FLUORINATED HIGHER CARBOXYLIC ACIDS

Perfluorinated carboxylic acids are corrosive liquids or solids. The acids are completely ionized in water. The acids are of commercial significance because of their unusual acid strength, chemical stabiUty, high surface activity, and salt solubiUty characteristics. The perfluoroaLkyl acids with six carbons or less are hquids the higher analogues are soHds (Table 1). 

The higher members of the series decrease the surface tension of aqueous solutions well below the point possible with any type of hydrocarbon surfactant, although in practice because of their strong acid character and solubiUty characteristics, more commonly salts and other derivatives are employed. A 0.1% solution of C F COOH has a surface tension of only 19 mN/m (dyn/cm) at 30°C (6). 

tlon There are five methods for the preparation of long-chain perfluorinated carboxyUc acids and derivatives electrochemical fluorination, direct fluorination, telomerization of tetrafluoroethylene, oligomerization of hexafluoropropylene oxide, and photooxidation of tetrafluoroethylene and hexafluoropropylene. 

Many of the perfluoroaLkyl carboxyUc acids were first prepared by the electrochemical fluorination (ECF) of the corresponding carboxyUc acids (7). In ECF acid chlorides are converted to the corresponding perfluoroacid fluorides as shown in equation 1 for octanoyl chloride. 

Perfluorinated acid fluorides containing heteratoms are also accessible by ECF. Long-chain perfluorinated acid fluorides produced by ECF containing nitrogen (10—12), oxygen (13), and sulfur (14,15) have been reported. The fluorinated mixed sulfonic acid—carboxyflc acid precursors are also known. ECF of hydrocarbon sultones has led to formation of FS02(CF2) C0F, where n = 2,3 (16). 

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FLUORINECOMPOUNDS,ORGANIC - FLUORINATED HIGHER CARBOXYLIC ACIDS] (Vol 11)... 

Electi ochemical fluorination of carboxylic acids produces perfluoroalkanoic acid fluorides in a low yield (about 10-20%) [31] and water as a by-product. Water forms explosive oxygen difluoride and causes oxidative degradation of the carboxylic acid. For acids with six or more carbon atoms, cyclic perfluoroethers are among the by-products formed. The anhydrides of carboxylic acids give per-fluorocarboxylic acid fluorides in a higher yield than the parent carboxylic acids. The electrochemical fluorination of carboxylic acids or their anhydrides is now obsolete. 

The yield of the perfluorinated product is higher when fluorinating a carboxylic acid chloride or fluoride, instead of the carboxylic acid itself or its anhydride [110] ... 

One of the most useful ways of introducing fluorine into organic compounds is the placement of the hydroxyl group in alcohols hydroxy compounds, and carboxylic acids Methyl alcohol reacts with anhydrous hydrogen fluoride at 100 500 °C in the presence of aluminum fluoride [60, 61], zinc fluoride [62] chromium fluonde [63], or a mixture of aluminum and chromium fluondes [64] to give a 20-78% yield of fluoromethane Attempted fluorinations of higher alcohols by this method failed [60]... 

The direct fluorination with elemental fluorine at — 78 "C of trimethylsilyl enol ethers derived from diketones results in the formation of the corresponding monofluoro diketones 11 in moderate yield. The trimethylsilyl ethers from cyclic diketones undergo smooth fluorination to give the enol forms, c.g. 12, and not the keto forms.Higher yields are generally observed for the analogous reactions of silyl derivatives of esters, carboxylic acids, malonates, dimethyl amides and lactones (Table 4). ... 

The reaction may proceed as homo- or cross-dehydrodimerization [105] and takes place with a wide range of substituted substrates such as higher alcohols, ethers, silanes, and partially fluorinated alcohols and ethers, but also with ketones, carboxylic acids, esters, amides, and amines [106]. Besides the formation of 1,2-diols from saturated alcohols, unsaturated substrates are also dimerized under hydrogen to form l,n-diols other than the 1,2-isomers [107]. The regio-selectivity of the diols is controlled by the formation of the most stable radical, which then dimerizes. 

Both the chlorine and fluorine sulfonyl compounds strongly deshield the ortho aromatic hydrogens. The chemical shifts observed for the two phenyl compounds are extremely similar. A comparison of the para substituted compounds indicates identical shifts for the two para substituted methyl compounds, higher field shifts for the chlorine substituted sulfonyl fluoride but lower field shifts for the corresponding carboxylic acid substituted sulfonyl fluoride compared to the corresponding sulfonyl chlorides. 

Liu and coworkers have recently published the synthesis of (3) and (4), as well as several other fluorinated pyrrole analogues (not shown), by aminofluorination of allenes the synthetic approach is shown in Scheme 3 [15], This strategy takes advantage of a selective, silver-catalyzed intramolecular fluorination reaction, but the approach works best with substrates that possess electron-withdrawing R -groups on the 3-position. Because of this limitation, the yield of (3) is significantly higher than (4), 80 % versus 28 %, respectively. 4-Fluoro-pyrrole-2-carboxylic acid (5a, Fig. 2), synthesized from a fluorinated proline, was explored as a potential intermediate on the route to (3), but only extensive decomposition products were observed when (5a) was subject to flash pyrolysis [16]. 

Much research has been done to determine the efficacy of SWNTs as NWs in molecular computers. One problem with SWNTs is their lack of solubility in common organic solvents. In their synthesized state, individual SWNTs form ropes from which it is difficult to isolate individual tubes. In our laboratory some solubility of the tubes was seen in 1,2-dichlorobenzene. An obvious route to higher solubility is to functionalize SWNTs by attachment of soluble groups through covalent bonding. Margrave and Smalley found that fluorinated SWNTs were soluble in alcohols while Haddon and Smalley were able to dissolve SWNTs by ionic functionalization of the carboxylic acid groups present in purified tubes. 

The yield of a perfluorocarboxylic acid fluoride is higher when a carboxylic acid chloride or fluoride is fluorinated instead of the carboxylic acid or its anhydride [48,49]. For example, acetic acid, acetic acid anhydride, and acetyl fluoride give trifluoroacetyl fluoride in 17%, 32%, and 76% yields, respectively [32,49] ... 

Electrochemical fluorination of dicarboxylic acid fluorides also gives a higher yield than fluorination of the parent acids. The lower yield given by the parent carboxylic acids has been attributed to decarboxylation, similar to the Kolbe reaction [31]. 

Aliphatic carboxylic acids and their anions are known to absorb in the ultraviolet region as a result of u-tt and tt-tt transitions. Mukerjee et al. [64] found that long-chain perfluorocarboxylates, such as perfluorooctanoate (e = 344 L/mol/cm at 25 °C), have higher molar absorptivities in the 205-230-nm region than perflu-oroacetate (e = 57 L/mol/cm at 25°C). The absorptivity of perfluoroalkanoates is sufficient for a quantitative determination of the fluorinated surfactant down to the 10 M concentration range using a 10-cm cell. Mukerjee et al. [64] observed that below the critical micelle concentration (cmc), perfluoroheptanoate and perfluorooctanoate solutions obeyed the Beer-Lambert law within 1%. A somewhat bet-... 

Perfluoroalkylfuran Derivatives Fluorination of 2,4-furandicarboxylic acid by sulfur tetrafluoride results in the mixture of products 45-47." The carboxylic group in position 4 is more reactive than in position 2 and the yield of the compound 46 higher than that of the compound 47. Decarboxylation of the acid 46 in the presence of copper powder in quinoline gives 3-trifluoromethylfuran. 

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