By James Ashenhurst
Reagent Friday: Raney Nickel
Last updated: February 10th, 2016
Named reagents have a slightly mysterious air to them, conjuring up (for me, anyway) the image of a lone scientist working long hours for an elusive goal, until they finally have that “Eureka” moment. In this vein, while “Rearden metal” may solely be a work of fiction, “Raney Nickel” is very real. Raney nickel is an alloy of aluminum and nickel, which has subsequently had much of the aluminum removed through a leaching process with sodium hydroxide (NaOH). The remaining alloy has a very high surface area and also contains hydrogen gas (H2) adsorbed on the nickel surface.
What it’s used for: Like palladium on carbon (Pd/C) and platinum on carbon (Pt/C), Raney nickel can be used for the hydrogenation of alkenes and alkynes. But what Raney nickel is used most for is its unusual property of reducing C-S bonds to C-H bonds. It’s this second application that can make Raney nickel uniquely useful. When combined with the formation of a thioketal from a ketone, this can serve as an alternative means of converting ketones to alkanes (just like the Wolff-Kishner reaction). For a real-life application of this reagent in the synthesis of erythromycin by Nobel Laureate R.B. Woodward, see the great discussion by B.R.S.M. here.
Here’s the second application of Raney nickel – as a catalyst for hydrogenation. Note that in this case we don’t necessarily need to add hydrogen gas (although it helps) – Raney nickel is usually obtained in its “activated” form, where the hydrogen is already adsorbed onto it.
How it works: “For our purposes” (love this phrase) it’s not so important exactly how Raney nickel works. There’s something mysterious about it: the aluminum is crucial for its activity, and Raney Nickel doesn’t behave the same once it’s been completely removed. To be honest I plead ignorance on exactly how Raney nickel works its desulfurizing magic, although the catalytic hydrogenation process is likely similar to those of Pd/C and Pt/C.
Real life tips: Perhaps a better description for Raney nickel is “Raney Mud”, because that’s what it looks and feels like. Raney nickel resembles a kind of mud or wet clay, and is actually stored in water. Determining the exact molar ratio of Raney nickel to use is also something of an art – rather than “moles”, typical procedures call for “teaspoons” [I’m not sure I’m aware of any other reagent that calls for this unit of measurement!] After dispensing (but before placing in the reaction vessel) the metal is then rinsed with water (to remove aluminum salts and ensure neutral pH). This can be something of a dicey prodedure since Raney nickel will spontaneously combust in air when traces of moisture are removed. Excess Raney nickel on benchtops, spatulas, weighing paper, etc. should be (carefully) destroyed with acid. There’s nothing like setting up your reaction and then, out of the corner of your eye, noticing little flames coming from traces of Raney nickel on your weighing paper. Disclaimer: this paragraph should not suffice as training in the use of this reagent, which can be extremely dangerous in the wrong hands.
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!
Image credit for Raney nickel