2714
LEONARD DOUBAND J. M. VANDENBELT
Vol. 69
1. The ultraviolet absorption of the monomers and polymers of a number of substituted vinyl aromatic compounds are reported for the range
2. The --Yravioletabsorption method has been successfully applied to the determination of the monomer ratio in copolymers of butadiene with many of these substituted vinyl aromatic compounds. 3. The relative intensities of absorption for various substituted polystyrenes have been compared with predictions based on a theory by Sklar.
250-300 :mp.
URBANA, ILLINOIS
The remaining series of monomers are so incomplete that the data are presented (Figs. 13-15) without any attempt a t correlation.
Summary
[ C O X T R I B U T I O N FROM THE
RESEARCH IdAIlORATORIES,
RECEIVED MARCH20, 1947
P A R K E , 1)AVIS ANI)
CO.]
The Ultraviolet Absorption Spectra of Simple Unsaturated Compounds. I. Monoand p-Disubstituted Benzene Derivatives BY LEONARD DOUBAND J. M. VANDENBELT Benzene exhibits absorption in two regions of the accessible ultraviolet. One is a highly resolved band of low intensity (E inax. = ca. 200) a t 240-60 mp. The other is an intense band (E max. = 7,400) in the neighborhood of 200 nip. The majority of studies reported in the literature have been concerned with the longer wave length absorption and have assumed that absorption of benzene derivat.ivesin this region is best explained by modification of this band. That the shorter wave length high intensity absorption might be displaced in derivatives to account for spectra near 250 nip has been recognized in a t least one instance. Surprisingly, however, there appears to have been no systeinatic study of the fate of this band in simple benzene derivatives although numerous isolated reports of high intensity short wave length absorption are on record.’-* Clearly it is of importance for the rationalization of the effect of substitution on benzene absorption to relate the maxima of a given derivative to one or the other of these parent bands and to cliscriminate from effects following the introduction of new chromophoric groups. This st.udy of the effect of substitution on the short wave length (ca. 200 mp) absorption of benzene in general corroborates and extends the concept that this band is capable of displacement to and beyond 250 mp. In addition, it would appear to provide a basis for a more logical inter(1) Wheland, “The Theory of Resonance,” John Wiley and Sons, Inc., New York, N . Y . , 1944, p. 151. (2) Dede and Rosenberg, Ber., 67, 1-17 (1934) (3) Flexsei-, Hammett and Diugwall, Tiiis J O U R N A L , 67, 2103 ( 1935). (4) Morton and McGookin, J . Chem. Soc,, 901 (1034). (5) Conrad-Billroth, Z . p h y r i k . C h e i n . . B19, 76 (1932); BIO, 222 (1933); B95, 139 (1934); B96, 217 (1934). (6) Kumler and Strait, T I I I JSO U R P I ’ A L ,65, 234’9 (19.13). (7) Kumler, i b i d . , 68, 118-4 (1040). (8) The aboveare afew representxtive sttidieson short wavelength benzenoid absorption. The f d l o w i n g sources may b e consulted for others: “Intdernational Criticxl Tableb,” Vol. V. McGraw-Hill Book C o . , Inc.. New York, N . Y . , 1929; Morton, “Absorption Spectra,” 2nd ed., Adam Hiker, London, 1942; Brode, “Chemical Spectroscopy.” 2nd ed., John Wiley and Suns, Inc.. New York. N . Y.. 1043.
pretation of benzenoid absorption than has heretofore appeared. Methods and Materials.-A Beckman quartz spectrophotometer with a hydrogen discharge tube source was used for all measurements. Extinction coefficients were calculated by dividing the observed densities by the molar cell concentration. Dilutions were prepared such that the peak absorption of each band fell on the density scale betwen 0.4 and 0.8. Readings in the difficulty accessible region below 220 rnp were obtained so far as possible a t a slit width of 1.0 mm.; if necessary the check switch position a t 0.1 was used. Calibration of the instrument with a mercury vapor arc indicated that the wave length scale in the ultraviolet region was correct to a few tenths of a millimicron. Reported wave lengths of band peaks are therefore taken directly from the highest instrument readings as the peak was read through in small increments; in the case of a broad band the center was estimated by symmct.ry from its graph. Liquids were redistilled before use. Solid coinpounds were recrystallized from a suitable solvent. Boiling and melting points were checked with the literature values as a confirmation of purity. Liquid samples such as benzene and toluene were weighed into stoppered flasks and dissolved in methanol. This solution was diluted into water and a L h i k containing the same amount of niethanol prepared for instrument reading. The methanol concentration was never greater than 2y0. General Absorption Characteristics.-Figure 1 gives curves of representative monosubstituted and @-disubstituted derivativesg The curves (9) The absorption spectra of the m a j u r i t y of the coinlmunds contained i n this study have been reported Ineviou4y a t leas1 in p:rrt. These have been repeated and recorded here Lrcnuse most of the earlier curves are unreliable in the range belnw 220-230 n i or ~ are nut reported a t all in this regiim. In a ~ l d i t i i ~ itih, e divrrsily of st11. veuts used in these prrvious studies reiidws coin~~arison exlreincly difficult. In this study the solvent is water, except in those few cilsc\ where a trace of methanol was used to facilitate solution of the compound. Water was chosen as the solvent because the condition\
Nov.. 1947
ULTRAVIOLET ABWKPTIONSPECTRA OF BENZENEDERIVATIVES
have been arranged in order of increasing wave length of the more intense band. Where two intense bands occur the more displaced band is used. The values of A max., E max., and Ah (displacement of the primary ban.d relative to the 203.5 mp band of benzene) of all the compounds included in this study are contained in Tables I and 11. The most displaced intense:bands are designated in the tables as the first primiary bands; where second intense bands occur, they are referred to as second primary bandslo The low intensity bands (exemplified by the 280 mp band of aniline) when present (absent in sulfanilamide, nitrobenzene and others) are referred to as secondary absorption. The values assigned X max. and e max. :are those of the highest points of the broad regions of absorption, no cognizance being taken of resolution within these regions. With the con:ipounds presented here, primary absorption is greater than e = 6 X lo3, while secondary absorption is never more than e = 2.6 X lo3,and usually much less. Thus the two types are easily differentiated. Obviously, where a compound is capable of existing simultaneously in two or more distinct molecular forms, this criterion for secondary absorption must be applied with care. Thus aniline, whose cation is transparent a t 230 m p , gives only feeble absorption a t this point when measured a t low PH ranges, although the molecular species gives intense absorption in this region. The monosubstituted derivatives (Table I) exhibit primary absorption over the whole range from t h a t of benzene a t 203.5 mp to that of nitrobenzene a t 268.5 mp, with a rather general increase of intensity with displacement. Secondary absorption in general is displaced approximately in proportion to the primary band and varies considerably in inten.sity, although there is some tendency to increase in intensity with displacement. The resolution exhibited markedly in the secondary, and slightly in the primary bands of benzene itself decreases with displacement of the bands. The disubstituted compounds (Table 11) show primary absorption starting well in the region common to the monosubstituted derivatives and extending to the edge of the visible region. When present, the secondary absorption tends to increase in intensity with displacement in similar fashion to that of the monosubstituted compounds, although. it seems somewhat more erratic. Frequently, secondary absorption appears to be absent. However, asymmetry of the single band in some of these instances suggests that the secondary may still be present fused with the first primary band although no distinct maximum can be identified. The primary band is distinctly greater than in the monosubstituted compounds, and, as governing ionization of acids and bases are best known and most easily controlled in this medium. A i il consequence, the absurptiun of molecular and ionic species can be assigned with considerable accuracy. (10) Attention has previously been called to the existence of these bands by Kulmer: he r’eferredtn them ns X’ hands.7
2715
_ _ _ _ ~ 200
280
300
440
mp.
Fig. 1.-Ultraviolet absorption spectra of representative mono- and @-disubstituted benzene derivatives arranged t o show progression of the bands. Each unit on the ordinate axis represents a n extinctioncoefficient e of 2000. The numbers refer to compounds listed it1 Tables I and 11.
the wave length increases, the intensity of absorption rises coincident with decreasing monochromatism of the band. The secondary absorption, on the other hand, retains the same magnitude in the two types. I t is interesting to note that with derivatives exhibiting more displaced absorption, a second band of relatively intense absorption appears in the shorter wave length region. For consistency, it has been designated in this paper as the second primary band. Its ratios with the primary band (lower part of colunin 10, Table 11) are very similar to those found by Kumler.lo The occurrence of the second primary band apparently is a gcneral property of benzenoid absorption. Although the instrument used in this investigation does not allow nieasureiiieiit a t the low wave lengths necessary to confirm this, it appears reasonable to assume that this band is present in most if not all simple benzenoid compounds
LEONARD DOUBAND J. M. VANDENBELT
2716
ULTRAVIOLET Compd no.
1 2
3: 4 6 6 7 8 9 10 11 12 13 14 15 16
TABLE I ABSORPTION CHARACTERISTICS OF h'fONOSUFlSTITUTED BENZENE DERIVATIVES Primary band
Secondary band
Compound
Amax., my
emax.
Xmu..mr
Benzene Aniline cation Toluene Chlorobenzene Broniobenzene Phenol Anisole Benzenesulfonamide Benzonitrile Benzoic acid anion Benzoic acid Aniline Phenol anion Acetophenone Benzaldehyde Nitrobenzene
203.5 203 206.5 209.5 210 210.5 217 217.5 224 224 230 230 235 245.5 249.5 268.5
7,400 7,500 7,000 7,400 7,900 6,200 6,400 9,700 13,000 8,700 11,600 8,600 9,400 9,800 11,400 7,800
254 254 261 263.5 261 270 269 264.5 27 1 268 273 280 287
Limiting attention to the more displaced (i. e., first) primary band, a progression of effect is obvious (Fig. 1). In contiguous compounds the appearance and order of intensity of the bands are similar, regardless of the diversity of substituent groups, and it would appear reasonable to interpret them all as being due to displacement of the 203.5 mp band of benzene. Since this point of view has not generally been recognized, and since in some instances primary absorption might be assigned. to unique chromophores formed by the substituent groups, further arguments for this concept will be considered in detail. T h a t absorption of this magnitude in the region under consideration cannot be assigned logically to the substituent groups themselves is borne out from a consideration of these groups (e. g., -Br,
10
8 0
I
2 6 X u
4
2
0
200
320
240
260
280
nip.
Fig. 2:-Ultraviolet absorption spectra of n-heptyl bromide (- - -) compared with bromobenzene (- -), and acetic acid (- - -) compared with benzoic acid (-).
-
Vol. 69
... ...
...
em=.
204 160 225 190 192 1450 1480 740
lo00 560 970 1430 2600
.. ..
..
L8./A,d.
1.25 1.25 1.25 1.25 1.24 1.28 1.24 1.22 1.21 1.20 1.19 1.22 1.22
.. .. ..
AA @PA -203.5)
AAcrlod.
0
0.55 2.1 6.1 5.8 8.0 12.6 13.3 (14.0) 20.5 21.2 (26.5) (26.5) 31.5 39.8 45.4 68.1
-0.5 3.0 6.0 6.5 7.0 13.5 14.0 20.5 20.5 26.5 26.5 31.5 42.0 46.0 65.0
(h
-23.5)
-OH,
-NH2, --COOH) when combined with alkyl chains. Even when these groups show absorption above 200 mp, the intensity is inadequate to account for that of the corresponding benzene derivative. Figure 2 gives the absorption curves of n-heptyl bromide compared with bromobenzene, and acetic acid compared with benzoic acid. In addition to being displaced, the absorption of these alkyl compounds would have to be increased about 40 and 300 times respectively to equal that of the benzene derivatives." The regularity in amount of displacement of both the primary and secondary bands in the derivatives relative to those in benzene is apparent from the A s m . / A p ~ . ratios shown in Tables I and 11. The ratios are calculated only for those compounds in which primary and secondary absorptions are separated widely enough in the spectrum to permit precise assignment of the position of the secondary maximum. When the secondary absorption is highly resolved, the wave length of the strongest resolved band is used. This similarity makes it appear extremely probable that both secondary and primary absorptions arise from the same chromophoric groups, rather than that the benzene substituents are acting independently as chromophores. There would be no reason to expect a uniform correspondence between two absorbing groups whose chromophoric properties are determined independently, and since the secondary bands have been associated with benzene it is reasonable to infer that the primary band also is benzenoid in origin. (11) It can he predicted that benzene derivatives whose substituent groups are themselves capable of intense absorption in this region may exhibit anomalous behavior. I n this paper, a deliberate attempt wus mode to exclude cornpounds of this type. However, the nitro derivatives may conceivably fall in this class. Since the alkyl nitrates absorb in the 220 mr region, while nitrobenzene absorbs a t 268.5 my, a displacement of almost 50 mp, it was felt that the two bands could be discriminated. Nevertheless, the exceptional behavior of the nitro compounds in some of the later devclopments in this paper may be a result of this feature of their absorption.
ULTRAVIOLET ABSORPTION SPECTRA OF BENZENEDERIVATIVES
Nov., 1947
2717
TABLEI1 ULTRAVIOLET ABSORPTIONCHARACTERISTICS OF DISUBSTITUTED BENZENEDERIVATIVES Fitst primary band Compd
17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61
62 63 64
.
Compound p-Toluidine cation p-Chloroaniline eation Sulfanilamide cation p-Bromoaniline cation 9-Aminophenol cation p-Aminobenzonitrile cation p-Chlorophenol p-Toluenesulfonamide p-Aminobenzoic acid cation #-Toluidine p-Tolunitrile 9-Chlorobenzoic acid anion 9-Toluic acid anion p-Chlorobenzonitrile p-Bromobenzoic acid anion p-Chloroaniline p-Bromoaniline p-Aminoacetophenone cation p-Chlorobenzoic acid $-Toluic acid p-Chlorophenol anion p-Hydroxybenzoic acid (1st) anion #-Bromobenzoic acid #-Anisic acid anion p-Hydroxybenzoic acid p-Methylacetophenone p-Anisic acid Sullanilamide p-Nitroaniline cation p-Br omoacetophenone p-Nitrohenzoic acid p-Aminobenzoic acid anion $-Dinitrobenzene p- Aminobenzonitrile 9-Nitrobcnzoic acid anion p-Hydroxyacetophenone p-Mcthoxyacttophenone $-Hydroxybenzoic acid (2nd) anion 9-Nitrochlorobenzene $-Hydroxybenzaldehyde p- Aminobenzoic acid p-Nitrotoluene p-Aminoacetophenone 9-Nitrophenol $-Hydroxyacetophenone anion $-Hydroxybenzaldehyde anion p-Nitroaniline $-Nitrophenol anion
Solvent 0.5 N HCI 2 N HC1 2 N HCl 0.1 N HCI pH 3 2 N HCI 0.1 N HCl 1 N HCI 2 N HCI pH 11 0 . 1 N NaOH pH 11 1 N NaOH 0.1 N NaOH 1 N NaOH 0.1 N NaOH . 1 N NaOH 2 N HCI 0.1 N HCI 1 N HCI 1 N NaOH pH 8 0 . 1 N HCI 1 N NaOH 1 N HCI fiH 5 0 . 1 A! HCI $H 7 2 N HCI 9H 6 0 . 1 N HCl 1 N NaOH 1 N NaOH 1 N NaOH 1 N NaOH 0.1 N HCI . 1 N HCI 1 N NaOH pH 6 0 . 1 N HC1 $ H 3.75 $H 6 0.1 N NaOH $H 3 0.1 N NaOH . 1 N NaOH 1 N NaOH 1 N NaOH
.
. .
. .
207.5 215.5 217.5 218.5 218.5 223 225 226 226.5 232 234 235 235 237.5 2.39 239 239.5 240 241 241.5 244
245 245.5 247 255 266
256.5 2!i8 258 258.5 2ci4 .5 265 266 270 274 275 276.5 280 280 283.5 284 285 311.5 317.5 324.5 330 381 402.5
.
Interaction of Groups in Band Displacement -If the substituent groups are divided into electron contributing (ortho-para directing) and electron attracting (meta directing) types. and arranged in order of increasing Ah values of the primary band from Table I. the following series are obtained Type I: Orthepara directing
.
< OCH
