Analytical Reactions Involved in the Spectrophotometric Determination of Alkylaluminum Compounds Using lsoquinoline George W. Heunisch Research and Development Department, Continental Oil Company, Ponca City, Okla. 7460 1
lsoquinoline is probably the most widely used reagent for the quantitative estimation of diethylaluminum hydride and triethylaluminum. The reactions involved in the analytical method, which involves quantitative addition of the alkyl aluminum compounds to a standard solution of isoquinoline, have been investigated from the viewpoint of the reaction products. NMR spectrometry has been used, and products suggesting complexation, as well as addition, have been found. Triethylaluminum simply complexes with isoquinoline, while diethylaluminum hydride reacts first to form a 1 : l addition product, which is capable of dimerization, followed by complexation of the 1:1 product with any available free isoquinoline. The complexed 1:l addition product is the only colored species and provides the basis for the analytical method.
While the primary analytical method used in our laboratory for the determination of mixtures of diethylaluminum hydride and triethylaluminum involves titration of the alkyl aluminum compounds with pyridine using phenazine end-point indicator ( I , 2 ) , the familiar isoquinoline method (3) is more generally accepted and is used for backup and confirmation. However, even though the isoquinoline method is widely accepted, doubt concerning its quantitative nature remains, largely because no comprehensive investigation of the analytical reactions has been reported. The work reported here is the result of a reaction study employing the extensive use of proton nuclear magnetic resonance, NMR. The reactions of isoquinoline with diethylaluminum hydride and triethylaluminum were first studied in 1955 when Bonitz ( 4 ) proposed that while triethylaluminum forms the simple complex shown in Equation 1, diethylaluminum hydride reacts to hydrogenate the 1-position as given by Equation 2. (C,H&Al
(CsHS),A1H
+ CgHTN
+ C9H7N
@
-
&H:N +Al(C,Hs), I
(1)
(2)
I1
In the presence of excess isoquinoline, Compound Il was suggested to react further as shown by Equation 3 to yield Complex 111, a red-colored species.
I11
Compound I1 was characterized, upon hydrolysis, by its 1,2-dihydroisoquinoline derivative. Neumann ( 5 ) , how(1) (2) (3) (4) (5)
D.E. Jordan.AnaL Chem.. 40, 2150 (1968). G. W. Heunisch,Anal. Chem., 44, 741 (1972). J . H . Mitchen, Anal. Chem., 33, 1331 (1961). F. Bonitz, Chem. Ber., 88, 742 (1955). W. P. Neumann, Justus Liebigs Ann. Chem., 618, 90 (1958)
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ever, in 1958, while investigating the hydrogenation of isoquinoline found that the 1,2-dihydro compound disproportionated readily to isoquinoline and 1,2,3,4-tetrahydroisoquinoline. The 1,2-dihydro compound could not be isolated from solution, which suggests that confirmation of Structure II by its 192-dihydroderivative is invalid. In 1960, Neumann (6) proposed the use of isoquinoline as an analytical reagent for the determination of diethylaluminum hydride and triethylaluminum in the presence of each other. In addition, he compared isoquinoline with benzalaniline and concluded that the latter was the preferred reagent. Since that time, considerable work (3, 7) has been done exploring the quantitative use of isoquinoline and refinement of the analytical procedure. The analytical method (3) as used in this laboratory involves successive injections of weighed aliquots of sample mixture into a measured volume of standard isoquinoline solution followed by measurement of the absorbance change in the colored solution after each injection. The sample weight is plotted us. the absorbance of the solution to produce a Bjerrum-type curve, the positive slope d which provides a measure of the diethylaluminum hydride concentration, while the negative slope is a measure of both the hydride and triethylaluminum.
EXPERIMENTAL Equipment a n d Reagents. Proton nuclear magnetic resonance (NMR) spectra were measured at ambient temperature with a Varian Model HA-100D Spectrometer using tetramethylsilane reference and benzene-de solvent. The electron paramagnetic resonance (EPR) Spectrum was measured at ambient temperature using a JEOLCO Model JES-ME-1X Spectrometer and benzene solvent. The diethylaluminum hydride and triethylaluminum used throughout this study were freshly distilled. The distillate fraction, rich in either diethylaluminum hydride or triethylaluminum as determined both by the phenazine titration ( I ) and by the gas volumetric technique (4, 6 ) was used. The highly air-sensitive material was stored under an inert atmosphere in a bottle equipped with a rubber septum. Otherwise, reagent grade chemicals were used. Isoquinoline-Alkylaluminum Product Solutions. A 1-ounce screw cap bottle equipped with a rubber septum was used for the reaction vessel, and a Teflon-coated magnetic stirring bar, approximately 0.1 g of Eastman reagent grade isoquinoline (accurately weighed), and 1.5 ml of benzene-de were added. With the rubber septum and cap in place, the solution was deaerated with nitrogen through syringe needles in the septum for a t least 20 minutes. The nitrogen inlet was removed, and diethylaluminum hydride was introduced into the isoquinoline solution uia a gastight syringe. The weight of diethylaluminum hydride added was appropriate to provide the necessary ratio of reactants. The reaction was allowed to proceed for about 20 minutes until a NMR tube was completely deaerated with nitrogen. After that time, the reaction solution was transferred uia a gas-tight syringe to the NMR tube, and about 1 ml of benzene-de was added to provide sufficient volume for measurement in the NMR instrument. The tube was quickly stoppered with a rubber septum and polyethylene cap. The same procedure was followed for the preparation of samples for the EPR measurement. N M R Spectra, isoquinoline: multiplet, 7.2; doublet 8.52 ( J = 5.8 Hz);singlet 9.34 ppm. 1:2 (mole ratio Al(C2Ha)2H:C9H7N)
(6) W. P. Neumann, Justus Liebigs Ann. Chem., 629, 23 (1960). (7) D. F. Hagen and W. D. Leslie, Anal. Chem., 35, 814 (1963).
product: multiplet 6.7-7.2; quartet 0.38; triplet, 1.34; doublets 5.71 (weak), 5.75, 7.94; singlets, 4.42 (weak), 4.54, 8.80 ppm. 1i1.5 product: multiplets, 0-1.5, 6.8-7.3; doublets, 5.77, 5.81, 5.95, 6.37, 8.00; singlets 4.02, 4.46, 4.58, 8.87 ppm (all peaks broadened). 1:l and 2:l products: multiplets, 0-1.5, 6.7-7.1; doublets, 4.00, 5.90, 5.97, 6.31, 6.39 ppm. A1(CzH5)3 > CgH7N product: multiplet, 7.0-7.5; quartet, 0.5; triplet, 1.5;doublet, 8.41; singlet, 9.09 ppm. EPR Spectra, 1:2 (colored species):no spectrum observed. Hydrolysis of Product Solutions. Reaction mixtures containing approximately 0.1 g of isoquinoline and enough alkylaluminum to give the desired ratio of reactants in 1.5 ml of benzene-dc were allowed to react with stirring for 30 minutes. After that time, the hydrolysis reagent, water or deuterium oxide-deuterated sodium hydroxide mixture, was carefully and slowly added while venting any liberated gases. Stirring was stopped and, upon separation of the layers, an aliquot of the benzene was taken for measurement of NMR spectra. The NMR tube was flushed with nitrogen to avoid oxidation of any air sensitive compounds. N M R Spectra, 1,2,3,4-tetrahydroisoquinoline:multiplet, 6.9; triplets, 2.49, 2.78 (J = 5.7 and 5.6 Hz, respectively); singlets, 1.38, 3.72 ppm. 2:l (mole ratio Al(CzHb)zH:CsH,N) product, HzO hydrolysis: multiplet, 6.7-7.5; triplets, 2.46, 2.74; doublet, 8.50 singlets, 1.50 (broad), 3.70, 9.13 ppm. 2 : l product, DzO hydrolysis: multiplet, 6.7-7.6; quartet, 2.76; triplets, 2.49, 8.45; singlets, 3.71, 9.10 ppm.
RESULTS AND DISCUSSION Triethylaluminum-Isoquinoline. The spectrum of isoquinoline in excess triethylaluminum is unchanged except for a slight upfield shift suggesting coordination (8) of the isoquinoline (compare with isoquinoline spectrum). The quartet a t 0.5 ppm (J = 8.4 Hz) and the triplet a t 1.5 ppm ( J = 7.8 Hz) are associated with the protons of the ethyl group on the electro-positive aluminum atom (9) and are not part of the isoquinoline spectrum. No other mole ratios have been evaluated because no reactions other than coordination were indicated in this initial spectrum. Diethylaluminum Hydride-Isoquinoline. More complex reactions have been produced by the action of diethylaluminum hydride on isoquinoline than was found in the triethylaluminum case. Reduction or covalent bond formation as well as coordination has been observed, and different products are observed a t different mole ratios. Mole Ratio 1:2. At mole ratio 1:2, isoquinoline is present in excess relative to the diethylaluminum hydride, and the product is the red-colored species, Compound III. Measurement by NMR shows unaltered, although coordinated, isoquinoline and ethyl groups bound to electro-positive aluminum (compare with isoquinoline and triethylaluminum complexed isoquinoline) as well as an additional spectrum. The doublet a t 5.75 ppm ( J = 6.7 Hz) and singlet a t 4.54 ppm are representative of a less aromatic, unsaturated species (8). The reduced form, Compound II, was expected ( 4 ) and should produce a singlet for the protons in the 1-position and doublets for those a t the 3- and 4-positions. The 1-position is sufficiently isolated from the other protons in the molecule that spin-spin coupling is unlikely, and a single line should be observed for those hydrogens. The 3- and 4-hydrogens, however, will couple with each other to produce two doublets. The singlet integrates to represent 2 protons, while the doublet represents 1 proton, and compares well with expected results. The doublet probably corresponds to the 3-position hydrogen atom, which is affected more by reaction a t the nitrogen atom than is the 4-position ( I O ) . The 4-position is not greatly affected and remains under the multiplet a t
7.2 ppm (10). It is probably influenced more by the neighboring, undisturbed phenyl group and the phenyl-C4-C8 bonds. The stabilities of both the triethylaluminum and the 1:l diethylaluminum hydride reduction product complexes are unknown. Two opposing electronic effects are measured by NMR in the formation of the complex. First, the resonance structure of the isoquinoline rings is disrupted, reducing the diamagnetically induced field of the aromatic system and producing an upfield shift (8). On the other hand, electron density is donated to the aluminum atom which should lower the electron density of the isoquinoline rings to produce a downfield shift. The overall spectral shift, then, will be qualitative and unpredictable. Stable complexes, however, are expected because neither any free, uncomplexed isoquinoline nor any appreciable broadening of the peaks, characteristic of an exchange mechanism ( 8 ) ,is observed in the spectra. A minor peak a t 4.42 ppm and shoulders on the doublet a t 5.71 ppm are probably the result of an impurity in the isoquinoline. An additional possibility is that the peaks represent a conformer of the 1:2 form. If inversion (8) of the nitrogen atom is greatly restricted or prevented, two stereo isomers are possible that give rise to individual NMR spectra. More will be mentioned concerning conformers in the 1:lcase. Because in the earlier work ( 4 ) a free radical was suggested, attempts were made, unsuccessfully, to measure an electron paramagnetic resonance, EPR, spectrum of the colored species. The absence of an EPR spectrum is conclusive evidence for the absence of a stable free radical in significant concentration. Mole Ratio 1 : I . At mole ratio 1:1, a rather unexpected NMR spectrum is produced. The multiplicity of peaks a t 5.95 ppm and 6.35 ppm was not expected and provides an example of an interesting observation unique to NMR. The multiplicity a t 1.0 ppm results from the T M S reference and ethyl groups attached to aluminum. The multiplicity a t 6.9 ppm integrates to 4 protons, while the four doublets (5.90-5.97 and 6.31-6.39 ppm) show proton each. Two protons are measured under the doublet a t 4.00 PPm. The reduced product, Structure 11, then, is present although with increased multiplicity. The increase in multiplicity suggests the presence of a greater number of nonidentical protons that, because of their close proximity in the spectrum, are very similar. Furthermore, a single proton split into two doublets suggests that the same proton has been measured in two different conformations. The reduced form as discussed in the 1:2 case is expected to produce a singlet for the 1-position and a doublet for both the 3-position and the 4-position. If, however, two stereo isomers of the molecule exist, a singlet and two doublets will be observed for each isomer.
i'i'
(8) L. M. Jackman and S. Sternhell, "Applications of Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry," 2nd e d . , Pergamon Press, New York, N . Y . 1969. (9) M. Witanowski and J. D. Roberts, J. Arner. Chern. SOC.,88, 737 (1966). (10) A. H . Gawer and B. P. Dailey, J. Chern. Phys.. 42, 2658 (1965). A N A L Y T I C A L C H E M I S T R Y , V O L . 46, NO. 8, J U L Y 1974
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The reduced isoquinoline molecule contains a nitrogen atom, which, unless restricted, will invert as shown for one conformation by Equation 4. In addition, the mole-, cule is capable of interconversion between chair conformations as indicated by Equation 5. As long as the inversions are in equilibrium and unrestricted, an average spectrum will be observed containing two doublets and a singlet much like that measured for the reduced fraction of the 1:2 compound. On the other hand, if the nitrogen inversion is restricted by, for example, complexation, two conformers will be measured. Furthermore, if the chair-chair interconversion is restricted, two additional conformers will be observed. The measured spectrum for the 1:l case is indicative of restricted inversion of the nitrogen atom. The two conformations produced are shown as the dimers by Structures IV and V.
Iv
v
The components of each dimer are equivalent, while the dimers are nonequivalent. The intensities of the NMR signals measured a t room temperature for each of the conformations are very similar and suggest that energetically the conformations are nearly identical. Mole Ratio 1:1.5. At mole ratio k1.5, halfway between 1:l and 1:2, spectra of the 1:l and 1:2 compounds were expected and have been measured. The multiplicity, however, of the l-,3-, and 4-positions on the reduced molecule indicates unrestricted inversion of the nitrogen atom because an average spectrum has been measured. The doublet, corresponding to the 1-position and centered at 4.00 ppm in the 1:l case, correlates with the singlet at 4.02 ppm in the 1:1.5 case. The AA' doublet pattern at 6.31 ppm and 6.39 ppm associated probably with the 4-position, which is influenced least by reaction a t the nitrogen atom, is a single doublet in the 1:1.5 case centered at 6.37 ppm ( J = 7.2 Hz). The 3-position proton is assigned to the pair of doublets in the 1:l case at 5.90 ppm and 5.97 ppm and shows a single doublet at 5.95 ppm (J = 7.2 Hz) in the 1:1.5 case. Unreduced isoquinoline exchanges with the reduced species and disrupts any complexation of the reduced species with itself. Because complexation is prevented at the nitrogen atom, inversion is unrestricted and an average spectrum is generated. Broadened peaks throughout the spectrum are evidence supporting the exchange mechanism. It should be mentioned that the exchange is apparently slow enough to allow measurement of the 1:2 component. The other peaks in the spectrum need little explanation. The multiplicity below 2 ppm is produced by ethyl groups attached to aluminum atoms and by the TMS reference. The singlet at 4.58 ppm and doublet a t 5.81 ppm (J = 6.6 Hz) derive from the reduced fraction of the 1:2 compound, while the doublet at 8.00 ppm ( J = 6.4 Hz) and singlet at 8.87 ppm result from the unreduced but complexed isoquinoline fraction. Hydrolysis of Mole Ratio 2:l. Hydrolysis of the 2:l product yields 1,2,3,4-tetrahydroisoquinoline and unreacted isoquinoline. The NMR spectrum of the hydrolysis 1020
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A N A L Y T I C A L CHEMISTRY, VOL. 46, N O . 8, J U L Y 1974
products compares directly with reference spectra of those compounds and are probably disproportionation products of 1,2-dihydroisoquinoline ( 5 ) . The peak at 1.5 ppm is broadened, which suggests a labile hydrogen, and is, therefore, assigned to a hydrogen attached to the nitrogen atom. Integration of the AB triplet pattern [2.46 ( J = 5.6 Hz) and 2.74 pprn ( J = 5.6 Hz)] shows that two protons are associated with each triplet. Because the two protons at positions 3 and 4 will couple with each other to produce two triplets, those protons have been assigned to the AB pattern. The singlet a t 3.70 ppm, which also integrates as two protons, is associated with the isolated 1-position. The peaks at 8.50 and 9.13 ppm represent free isoquinoline. Because the hydrogen a t the nitrogen atom might stem from the disproportionation rather than from hydrolysis itself, deuterium oxide, D20, was used in place of water for the hydrolysis. The most noteworthy change in the NMR spectrum of the products is the absence of a peak near 1.5 ppm, and suggests that a deuterium atom has replaced the hydrogen atom attached to the nitrogen, perhaps by direct hydrolysis. The result, however, is not conclusive. The deuterium substituted at the 4-position of the free isoquinoline suggests an interesting intermediate for the disproportionation. The proton in the 3-position (8.45 ppm) is measured as a triplet ( J = 3.2 Hz)'and implies that the 4-position is deuterated. The multiplicity of the peak at 2.76 ppm, which is considered to represent the 3position on 1,2,3,4-tetrahydroisoquinolinesuggests, furthermore, that the 4-position of the tetrahydrocompound is deuterated. It should be mentioned that assignment of peak position with regard to the tetrahydroisoquinoline is implied by its position in the NMR spectrum and should not be taken as proof. However, if the assignment of peaks is accepted, it follows that the hydrogen or deuterium attached to the nitrogen atom adds to the double, bond in 1,2-dihydroisoquinoline at the 4-position, while the 1-hydrogen attacks the 3-position. Furthermore, the hydrogen or deuterium at the nitrogen has an approximately equal likelihood of staying with the nitrogen or exchanging with the proton at the 4position. Otherwise, no deuterium would be found on the free isoquinoline fraction. For a period of time, then, the N-hydrogen or deuterium and the hydrogen at the 4-position are chemically equivalent. Such an arrangement can exist through the bimolecular intermediate, Structure VI.
VI The triplet at 2.49 ppm representing the 4-position hydrogen is equivalent to 1 proton upon integration and shows that one of the two atoms at that position is a hydrogen while the other is a deuterium. Theoretically, 1h of a hydrogen equivalent should be found on the nitrogen at 1.5 ppm, but dissociation forbids its accurate measurement. Mole Ratio 2 : l . At mole ratio 2 : l diethylaluminum hydride is present in excess and no change relative to the 1:l case is observed. Only the 1:l reduced compound, 11, and excess diethylaluminum hydride are measured. Discussion of Analytical Method. The positive slope of the analytical curve ( 3 ) is ideally suited for the determination of diethylaluminum hydride because the absorb-
ance of the solution is caused solely by the 1:2 product, Structure III. That portion of the curve is much like a Beer-Lambert law plot and can be used in practice to calculate the concentration of hydride material directly. Of course, a standard solution of known diethylaluminum hydride content and a minimum of other impurities must be used to construct a working plot. Mixtures of diethylaluminum hydride with triethylaluminum do not complicate the analysis significantly. The diethylaluminum hydride reacts first, quantitatively and rapidly, with the excess isoquinoline to form the 1:l product, Structure II, which in turn coordinates with an unreacted isoquinoline molecule to produce a red color. At the same time, any triethylaluminum present coordinates with another unreacted isoquinoline molecule without color generation. As long as excess isoquinoline exists uncoordinated, no significant competition between the 1:1 diethylaluminum hydride product and triethylaluminum will exist, and no deviation from linearity will be observed. When, however, the free isoquinoline is consumed,
either by reduction or coordination, a competition for the coordinated molecules will result and curvature may be produced. Each of the reactions given by Equations 1, 2, and 3 occur during the positive slope portion of the analytical curve. The negative slope portion of the curve follows similarly in that additional diethylaluminum hydride reacts either with the isoquinoline molecules coordinated with the 1:l product, Compound 111, to disrupt the colored complex directly or reacts with the triethylaluminum complexed isoquinoline molecules with subsequent abstraction of the isoquinoline on the 1:1 product by the freed triethylaluminum to disrupt the colored complex indirectly. Highly stable complexes are necessarily involved. No extraneous or unexplainable products that would indicate nonquantitative reactions have been observed for any of the discussed reactions. Received for review August 2, 1973. Accepted February 22, 1974.
Wear Metal Determination by Plasma Jet Direct Current Arc Spectrometry P. M. McElfresh' and M. L. Parsons2 Department of Chemistry, Arizona State University, Tempe, Ariz. 8528 7
A spectrographic method has been developed for the determination of wear metals in used oils by means of the plasma jet dc arc. The method incorporates force feeding of the oil samples (diluted 1:l with xylene) into an all-He arc and a photographic detection system. Elements determined included AI, Cr, Cu, Fe, and Mg with limits of detection of 0.01, 0.11, 0.08, 0.40, and 0.003 ppm, respectively. The method has been evaluated for accuracy and precision by determination of standard and actual samples, and areas for improvement have been identified.
The determination of trace metals in used lubrication oils has become a useful diagnostic tool for monitoring frictional wear in many systems. The Navy, the Air Force, several railroads, and large trucking firms use trace wear metal determination effectively in the maintenance of their equipment. Atomic absorption spectrometry (AAS) and spark atomic emission (SAE) with a rotating disk electrode are the most widely used techniques for this analysis. Three problems are presented by the oil in these methods; viscosity causes slow sample aspiration (1-3), the combustion of the oil upsets the fuel to oxidant ratio (in AAS), and the oil residue can clog the rotating disk (in SAE). Present address,
T e x a s I n s t r u m e n t s Corp.,
Dallas,
Texas
75222. A u t h o r t o whom a l l correspondence s h o u l d b e addressed (1) S.Slavin and W. Slavin, At. Absorption Newslett., 5, 106 (1966) (2) A. J. Mitteldorf. SpexSpeaker, 13, 1 (1968). (3) D. L. Fry, Appl. Spectrosc., 10, 65 (1956).
The viscosity effects (4-6) are decreased by the addition of MIBK or xylene as solvent. Another major problem is that not all of the metal is necessarily dissolved in the oil; it can be present as particles ranging to several k m in size. The large particles tend to pass through flames unaffected ( 2 ) or settle out in the sample boat of the rotating disk ( 3 ) . A spectroscopic source which has shown ability to eliminate difficult matrices is the dc arc plasma jet (7-16). The plasma jet typically operates in the 7000-10,000 "K temperature range and is designed so liquid samples can be introduced directly into the high energy dc arc. It was felt that this source could be utilized to overcome the problems mentioned above; therefore, it was decided to explore the feasibility of developing a method adaptable to wear metals using this source.
(4) R. Smith, C. M. Stafford. and J. D. Winefordner, Can. Spectrosc.. 14, 2 (1969). (5) R . L. Miller, L. M. Fraser, and J. D.Winefordner, Appi. Spectrosc., 25, 477 (1971). (6) S. Sprague and W. Slavin, At. Absorption Newslett., 4, 367 (1965) (7) M . Margoshes and 8. F. Schribner, Spectrochim. Acta.. 15, 138 (1959). (8) L. E. 0wen.Appl. Spectrosc., 15, 150 (1961). (9) A. J. Mitteldorf and D. 0. Landon, Spex Speaker, 8, 1 (1963) (10) K. Hirokawaand H . Goto, Bull. Chem. SOC.Jap., 42, 693 (1969). (11) G .Collins and C. A. Pearson. Anai. Chem., 36,787 (1964). (12) J. Szivek, C. Jones, E. J. Paulson, and L. S. Valberg, Appl. Spectrosc., 22, 196 (1968). (13) R. J. Heemstra and N. G. Foster, Anal. Chem., 38, 492 (1966) (14) R . J. Heemstra, Appl. Spectrosc., 24, 568 (1970). (15) P. A. Serin and K . H . Ashton, Appl. Spectrosc., 18, 166 (1964) (16) R . Lerner, Spectrochim. Acta., 20, 1619 (1964). A N A L Y T I C A L CHEMISTRY, VOL. 46, N O . 8, JULY 1974
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