[CONTRIBUTION O F THE DEPARTMENT O F CHEMISTRY, P U R D U E UNIVERSITY ]
l'ltraviolet Absorption Spectra of Nitroparaffins, Alkyl Nitrates, and Alkyl Nitrites' HERBERT E. UNGN.4DE2
AND
ROBERT A. SMILEY3
Received March 26. 1966 Ultraviolet absorption spectra have been determined for a series of nitroparaffins, alkyl nitrates, and alkyl nitrites. The relationship between structure and absorption spectra has been discussed.
During an investigation of the chemistry of nitroparaffins, alkyl nitrates, and alkyl nitrites, the ultraviolet absorptiori spectra of a number of these compounds were determined. The experimental results are summarized and interpreted in the present, communication. -Vitroparafins. Aliphatic nitro compounds are characterized by a broad absorption band of low intensity in the region of 270-284 mp and a second band of greater intensity which lies below 200 mp.4-7 The bands are usually well separated and distinct except in those nitro compounds where both oxygens of the nitro group are bonded with adjacent hydroxyl groups or one oxygen with an S-H group. In these cases the low-intensity band occurs only as a shoulder or inflection.* A maximum at 366 mp, which is occasionally ascribed to the nitro group,y,lohas not been observed in these compounds aiid presumably refers to one of the weak bands in r~it~robenzene.' The wavelength of the low-intensity transition of the aliphatic nibro compounds is relatively independent of the type of alkyl group.6 The over-all variation in the 21 compounds examined is only 7 mp (Table I). Within this range the primary nitro paraffins absorb at shorter wavelengths (274-278 mp), the tertiary compounds near the upper limit (278-281 nip), and the secondary compounds in between. The molar absorptivities, in general, have a tendency to increase with molecular weight. Nitro
esters fit into the series, as do cyclic nitroparaffins. The behavior of the tertiary compounds and of 2nitro-2-chloropropane makes it unlikely that steric effects play any considerable role in a-chloro nitroparaffins, as was proposed for chloropicrh6
(1) This investigation was supported by the United States Air Force under contract No. AF 18 (600)-310 moni. tored by the Office of Scientific Research, Air Research and Development Command. (2) Present address: Los Alamos Scientific Laboratory, Los Alamos, N. M. (3) Present address: E. I. du Pont de Nemours & Co., Gibbstown, N. J. (4) Braude, Ann. Repts. on Progr. Chem. (Chem. Soc.
The 280 mp absorption band of aliphatic nitro compounds is shifted to the blue on increasing the polarity of the solvent. Thus nitromethane absorbs at 277.6 mp in heptane, at 274 mp in ethanol, and a t 268.8 mp in water while in 90% sulfuric acid the band occurs a t 252.5 mp.'J1 Alkyl nitrates. Various alkyl nitrates which have been investigated, show a weak band in ethanol at 270 mp as a shouIder (Fig. 1). The position of the band and the shape of the curve are little affected by varying the alkyl group, while the molar absorptivities at 270 mp for eight compounds vary from 15
London),42,105 (1945).
(5) Gillarn and Stern, Electronic A bsorptwn Spectroscopy, Edward Arnold Publishers, London, 1954, p. 112. (6) Haszeldine, J . Chem. SOL,2525 (1953). (7) Bayliss and McRae, J. Phys. Chem., 58, 1006 (1954). (8) Urbanski, Bull. acad. polon. sci., Classe ZZZ, 1, 239 (1953); Urbanski and Ciecierska, Roczniki Chem., 29, 11 (1955). (9) Brode, Chemical Spectroscopy, John Wiley and Sons, YewYork, N. Y., 1939, p. 130. (IO) Coggeshall, Anal. Chew., 22, 389 (1950).
TABLE I ABSORPTIONSPECTRAOF NITRO COMPOUNDS IN ETHANOL 1-Nitrobutane" 1-Nitropentane" 1-Nitrohexane 1-Nitroheptane 1-Nitrooctane 1-Nitro-2methylpropane 1-Nitro-3-methylbutane" Cyclohexylnitromethaneb 2-Nitropropanee 2-Nitrobutane 2-Nitrooctane Nitrocyclopentaned Nitrocycloheptaned Ethyl a-nitrobutyrate Ethyl a-nitrovalerate Ethyl a-nitrocaproate 2-Nitro-%methylpropaneb 1-Nitro-1-methylcyclopentane* l-Nitro-l-methylcyclohexaneb 2-Nitro-2,4dimethylheptaneb 2-Nitro-2-chloropropene
I" I" I" I" I" I" I" I"
I1" I1" I1" 11" I1" I1" I1" I1" 111" 111" IIIO IIIO
111"
276 276 275 274 276 277 275 278" 276 279 277 277 276 278 278 279 278 279 280 280 281
24 26 28 30 29 31 29 41 27 24.5 34.5 31 39 30 31 32 25 31 32 32 31
Taub, PhD thesis, Purdue University, 1952. Rimmer, 278, PhD thesis, Purdue University, 1953. I n ether A,, Mooberry, PhD thesis, Purdue University, 1954. E**.
(11) Bayliss and Brackenridge, ,J. Am. Chhov. Soc., 77, 3959 (1955).
998
994
UNGNADE AND SMILEY
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'LogE
2
I
FIG. ULTRAVIOLET ABSORPTION SPECTRA: (1) 2-butyl nitrate in ethanol; (2) Z-nitrobutane in ethanol; (3) 2-butyl nitrite in ether.
to 37 without any obvious relationship to structure, except that 11" alkyl nitrates of larger molecular weight have higher values (Table 11).The absorption intensities and wavelengths agree with other measurements. 4,6 300
350
400mv
TABLE I1 ABSORPTIONSPECTRA I N ETHER: 95% ETHANOL FIG.2.-uLTRAVIOLET (1) isobutyl nitrite; (2) sec-butyl nitrite; (3) tert-butyl Type E Z ~ Omic nitrite.
ABSORPTION SPECTRA O F ALKYL N I T R A T E S I N
Compound Cyclopentylmethyl nitrate" Cyclohexylmethyl nitrate" l-Octyl nitrate 2-Butyl nitrate Cyclohexyl nitrate" 4-Heptyl nitrate 2-Octyl nitrate 5-Methyl-Zoctyl nitrate a
I" I" I" 11"
I1 I1
O
O
11" I1
16 20 15 17 22 28 37 33
Rimmer, PhD thesis, Purdue University, 1953.
Alkyl nitrites. Alkyl nitrites exhibit two maxima above 200 mp, a high-intensity band at 228 mp and a low-intensity transition with six or seven vibrational fine-structure bands centered at around (Fig. 1). The solution 375 mp ( E about 70)6J112v13 spectra of alkyl nitrites in the region of 300400 mp are characteristic for each type and permit the rapid identification of primary, secondary, and tertiary nitrites (Fig. 2). Within each group of nitrites the wavelengths of the fine structure bands chazlge little except when the electronic character of the alpha carbon is changed, an increase in the electron density of this carbon producing a shift of all bands to the red and an increase in the absorption intensity. The molar absorptivities tend to increase with the molecular weight of the alkyl group (Table 11). As in the case of the nitro compounds, the weak ~-
(12) Kuhn and Lehmann, 2. physik. Chem., B18, 32 (1932). (13) EIaweldine and Jander, .I. Chem. SOC.,691 (1954).
transition in the alkyl nitrites may be regarded as blue shift bandI4 since the band farthest to the visible of tert-butyl nitrite, for instance, is shifted from 398.2 mp in vapor15to 395 mp in ethanol, and to 383 mp in water (Table 111). Changes in the polarity of the solvent probably also change the ratio of cis- and trans-isomers.l6 Applications. The ultraviolet absorption spectra of nitroparaffins, alkyl nitrates, and alkyl nitrites differ sufficiently (Fig. 1) so that they can be used for identification purposes for pure compounds. Recently the ultraviolet absorption above 400 mp has been utilized for determining alkyl nitrites in Victor Meyer reaction mixtures also containing alkyl nitrates and nitroparaffins. l7 Since nitro compounds and nitrates do not absorb in this region (Fig. l),the ether solution of the products, made up to a definite volume, can be read directly after removing the silver salts from the reaction mixture. Some caution is necessary with this method since it has been observed that nitrogen dioxide in ether absorbs in the same region as the nitrites, having
(14) McConnell, J. Chem. Phys., 20, 700 (1952). (15) Tarte, J. Chem. Phys., 20, 1570 (1952). (16) Hasaeldine and Mattinson, J . Chem. Soc., 4172 (1955). (17) Kornblum, Taub and Ungnade, J. Am. Chem. SOC., 76, 3209 (1954); ,Kornblum, &n.iley, Ungnade, White, Taub, and Herbert, J . Am. Chem. Soc., 77,5528 (1955).
SEPTEMBER
1956
995
U. V. ABSORPTfON YPECTB.4 OF NITHOPArLtFFIXS
TABLE I11 ULTRAVIOLET ABSORPTION SPECTRA O F ALKYL XITHITESIS ETHER Isobutyl n-H~yl" n-Ht:ptyl n-Dc:cyl" 2-Phenyl-1-propyl Benzyl p-Methylbenzyl p-Methoxybenzyl 2-Propyl
X 313 e 22.6 X 314 X 314.5 e 23.7 X 314.5 X 315 e 29.8 X 315 E 23.4 X 315b e 28.2 X 318 e 11.0 X e
2-Butyl
x
2-oc tyl
X
1-Phenyl-2-propyl
X
tert-Uutylc
X
tert-Amyl
X
e e e e E
323 29.0 323.5 323.5 32.4 323.5 323 34.3 325.5 27.6 327 31.8 328 45.7 327 15.8 327 18.1 326 19.7 325 21.5 329 11.5 329 11.6
332,5 44.8 333.5 333 51.4 333 332.5 49 336 39.1 337 45.1 339.5 55.6 337 25.2 336 26.9 336 29.6 336 30.5 340 21.4 341 21.5
343 64.7 344 344,5 73.0 344 344 69.5 346 54.2 348 61.1 351 71.0 347 38.6 346 40.8 346 44.7 346 45.4 352 40.5 353 38.1
355 76.1 356 356 84.7 356 355 82.9 357 63.1 359 71.6 361 80.4 359 50.5 357 54.3 357 60.2 356 61 365.5 61.0 365.5 57.3
368 64.7 368.5 369 69.0 369 368 69.4 370 55.8 370 63.3 373b 72.7 373 49.3 369 55.8 360 61.6 368 65.6 379.5 68.6 380 63.7
382 31.6 382 382.5 32.9 383 382 40.8 385b 28.5
335 339
346 351.8
357 365.8
369 381. I
383 398.2
387 28.0 382 35.2 382 39.1 380 46.1 395 43.6 394.5 40.0
The exact concentration was not determined in this case.
* Shoulder. e
I n water In vapord Tarte, J . Chem. Phys., 20,1570 (1952).
3276
-
-
-
300
35 0
400
,,,
.
that they appear as shoulders or separate bands in mixtures of nitrites and nitrogen dioxide in ether (Fig. 3). The interfering absorption bands can be detected by scanning the region between 300 and 400 mp with a recording spectrophot,ometer. In view of the great differences in the spect'ra of primary, secondary, and tertia,ry nit'rites, it has been possible to ascertain that isobutyl nitrite is identical whether it is prepared from the alcohol by means of sodium nitrite and aluminum sulfate'* or by use of nitrosyl chloride in pyridine, and that it is formed from isobutyl halides and silver nitrit'e without isomerization. The spectra of the isomeric phenylpropyl nitrites differ sufficiently so that t'hey can be used for determination of mixtures. As litt'le as 5% (or less) of the secondary nitrite can be detected in mixtures of known concent'ration. Since the ultraviolet spectra of the nitrites from the Victor Meyer reactions'g are identical wit,h those prepared from the alcohols by reaction with nitrosyl chloride in pyridine, it may be concluded that no appreciable isomerization occurred. Rearrangements of nitrites have not been encountered in any of the Victor Meyer reactions investigated.
FIQ, 3.-ULTRAVIOLET ABSORPTION SPECTRA IN ETHER: (1) NOS; (2) tert-butyl nitrite; (3) tert-butyl nitrite containing NO,.
five fine-structure bands at 337, 348, 359, 373, and 387 m ~ These . absorption bands are sufficiently displaced with respect to the nitrites SO
(18) Chretien and Longi, Compt. rend., 220, 746 (1946). (19) Kornblum, Chalmers, and Daniels, J . Am. Chem. sot., 77, 6654 (igjj); Blackwood, Ph.D. thesis, Purdue University, 1955.
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Starting material. The preparation and separation of the nitro compounds, nitrates, and nitrites, have been described previously in detail.17.1* The purity of each of the substances was established by means of infrared absorption spectra. Nitrogen dioxide in ether was obtained by dissolving purified nitrogen tetroxide in anhydrous ether and diluting successively until the absorbencies were brought into range. Ultraziolet absorption spectra for alkyl nitrates and nitro
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compounds were determined in 95% ethanol, those for alkyl nitrites in anhydrous ether to avoid ester interchange reactions with alcohol. The measurements were made in 1-cm. cells a t 25" with a Cary recording spectrophotometer.2o
WESTLAFAYETTE, INDIANA (20) hIeasurements by Robert P. Curry, Purdue University.
