Question

How do you remember chemical reaction reagents, solvents, and products?

Answer 1

I personally don't put forth the effort to memorize them via flash cards, or something of that sort.

Instead, I commit the mechanisms to muscle memory in such a way that I can imagine a reaction mechanism in my head by association with the goal of the reaction itself, and I determine the reactants necessary from there.

So, for example, there are quite a few bromine reactions you may have had to learn:

• `"Br"_2,` `"CH"_2"Cl"_2` (bromination of alkene/alkyne)
• `"Br"_2,` `"H"_2"O"` (intercepted bromination of alkene/alkyne by `"H"_2"O"`)
• `"HBr"` (hydrobromination)
• `"HBr",` `"NBS",` `"a peroxide"` (radical hydrobromination on allylic carbon)
• `"PBr"_3` (`"OH"->"Br"` in alcohols, carboxylic acids, etc)
• `"Br"_2, "FeBr"_3` (aromatic bromination)
• `"Br"_2, "PBr"_3, "H"_2"O"` (bromination of alpha-carbon on carboxylic acid; HVZ reaction in acid or base)

...amongst others.

How I reason this out is by branching out from the least complex one and relating them to each other.

• Bromination is just the basic nucleophilic attack of bromine, forming the cyclopropane analog and then breaking that to give an anti-addition product.

• Intercepted bromination then, is bromination that goes halfway, and then water intercepts and breaks the ring on the cyclopropane analog before bromine can (because water is a stronger nucleophile), and you still get anti-addition, but `"OH"` is one of the new substituents instead of `"Br"`.

• Hydrobromination is like a variant where one bromine is instead a hydrogen that adds Markovnikov.

• Radical hydrobromination builds off of hydrobromination, and instead adds anti-Markovnikov (I associate peroxides with high reactivity and thus radical reactions. Then I say "radical = weird = opposite to expectations" and assert anti-Markovnikov). Not only that, but the bromine adds onto the allylic carbon.

• I think of using `\mathbf("PBr"_3)` as in a class of useful reactions that convert alcohols to something more reactive, so you could associate `"PBr"_3` with `"SOCl"_2`, for instance. Grouping usually helps.

• I associate aromatic bromination with the `"FeBr"_3` being a weird catalyst. When would you ever need to use transition metals to catalyze a normal reaction, right?

  Think about it---when have you ever needed to know the arrow-pushing mechanism for how a transition metal participates? Those are elusive elements. The only ones you may have used are `"Cr"`, `"Mn"`, `"Ni"`, `"Pd"`, `"Cu"`, `"Fe"`, and `"Zn"`, and my textbook has shown mechanisms involving `"Cr"` (chromic acid oxidation), `"Cu"` (Gilman reagents), and `"Fe"` (aromatic halogenation). The rest were usually said to participate somehow.

• I actually have the hardest time remembering the HVZ reaction, but I do remember that `"PBr"_3` converts `"OH"` to `"Br"` on alcohols, carboxylic acids, etc., so I associate that with the water in reaction.

  Reacting water with a carboxylic acid would be pointless if the `"OH"` on the carboxylic acid was still an `"OH"`---water has an `"OH"` to contribute too, so it would reform the reactant! So it must have been that the `"OH"` was converted to `"Br"` by `"PBr"_3`. In fact, that is the first step to that reaction!