Functional group tagging - revisited

Question

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.

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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 halides.

• 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 halides 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.

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n.b. I wouldn't necessarily want to scrap all of them, though. For example, I think that heterocyclic-compounds 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 halides. However, I would prefer renaming it to heterocyclic-chemistry to reflect the overall philosophy.

Answer 1

Proposal of a different tagging system

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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 hybridisation + halides. A question comparing the C=O wavenumbers in carbonyl compounds may be tagged as ir-spectroscopy + carbonyl-compounds.

   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 S_N2 reaction on chloromethane should not be tagged with halides, simply because the substrate is a halide. A question about the aldol reaction should be tagged with aldol-reaction instead of carbonyl-compounds.

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2. For questions about reactions, the top-level tag should be organic-chemistry; 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.

   • nucleophilic-substitution for S_N1, S_N2, S_N2' processes; excluding S_NAr
   • acyl-substitution for all substitutions at carbonyl groups
   • elimination for all eliminations
   • c-c-addition for all additions to C=C or C≡C
   • c-x-addition for all additions to C=X or C≡X
   • enolate-chemistry for all reactions of enolates
   • aromatic-chemistry for all reactions of aromatic carbocyclic compounds. Note that this supersedes an earlier post, where it was proposed that questions about heterocyclic chemistry should also be tagged with aromatic-chemistry.
   • heterocyclic-chemistry for all reactions of aromatic heterocyclic compounds
   • organotransition-metal-chemistry for all reactions catalysed by transition metals, including but not limited to cross-coupling [n.b. this was changed from organometallic-catalysis, see comments for further discussion]
   • rearrangements for all rearrangements
   • organic-reduction and organic-oxidation for all redox processes where the emphasis is on an organic transformation (as opposed to redox which is for questions about the general concept)
   • pericyclic for pericyclic reactions

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

   • aromaticity should be used for questions about Hückel's rule, aromaticity, and anti-aromaticity.
   • stereoselectivity should be used for questions about stereoselective processes. Currently we are using a combination of selectivity (unhelpful, broad tag) and stereochemistry (should be used only for questions about stereochemistry of molecules).
   • Logically it makes sense to therefore also make regioselectivity and chemoselectivity.

Answer 2

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 reaction and homework, 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 aromaticity (questions about the concept) from aromatic-compounds (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 aromatic-compounds from questions tagged with heterocyclic-compounds, unless there is also a component involving carbocyclic rings (e.g. comparison of pyridine with benzene)

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

4. Make and use organotransition-metal-chemistry.