This Month's Puzzler:
We manufacture 97%-pure ethylnylcyclopropane (ECP, also known as cyclopropyl acetate — CAS No. 6746-94-7). The batch ECP process involves these primary steps: reaction with an alkyl lithium compound; blowing in NH4Cl with high purity N2 to form the ECP; pumping the product solution through a bank of filters; quenching the alkyl lithium with cold heptane and 2-propanol; washing with chilled deionized (DI) water, followed by filtering; decanting in a bed filled with random packing to remove the salts, separating the organic and aqueous phases; and, finally, distilling the organic phase to separate the ECP (distillate) from the 2-propanol. The wastewater, which still contains a trace organic phase, undergoes batch vacuum distillation to remove the trace. The wastewater then passes through our new trickle-bed air bio-filter before discharge to the city sewer water plant.
Unfortunately, the new manager made some changes to reduce costs: 1) using air instead of N2; 2) replacing deionized water with plant (well) water; 3) bypassing the chilled water cooler for the quench; and 4) cutting back the 2-propanol to almost zero. He was following the advice of an experienced operator. The first campaign didn’t go well.
We had organic carry-over from the biofilter into the city sewer; the stream also contains n-hexyl lithium. In addition, our product quality suffered; we only achieved about 88% purity after distillation. What did we do wrong? And can we get rid of the 2-propanol without substituting something worse?
Adopt A Different Process
The synthetic method implied by the use of alkyl lithium is dimetallation of 5-chloro-1-pentyne, followed by cyclization:
Step 1: Cl-CH2-CH2-CH2-C=C-H + R-Li → Cl-CH2-CH2-CH2-C=C-Li + RH, where R is the alkyl group.
Step 2 is a second deprotonation: Cl-CH2-CH2-CH2-C=C-Li + RLi → Cl-CH2-CH2-CHLi-C=C-Li + RH
Step 3: Heating completes the reactions in Steps 1 and 2, and cyclization forms the cyclopropane moiety by eliminating LiCl:
Cl-CH2-CH2-CHLi-C=C-Li → LiCl + Li
Step 4: Quench the acetylenic anion with NH4Cl. Blowing in NH4Cl as powder with nitrogen gas seems odd and undesirable because DI water or well water was added later. This step will be quite exothermic, so slow addition of ammonium chloride dissolved in water would be quite satisfactory. Nitrogen or air should not be used because either of these would tend to carry away the fairly volatile cyclopropyl acetylene (CPA) product. The problem description notes n-hexyl lithium was “carried over,” even though the reaction was quenched with NH4Cl, isopropanol and water. (Certainly any hexyl lithium would have been totally hydrolyzed to n-hexane and lithium product, but thanks for the hint.)
Selecting appropriate raw materials can simplify the purification of CPA. A better lithium alkyl than n-hexyl would be n-butyl because butane formed by protonation could be flashed off. And better than n-butyl lithium in hexanes is n-butyl lithium in cyclohexane. This allows the CPA to be distilled more cleanly from the higher boiling solvent — no hexanes present. CPA boils at 52°C, butane at -0.5°C, n-hexane at 69°C and cyclohexane at 81°C.
This batch preparation yields an aqueous solution of lithium chloride and ammonia under an organic layer. If the NH4Cl was added as a 25% solution in water (nearly saturated), the amount of water added thereby will be enough to dissolve all the lithium chloride produced (it’s about twice as soluble). The requirement for washing, filtering and decanting depends on the side reactions that occur but typically a simple filtration will clear up the aqueous layer enough to flash off any CPA and cyclohexane present without presenting a foaming problem. (The high ionic content makes it unlikely that a significant amount of organics will be dissolved in the water, so eliminating the flash-off step might reduce costs.) The organic layer will be a bit more complicated: estimated composition will be 5–8% CPA and a little butane, but mostly cyclohexane. Whatever yield of CPA is lost to side reactions will probably generate higher-molecular-weight tars but these probably will be soluble in the cyclohexane.
Disposal of the organic residue after distillation will be fairly inexpensive as it will be useful as fuel (high BTU content). The water layer may be a challenge and an opportunity: As a fairly concentrated lithium chloride solution, it might have salvage value. Its high concentration of salts may make its purification by biological approaches less satisfactory. Separating the output into just four products: 1) CPA, 2) aqueous layer, 3) organic layer and 4) a little precipitate (from the aqueous filtration) minimizes waste generation and handling.
I would tell the new manager:
1. Use butyl lithium in cyclohexane, not n-hexyl lithium in hexane.
2. For the quench, dissolve NH4Cl in water, about 25%, and drip it in slowly with cooling; don’t use nitrogen or air.
3. DI water or potable well water is suitable for dissolving NH4Cl.
4. Use cooling, especially during the quench — a chiller or ice could work. Cooling the reactants at the beginning of the reaction will reduce the side reactions.
5. Isopropanol is neither necessary nor desirable for quenching, as it tends to unite the aqueous and organic layers. Plain water would quench the reaction but lithium hydroxide is not very soluble; so the addition of NH4Cl provides acidic chloride — but then NH3 is a product and the aqueous layer will be alkaline and smell of ammonia.
Jim Gaidis, technical director