In a blatant plug for the Reagent Guide and the Reagents App for iPhone, each Friday I profile a different reagent that is commonly encountered in Org 1/ Org 2.
What it’s used for: For our purposes, zinc amalgam has one important use: in the Clemmensen reduction of ketones to alkanes.
Similar to: The reagent has essentially the same effect as the Wolf-Kishner reaction, although it is done under acidic conditions.
Examples:
Zinc amalgam (Zn-Hg) is most commonly used in the Clemmensen reduction, which takes ketones adjacent to aromatic rings down to the alkane. This can be useful in, say, after the Friedel-Crafts acylation when you want to obtain a straight chain alkane that would otherwise rearrange.
Note that the Clemmensen isn’t as effective on ketones that aren’t adjacent to aromatic systems, a sign that the reaction is probably proceeding through a carbocation or other electron-deficient intermediate that is stabilized through resonance with the aromatic ring.
How it works:
Amalgams are alloys of mercury and other metals. They are among the oldest of reducing agents, but their precise mode of action remains somewhat mysterious. I remember reading at some point that their mode of action is not unlike that of an electrochemical cell. In the Clemmensen, addition of acid protonates the ketone, and electrons from the Zn(Hg) are delivered to the carbon; the new hydrogens come from the acid, while the oxygen is eventually expelled as water.
Real life tips: Although I’ve never prepared zinc amalgam, for a good time, try making sodium amalgam through dropping liquid Hg into a beaker of molten Na in mineral oil. Watch that you don’t get splattered. Shulgin uses aluminum amalgam as the reductant of choice for taking ketones and aldehydes to alcohols. In PIKHAL I recall him describing the preparation of aluminum amalgam through cutting conventional aluminum foil into small squares and adding a solution of mercuric chloride in water.
P.S. You can read about the chemistry of Zn(Hg) 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!
Imagine having a comprehensive online guide to help you solve your own problems in organic chemistry. That's my mission with this site. After earning a Ph.D. at McGill and doing a postdoc at MIT, I applied to be a professor. That didn't work out. So I decided to teach organic chemistry anyway. Master Organic Chemistry is the site I wish I had when I was learning the subject. 
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{ 12 comments… read them below or add one }
Wow, is like you’re doing all the reagents I’ve learned this semester, thanks for reminding me of this one for my final.
I’ve actually performed traditional Clemmensen reductions a couple of times. They are very straightforward experimentally. Sometimes they work, and sometimes they don’t. That is organic synthesis in a nutshell. If we are talking about reducing aromatic ketones down to the alkane, I’ve had much better success with either Wolff-Kishner or with alane (AlH3 – prepared in situ from LAH and AlCl3).
Regarding the mechanism for Clemmensen, it is indeed elusive. I’m fairly certain however that protonation of the ketone is NOT the first step. You would expect the alcohol as an intermediate (partially reduced ketone) if that was the case. It has been shown numerous times that a) not even trace amounts of alcohol are produced and b) alcohols when subjected to Clemmensen conditions are not reduced down to the alkane, which tells us that the mechanism is something else. I think that the foremost candidate today is a zinc-carbene intermediate, as depicted here. N.B. the single-electron transfer in the first step.
Other than that, great post. Any student must know you cannot take any exam in organic chemistry, at any level, not knowing about Clemmensen. It’s one of those reactions you absolutely must know of. (But not the exact mechanism, because it is still unknown.)
Cool! That’s strange, like I said I would have definitely expected the alcohol as a byproduct. I guess that means that benzylic alcohols wouldn’t be good substrates for the Clemmensen?
My link above appears to be defunct; here is the full path to the proposed zinc-carbene mechanism again
http://www.organic-chemistry.org/namedreactions/clemmensen-reduction.shtm
Yes, I am quite sure that benzylic alcohols are inert towards Zn(Hg). To reduce these down to the alkane, one popular two-step method is i) conversion to the chloride with SOCl2 (which you just wrote about!) followed by ii) reduction with Raney-Ni/H2.
Alternatively i) oxidation to the ketone followed by ii) Zn(Hg) or ii) Wolff-Kishner or ii) AlH3
My profesor writs only Zn when writing the reactions, does he mean ZnHg?
For the Clemmensen, it should be Zn amalgam, which is Zn-Hg.
No need for Zn amalgam you can use Zn powder
There’s no mechanism for clemmenson reductiön of aromatic ketone
The mechanism is somewhat mysterious for this reaction.
Can you tell that what happens if pyruvic acid is treated with Zn-Hg . Or for that matter what happens when any carboxylic acid is treated with Zn-Hg?
Hi I don’t know why but for some Reason when I go and click the icon for the app for the IPhone it does not take me to the Application Do you know why?
Fixed now. Thanks for pointing that out!