Chapter 21 Oxo compounds

Chapter 21 Oxo compounds

Chapter 21 0 x 0 compounds J. CHURA~EK CONTENTS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aliphatic and c...

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Chapter 21

0 x 0 compounds J. CHURA~EK

CONTENTS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aliphatic and cyclic aldehydes and ketones . . . . . . . . . . . . . . . . . . Separation of aldehydes and ketones in the form of their derivatives . . Ion-exchange chromatography of free aldehydes and ketones . . . . . . Other chromatographic methods of separation of carbonyl compounds

Quinones

......................................

Applications in lignin chemistry References . . . . . . . . . . . .

........................ ........................

........... ........... ........... ........... ........... ........... ........... ...........

455 456 456 451 458 459 461 463

INTRODUCTION The use of liquid chromatography for the systematic separation of free aldehydes and ketones is not extensive. Most papers deal with applications and mainly concern the isolation and purification of synthetic products. When aldehydic substances are chromatographed on alumina, it should be borne in mind that they may undergo some catalyzed reactions in alkaline media and form various intermediates. These reactions restrict the general utilization of the liquid chromatography of aldehydes, especially if alumina is used as a sorbent. Carbonyl compounds are most often separated in their hydrogen sulphite form by ion-exchange chromatography on basic resins. The reactivity of the carbonyl compound with hydrogen sulphite ions is made use of, which leads to the formation of a-hydroxysulphonic acids. In the liquid chromatography of 0x0 compounds, their derivatization is also made use of: mainly oximes and 2,4-dinitrophenylhydrazones are prepared. The section on quinones includes the liquid chromatography of simple quinones, anthraquinones, ubiquinones and plastoquinones. Liquid chromatography is used mainly for the purification and preparation of synthetic compounds, or for the isolation of active components from natural material. In future, extensive use of the high-speed liquid chromatography of these substances on modern sorbents is to be expected because often they cannot be separated by gas chromatography owing to the low thermal stability of some quinones. The application of liquid chromatography in lignin chemistry is topical at present. Chromatographic papers in this field can be divided into two main groups. The first group deals with the fractionation of polydisperse lignin derivatives and the determination of the molecular-weight distribution on the dextran gel Sephadex. The second group, dealing with the separation of lignin derivatives (mainly.ligninsulphonic acids), References p.463

455

456

O X 0 COMPOUNDS

uses ion exchangers in which molecular sorption plays a role in addition to ion exchange. This chapter also covers papers that concern the identification of lignin derivatives resulting from degradation processes.

ALIPHATIC AND CYCLIC ALDEHYDES AND KETONES Aldehydes and ketones can be separated chromatographically whether they are in the free state or in the form of derivatives. Although carbonyl compounds represent nonionogenic compound:, most investigators use ion-exchange chromatography for their separation. Very often the problem of the group separation of aldehydes and ketones from organic acids and other substances can also be solved. The acids are bound quantitatively on to a column of the strongly basic anion exchanger Amberlite IRA-400 (HCOJ). Aldehydes and ketones pass into the filtrate and the acids are then desorbed with sodium carbonate solution (Gabrielson and Samuelson, 1952a). For the separation of ketones from alcohols, quantitative sorption of ketones on anion-exchange resins in the hydrogen sulphite form can be used, while alcohols pass into the eluate. Ketones can be eluted with hot water or with a solution containing a mixture of carbonate and hydrogen carbonate (Gabrielson and Samuelson, 195213). Samuelson described a complete analysis of a mixture of acetic acid, ethanol, furfural, and acetone, based on the following principle. Acetic acid is bound on a column of anion exchanger in the hydrogen carbonate form, and acetone and furfural are retained on a column containing the anion exchanger Amberlite 1RA-400 (HSO;) (0.12-0.30 mm). They are then.eluted selectively with water and 1 N sodium chloride solution. Acetic acid is eluted with alkali, filtered through a cation exchanger in the H+ form and titrated with alkali; acetone and furfural are determined photometrically by reaction with salicylaldehyde and orcinol, and ethanol is determined by measuring the density of the effluent from the hydrogen sulphite column. A column of anion exchanger in the cyanide form on to which aldehydes and ketones are bound via the formation of addition compounds was not found to be advantageous for their mutual separation, but it can be used for their separation as a group from other types of compounds (Gabrielson).

Separation of aldehydes and ketones in the form of their derivatives Liquid chromatography can be used successfully for the determination of carbonyl compounds in the form of derivatives with hydroxylamine, and the oximes formed can be determined by ion-exchange chromatography. A strongly basic anion exchanger was used with water as eluent (Ebel). Using high-speed liquid chromatography, 2,4-dinitrophenylhydrazones of aliphatic aldehydes and ketones can be separated. The derivatives are orange coloured and absorb strongly in the UV region, producing a strong response during detection. Using a 3 m X 2.1 mm column, packed with Corasil I1 activated by heating in vacuo for 3 h at 110°C, and a mixture of n-heptane and 3% of ethyl acetate, separations were achieved within ca. 35 min (Carey and Persinger) (Fig. 21.1).

ALIPHATIC AND CYCLIC ALDEHYDES AND KET0NT.S

457

I E 0 N ln

8z 4

rn R

$m a

J

Fig. 21 . I . Separation of 2,4-dinitrophenylhydrazones o f aliphatic aldehydes (Carey and Persinger). Column: 3 m X 2.1 mm. stainless steel. Sorbent: Corasil I 1 activated for 3 h at 110°C. Eluent: 9 7 : 3 (v/v) n-heptane-ethylacetate. Operating conditions: flow-rate, 1.7 ml/min; pressure, I 175 p.s.1. Detection: absorption at 254 nm. Peaks: 1 = butyraldehyde; 2 = propionaldehyde; 3 =acetaldehyde; 4 = formaldehyde (all 2.4dinitrophenylhydrazones).

Ion-exchange chromatography of free aldehydes and ketones For the chromatographic separation of aldehydes and ketones, strongly basic anion exchangers in the hydrogen sulphite form were used. On such exchangers, carbonyl compounds are sorbed via the formation of complex a-hydroxysulphonic acids. On the basis of the differences in stability of these complexes, their chromatographic separation can be achieved by using water or carbonate buffers as eluents (Cabrielson and Samuelson, 1950; 1952~).Complexes of aldehydes are more stable than complexes of ketones. The most stable compound is given by formaldehyde, and acetaldehyde, furfural, benzaldehyde, salicylaldehyde, vanillin, glyoxal, acetone and methyl ethyl ketone also form stable, strongly retained complexes. Some ketones can be eluted selectively with hot water and so separated from aldehydes, which remain on the column and which can be eluted with salt solutions (for example, 1 M sodium chloride solution). Thus, for example, acetaldehyde and furfural were separated from acetone and methyl ethyl ketone. Huff separated a mixture of lactic aldehyde, acetone, pyruvic aldehyde and a mixture of formaldehyde and acetaldehyde on a column of Dowex (HSO;) by using gradual elution with hydrogen sulphite solutions of increasing concentration. A mixture of carbonyl compounds was separated chromatographically on a 41 X 1.1 cm Dowex-1 (HSO;) (150-300 mesh) column using the same procedure as above (Christofferson, 1965). After the removal of hydrogen sulphite from the eluate fractions with iodine, the concentration of the carbonyl compounds could be determined photometrically in UV References p.463

458

O X 0 COMPOUNDS

TABLE 21.1. SOLUBILIZATION CHROMATOGRAPHY OF KETONES (SHERMA AND RIEMAN) Column: 20 X 2 cm. Cation exchanger: Dowex 50-X8(H') (200-400 mesh). Flow-rate: 0.4cm/min. Temperature: room temperature. Detection: fractions of 5 rnl were mixed with 5 mI of 0.1 M hydroxylamine hydrochloride and the pH determined after 5 min. The difference between this pH and that of a fraction containing no ketone is proportional t o the amount of ketone present. Samples consisted of 0.2 mrnole of compound dissolved in 1.0 ml of at least 50% of eluent. Ketone

Mobile phase ~~

Aqueous methanol

Methyl n-butyl ketone Methyl n-amyl ketone Methyl n-hexyl ketone Methyl isobutyl ketone Methyl n-heptyl ketone Methyl n-octyl ketone Me thy I n-nony 1 ketone Ace tophenone

Aqueous ethanol

2.0 M

4.0 M

1.OM

2.0 M

4.14 5.95 10.1 3.21 17.1 33.6 63.1

3.04 4.00 6.94 2.76 11.6 21.3 39.6 9.56

4.33 6.13 11.3

3.58 4.96 8.16

12.1

light (Christofferson, 1964). Mixtures containing acetaldehyde, formaldehyde, 5-hydroxymethylfurfura1;furfural and vanillin in amounts of 0.05-0.1 mmole could be separated sharply and the relative error was less than 10%. Ketones were separated on cation-exchange columns of Dowex 50-X8(H') (200-400 mesh) by gradual elution with aqueous methanol or ethanol of increasing concentration (Sherma and Rieman). From Table 21.1, it is evident that a successful separation of various methyl ketones was achieved. The separation of aldehydes and ketones by salting-out chromatography on anion-exchange resins can also be recommended (Breyer and Rieman, 1958; 1960). A good separation of a series of carbonyl compounds is shown in Table 21.2.

Other chromatographic methods of separation of carbonyl compounds Bell et al. separated aromatic ketones from cigarette smoke. By extracting the condensate, acetonitrile fractions were obtained with derivatives of fluoren-9-one, and these fractions were then fractionated on an alumina column by gradient elution. The polarity of the system was gradually increased. The eluent was n-hexane to which benzene, diethyl ether and methanol were added gradually. The final identification of the components was carried out by paper or gas chromatography. In Table 21.3, elution data are given for some carbonyl compounds (and esters), which were separated by gel chromatography on styrene-divinylbenzene gel (Hendrickson).

459

QUINONES

TABLF 21.2 DISTRIBUTION RATIOS OF CARBONYL COMPOUNDS IN SALTINC-OUT CHROMATOGRAPHY (BREYER AND RIEMAN, 1958) Column: 25 X 2 cm. Cation exchanger: Dowex 1-X8 (SO:-) (200-400 mesh). How-rate: 0.2-0.8 cm/min. Tempcraturc: room temperature. Detection: by the differential pH method of Roe and Mitchell in which 5 ml of 0.1 Mhydroxylamine hydrochloride is added to each fraction and the pH is determined after 5 min; the amount of carbonyl compound is related to the pH. Alternatively, fractions are mixed with 5 ml of 0.1 N sodium dichromatc in conc. H,SO,, diluting with 25 ml of water and measuring the absorbance of the resulting Cr(l11). Samples consisted of 0.050-0.1 00 mmole. Compound

Mobile phase Water

Formaldehyde Acetaldehyde Acetone Acetoin Diacetyl 2.5-tIexanedione Diacetone alcohol Propionaldehyde Methy I ethyl ketone Cyclopentanone 2,3-Pen tanedione 2,4-Pentanedione Methyl isopropyl ketone Bu tyraldehyde Methyl rz-propyl ketone Diethyl ketone Cyclohexanonc

Ammonium sulphate solution

0.5 M

I .O M

2.0 M

3.0 M

4.0 M

1.05 0.80 0.70 0.76 1.25 1.06 0.78 1.37 1.28 1.70 1.92 1.90 2.12

0.93 0.98 0.93 0.96 2.06 1.47 1.36 1.86 2.00 2.59 2.59 2.73 3.59

1 .oo

1.22 1.53 I .38 2.20 2.56 2.3 1 2.50 3.02 4.01 4.49 4.34 5.76

0.98 P.52 3.00 2.91 4.60 6.87 7.31 4.58 8.21 8.83 12.8 12.9 16.5

0.97 2.18 6.12 6.1 3 8.50 22.4 22.5 7.70 21.1

0.88 2.92 10.9 10.8 13.9

2.82 2.5 1

4.42 4.56

5.8 I 7.52

-

-

-

19.6

2.52 3.09

4.42 5.17

7.16 8.61

19.6

-

-

-

-

21.8 -

-

-

-

QUINONES Naturally occurring quinones can be isolated from most natural materials by first extracting them with a neutral or alkaline extractant and then fractionating the extracts by column chromatography. Alkali-soluble fractions can be further fractionated on a column packed with deactivated silica gel by elution with benzene (Thomson and Burnett, 1967). Some synthetic quinones of the anthraquinone type were separated by high-speed liquid chromatography on modern sorbents, such as Corasil/CI8 or Permaphase ODs. In the first instance, methanol-water (1 : 1) was used as the polar mobile phase at a pressure of 1200 p.s.i.g. (Fig. 21.2) (Waters Ass.). In the second instance, the separation was carried out at an elevated temperature (60°C) using a methanol-water mixture (45:55) ’ at a pressure of 450 p.s.i.g. (DuPont) (Fig. 21.3). Table 2 1.4 gives a review of some recent applications in which liquid chromatography is used primarily for purification and preparative purposes. References p.463

460

O X 0 COMPOUNDS

TABLE 21.3 GEL CHROMATOGRAPHY OF CARBONY L COMPOUNDS (HENDRICKSON) Column: 12 ft. X 3/8 in. Gel: 40 A styrene-divinylbenzene gel, permeable to alkanes of mol. wt. Mobile phase: benzene. Flow-rate: 1.O ml/min. Temperature: 24°C. Detection: RI.

< 450.

Compound*

Elution volume (V,, ml)

Peak width (ml)**

Ace tone* * * Acetone * * * Ace tone* * * n-Butyraldehyde Methyl ethyl ketone Ethyl acetate Methyl isobutyl ketone Dimethyl terephthalate n-Heptaldehyde Dimethyl adipate Dimethyl sebacdte

(75.6) 115.3 101.5 110.4 110.3 108.6 102.2 100.6 98.1 94.2 73.8

(4.66) 4.34 4.52 4.46 4.10 4.34 4.40 5.30 6.40 4.17 4.62

*Sample sizes were typically 0.1 ml of 4% solute in benzene. **Peak widths were obtained by drawing tangents t o each side of the curves and reporting the number of millilitres of eluent a t the base of the triangle. ***Compound run more than once.

TABLE 21.4 SURVEY OF CHROMATOGRAPHIC PROCEDURES APPLICABLE TO THE SEPARATION OF ALIPHATIC AND AROMATIC O X 0 COMPOUNDS Substances chromatographed

Sorben t

Mobile phase

Reference

Cyclobutanone derivatives

Alumina, act. I

Dichloromethane and methanol

Huisger and F:ei le r

Bis-diazoke t ones

Florisil (60-100 mesh)

Benzene-n-hexane (1:l)

Bien and Ovadia

Dichlorobenzophenone

Florisil

Light petroleum (b.p. 60-80°C) -diethy1 ether (9:l)

Morgan

6,6-Diarylbicyclo[ 3.1.01 hexan-2-one and its derivatives

Silica gel, Celite, silicic acid

n-Hexane-1% diethyl ether

et QI.

Trihydroxycyclo-2,6dien-5-one (purification)

Silicic acid, Dowex AB I-X2

Diethyl ether-benzene (9: 1)

Shaw and Smith

Zimmerman

46 I

APPLICATIONS IN LIGNIN CHEMISTRY TABLE 2 1.4 (corztinued) Substances chromatographed

Sorbent

Mobile phase

Reference

Tropolones (purification)

Silica gel

Chloroform-benzene (1:l)

Forbes and Criffiths

Adaniantanone (preparation)

Silica gel

Light petroleum (b.p. 60-8O0C)-acetone (5:2) and others

Snatzke and Eckhard t

Anthraquinones in natural materials

Silica gel

Benzene

Thomson and Burnett (1 968b)

Florisil

Benzene

Silica gel G

Benzene-light petroleum (b.p. 60-80°C) (1:4)

Thomson and Burnett f 1968~)

Florisil

Light petroleum (b.p. 60-80°C)

Thomson and Burne t t (1968a)

Magnesium oxide, silica gel

Chloroform

Thomson and Brown

Anthraquinone derivatives (purification)

Cristol and Caspar

Alumina

Acid alumina

Benzene

Bredercck et al.

Benzoquinone derivatives (purification)

(a-, p-, y-) Isomers of

rubromycin derivatives of napthoquinone (isolation)

Silica gel

Silica gel C and oxalic acid

Ethanol (extraction from the column with water) Chloroform Chloroform-acetone

(97: 3)

Teuber and Die trich Cuntze and Musso Brockmann and Zeeck

Chloroform-benzene ( I : 1)

APPLICATIONS IN LIGNIN CHEMISTRY In lignin chemistry, liquid chromatography is often used for the fractionation of polydisperse lignin derivatives, mostly on Sephadex gels (Kirk et al. ). For determining molecular-weight distributions, Sephadex G-100 was found t o be the most suitable resin and, when the formamide system was applied, enabled a good separation of single fractions t o be achieved. Aqueous extracts of lignin derivatives were fractionated successfully on a References p.463

O X 0 COMPOUNDS

462

1

J-. I

0

I

I

5

I

10

I

TIME,MIN

I

20

1 I 0

\

I

5 TIME,MIN

I 10

Fig. 21.2. Separation of quinones (Waters Ass.). Column: 4 ft. X 2.3 mm I.D. Sorbent: Corasil/C,, (reversed phase). Mobile phase: methanol-water (lSO:SO, v/v). Pressure, 1200 p.s.i.g. Peaks: 1 = p-quinone; 2 = 1,4-napthoquinone; 3 = anthraquinone; 4 = 2-methylanthraquinone; 5 = 2ethylanthraquinone; 6 = 2-ferf.-butylanthraquinone. Fig. 21.3. Separation of substituted anthraquinones (DuPont). Column: 1 m X 0.083 in. I.D. Sorbent: Permaphase ODS. Mobile phase: 45:55 (v/v) methanol-water. Operating conditions: column temperature, 60°C; pressure, 450 p.s.i.; flow-rate, 1 cni3/min. Detection: UV photometer. Peaks: 1 = 9,lOanthraquinone; 2 = 2-methyl-9,lO-anthraquinone; 3 = 2-ethyl-9,lO-anthraquinone; 4 = I ,4-dimethyl9,lO-anthraquinone; 5 = 2-fert.-butyl-9,lO-anthrdquinone.

polyamide column using aqueous methanolic solutions for elution (Hostettler and Seikel, Seikel er d.).I t was observed that the gel permeation chromatography of wood components, such as hemicelluloses and lignins, is more effective than column electrophoresis. A very positive effect on the separation of these substances in buffered systems is exerted by the presence of carboxyl groups in the gel, and for this reason the polyacrylamide gel Bio-Gel P and similar gels can be recommended for the fractionation (Simonson).

REFERENCIiS

463

Very often, tasks connected with the fractionation of lignin sulphonates have t o be performed. These substances can be isolated from sulphite liquors (wastes) by means of hexamminocobalt trichloride and conversion into their barium salts on an ion-exchange column (Alekseev et d.). Using Sephadex G-75 and G-100, fractions of molecular weight up t o 100,000 can be obtained. The mechanism of the fractionation of calcium and lithium lignosulphonates by gel chromatography was investigated on a column packed with Sephadex G-25 and C-50 using water, dioxane-water and aqueous solutions of calcium and lithium chlorides as eluents (Stenlund). On Sephadex G-50, lignosulphonates up t o a molecular weight of 40,000 can be well separated, and 011 Sephadex G-75 up to a molecular weight of 80,000 (Forss and Stenlund). When ligninsulphonic acids are sorbed on ion exchangers, molecular sorption occurs in addition t o ion exchange (Seidl). For sorption, the weakly basic anion exchanger Lewatit MP-60 was found t o be the most suitable. The sorption of these acids is partly irreversible. The most efficient desorption agent was a solution of 2 Msodium chloride plus 1.5 M sodium hydroxide. Anion exchangers with a microporous or visibly porous structure were equally efficient. A very common task consists in the separation and isolation of degradation products of lignin derivatives obtained either by degradation with thioacetic acid (Nimz, 1969b) or alkali (Johansson and Miksche), or by new degradation procedures (Nimz, 1969a). In the last instance, Sephadex LH-20 is used as sorbent and dimethylformamide as eluent. In other instances, columns packed with silica gel and eluted with acetone and n-hexane (or cyclohexane) are used.

REFERENCES Alekseev, A. D., Ashina, I. V., Reznikov, V. M . and Sukhaya, T. V., Obshch. Prikl. Khim., (1969) 226;CA., 73 (1970) 132105e. Bell, J . H., Ireland, S. and Spears, A. W . ,Anal. Chem., 41 (1969) 310. Bien, A. and Ovadia, D., J. Org. Chem., 35 (1970) 1028. Bredereck, K., Sornrnerrnann, F. and Diarnantoglon, M., Chern. Ber., 102 (1969) 1053. Breyer, A. and Kiernan, W., Anal. Chim. A c ~ Q18 , (1 958) 207. Breyer, A. and h e m a n , W.,Talanta, 4 (1960) 67. Brockrnann, H. and Zeeck, A., Chem. Ber., 103 (1970) 1709. Carey, M . A. and Persinger, H. E.,J. Chromatogr. Sci., 10 (1972) 537. Christofferson, K., Anal. Chim. Acta, 31 (1964) 233. Christofferson, K . , Anal. Chim. Acta, 33 (1965) 303. Cristol, S. J . and Caspar, M. L.,J. Org. Chem., 33 (1968) 2020. Cuntze, U. and Musso, H., Chem. Ber., 103 (1970) 62. DuPont, Product Bulletin, Liquid Chromatographs, No. 820 PB4, DuPont, Wilmington, Del., 1971. Ebel, S., Arch. Pharm., Berl., 300 ( I 967) 472. Forbes, E. J. and Griffiths, J . , J. Chem. Soc., C , (1968) 575. Forss, K. and Stenlund, B., Pap. Puu, 5 1 (1969) 93 and 97; C.A., 70 (1969) 8 8 9 6 5 ~ . Gabrielson, G.,J. Appl. Chem., 7 (1957) 533. Gabrielson, G. and Samuelson, O., Sv. Kern. Tidskr., 62 (1950) 214. Gabrielson, G. and Samuelson, O . , A C ~Chem. Q Scand., 6 (1952a) 729. Gabrielson, C. and Sarnuelson, O., Acta Chem. Scand.. 6 (1952b) 738.

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O X 0 COMPOUNDS

Gabrielson, G. and Sarnuelson, O.,Su. Kem. Tidskr., 64 ( 1 9 5 2 ~ )150. Hendrickson, I. G . ,J. Chromatogr., 32 (1968) 543. Hostettler, F. G. and Seikel, H. K., Tetrahedron, 25 (1969) 2325. Huff, E., Anal. Chem., 31 (1959) 1626. Huisger, R. and Feiler, L. A., Clzem. Eer., 102 (1969) 3391. Johansson, B. and Miksche, G. E., Acta Chem. Scund., 23 (1969) 924. Kirk, T. K., Brown, W. and Cowling, E. B.,Eiopolymers, 7 (1969) 135; C.A., 71 (1969) 1 4 3 4 8 ~ . Morgan, N. L., Bull. Environ. Contam. Toxicol., 3 (1968) 254; C.A., 69 (1968) 85459d. Nirnz, H., Chem. Eer., 102 (1969a) 799. Nirnz, H., Chem. Eer., 102 (1969b) 3803. Roe, H. and Mitchell, J., Anal. Chem., 23 (1951) 1758. Saniuelson, O., 2. Elekrrochem., 57 (1953) 207. Seidl, J.,Chem. Prgm., 16 (1966) 273. Seikel, M. K., Hostettler, 1 . D. and Johnson, D. B., Tetrahedron, 24 (1968) 1475. Shaw, S. J . and Smith, P. J . , J . Chem. Soc., C, (1968) 1882. Sherrna, J. and Rieman, W., Anal. Chim. Acta, 19 (1958) 134. Sirnonson, R., Su. Papperstidn., 70 (1967) 711; C A . , 68 (1968) 51090r. Snatzke, G. and Eckhardt, G., Chem Ber., 101 (1968) 2010. Stenlund, B., Pap. Puu, 52 (1970) 197; C.A., 73 (1970) 26793t. Teuber, H. J. and Dietrich, M., Chem. Eer., 100 (1967) 2908. Thornson, R. H. and Brown, P. N., J. Chem. SOC.,C, (1969) 1184. Thornson, R. H. and Burnett, A. R., J. Chem. Soc., C, (1967) 2100. Thornson, R. H. and Burnett, A. R.,J. Chem. SOC..C, (1968a) 850. Thornson, R. H. and Burnett, A. R., J. Chem. Soc., C , (1968b) 854. Thomson, R. H. and Burnett, A. R.,J. Chem. Soc., C, ( 1 9 6 8 ~ )2437. Waters Ass. Inc., Firm Prospects-CorasillCls, Waters Ass., Frarningharn, Mass. Zirnrnerrnan, H. E., Crumrine, D. S., Dopp, D. and Huyffer, P. S., J. Amer. Chem. Soc., 9 1 (1969) 434.