Sunday, November 29, 2015

Molten Salt Bath, Anyone?

While paging through Organic Reactions, Vol 1 in the chemistry library*, I encountered some unusual conditions in a review of the Elbs reaction. If you're not familiar, it's a high-temperature pyrolysis** of ortho-tolyl ketones to produce polycyclic aromatic hydrocarbons (PAHs). Products produced from the reaction find use as analytical standards for oil processing, in studies of DNA intercalation, and as molecular wires.

PAH prepared by heating at 500 oC
Bonus: Roman numerals in older reviews? Classy, but confusing.
Source: Organic Reactions 1, p. 154
As alluded to above, the chemistry itself wasn't what caught my attention, but rather this sentence:
"The flask is charged with 152 g of the crude ketone and heated in a nitrate-nitrite bath (care!) a430 ± 5 oC."
Four hundred degrees! During all my years in lab, I can't remember heating reactions past about 300, and those were with machined blocks of aluminum on an ancient Thermo hotplate.*** 

I realize that pyrolysis technologies have advanced in 70 years' time - FVP permits higher temperatures for much shorter residence times - but the concept of a molten mixture of sodium nitrate and potassium nitrite seemed both dangerous and alluring.

This is either Hawaii's Mt. Kilauea, or a molten salt bath. Or both.
Source: thelandofshadow.com

Looking through SciFinder, it seems that molten salt baths still find regular use in case-hardening and nitriding of steelwork, along with applications in cleaning organic residues off polymer extrusion dies. For extreme data-philes, the Molten Salts Research Center thoroughly analyzed a variety of salt mixture properties in a 150-page reference. Park Thermal International has published what seems to be a very conservative safety manual for nitrate-nitrite baths, and with good reason: aluminized leather aprons, tinted visors, and gauntlets help protect workers from glowing pools of salts at temperatures just below "decomposition...with extreme explosive violence."

Though I've never worked with molten salts, that doesn't mean none of my readers have. I see Milkshake has softened a 2L round-bottom flask by heating up to 380 with a graphite flake dry bath. How about you, Chemjobber?

Or are molten salt baths potential entrants in Derek's "Things I Won't Work With"?

Update (29 Nov): A commenter points out some previous Milkshake high-T campaigns.

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*Real books! Complete with yellowing pages, musty smells, and indexes full of names like Fuson, Fieser, and Bachmann.
**Can't get enough Elbs? Curious readers are directed to Name Reactions for Carbocyclic Ring Formations, 2010, Jie Jack Li and Timothy T. Curran. Review starts on p. 324.
***Obviously not counting use of a butane torch to pull pipettes, heat sieves and salts, etc.

4 comments:

  1. See also from Milkshake:
    https://orgprepdaily.wordpress.com/2007/11/03/high-temperatures/

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  2. I had to do some high temperature solid state reactions in order perform ring closure reaction in large PAHs during my grad school days. Often had to get the reaction up to 380C to see cyclization, but it worked reliably and I got good yields.

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  3. ran a few preparations of guanidine nitrate by the dissolution of calcium cyanamide in molten ammonium nitrate.

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  4. I haven't used molten salts as a bath, but I've used them as a solvent! Molten NaOH to be exact (mp = 318 degrees C). It was heated in a stainless steel container directly over a bunsen burner flame.

    ReplyDelete