Reduction of aromatic fluorine compounds

Reduction of Aromatic Fluorine Compounds Bruce H. Campbell Department of Chemistry, Uniaersity of Southern Mississippi, Hattiesburg, Miss. 39401

The reduction of carbon-fluorine aromatic compounds has been studied on mercury electrodes. Three skeletal types were chosen, benzene, biphenyl, and naphthalene. Fluoride was found as a product of controlled potential electrolysis of the compounds studied and the reactant minus the fluorine was also found. The mechanism of reduction is formulated as ECE where each electron transfer step involves one electron and the chemical step is protonation. As was expected, the lifetime of the species produced by the first reduction step increases as the aromaticity increases. This was proved by the increased reversibility of the reduced molecule as the aromaticity increased.

potentials. Also, greater stabilization of the reduced product would result from the more extensive pi system, as compared to the benzene series. The naphthalene series was chosen as the next step in a more extensive pi system. EXPERIMENTAL

THEGENERAL BEHAVIOR of carbon-halogen bonds undergoes two different modes of reaction when reduced ( I ) depending on whether the adjacent carbon has a halogen bond to it, and if the carbon is saturated. If adjacent carbons are not halogenated, the result of the reduction is the replacement of the halogen by a proton with the subsequent release of halide. When the adjacent carbon is also halogenated, the two halogen atoms are lost as halides with the formation of a double bond between the two adjacent carbons. However, if a double bond already exists between the two adjacent carbons that are bonded to the halogens, reduction leads to replacement of the halogen atoms, released as anions, by protons. The general classifications of the carbon-halogen reduction was shown to be ECE ( 2 ) where the chemical reaction is protonation leading to a more easily reduced species which immediately undergoes reduction at the electrode. This mechanism is known as a solvent assisted reduction. Bartle and Eggins (3) have investigated the reduction of some fluorinated compounds in 75 dioxanelwater. They conclude that the overall reaction is addition of a proton after the initial electron transfer, leading to a species that is reducible at a potential more cathodic than the initial reduction potential. The end result of their study was the same as the general mode of reduction explained above with the difference being the number of electrons exchanged per molecule and the ease of reduction of the protonated product. The reduction of fluoro-aromatic compounds in dimethylformamide was studied in this project. This media was specifically chosen to provide a system that would dissolve the polar compounds and to provide a system in which the available proton donor concentration is quite low. The compounds studied fall into three groups. The first two groups-fluorinated benzenes and fluorinated biphenylswere chosen for the following reasons. The benzene series represents a simple case structurally, and the effects of electron withdrawing groups, such as fluorine, on the benzene ring are well known. The biphenyl series was chosen to compare the effects of reducing fluorine on one of the rings with that of reducing fluorine on both rings. The electron-withdrawing effect of fluorine from one ring to another would then be easily compared by examining the differences in reduction (1) S. Wawzonek, ANAL.CHEM., 24, 35 (1952). (2) M . Von Stackelberg and W. Stracke, Z. Electrochem., 53, 118 (1949). (3) W. W. Bartle and R . R . Eggins, J. Polarogr. SOC.,12: 89 (1966).

The apparatus used for the study of biphenyl and benzene series has been previously described (4). The experiment used for the study of the 2-fluoronaphthalene is as follows: a Wenking potentiostat, Model 61 RS, serving as the power source, was driven by a signal from a chopper-stabilized Heath polarograph and a Wavetek Model 112 wave generator. The signals were added by a Philbrick-Nexus operational amplifier, Model 1303. The voltammetric curves were recorded on a Fisher Recordall series lOOXY recorder. Potentials were measured with a Honeywell Digitest digital voltmeter, Model 333R. The electrochemical cell was composed of a reference compartment separated from the solution bulk by an asbestos fiber sealer in the compartment tip, an auxiliary electrode compartment separated from the solution bulk by a fritted disk, and ports for both deaeration of the solution and maintaining internal pressure of nitrogen gas. Deaeration of the solution was continued for a minimum of 45 minutes. The auxiliary electrode was platinum, the working electrode was either platinum or mercury, and the reference electrode was a saturated calomel electrode (SCE). All fluorinated compounds were from Aldrich Chemical Company; the supporting electrolytes, tetrabutylammonium perchlorate or iodide, were from Southwestern Chemicals, and the lanthanum chloranilate was from Fisher Scientific Company. The gas chromatograph was an F & M Model 609 with a flame ionization detector. RESULTS AND DISCUSSION

This section is divided into four parts, the first part covering the benzene skeletal fluoro-aromatics, the second part the biphenyl fluoro-aromatics, the third part deals with the reductions of 2-fluoronaphthalene, and the last part is a summation of similarities of the first three parts as well as general discussion. The fluorobenzenes studied are, pentafluorobenzene, 1,2,4,5-tetrafluorobenzene, 1,2,3,5-tetrafluorobenzene, and 1,3-difluorobenzene. Data from cyclic voltammetry of pentafluorobenzene are in Table I. A typical cyclic voltammogram of pentafluorobenzeneis shown in Figure 1. The cyclic voltammetry of both 1,2,4,5-tetrafluorobenzene and 1,2,3,5-tetrafluorobenzenewas examined from ca. 50 mV/sec to 300 mV/sec. Both tetrafluorobenzenes gave a decrease in iplcl'2 of the first reduction wave as the scan rate was increased ( 2 0 x and 4 decreases, respectively). Three reduction waves for 1,2,4,5-tetrafluorobenzenewere found with peak potentials of -2.40 i 0.03, -2.53 k 0.04, and -2.65 i 0.05 V as. SCE. Only two reduction waves were observed for 1,2,3,5-tetrafluorobenzenewith peak potentials of -2.65 i 0.02 and -2.77 f 0.02 V us. SCE. Polarography of 1,3difluorobenzene showed a half-wave potential of -2.82 V cs. (4) K. S. V. Santhanarn and A. J. Bard, J . Amer. 88, 2669 (1966).

Cliem. Soc.,

ANALYTICAL CHEMISTRY, VOL. 44, NO. 9, AUGUST 1972

1659

Table I. Cyclic Voltammetry of Pentafluorobenzene“ Scan rate, Wave l*+ mV/sec Ep, V Epil, V ip/u112 48.1 -2.31 -2.23 100 72.7 -2.33 -2.25 98 91.4 -2.35 -2.25 98 203 -2.39 -2.28 98 298 -2.39 -2.29 98 a 2.7mM in a DMF-O.1M TBAI solution. * A = 4.14 X 10-2 cm2. ip/v1‘2C,dropped from 43 to 36 going from 0.90 to 2.7mM.

Compound 4-Fluorobiphenyl 4,4 ’-Difluorobiphenyl Decafluorobiphenyl At lowest scan rate. At increasing scan rate.

60-

40-

20-

(E. -2.4

-2.6

yL

S.C.E.)

-2.8

Figure 1. Cyclic voltammogram of pentafluorobenzene, C = 2.7mMin a DMF-O.1M TBAI solution, Curve 1, u = 91.4 mV/ sec, Curve 2, u = 48.1 mV/sec, A = 4.14 X loFzcmz SCE, a n average Tomes criteria of 110 mV and a n id/conc of cu. 22 pA/mM (The flow rate of Hg times the drop time was approximately 5 mg). Because solvent assistance is an important part of the electro-reduction of halogen-containing organic compounds (2), the proton concentration was varied in the various solutions. (The variation of proton concentration was affected by the addition of hydroquinone, HzQ). If protonation is part of a solvent-assisted reduction, a n increase in proton availability would lead to the more complete addition of another electron. This would be shown by an increase in the current (wave height increase) and possibly by the appearance of a new wave. The “new” wave would arise because protonation accelerated the reaction sufficiently for the new wave to become noticeable in the time span of the experiment. Without the presence of the added proton donor, the reaction producing the substance responsible for the “new” wave would be too slow, and thus not enough of the substance would be produced to give a noticeable wave. The addition of H2Q caused an increase in the reduction wave current. No shift in wave potentials was measured, and 1660

Wave 3 Em V -2.62 -2.64 -2.65 -2.68 -2.70

Table 11. Cyclic Voltammetry of Fluorobiphenyls Increase in peak potential Increase of per decade half-peak, Scan rate increase in peak potential range, mV/sec Peak potentia1,a V scan rate, mV differences,*mV 50-300 -2.64 80 50-80 50-200 -2.66 80 60-90 12-190 -1.73 50 30-50

microamps

-2.2

Wave 2 Em V -2.48 -2.48 -2.48 -2.51 -2.52

Wave 4 Ep,

V

-2.76 -2.77 -2.78 -2.81 -2.82

ip/v1‘2C 1~A.sec1’2 V1/2.mM

Wave 5 Ep,

V

-2.91 -2.93

...

... ...

Decrease in i p / v l / 2 with increase in scan rate,

71

0

120 75

40 40

no new waves appeared. However, the height of the second wave increased. The situation upon proton donor addition fits very well with what was expected for a solvent-assisted reduction. Controlled potential coulometry was attempted in order t o find the value of napp. No direct results were obtained because of the regenerative or catalytic behavior observed. The term “regenerative behavior” is used here because the compound undergoing reduction, the pentafluorobenzene or a product of pentafluorobenzene, is being regenerated (5). This term is more applicable than catalytic because it is desirable to make the subtle distinction between studying pentafluorobenzene rather than the compound causing the regeneration. Since the final steady state current in coulometry was cu. 10% of the initial value, an indirect method of obtaining nappwas used. This method involved passing a measured amount of coulombs through the solution and subsequently measuring the, height of the reduction wave with cyclic voltammetry. Using this method, the number of moles apparently electrolyzed as measured by the number of coulombs passed should be greater than the number of moles electrolyzed as measured by the reduction in wave height. Any disparity is then due to the regenerative behavior. One experiment was run with the following results. After passing 1.34 Faradays per mole, the pentafluorobenzene reduction wave height was decreased by 5 5 %. If naps was one, and considering the ca. 10% regeneration reaction, the wave height should have decreased by 80 + 90%. If nappis two, a 10% regeneration rate would give a wave decrease of cu. 50 to 60% whereas a higher nappwould give a lower percent decrease in wave height. Thus, nappis probably two. The reduced solutions of pentafluorobenzene were analyzed with vapor phase chromatography (VPC) with a Carbowax 20M column on a Chromosorb W support. A new peak in the chromatogram was identified as 1,2,4,54etrafluorobenzene. VPC retention times of the other isomers of tetra(5) A. J. Bard, University of Texas, private communication, 1966.


fluorobenzene, the three isomers of trifluorobenzene, and those of difluorobenzene were obtained. The results eliminated these compounds as being major or measurable products of the reduction of pentafluorobenzene. Notice that the peak potential of the second reduction wave of pentafluorobenzene is more negative-by ca. 80 mV-than the first peak potential of 1,2,4,5-tetrafluorobenzene.This negative shift of potential is expected when the product (1,2,4,5-tetrafluorobenzene) of a previous reaction (reduction and protonation of pentafluorobenzene) is electrolyzed (6) and is further evidence that 1,2,4,5tetrafluorobenzene is a product of the reduction. Another reduced solution of pentafluorobenzene was evaporated under vacuum. The solid residue was dissolved in water and tested for fluoride with lanthanum chloranilate. This highly specific test for fluoride was positive. Another portion of the solid residue was analyzed by mass spectrometry. The mass spectrogram contained mje parent peaks at 185, 142, and 100, among others. These are the same as the mje parent peaks for tributylamine. The tetrabutylammonium salts did not give these peaks under the conditions used for the mass spectrogram. Since the tetrabutylammonium ion did not give the tributylamine peaks, the tributylamine must have been present in the solution before the mass spectrogram was obtained. A blank solution containing tetrabutylammonium ion was electrolyzed, evaporated to a solid, and analyzed with the mass spectrometer. No tributylamine peaks were observed. This left the reduction of pentafluorobenzene as the agent respondible for the production of tributylamine from tetrabutylammonium ion. The procedure hypothesized is the familiar Hofmann degradation (7), whereby a proton is abstracted from a beta carbon in a tetraalkylammonium ion, leaving trialkylamine as one of the products. The general reaction thus appears to be the initial reduction of one electron followed by protonation and subsequent reduction by a second electron at the same potential as that for the addition of the first electron. If no proton is added to the solution as such, the reduced pentafluorobenzene obtains its needed proton from the supporting electrolyte, tetrabutylammonium ion. The other fluorobenzenes were not studied as extensively as the pentafluorobenzene. They were mainly used in analysis confirmation and for comparison of peak potentials for the separate compounds with the peak potentials of the pentafluorobenzene reduction waves. The fluorobiphenyls studied are, 4-fluorobiphenyl, 4,4’difluorobiphenyl, and decafluorobiphenyl. The cyclic voltammetric behavior is given in Table 11. Note that 4-flUOrObiphenyl shows some Nernstian behavior at small concentrations of the proton donor, HzQ, Table 111. All three fluorobiphenyls showed an increase in wave height when the proton donor H2Q was added to the solutions. Cyclic voltammetry of 4-fluorobiphenyl and 4,4’-difluorobiphenyl, after addition of HzQ, showed a second reduction wave appearing at potentials more cathodic than the original wave. The potential of the second wave drastically shifted more cathodic as the scan rate was increased, and totally disappeared at scan rates higher than 100 mV/sec (See Table IV). All the reduction behavior observed when the proton donor was added to the solutions fits very nicely with that predicted for a solvent assisted reduction. The reduction behavior parallels that (6) R. S.Nicholson and I. Shain, ANAL. CHEM., 36,706 (1964). (7) R. Q. Brewster, “Organic Chemistry,” 2nd ed., Prentice-Hall, Englewood Cliffs, N.J., 1958, p 273.

Table 111. Anodic to Cathodic Peak Current Ratio of 4-Fluorobiphenyla Concentration Concen4-fluorotration biphenyl, H2Q, Scan rate, Epc - Epor mM mM V/sec ipa/ipcb’c mV 0.0 0.203 0.57 40 1.o 1.o 0.0 0.298 0.60 50 0.7 1 .o 0.0 12.6 .. 0.31 0.0 0.203 70 1.61 1.61 0.0 0.30 0.298 80 1.61 0.0 12.6 0.5 1.61 1.3 0.203 0.39 10 0.31 EO 1.61 1.3 0.298 1.61 18 0.203 0.0 .. 1.61 18 0.298 0.0 .. 1.61 18 12.6 0.0 .. a In a DMF-O.1M TBACIOl solution. * A = 4.14 X 10-2 cm2. iPa calculated by the semi-empirical method of Nicholson (6). I .

for the reduction of pentafluorobenzene except for the Nernstian behavior seen for 4-fluorobiphenyl. Controlled potential coulometry of all of the fluorobiphenyls showed regenerative behavior both in the presence and absence of H2Q. Cyclic voltammetry of the reduced solutions showed a new reduction wave for the decafluorobiphenyl solution at -2.92 V. In each of the three solutions, the original reduction wave was decreased in height when measured after coulometry was halted. The reduced solutions were evaporated under vacuum to a solid residue. Positive tests for fluoride ion were obtained with lanthanum chloranilate. Mass spectrometry of the reduced decafluorobiphenyl residue gave parent peaks at 331, 185, 142, and 100. The last three peaks correspond to tributylamine peaks. Identification of the peak at 331 inje has not been made. Identification of gas chromatographic peaks was by comparison of retention times and co-chromatography of the unknown peaks and those peaks resulting from unknown compounds. Reduced 4,4’-difluorobiphenyl solutions showed new peaks that correspond to those of 4-fluorobiphenyl and biphenyl as well as one corresponding to tributylamine when HzQ was absent from the solution. Reduced 4-fluorobiphenyl solutions showed a new peak identified as biphenyl. Reduced solutions of decafluorobiphenyl gave a chromatographic peak corresponding to that for tributylamine. The lanthanum chloranilate test for fluoride was positive when performed on the solid residue remaining after vacuum evaporation of the reduced solutions. The last series of fluoro-aromatics investigated was that of fluoronaphthalenes. Only one compound of this series, 2fluoronaphthalene, was studied. Cyclic voltammetric data for 2-fluoronaphthalene are given in Table V. When hydroquinone was added, the variation in peak potentials went to about 60 mV for a ten-fold change in scan rates, ip/v112C values increased to 2.0 times as large as when no hydroquinone was present. The anodic wave disappeared, and a new cathodic wave appeared at -2.59 V. Polarographic data were obtained and Tomes criteria were 60 mV when no hydroquinone was present. The reduction behavior is more Nernstian than any other fluoro-aromatic studied and the data, Ep - E,,z and especially E314 are close to the values normally expected for a one-electron reduction (6).


1661

~~~~

~~~

~

Table IV. Effect of Hydroquinone on Cyclic Voltammetry of 4Fluorobiphenyl' Scan rate. Wave 1 mV/sec E p ,V Epi2, V !,,",PA -2.57 23.5 23.1 -2.64 -2.57 27.0 33.8 -2.64 -2.65 -2.58 31 .O 48.1 -2.66 -2.59 40.0 91.4 -2.68 -2.61 54.5 203 -2.71 -2.62 64.0 298 a 1.61mM plus 18mM H2Qin a DMF-O.lMTBAClO4 solution. A = 4.14 X cm*.

Em V -2.69 -2.72 -2.83

i,/u1/2

155 145 135 131 121 113

Wave 2 i,, P A

6.0 5.5

2.0

ip/o11 2

40 30 7.4

Table V. Cyclic Voltammetry of 2-Fluor~naphthalene~ Scan rate, mV/sec 20

Ep,c,

Ep,~.V o

-2.410 -2.420 -2.414 -2.416 -2.425

50

0

V

-2.325 -2.333 -2.320 -2.325 -2.325

100 200 300 1.66mM. An increase to 6.76mM caused a decrease of 3 %.

The values of i p , u / i p , in c Table V are of interest in following the variation of reduction behavior as a function of added proton donor concentration. The fraction begins very close to the value expected for a Nernstian case and gradually decreases as HzQ concentration is increased. Also, the value of i p , C / ( o / l / *is C )almost doubled by the presence of excess HzQ. Both of these facts suggest that enough HpQ was added to the solution to enable the reduction to proceed through the ECE mechanism instead of stopping after the first electron transfer. The polarographic current data was graphed us. height of mercury in three cases and the values of the slope were: 0.44, 0.50, and 0.45, again suggesting a became broadened diffusion controlled reduction. EZiawith increasing amounts of HzQ, showing a non-Nernstian step, probably the proton transfer. Thus, the general mechanism for the reduction of 2-fluoronaphthalene is probably ECE with protonation being the chemical step. CONCLUSIONS

All of the data indicate a loss of fluoride upon reduction of the compounds studied. RF

+ 2e + H + $

RH

+ F-

and although controlled potential electrolysis did not yield a directly determined value of napp,the reduction data of 4fluorobiphenyl and 2-fluoronaphthalene show the same trends of behavior as the theoretical cases shown by Shain and coworkers (6, 8) for an ECE mechanism. The individual reactions could be given as :

+ e $ RF.RF.- + H + $ R F H . R F H . + e $ RFHRFH- $ R H + FRF

Alternative mechanisms would be the loss of the fluoride ion at step two or three. No definite evidence exists for its loss (8) R. H. Wopschall and I. Shain, ANAL.CHEM.,39, 1513 (1967). 1662

I P A

PA

6.6 13.5 17.3 23.9 31.6

Ep.,,

V

None -2.298 -2.312 -2.310 -2.307

ip,a/ip,c 1.03

1.10 1.04 0.99

ipb/(V I i 2c)

28 30 33 32 34

at the fourth step. The outlined process seen above is a familiar one in reduction of organic molecules-a process whereby the reduced substance is protonated and forms a species more easily reduced than the original species (a solventassisted reduction). As written above, the reactions are actually an ECEC type, but with the inability to differentiate, on the basis of this data, between an ECE, ECEC, and an ECCE (in the ECCE case one of the chemical reactions is ratedetermining), the general case of ECE is used for descriptive purposes. The species responsible for the regenerative behavior observed in controlled potential electrolysis is probably a reduction product, such as biphenyl in the fluorobiphenyl reductions. This is postulated from the known behavior of biphenyl and the fact that often the original compound was absent in the solutions after coulometry. The absence of the original compound was noted by either cyclic voltammetry or polarography and by VPC. The observed path of reduction of pentafluorobenzene (to the 1,2,4,5-tetrafluorobenzene) is paralleled by the nucleophilic attack of ammonia at elevated temperatures (9). The observed pathway is also what would be expected for an attack on a compound containing fluorine, attack at a meta position. Examination of pentafluorobenzene shows the 3 position is meta to two fluorine-carbons while no other position is meta to two fluorine-carbons. Thus, the 3 position should be more electron rich and the fluorine at that position would be more easily removed as the ion. If any of the other isomers of tetrafluorobenzene were produced, their concentrations were quite minor. The similarity of reduction potentials of 4-fluorobiphenyl and 4,4'-difluorobiphenyl indicates that little of the electrophilic character of the fluorines is passed from one ring to the other. This may be caused by the plane of the rings not being parallel, thus not allowing transferring of pi electron effects (IO,11). (9) G. M. Brooke, J. Burdon, M. Stacey, and J. C. Tatlow, J . Chem. SOC..1960, 1768. (10) H. H. Jaffe and 0. Chalvet, J . Amer. Chem. SOC.,85, 1561 (1963). (11) L. 0. Wheeler and A. J. Bard, J. Phys. Chem., 71, 4513 (1967).


This formation of tributylamine from the tetrabutylammonium is the familar Hofmann degradation (8) whereby a proton is abstracted from the tetrabutylammonium ion by another solution species. Its prevention could be accomplished by using a tetraalkylammonium salt that doesn’t have hydrogen on the carbons beta to the nitrogen, by using another cationic type salt, or by adding a proton donor, such as H2Q, to the solution. The first type is generally unsuitable because of the limited solubility of tetramethylammonium ion

and the expense of synthesizing a tetra-t-butylammonium compound. However, the presence of a tetraalkylammonium ion is not always detrimental because it acts as a very weak acid and thus affords an estimation of the strength of a base, upon the removal or non-removal of a proton from the tetraammonium ion. RECEIVED for review October 4, 1971. Accepted March 29,

1972.

High Sensitivity Thermochemical Analysis Earl B. Smith, Charles S. Barnes, and Peter W. Cam1 Department of Chemistry, University of Georgia, Athens, Ga. 30601

A lock-in amplifier and linear ramp generator system has been developed which permits an rms temperature resolution of 3-4 / * O C . This temperature resolution approximates the theoretical Johnson noise limit of the signal source. The system was employed to improve the sensitivity of thermometric analysis. In a 100-ml calorimetric vessel, solutions of 15 pM perchloric acid have been titrated with sodium hydroxide with a precision and accuracy of better than 10%. This reaction generated 20 millicalories of heat and caused a temperature change of approximately 0.2 m°C. At higher concentrations ( 1 . 5 m ~ )the optimum precision achieved was 0.2%, which is the limiting precision of the volume delivery device.

SINCETHE INTRODUCTION of the thermistor in thermochemical analysis by Linde, Rogers, and Hume ( I ) , the techniques of thermometric titration and enthalpimetric analysis (2) have been applied to a very wide variety of aqueous and nonaqueous chemical reaction systems (3-8). Because of the fundamental nature of the primary process ultimately responsible for generating the signal, thermochemical techniques are among the most universally applicable methods of chemical analysis. They are, in our experience, subject to two very serious limitations which have restricted their acceptance. First, as with all linear titration methods, they do not provide the same high degree of precision obtained with other stoichiometric chemical methods such as potentiometric titrations To whom requests for reprints should be addressed. (1) H. W. Linde, L. B. Rogers, and D. N . Hume, ANAL.CHEM. 25, 404 (1953).

(2) J. C. Wasilewski, P. T-S Pei, and J. Jordan, ibid., 36, 2131 (1964). (3) S. T. Zenchelsky, ibid., 32, 289R (1960). (4) J. Jordan, “Thermometric Enthalpy Titrations” in “Treatise on Analytical Chemistry,” I. M. Kolthoff and P. J. Elving, Ed., Part 1, Volume 8, Interscience, New York, 1968. ( 5 ) H. J. V. Tyrrell and A. E. Beezer, “Thermometric Titrimetry,” Chapman and Hall, London, 1968. (6) L. S. Bark and S . M. Bark, “Thermometric Titrimetry,” Pergamon Press, Oxford, 1969. (7) L. D. Hansen, R. M. Izatt, and J. J. Christensen, “Applications of Thermometric Titrimetry to Analytical Chemistry,” in “Modern Titrimetry,” J. Jordan, Ed., Marcel Dekker, New York, N.Y., in press. (8) P. W. Carr, “Analytical Aspects of Thermometric Titrations,” in “Critical Reviews of Analytical Chemistry,” L. Meites, Ed., Chemical Rubber Co., Cleveland, Ohio, in press.

or coulometry. At best, thermometric titrations are precise to only a few tenths (0.2-0.5%) per cent (4, 8). For this reason, other techniques are frequently preferred. A second major deficiency is the very poor sensitivity of the method in comparison to other titration methods such as amperometry and photometry. The method is seldom applied below the millimolar level. There are outstanding exceptions to the sensitivity cited above; all of these, however, involve reactions which are either directly or indirectly very highly enthalpic, e.g. Everson ( 9 ) has successfully analyzed dilute solutions of butyllithium by a reaction with butanol which liberates 53 Kcal/mole of heat. In this instance, the sensitivity is better than 1 mM. The objective of this work was to provide a general solution to both of the above problems. In an attempt to solve the above problems three devices were used. First, a lock-in or phase sensitive amplifier was used to enhance the signalto-noise ratio. Second, a ramp function generator was constructed to null out the base-line drift caused by stirring and poor adiabaticity. Third, the volume of titrant added was monitored via a linear position transducer. Two reactions were studied, namely the titration of perchloric acid and of potassium acid phthalate with sodium hydroxide. This was done advisedly in order to employ reactions whose enthalpies are typical rather than exceptional, avoid end-point curvature due to incomplete equilibria, and avoid all problems associated with reaction slowness which might be encountered at low concentration with other reaction types. EXPERIMENTAL

Reagents. All materials used in this work were reagent grade or primary standard quality. Sodium hydroxide solutions were prepared in the usual fashion from 50% stock solution, stored in polyethylene containers, and protected from atmospheric carbon dioxide by ascarite. Perchloric acid solutions were freed of carbon dioxide by boiling and maintained under a blanket of prehumidified nitrogen during the course of all titrations. Potassium bipthalate was oven dried at 110 “C for 24 hours prior to use. All solutions were standardized by the best accepted procedure. Apparatus. The thermometric titrations reported in this work were carried out in an air thermostat described in a previous communication (10). The special purpose lock-in (9) W. L. Everson, ANAL.CHEM., 36,854 (1964). (10) P. W. Carr, Thermo Chim. Acta, 2, 505 (1971).


1663

Reduction of aromatic fluorine compounds

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