By James Ashenhurst
Reagent Friday: Sodium (Na)
Last updated: March 8th, 2019
Today’s topic, sodium metal is a pretty notorious reagent. You might know it from Youtube as “That solid that you drop in water and blows things up, a property it shares with potassium and other alkali metals.
Sodium has lots of uses in organic chemistry. Sodium metal has but a lone electron in its valence shell, and gives it up quite easily to any molecule that will accept. That includes water: what’s happening when you drop sodium metal into water is that H2O is being reduced to hydrogen gas, forming sodium hydroxide in the process. Sodium can also be used to reduce many types of organic compounds such as alkynes and aromatic compounds among many others not discussed here.
What it’s used for: a strong reducing agent, will reduce alkynes to trans-alkenes. Will also form hydrogen gas when added to alcohols, resulting in alkoxides. Will also reduce aromatic groups to alkenes (the Birch reduction). Sodium metal will generally not reduce alkenes.
Let’s look at the first example: reduction of alkynes to trans-alkenes. Sodium will dissolve in liquid ammonia (boiling point –33 °C) producing a beautiful deep blue color. When alkynes are present, they will be reduced to the trans (i.e. E) alkene. This makes Na/NH3 a useful companion to the Lindlar reduction of alkynes, which gives cis-alkenes.
As mentioned previously, sodium metal will also convert alcohols to alkoxides (remember: alkoxides are the conjugate bases of alcohols). In the process this liberates hydrogen gas, so it must be done with great caution and advisably under an oxygen-free atmosphere. This is a useful way to make the conjugate base of an alcohol without worrying about the conjugate acid hanging around in solution, since the only byproduct is hydrogen gas.
Finally, sodium metal in ammonia (NH3) containing several equivalents of t-butanol will reduce aromatic compounds to (non-conjugated) dienes. This useful reaction is called the Birch reduction. Note that the pattern depends on whether the substituent on the aromatic group is electron donating (such as OCH3) or electron withdrawing (CO2Me).
How it works – we’ll just discuss the alkyne reduction here for now, but reduction of the alkyne by sodium results in breakage of the C-C double bond and formation of an anion adjacent to a radical. The radical that is formed can interconvert between its cis and trans form, but the trans is generally more stable due to steric factors. The anion is then protonated by NH3 (the only acid present in solution) to give the vinyl radical, which is then reduced by a second equivalent of Na to give a second anion. This is then (you guessed it) then converted to the alkene by protonation with a second equivalent of NH3. So the net process gives a trans-alkene and two equivalents of NaNH2.
Real life tips. Since sodium metal is a solid, many of its reactions are surface-area dependent. That means that it is most effective as a reagent when cut up into fine strips or wires rather than chunks. As you can imagine, a reagent that reacts violently with water is potentially very hazardous. Sodium is notoriously dangerous when used to remove traces of water from solvents in distillation setups: chunks can be passivated with a surface layer of NaOH, and scratching the surface of the material during quench will then expose reactive sodium metal, which can then react rapidly with the quenching agent, leading to explosions and fires. Potassium is even worse for this. Do a Google search for “solvent still fire” and you will get the idea.
P.S. You can read about the chemistry of Raney Nickel and more than 80 other reagents in undergraduate organic chemistry in the “Organic Chemistry Reagent Guide”, available here as a downloadable PDF. The Reagents App is also available for iPhone, click on the icon below!