RESEARCH
Fluoro Alcohols Show High Solvency Power Strong hydrogenbonding donor capability of new compounds enables them to dissolve many polymers Members of a new family of solvents are in various stages of research at Du Pont. The group consists of a number of secondary and tertiary fluoro alcohols and diols synthesized by chemists at the company's Central Research Department. Development studies of some of these compounds are under way by the Organic Chemicals Department's research section. The alcohols are strong hydrogenbonding donors, a property that accounts for their high solvency, say Dr. W. J. Middleton and Dr. R. V. Lindsey, Jr. [/. Am. Chem. Soc, 86, 4948 ( 1964 ) ]. Compounds that contain receptive sites for hydrogen bond formation dissolve easily in the alcohols. Among such compounds are polymers such as polyformaldehyde, nylon and other polyamides, polyacrylonitrile, polyvinyl alcohol, and polyesters. The fluoro alcohols (especially fluoro ketone hydrates) are also solvents for natural products containing amide, amino, ester, alcohol, or ketone groups. Commercial Delrin acetal resin (a polyformaldehyde) is extremely soluble in the compounds at room temperature. For instance, a 207c solution of Delrin can be prepared at 25° C. in aqueous hexafluoroacetone hydrate (buffered with triethylamine to prevent degradation of the polymer). No low-temperature solvent for Delrin was formerly available; it's generally dissolved in hot p-chlorophenol. A low-temperature solvent could find use as an adhesive for Delrin surfaces and for reclaiming scrap polymer. The fluoro alcohols stem from Du Pont's fluorine chemicals program. Progenitor of the new compounds is 32
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SOLVENT. Dr. W. J. Middleton of Du Pont's Central Research Department examines the paste made by dissolving Delrin in hexafluoroacetone sesquihydrate. The compound is the first low-temperature solvent for Delrin (a polyformaldehyde)
hexafluoroacetone. This compound and its sesquihydrate are now available as development products from Du Pont and Allied Chemical. Du Pont's price is $15 a pound. Hexafluoroacetone, ( C F 3 ) 2 C O , is a reactive gas that undergoes many addition reactions. It combines with many active hydrogen compounds to give substituted hydroxyl compounds. The acetone can also be used to introduce ( C F 3 ) 2 C O H groups into organic molecules. Fluorine-containing monomers can be made and used in pol-
ymer syntheses; C F 3 groups increase oil and water repellency of polymers. The sesquihydrate, ( C F 3 ) 2 C O L 5 H 2 0 , possesses the solvency powers that prompted Dr. Middleton and Dr. Lindsey to synthesize other fluorinecontaining compounds. The sesquihydrate is a liquid at room temperature and has a pH of less than 1. Acidic. Aliphatic alcohols containing a high percentage of fluorine are more acidic than are the corresponding fluorine-free alcohols. Such compounds should thus be strong hydro-
gen-bonding acids. The most pro nounced effects, the Du Pont chemists reasoned, would be found in alcohols containing fluorine bonded directly to the alpha carbon. One such alcohol, perfluorocyclobutanol, had been prepared a few years ago at Du Pont. Nuclear mag netic resonance studies show that it is internally hydrogen bonded. This al cohol, however, is thermally unstable. The effort was then centered on preparing alcohols containing two or three perfluoroalkyl groups attached to the alpha carbon. The more stable secondary and tertiary fluoro alcohols and diols were then made. Their main characteristic is their ability to function as strong hydrogen-bonding donors. A number of the secondary fluoro alcohols are made by reducing the cor responding fluoro ketones. For in stance, catalytic and chemical reduc tion of hexafluoroacetone gives 2-Hhexafluoro-2-propanol in high yield. (This compound was also prepared by chemists in the U.S.S.R. by reduction of the acetone with sodium borohydride; it can also be made by catalytically reducing the acetone.) The Du Pont group uses lithium aluminum hy dride in tetrahydrofuran (THF). The reaction product is a 1:1 complex of the alcohol with THF; fuming sul furic acid frees the alcohol from the complex. A number of other fluoro alcohols were made the same way, in cluding those from 1,3-dichlorotetrafluoroacetone, 1,1,3,3-tetrachlorodifluoroacetone, and bis(perfluoroisopropyl) ketone. Photolytic. Perfluoropinacol, a diol that resists the pinacol-pinacolone re arrangement, was made by a photoinitiated bimolecular reduction of hex afluoroacetone with isopropyl alcohol. The Du Pont workers believe that per fluoropinacol is the first nonaromatic pinacol made by a photolytic reduc tion. The compound can also be made by reducing hexafluoroacetone with magnesium amalgam in ether, but the yields from such a reaction are very low. Although its solvency power is among the greatest of the series, perfluoropinacoFs potential usefulness is limited by its toxicity. One drop of the compound on the skin of a guinea pig is enough to kill the animal. A four-hour exposure to 50 p.p.m. of the vapor in air is lethal to rats. The hydrated fluoro ketones can be viewed as fluoro alcohols. They are
actually gem diols (two hydroxyls on one carbon atom). Well-defined hy drates of ketones and aldehydes are known, but not definite 1:1 hydrates of fluoroacetones; several indefinite hydrates appear in the literature. The Du Pont chemists made several fluorine-containing gem-diols by add ing stoichiometric amounts of water to fluoroacetones. Other ketones are made by reduc ing chloro ketones with triethyl phos phite (a reaction which extends the work of Du Pont's Dr. H. E. Simmons and Dr. D. W. Wiley). For instance, 1 - chloro - 1,1,3,3 - tetrafluoroacetone was made this way from 1,3-dichlorotetrafluoroacetone. 1,1,2,2-Tetrafluoroacetone and related compounds can be made the same way. Perfluoro ( 3-H,2,3-dimethyl-2-butanol ) is made by base-catalyzed hydration of tetrakis ( trifluoromethy 1 ) ethylene. Abnormal. All of the secondary and tertiary fluoro alcohols studied are strongly acidic because of the cumulative inductive effect of the fluo rine atoms. However, the fluoro diols seem to be abnormally acidic. Per fluoropinacol, for example, has a pK a in water of 5.95, lower than that of any other reported saturated alcohol. This is a 3000-fold increase in acidity over perfluoro-fert-butyl alcohol (pK a of 9.52). Inductive effects alone do not ex plain the increased acidity. There fore, the Du Pont chemists propose that the unusual acidity is due to strong intramolecular hydrogen bond ing that could stabilize the anion.
CF3 ι CF,-C 3
CF3 I C-CFa
I \
I
Θ
Η'*
Molecular models suggest to them that such an intramolecular hydrogen bond is sterically very favorable. A similar theory has been proposed by others to explain the abnormal acidity of some dicarboxylic acids. The gem-diols also seem unusually acidic. Factors other than inductive effects must also be important in this case. A possible explanation put forth by Dr. Middleton and Dr. Lindsey is that stabilization of the anion is due to a strong intramolecular hydrogen bond or to a complex that's hydrogenbonded to a molecule of water.
The stable (distillable or recrystallizable) 1:1 complexes formed by the secondary and tertiary fluoro alcohols with hydrogen-bond acceptors repre sent further evidence for the strong hydrogen-bonding capabilities of these alcohols. For example, the hexafluoro-2-propanol/THF complex boils at 100° C , which is 30° C. higher than the boiling point of either component. A similar complex of perfluoropina col with dioxane melts at 80° C. This is 54° C. higher than the pinacors melting point and 70° C. higher than dioxane's. Stable complexes can also be made from one or more of the fluoro alcohols with solvents such as acetonitrile, tetrahydrothiophene, and pyridine.
BRIEFS The "photon echo/' a previously un known radiation phenomenon, has been observed by Dr. S. R. Hartmann, Norman Kurnit, and Dr. Isaac Abella of Columbia University, New York City [Phys. Rev. Letters, 13, 567 (1964)]. The phenomenon is an in tense burst of radiation emitted by a ruby crystal which has been previously subjected to two short, intense light pulses from a ruby laser. The "photon echo" can be observed on an oscillo scope connected to a photomultipher monitoring the radiation from the ruby crystal sample. The phenomenon should have wide application in solidstate physics and in computer work. NOV. 3 0, 196 4 C&EN
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