(11) Litchfield, C., Reiser, R., Isbell, A. F., Feldman, G. L., Ibid., 41, 52
(1964). (12) RlcGee, J., ANAL. CHEM. 31, 298 (1959). (13) Nichols, P. L., Jr., J . Am. Chem. sot. 74, 1091 (1952). (14) Privett, 0. S., Nickell, E. C., Lipids 1, 98 (1966).
(15) Scholfield, C. R., Jones, E. P., Butterfield, R. O., Dutton, H. J.,
ANAL.CHEW35, 1588 (1963).
Agricultural and Food Chemistry, 148th Lleeting ACS, Chicago, Ill., AugustSowakowska, Janina, Selke, E., DutSeptember 1964. The Sorthern Labton, H. J.2 J . Am. oil C‘htmhts’ SOL oratory is headquarters for the Northern 38, 208 (1961). rtilization Research and Development ( l ~ ~ ~ C $ Il.It;,, l ~ ~ ~ t o n ~ ~ Division, Agricultural Research Service, U. S. Department of Agriculture. Men(1960). tion of trade or company names is for RECEIVEDfor review June 27, 1966. identification only and does not imply Accepted August 22, 1966. Division of endorsement by the Department. (16) Scholfield, C. R., Jones, E. P.,
~~~~k~7N,skzRf
Structure-Rf Correlations in the Thin Layer Chromatography of Some Basic Drugs WINSTON W. FIKE’ Cuyahogo County Coroner’s Office, Cleveland, Ohio 44 7 06
b Rf values for 140 basic drugs in five chromatographic systems have been obtained. Use of these data for the identification of the particular drugs listed is briefly discussed. Correlations of a more general nature between R, values in the five systems and the presence of particular chemical groups in these compounds are made. Steric hindrance around the group responsible for the bonding to silica in a particular system, the basicity of the compound, and the presence or absence of a pyridyl ring influence R, values to the greatest extent. A comparison has also been made of Rf values from the literature for the more common alkaloids and narcotics.
V
WORKERS have reported thin layer chromatographic studies of restricted groups of drugs-e.g., local anesthetics, analgesics, tranquilizers, etc. (5, 11, 13). Recently thin layer data for 140 drugs, many of which had not previously been reported, have appeared (16). These data were presented in a manner suitable for use in the rapid identification of a drug in biological media. I n this paper these compounds have been divided into two groups, phenothiazines and all others. This division has been made since the phenothiazines form a chemically distinctive group and give color reactions characteristic of them alone. The present emphasis is on correlations between the chemical structure of the drugs and their chromatographic properties. ARIOUS
EXPERIMENTAL
Reagents, D r u g Standards, and Apparatus. These were a s reported
earlier (’7). Chromatographic Systems. Systems I, 11, a n d I11 consisted of silica
Present address, The Wm. S. Merrell Co., Cincinnati, Ohio, 46235.
gel plates prepared with 0.1M potassium hydroxide with solvents cyclo(75: 15: hexane-benzene-diethylamine 10), methanol, and acetone, respectively. Systems IV and V consisted of silica gel plates prepared using 0.1Ji sodium bisulfate with solvents methanol and 95% ethanol, respectively. Spray Reagents. A 1% solution of iodine in methanol was used as a general locating agent because it gave a positive test with all t h e drugs. Other sprays used were Dragendorff’s and Mandelin’s reagents prepared as noted previously ( 4 ) ; a 0.5% aqueous solution of Fast Red G, the stabilized diazonium salt of p-nitroaniline, and Forrest’s acidic ferric chloride (FPX) reagent used for phenothiazine detection (13). A 5y0 aqueous solution of sodium nitrite was commonly used as an overspray following Dragendorff’s reagent. This combination releases iodine and proved more effective in detecting small amounts of many of the drugs than the solution of iodine in methanol; it also produces a more persistent spot with some of the drugs. Chromatographic Procedure. T h e R f d a t a were obtained using the same procedure given previously (7) except that silica gel G slurries were used to prepare the plates (8). RESULTS AND DISCUSSION
Average Rf values of the drugs in the five systems are reported in Tables I and 11. Each value is the average of at least three determinations on separate plates. These chromatographic systems enable one to identify a large majority of the drugs satisfactorily when used in conjunction with spray reactions. KO attempt has been made here to identify unequivocally all the drugs listed. Rather, an attempt has been made to develop data and structural correlation which should be useful as a n aid in identifying a large number of basic organic compounds. System 111 mould be the best system for a preliminary screening of the drugs
as it gives the most even distribution of R, values of those reported. The use of ultraviolet light and the sprays listed would provide further characterization. Mandelin’s reagent and the Fast Red G spray gave distinctive colors with some of the drugs. Fast Red G gave orange or pink colors with those compounds containing either aniline or phenolic groups and a yellow color with the primary amines. The colors produced by the antihistamines with Mandelin’s reagent have been reported earlier ( 7 ) . Because some of the colors produced are not completely reproducible, they have not been reported. Relative R, values were not calculated. Mepivicaine and dyclonine would both be suitable as reference compounds since their solutions are stable and each has R, values between 0.30 and 0.70 in all five systems. Structure-R, Correlations in the Non-Phenothiazine Drugs. SYSTEM
I. I n this system all t h e very weak bases have low R,’s. -4mide groupse.g., in caffeine and iproniazidhydroxyl groups a s in 3-pyridinemethanol, a n d in papaverine t h e four methoxyl groups probably account for these low values where present. T h e attraction of oxygen-containing groups in steroids for silica has been noted earlier (14). It is noteworthy t h a t neither the ester nor the keto group of themselves produce a low R,. Piperocaine with an ester group and diethylpropion with a keto group have RrJsof 0.63 and 0.76, respectively. Three compounds containing hydroxyl groups are exceptions to the above generalization. Benactyzine, pipradrol, and procyclidine have Rf values of 0.63, 0.74, and 0.78, respectively. Each has its hydroxyl group on a highly hindered tertiary carbon atom where i t would be sterically prevented from hydrogen bonding to the silica. Steric hindrance of the bonding between VOL 38, NO. 12, NOVEMBER 1966
0
1697
a n anilino group and silica is exemplified by the three structurally similar compounds-procaine, tetracaine, and metabutoxycaine. Procaine, with a n unsubstituted para amino group, has a n Rj of 0.05, tetracine with a n n-butyl group on the anilino nitrogen atom has a n Rj of 0.18, and metabutoxycaine with a butoxy group ortho to the anilino group has a n Rj of 0.30. Piperocaine, a compound of similar structure b u t without the aromatic amino group, has a n Rj of 0.63. The basicity of the amino group mould not appear to be involved. To check this latter point, several aniline compounds were chromatographed in System I. The results are tabulated in Table 111; the pK, values of the compounds are also listed. The PI S-H linkage while the low Rj for p-nitro-aniline is due to the strong attraction between the nitro group and silica. I n several cases the oxygen-containing groups cannot be the sole or even the chief cause of low Rj values in System 1. The comparison noted earlier of piperocaine with procaine illustrates this point. The only significant difference between these two drugs is the aromatic primary amino group in procaine. The low Rj of pentylenetetrazole, which lacks a n y oxygen atoms, is best explained by considering the tautomeric form I1 below as more likely than the usually written form I. Structure I1 is consistent with that accepted for pyrrole
dHz-C
N
\/ N
I
I
CH=C
I
I1
N
\ / N
H I1 The apparently strong attraction between silica and the aromatic > K-H linkage in System I is not shared by this linkage in aliphatic and heteroparaffinic amines. The range of Rj values for 10 primary and secondary aliphatic and
1698
ANALYTICAL CHEMISTRY
T a b l e 1.
Drug Adiphenine 2-Aminoheptane Amitriptyline Amolanone Amphetamine Antazoline Atropine Azacyclonol Benactyzine Benzocaine Benzphetamine Betazole Bromodiphenhydramine Buclizine Butacaine Butamben Butethamine Caffeine Carbetapentane Car binoxamine Chlorcyclizine 2-C hloroprocaine Chlorothen Chlorpheniramine Cinchonine Clemizole Cocaine Codeine Covatin Cyclizine Cyclomethycaine Cyclopentamine Desimpramine Diacetylmorphine Diazepam Dieth lpropion Dih y &ocodeinone Ilih dromorphinone Dipgenh ydramine LXphenylpyraline U y c1on iI 1e Ephedrine Epinephrine Ethoheptazine Guanethidine Hydroxyamphetamine Hydroxyzine Imipramine Iproniazid Isoamylhydrocu preine Isocarboxazid Levallorphan Lidocaine Meclizine Meperidine Mephen terrnine Mepivacaine Metabutox y caine Metaraminol Methadone Methamphetamine Methaphenilene Methenamine Methoxamine Methoxyphenamine Methylhexaneamine Methylphenethylhydrazine Meth lphenidate MorpKine Naphazoline Nialamide Nicotine Nikethamide Norepinephrine Nylidrin Orphenadrine Papaverine Pentylenetetrazol Phenacaine Phendimetrazine Phenelzine Phenindamine Pheniramine Phenmetrazine
System I 65 53 72 64 34
System 11 65 22 50 53
9 2 63 6 79 19 50 73
11 8
8
8
7 5 4 63
29
49 3 43 38 10 33 58 7 58
55 66
52
26 22 33 76 6 5 52 42 54
8 1
59 0
3
8
61 4 12 28 31 39 69 55 50 37 30
1
76 46
55
2
13 42
38 56 55 2 5 0 53 24 2 5 60
11 13
12
54 51
55 40 24
28 11
69 68 70 20 37 72 63 68 58 55 37 21 44 53 37 19 37 67 57 28 54 46 45 10 18 39 65 69
17 15
37 25 51 18 7 29
3 29 59 35 64 60 71 60
70 71 48
15 62
60 33 37
18
46 16 28 21
25 72
56
28 8
55 52 64 21
68 48 62 63 63 61 72
53 18
44
Thin l a y e r Chromatographic
System 111 70 36 34 75 33 6 1 1 72
73 85 47 26 84 71 76 49 42
30 6 30 52
28 6
11
61 64 6 52 27 42
2 3 20 62
78 4 3 26 11
46 2
11 11
0 36 39 18 34 22
70 53 69 84 21 3
57 67 44 43 4 40 0 53 4 39 73 35 4 1
9
29
44 15 64 32 53 67 76 41 75 35 5 11
R, values ( X 100) in System System IV V 46 25 60 56 41 28 51 35 59 53 61 38 31 14 60 44 57 28 63 57 54 49 38 17 48 33 75 64 59 51 63 60 58 47 48 34 44 18 5 43 46 15 '8
22 47 26 24 52 41 46 52 62
24 65 44 11
13 45 44 46 54 62
39 15 59 56 39 34 34 65 63 47 74 49 52 43
43 68 51 50 44
14 63 53 59 60 54 23
43 35 8 38
66 66 45 62 53 61 28 62 41 6 49
0
19 26
2 1
9 27 10 10
29
16
29 40
45 9 54 27 3
5
25 23 31 42 40 19 5 21 25 25 22 25 61 47 23 60 24 39 30 23
21 25 39 24 4
56 42 57 65 45 10 30 23
2 25 50 68 22 21 51 55
15
49 25
1
37
Data for 1 1 2 Basic Drugs various solvent systems" Drugs
Phenylephrine Phenylpropanolamine Phenyltoloxamine Phenyramidol Piperidolate Piperocaine Pipradol Pivalylbenz hydrazine Pramoxine Procaine Procyclidine Propoxyphene Prop ylhexedrine 3-Pyridinemethanol Py ri1amine Pyrrobutamine Quinine Scopolamine Strychnine Tetracaine Thenyldiamine Thenylpyramine Thonzylamine Tranylcypromine Tripelennamine Triprolidine Yohimbine Zoxazolamine Plate coated using:
Table 111. R I Values in System I and pK, Values of Some Anilines
System I 5 9 46 14 70 63 74 29 52 5 78 72 54 3 42 62 5 9 13 18
System I1 21 35 53 68 65 45 44 72 61 52 36 62 15 60 33 39 47 54 17 50 32 36
System I11 33 50 33 66 65 42 66 67 55 47 39 60 3 40 24 34 11
33 3 28 25 27 27
47 47 41 38 57 58 55 27 50 35 13 41 45 61 68 9 64 64 4 0.1M 0.1M O.1M NaOH NaOH NaOH Solvent: CycloMethanol Acetone hexanebenzenediethylamine (75: 15: 10 v./v.) a Rj values are averages from three or more difierent plates.
Table 11.
System System IV V 60 45 58 56 42 18 57 40 42 25 47 29 59 51 69 59 48 28 39 18 52 41 54 30 54 46 30 5 1 12 40 59 37 14 13 34 7 17 14 38 12 1 2 14 12 29 56 58 12 3 2 18 1'1 55 66 59 0.1M 0.1M KHSOa KHSOd Methanol 95% Ethanol
Thin Layer Chromatographic Data for 28 Phenothiazines
R, values ( X 100) in various solvent systemsa System I System I1 System I11 System IV System V 3 51 1 15 5 2-Chloro-lO-(3-N-pyrrol62 41 37 49 34 idinoprop y1)-phenot,hiazine Chlorpromazine 57 44 37 44 26 Dimethoxanate 24 31 13 33 13 Ethopropazine 68 82 62 40 25 Fluphenazine 6 25 60 15 6 Isothipendyl 51 47 32 36 18 Mepazine 55 49 37 43 21 Methdilazine 44 29 14 39 18 Methopromazine 46 37 26 40 19 Methotrimeprazine 56 56 65 46 23 Methylpromazine 53 28 39 42 23 Perphenazine 6 20 57 12 4 Phenothiazine 16 71 85 67 71 Pipamazine 0 60 32 48 27 Prochlorperazine 46 41 14 7 2 Proketazine 4 57 17 12 4 Promazine 50 36 25 39 20 Promethazine 46 47 37 45 23 Propiomazine 42 59 53 40 26 Prothipendyl 53 37 20 24 15 Pyrathiazine 52 44 46 43 26 Thiethylperazine 44 47 14 8 2 Thiopropazate 44 66 67 30 11 Thioridazine 52 31 45 41 27 Trifluoperazine 45 49 19 2 10 Triflupromazine 57 52 48 48 3i Trimeprazine 64 55 62 44 22 Conditions-Le., plate coating and solvents-ame as for Table I. R, values are averages from three or more different plates. Drug Acetophenazine
-.
Compound p-Nitroaniline p-Chloroaniline p-Methoxyaniline Aniline 2,6-Dimethylaniline N-Methylaniline N,N-Dimethylaniline
Rl 0.05 0.19 0.25 0.31 0.48 0.53 0.73
PKCl 2.38 3.92 5.32 4.69 3.97 5.0 5.18
heteroparaffinic amines, excepting those with hydroxyl groups, is from 0 34 to 0.55. Within this group of compounds, methylphenidate, R, 0.55, has a n ester group and methoxyphenamine, R, 0.42, has a n ether group. The nine compounds containing a n aromatic primary or secondary amino group all have RI values below 0.20 except the sterically hindered meta-butoxycaine, R f 0.30. An ester group is the only additional polar group in several of these latter compounds. It would appear that the attraction between silica and the aromatic > N-H linkage in this nonpolar solvent is similar in nature to that between the neutral hydroxyl group and silica. S Y ~ T E11. M RI values in this system are closely related to basicity and to the size of the groups around the basic nitrogen atom. When the steric lactors are similar in two compounds, then the strongest base has the lowest RI value as expected on a n acidic substrate. The very weak bases have high Rj values in this system; the polar methanol molecules readily displace these weakly bonded compounds from the silica. When the RJ values in System I1 are arranged from lowest to highest, the lowest values are associated with secondary aliphatic amines as a group and the single guanidino compound, the strongest bases. However, two secondary amines, butethamine and nylidrin, each of which has two large groups on the nitrogen atom, have Rl's of' 0.59 and 0.68, respectively. Pipradrol, RI 0.44 and azacyclonol, R f 0.08, are 2and 4-benshydrol piperidine, respectively. Azacylonol exhibits the strong bonding expected in a piperidine compound, while in pipradrol the large benzhydrol group in the 2-position interferes with this bonding. Chalmers, et al. (2) attributed the lower RI values for a particular group of compounds within a series of pyrroliaidine alkaloids to their higher pK, values. Their chromatographic system was identical to System I1 of this work. The primary aliphatic amines have as a group the next lowest Rl values followed by the tertiary amines. The basicity of these two groups with respect to the hydrogen ion overlap b u t the primary amino nitrogen is significantly less sterically hindered from bonding to the silica. Among the tertiary aliphatic amines, the dimethylVOL. 38, NO. 12, NOVEMBER 1966
* 1699
Table IV.
Rf Values of Some Primary Amines and Their N-Methyl Derivatives in System II
Primary amines Compound Amphetamine Metaraminol Norepinephrine Phenylpropanolamine
N-Methyl derivatives Compound Methamphetamine Phenylephrine Epinephrine Ephedrine
R, 0.28 0.33 0.21 0.35
amino compounds as a group have lower R,’s than those drugs with larger alkyl groups on the nitrogen atom. I n Table IV, the R, values in System 11 of four primary amines are compared with those of their N-methylated derivatives. I n each case, the more strongly basic secondary amine has a significantly lower R, value. A hydroxyl group beta to a n amino group, although a base weakening substituent, has little or no effect on R f values, cf. phenylpropanolamine us. amphetamine and ephedrine us. methamphetamine in Table 111. The bonding of the hydroxyl group to silica evidently counteracts the decreased basicity of the amine function. A morpholino ether linkage produces a n increase in R, presumably due to decreased basicity-e.g., scopolamine has an R, of 0.54 and atropine one of 0.11. Functional groups which have little effect on R, values in this system are the ether groups, halogen atoms, and thienyl groups as compared to phenyl groups. Pyridyl groups decrease R, values when substituted for phenyl groups. Carbinoxamine and thenylpyramine which contain pyridyl groups, have Rr’s of 0.21 and 0.36, respectively; their respective phenyl analogs bromodiphenhydramine and metaphenilene
Table V.
RI 0.18 0.21 0.07 0.18
have R f values of 0.33 and 0.46, respectively . SYSTEM111. The major difference between R, values in this system and System I1 is the higher R, values for the primary aliphatic amines as a group. This is probably due to the formation of neutral azines in a n equilibrium reaction. Of the 10 primary amines in Table I, eight have significantly higher R, values in System I11 than in System 11. The two exceptions are norepinephrine with three hydroxyl groups and tranylcypromine which behaves as a much weaker base in System I1 than the other primary amines. For other aliphatic amines, the R, values are lower in System I11 than in System I1 except for those compounds in which steric hindrance around the nitrogen atom exists. Compounds which can readily bond to the silica are less easily displaced by acetone than by the more polar methanol. Almost without exception, the dimethylamino tertiary aliphatic amine derivatives have lower R, values in System I11 than in System 11. The tertiary aliphatic amines with larger alkyl groups on the nitrogen atom, however, have roughly equivalent values in the two systems. Similarly, in the cyclic nitrogen compounds with either a pyrrolidine or
piperidine ring present, a significantly lower R, value in System I11 compared to System I1 is noted only with those compounds where a methyl group is attached to the ring nitrogen atome.g., meperidine and nicotine. Compounds with large alkyl groups on the nitrogen atom have equivalent R, values in the two systems with the exception of tripolidine which contains a pyridine ring, see below. The more strongly basic secondary aliphatic amines have, as a group, lower R f values than the tertiary aliphatic amines as in System 11. The R, values of most of the secondary amines are about 0.10. Compounds having sterically hindered nitrogen atoms-e.g., butethamine-again have much higher R, values. The importance of the steric factor has been noted previously by Feltkamp and Koch (6) who found that 2-alkyl substituted cycloalkyl amines had higher R, values than the corresponding 3- and 4- alkyl derivatives. Their chromatographic system contained acetone, ammonia, andligroin. The attraction of silica for pyridyl groups, noted for System 11, exists in System I11 also. I n addition to the R, differences noted for the same two pairs of compounds discussed under System 11, four weak bases containing pyridyl rings, iproniazid, nialamide, nikethamide, and 3-pyridine-methanol, have R I values of 0.44 or less. The remaining weak bases all have Rj values of 0.66 or more. This includes phenyramidol which has a 2-amino pyridine group present. X nitrogen atom in the 2 position seems to negate the special attraction between silica and the pyridine ring otherwise noted, cf., pheniramine R, 0.06 and tripelennamine R, 0.27. SYSTEMS IV AND V. Rf values in System V are almost always less than
Comparison of Rf Values of Some Alkaloids and Narcotics in Polar Solvent Systems (18)
Drug/reference This work Atropine 0.11 Cocaine 0.57 Codeine 0.28 Dihydromorphinone 0.15 Dihydrocodeinone 0.17 0.48 Meperidine Methadon 0.37 Morphine 0.28 Nicotine 0.52 n$i;te: Scopalamine Strychnine Plate coated using: Solvent:
0.62 0.47 0.54 0.17
0.1M NaOH Methanol
(17)
(9)
0.17 0.61
0.17 0.62 0.34 0.16 0.18
0.26 0.14
0.35
0.36 0.18 0.23 0.50 0.73
0.70 0.43 0.52 0.22
0.1M NaOH Methanol
(1 )
0.30 0.11 0.17 0.42 0.34 0.29
0.27 ..~ 0.16 0.22 0.62 0.86 0.24 0.74
0.56
0.18
2 0.20 0.80 0.24
0.18 0.48
0.25 0.60
0.64
0.69
0.23 0.37 0.20
0.35 0.52
0.40
1
0.78
HzO
0.19 Hz0
Hz0
Hz0
HzO
Hz0
Methanol
Methanol-
Ethanolpyridine-
Ethyl
Benzeneacetoneether-
Benzeneacetoneether-
triethanolamme (50:50: 1.5)
ANALYTICAL CHEMISTRY
(4)
0.08
0.57
acetone-
1700
(18)
dloxane-
water
(10:4:5: 1)
acetate-
dimethylformamide (1:3)
10% NH3 (4:6:1: 0.3)
25% NHa (416~10.3)
Table VI.
Compound Promazine Methylpromazine Methopromazine Chlorpromazine Triflupromazine
R, Values of Some Closely Related Phenothiazines in Systems II and 111 R / values 2-Substituent IC-Substituent System I11 System I1
Section A -CHrCHz-CHrN (CH3)z -CHz-CHrCHz-N (CH3)z -CHz-CHz-CHz-N(CHs)z -CH2-CHz-CHz-N (CH3)Z -CHz-CHrCHrN(CHOz Seetion B
...
-CH3 --CHI
-c1
0.25 0.28 0.26
0.36
0.39 0.37
0.37
-CF3
0.44 0.52
0.48
c1
0.41
0.37
*..
0.47
0.37
Trimeprazine
...
0.55
0.62
Ethopropazine
...
0.62
0.82
2-Chloro-lO-(3-h'pyrrolidino propyl) phenothiazine Promethazine
-CHrCHrCHrN CHa -CHr
those in System IV. This is in agreement with the usual observation that Rf values decrease as one uses solvents of higher molecular weight in a homologous series. -4s noted earlier ( 7 ) , those compounds which contain a pyridine ring have unusually low Rf values in these acidic systems. The pyridine ring would appear to be strongly attracted to silica gel in both alkaline and particularly in acidic systems. Previous work by Schron (16) with coal tar bases shows this same effect. The pyridine bases had the lowest R f values of those compounds tested in both neutral and acidic solvents. The quinoline compounds ranked next above them, followed by aromatic amines with the highest R f values. All of the aliphatic primary am'res, with three exceptions, have R/ values close to 0.60 in both systems. Two of the exceptions have phenolic groups present and show a drop of about 0.40 unit as one goes from System IV to System V. The third exception has a diazine ring structure in addition to the primary amino group. Other compounds which have similar Rf values in the two systems are the weak bases, except those containing pyridine rings, and those with sterically hindered basic nitrogen atoms-e.g., nylidrin. The secondary amines, nylidrin excepted, have Rfvalues of 0.55 =t0.07 in System IV and 0.43 =t0.06 in System V. Alkaloids and Narcotics. Because these compounds have been reported by numerous investigators, a comparison of d a t a in similar systems was thought desirable. I n Table V, it can be seen t h a t System I1 gives essentially t h e same R/ values and separations as attained by the more complex solvent systems used b y others. Reproducibility of R/ values between this work and that of Waldi,
A
H-N(CHs)z
C_l
et. al. (17) is also quite satisfactory.
The Rj values for all alkaloids in System I of the present work are intermediate in every case between those of Systems 11, cyclohexane :diethylamine (9: 11, and System 111, cyclohexane: chloroform :diethylamine (5 :4:1), used by Waldi et. al. This would be expected as the polarity of System I is intermediate between those of the other two. Structure-Rf Correlations in the Phenothiazines. T h e same generalizations given above apply to this group of drugs as well. I n System I , t h e four compounds which contain hydroxyl groups and the two conlinkage all have taining the --N-H low R, values. iiside from these compounds, System I is poor in its resolving properties for the phenothiazines. I n Table VI R f values of several groups of related compounds in Systems I1 and I11 are tabulated. I n section A of the table, the effect of the 2-substituent on R/ is noted; the chlorine atom and the trifluoromethyl group produce a n R/ increase in both systems. This increase may be due to the decreased basicity of the phenothiazine portion of the molecule produced by these electron-withdrawing substituents. I n Section B of the table the increase in Rf is due to the increase in steric hindrance around the nitrogen atom. It is noteworthy that the number of carbon atoms alone does not determine Rf values because the dimethylamino compound, chlorpromazine, has essentially the same R f as 2-chloro-10(3-N-pyrrolidino-propyl) phenothiazine. Ethopropazine, in which the two ethyl groups are not joined to form a ring, has a considerably higher R/ in these systems than its dimethyl analog,
promethazine. The greater sensitivity of System I11 to steric hindrance around the nitrogen atom, compared to System 11, is shown by the increased range of Rj values in this solvent exhibited by the compounds in Section B of Table VI. I n System IV, the two 1-aza derivatives of phenothiazine, isothipendyl and prothipendyl, have lower Rfvalues than the corresponding phenothiazines, promethazine and promazine, respectively. This decrease in Rf for the 1-aza compounds is similar to that noted earlier in the pyridine ring containing compounds in acidic systems. I n the alkaline media of Systems 1-111, these 1-aza nitrogen atoms produce essentially no change in Rj. Those compounds containing a piperazine ring have R/ values below 0.20 except thiopropazate which has large groups on both nitrogen atoms. These low values for the piperazine ring compounds is in contrast to the normal values found for cyclizine, etc., in the nonphenothiazine drugs. This difference may be due to the greater separation of the piperazine ring from the aromatic residues in the phenothiazines or a combined effect of the two nitrogen containing rings. CONCLUSIONS
An attempt has been made through numerous examples to illustrate some of the relationships existing between the R/ values of these basic drugs on silica gel and their chemical structure. The organic bases cannot be handled by the approach of Martin (IO) using R, values because adsorption effects on the acidic silica outweigh partition effects. Certain empirical correlations, however, have been noted between a compound's structure and its R! values in the five chromatographic systems. These correlations should prove helpful VOL 38, NO. 12, NOVEMBER 1966
1701
in identifying unknown nitrogenous organic compounds. ACKNOWLEDGMENT
The author is indebted to Irving Sunshine for his encouragement to proceed with the collection and analysis of the data and to Halle Landesman for her capable technical assistance. LITERATURE CITED
(1) Baumler, J., Rippstein, S., Pharm. Acta. Helv. 36, 382 (1961). (2) Chalmers, A. H., Culvenor, C. C. J.,
Smith, L. W., J. Chromatog. 20, 270 (1965). (3) Clark, J., Perrin, D. P., Quart. Rev. (London) 18, 295 (1964). (4) Eberhardt, H., Norden, O., Arzneimittel-Forsch 14, 1334 (1964). (5) Emmerson, J. L., Anderson, R. C., J . Chromatog. 17, 495 (1965). (6) Feltkamp, H., Koch, F., Ibid., 15, 314 (19641. (7) Fike, W., Sunshine, I., AKAL.CHEW 37, 127 (1965). (8) Fike, W., Sunshine, I., J . Chromatog. 18, 405 (1965). (9) Machata, G., Mikrochim. Acta 1960, 74
(Iij’Martin, A. J. P., Biochem. SOC. Symposia, (Cambridge) 3, 4 (1950). (11) Mellinger, T. F., Keeler, C. E., J . Pharm. Sci. 51, 1169 (1962).
(12) Mule, S. J., ANAL. CHEY.36, 1907 (1964). (13) Noirfalise, A., J . Chromatog. 20, 61 (1965). (14) Randerath, K., “Thin-Layer Chromatography,” pp. 112-16, Academic Press, New York, 1965. (15) Stahl, E., Ibid., pp. 303-04. 116) Sunshine. Fike. W. W.. Landesman. H.. J . Forensic SA. in Dress. ( l 7 j ’Waldi, D., Schnack; -z, K., Munter, F., J . Chromatog. 6 , 61 (1961). (18) Zarnack, J., Pfeifer, S., Pharmarie 19, 216 (1964). ~
RECEIVEDfor review July 18, 1966. Accepted August 29, 1966. Work supported by funds from Grant R.G. 9863 of the National Institutes of Health, U.S. Public Health Service.
Electronic Spectra of 8-Mercaptoquinoline PAUL D. ANDERSON and DAVID M. HERCULES Departmenf of Chemistry and laboratory for Nuclear Science, Massachusetts lnstitute of Technology, Cambridge, Mass. The electronic spectra of 8-mercaptoquinoline have been studied in aqueous solution and in nonaqueous solvents. The molar absorptivities of the various species have been redetermined and causes for disagreement with values of earlier investigators discussed. The blue color of 8-mercaptoquinoline has been shown to arise from a charge-transfer transition of the zwitterionic form, the analogous transition in 8-hydroxyquinoline being less evident and blue-shifted. The absorption spectra of 8-mercaptoquinoline are compared to those of 8-hydroxyquinoline and quinoline; differences between these spectra are discussed. Results of SCF-CI molecular orbital calculations support the spectral interpretations. The fluorescence of 8-mercaptoquinoline has been studied in nonaqueous solvents. Fluorescence spectra, both in wavelength and intensity, vary as a function of solvent. These and other observations are consistent with fluorescence from the second excited state of the 8-mercaptoquinoline zwitterion. Also, fluorescence spectra of the dihydrate of the mercaptan and of the cadmium(l1) and zinc(l1) chelates are reported.
these differences ( I , 3-5, 1 4 , and some tentative assignments of the origins of the absorption bands of 8MQ have been suggested. However, to date there have been no detailed studies of the spectral characteristics of the mercaptan. The present investigation was undertaken to provide a complete investigation of the electronic spectra of SMQ, with a particular interest in differences of spectral behavior between 8MQ and 8HQ. The aqueous absorption spectra have been investigated as a function of pH and the molar absorptivities for 8MQ redetermined. The spectra of 8MQ and 8HQ are compared in detail and indicate that a larger extent of zwitterion formation in 8MQ is responsible for most of the spectral differences. I n addition, 8MQ fluoresces and forms a fluorescent dihydrate, and some of its metal chelates show strong fluorescence. EXPERIMENTAL
The mercaptan was prepared by the method of Corsini, Fernando, and Freiser (11) using the following reaction sequence :
Gq (&’ S
M
contrasts are exhibited by 8-mercaptoquinoline (8MQ) to 8-hydroxyquinoline (8HQ) , the most striking of which are the colors of the mercaptan and its chelates with metal ions. While 8HQ is a colorless solid and most of its chelates are light yellow, 8MQ is a deep blue liquid which forms a red crystalline hydrate, and its chelates show a variety of colors. Some spectral data have been reported which point up ANY
1702
ANALYTICAL CHEMISTRY
00
1) Ha
+ H3P02 2) NeulmIiralian*
90 0 SH
The disulfide was obtained from the Dojin Pharmaceutical Co., Kumamotoski, Japan. All solutions used were deaerated by bubbling with nitrogen, and the entire procedure was carried out under a nitrogen atmosphere. ,4 portion of the freshly prepared mercaptan was diesolved immediately in deaerated dis-
02 1 39
tilled water to prepare a n aqueous stock solution; the remaining mercaptan was dehydrated by drying overnight in a vacuum desiccator over sodium hydroxide pellets, and was then vacuum distilled (ca. 1 mm. Hg) into tubes which were sealed under vacuum. The mercaptan was stable indefinitely when stored in this manner. The aqueous stock solution was analyzed to determine the mercaptan concentration by titration with iodine solution and a n amperometric dead stop end point. The precision was better than =kl% of the determined concentration. Aliquots of the stock solution were then diluted to the appropriate concentrations with buffers of the desired pH. Hydrochloric acid, sodium hydroxide, acetic acid, and sodium bicarbonate were used to prepare the buffers. The pH of each mercaptan solution was measured after dilution. Aqueous absorption spectra were obtained at p H -1, 5.2, and 13 to determine the molar absorptivities of the acidic, neutral, and basic forms, respectively, of the mercaptan. Spectra were also obtained at pH’s about 2.0 and 8.4 (corresponding to the pK, values) to determine the values of the acid-base equilibrium constants. Absorption spectra were obtained on a Cary Model 14 recording spectrophotometer. A Leeds and Northrup Model 7664 pH meter was used for pH measurement. Data processing calculations were done on the M.1.T. Laboratory for Nuclear Science I B M 7044 computer. For nonaqueous studies, sealed tubes of anhydrous mercaptan were opened as needed, and the mercaptan was either dissolved in chloroform t o prepare a stock solution or dissolved directly in the desired solvent. When chloroform stock soiution was used, aliquots of the stock solution were diluted with the desired solvent before absorption spectra were obtained.
