being an expert in the chemistry of a functional group seems implausible to me.

3. A functional group tag invariably ends up being applied on many questions which are fundamentally unrelated.

It's not fair for me to criticise without suggesting an alternative, so I'd suggest to use class of reactions. Many organic texts already adopt such a classification, so there is ample precedence. Two examples I like are Clayden 2ed (TOC) and Carey/Sundberg 5ed (part A TOC, part B TOC), and I could envision adopting these (with slight modification). I personally think that this addresses the three problems above in a much better fashion.

I'll give one example of why I think functional group tagging is bad. I could write more, but I think one is sufficient to put the point across. Consider our current tag .

• I don't think I've seen anybody who claims to be an expert on halides (both inorganic and organic, mind you).
• I don't think I've seen any books exclusively on halides.
• Even if we restrict it to organic chemistry, halides have multiple modes of reactivity: substitution/elimination, Pd cross-coupling, lithium-halogen exchange, ...
• Fundamentally these are not conceptually related to each other and it is hard to imagine that anybody would be interested in all of these at the same time.
• Hence, the tag is not successful in linking people to content that they are interested in. From a student's point of view, he is not keen on learning all reactions of halides at one go; the same can be said of the expert, who is not keen on reading through a mishmash of questions.
• Instead, I'd propose having one tag for substitution+elimination (as these two processes often compete with one another) and one tag for cross-couplings. Lithium-halogen exchange can probably be covered under organometallic chem.

n.b. I wouldn't necessarily want to scrap all of them, though. For example, I think that should be retained, since heterocyclic chemistry is traditionally taught as a subfield of organic chemistry, and it does not have the problems described above for . However, I would prefer renaming it to to reflect the overall philosophy.

• I agree. I've thought that the functional group tagging was kind of strange, but admitted to myself that there are certain things that happen to different functional groups. I feel that tags that describe the "things" that are happening do a better service than the functional group tags. – Melanie Shebel Aug 24 '17 at 21:54
• Is this actually happening? Or are you proposing it to see if there's support? I like the idea (as stated on the other Meta post), but if you are proposing we do this, it would perhaps be instructive if you gave some instructions/examples of how the tags should be changed. – NotEvans. Aug 25 '17 at 8:49
• @NotEvans. This post is generally to gauge community opinion. I want to see it happen, but we obviously cannot retag several thousand questions at once. I think we should firstly decide on what is a good set of tags that adequately categorises all organic reactions, and I was going to try and come up with something (mostly based on the textbooks I listed), but I've also been spending time answering questions on main site so this was sort of put on hold for a couple of days. Once this set of tags has been chosen, I guess we (slowly) go through the organic questions and replace the tags. – orthocresol Aug 25 '17 at 8:57
• – Martin - マーチン Sep 21 '17 at 8:18

## Proposal of a different tagging system

1. Functional group tags need not be scrapped entirely, but their usage should be restricted.

Questions about the properties of functional groups may be tagged as such. For example, a question about comparing the C–Cl bond lengths in chloromethane vs. chlorobenzene may be tagged as + . A question comparing the C=O wavenumbers in carbonyl compounds may be tagged as + .

However, questions about the reactions of functional groups should not be tagged as such, since the classification of reactions by functional group is problematic. A question about a SN2 reaction on chloromethane should not be tagged with , simply because the substrate is a halide. A question about the aldol reaction should be tagged with instead of .

1. For questions about reactions, the top-level tag should be ; the second-level tag should be one (or at most two) of the following.

I have drawn up a comparison of old vs new tagging systems here.

1. For questions about concepts, some new tags should be drawn up:

• should be used for questions about Hückel's rule, aromaticity, and anti-aromaticity.
• should be used for questions about stereoselective processes. Currently we are using a combination of (unhelpful, broad tag) and (should be used only for questions about stereochemistry of molecules).
• Logically it makes sense to therefore also make and .
• +1 for 1st and 3rd, especially 3rd’s second bullet point. Also for $90~\%$ of 2nd, except that I don’t agree with the name organometallic-catalysis. For example in a typical Suzuki reaction, the actual catalytic species is an inorganic palladium(0) complex while the only organometallic species entering the reaction is a boronate which is a stoichiometric reactant. Only if you are using in-situ organometallic species generation can that be properly called organometallic-catalysis. – Jan Aug 27 '17 at 14:43
• @Jan you have an interesting point. Just so that we're on the same page, my intended scope of this tag would include (not exhaustive) Pd and Cu couplings, Tsuji–Trost, Wacker, Rh reactions, Au reactions, Noyori/Wilkinson hydrogenation, olefin metathesis. The intermediates in such catalytic cycles are certainly organometallic, so one option is to just define this carefully in the tag wiki. I haven't come up with a good alternative - something like organometallic-chemistry would theoretically encompass inorganic reactions [...] – orthocresol Aug 27 '17 at 15:04
• In general the delineation of the inorganic/organic border here is very tough and was something I wanted to look at separately, but no harm in thinking about it now. Maybe an option is to take the title of Hartwig's book - organotransition-metal-chemistry (31 chars) or organotransition-metal (22)? – orthocresol Aug 27 '17 at 15:05
• Organotransition-metal-chemistry is perhaps the most accurate/descriptive, though perhaps with the others as synonyms as it's not the most intuitive thing to choose – NotEvans. Aug 27 '17 at 18:50
• As an aside, and because I don't want to write a whole meta post arguing for it. If we have stereoselectivity, can regioselectivity become a thing, because, you know, its nice to have things in nice sets! – NotEvans. Aug 28 '17 at 20:08
• @NotEvans. chemoselectivity as well? Oddly enough, these two (regio/chemo) were on my list of things to discuss as well... – orthocresol Aug 28 '17 at 20:25
• @orthocresol- the whole lot! I think there are easily enough questions to justify it (plus it saves silly tag + tag = the actual tag we want situation). Plus if things are getting re-tagged anyway to accommodate the above then its barely extra work :) – NotEvans. Aug 28 '17 at 20:43
• Split organic-redox into organic-reduction and organic-oxidation? While red cannot happen without ox, the methods achieving either the one or the others on a functional group tend to vary significantly, iirc. – Martin - マーチン Sep 21 '17 at 7:58
• @Martin-マーチン I thought about that, my own conclusion was to not include it in the end. Mainly because there will be edge cases of redox reactions where both reactants are organic. I appreciate that we could easily lump both tags on, though. Maybe that would be for the better – orthocresol Sep 21 '17 at 22:11

Well, as of today (21st Sept) I think the best way forward is to go slightly more slowly. As far as cleanup work is going, there is still a lot to be done for and , so to add on to that by essentially retagging all org-chem questions is far, far too much.

So, let's go slowly and adopt some of the easier things to do. Since I left this here for four weeks and no real objections were raised, I assume that nobody has intrinsic objections to any of these:

1. Separate (questions about the concept) from (questions about the chemistry and reactions of aromatic compounds). Most questions should clearly fall into one of these two categories, but if there is uncertainty, feel free to discuss it in chat.

2. Remove from questions tagged with , unless there is also a component involving carbocyclic rings (e.g. comparison of pyridine with benzene)

3. , , and have already been created and should be used. Eventually, the broad tag should be cleared out.

4. Make and use .

• ortho, if you need any SEDE queries to help with this, let me know. – hBy2Py Sep 24 '17 at 13:41
• Thanks for the offer, @hBy2Py! – orthocresol Sep 24 '17 at 14:00