I
I
George M. Rubottom University of Puerto Rico Rio Piedros, 00931
The Effects of Arvl Substituents ~ IIR I, NMR, and Mass spectr. of N - l - ~ ~ t ~ ~ b e n ~ a . i d e ~
The value of relatine research techniques to the elementary organic laboratory has been generally accepted as a relevant teaching method (1). We have developed the following experiment to give our students the opportunit~to carry out a high yield synthetic reaction coupled with an analysis of the effects of substituents on the ir, nmr, and mass spectral properties of the compounds prepared. The exneriment is introduced in the second semester of our basic organic course for chemistry majors. Since the students have been exposed to introductory concepts of spectral techniques in both the classroom and laboratory prior to this experiment, they have the background necessary for makinitheir spectral-analysis meaningful. In the pre-laboratory lecture it is explained to the students that nitro groups function a s electron withdrawing substituents while methoxy groups are capable of strong electron donation a s a consequence of the nonbonded electron pairs. It is then pointed out that the influence of these substituents should he noticeable in the spectral properties of compounds to which these groups are attached. The purpose of the experiment will be to prepare (111, N-t-butylbenzamide, (I), N-t-butyl-p-nitrobenzamide, and then study and N-t-butyl-p-metboxybenzamide, (El), the effect of the substituents on spectral properties. n
0
0
The teaching assistant synthesizes, (I) while the students. workine. in pairs, synthesize (11)and (111). The reaction is straigitforkard and highly pure products are obtained. At this juncture, each student obtains an infrared spectrum of the pure amide he has prepared and observes as the teaching assistant runs an nmr on a representative class sample of each compound. At the same time, the students observe as mass spectra are obtained for the amides. Xerox copies of the nmr spectra and mass spectra (line drawings including metastable transitions) are given to each student pair. Copies of the ir, nmr, and mass spectra of (I) are presented to the students as data for reference. The students then retire to the library and attempt to assess the role of substituents on the spectra of their aryl substituted N-t-hutylbenzamides. To facilitate the library work, a list of key references is provided (2-41. The procedure for the preparation of the aryl substituted N-t-butylhenzamides (5) (eqn. (1)) is outlined in the experimental section. O
0
R
616
/
- H.
NO.. OCH,
Journal of Chemical Education
Experimental A of 0,01 of aroyl ehloridel in 20 ml of dry ether is caoled to O-C by means of an ice bath, To this stirred solution (magnetic stirrer) is then added a solution containing 0.01 M (1.4 ml) of triethyl amine and 0.01 M (1.05 ml) of t-hutyl amine in 30 ml of dry ether. After the addition is complete (-2 min), the mixture is stirred at room temperature for 3 hr. The resulting slurry is then filtered under vacuum and the filter cake washed with 50 ml of dry ether. The filtrate and washings, containing the amide, are then stripped of solvent, in uacuo, to afford e crystalline residue of crude amide. Purification of a nortion of the crude amide is then effected by vacuum suhlima~on.~~3 The percent and melting points for the pure aryl substituted N-tyields (70)~ butylbenzamides are listed as follows
N-t-hutylbenzamide, (I). (98.5%), mp 134.8-135°C (Ref (6), 12d°C\ --.-,. N-t-h~t~I.~-nitrobenzamide, (II), (68%). mp 160-160.5'C (Ref 171 , .,, lRIDC\ -,. N-t-butyl-p-methoxyhenzamide, (III), 97.5%). mp 114-114.5'C (Ref (7), 116°C).
Spectral Analysis infrared The most profound substituent effect observed in the infrared spectra (CHC13) involves the carbonyl stretching frequency. Relative to the "normal" value of 1665 cm-l observed for (I), the band for (11) is shifted +10 cm-I to 1675 cm-1, while the carbonyl frequency of (DI)reveals a -15 cm-I shift to 1650 cm-*. These shifts are related to the ability of the substituent to interact electronically with the carbonyl functionality in such a way a s to either withdraw electron density from it ( N o d or to increase the electron density in the group (OCH3) (261. The somewhat larger shift observed in-(111) is probably a consequence of the strong resonance interaction of the p-methoxy group with the carbonyl, as indicated in structure (IV).
'
Benzc,yl chlrmdr, p-n~rmhenmyl chlondr, and p-rnethnxubenrovl chluridr tp-aniiy l chloride 3re comrnerriall) avs~lahle
from Aldrich at law cost. ZInorder to save time, a small portion of crude amine is sublimed. Wrystallization of the crude amides from ether-pentane mixtures also serves as a purification technique. 'Represents material obtained when all of the amide is purified.
smolloj se paz!remmns are sap!rne aql roj sa!auanbarj I! a!ls!ra?aeieqa i a q l o '(8) pa~iasqos! d!qsuo!yqar reaug e pue sanlen o TaurureH q ? ! ~paplarioa uaaq seq ?j!qs [Luoqrea aq7 'sauouaqdola3e palnl!pqns-d jo asea snoXoleue aq? UI
ep!uei.uaqAXOqlau-d-(A1nq-l-~'3 pue :ap! -uezuaqo~l!u-d-~Alnq-~-~ '8 :ep!uezuaql6inq-i-~'t,+ W ss paleu6!sap s! leln3a!ou a41 :,w Aq palouap ale suo!i!sueu alqeLse1aw 1 a m ! j u! io ( ~ U ! M W aull) P u n w a d s sseu le!ved z em6!3 UO!
U M ~ E s ~ u n o d w oaqi ~
3 - . : $
(l.j)
mi,
c q l a (,f W i.l - 1-:.-1.
,.-W
is in contrast to the rearrangement with double hydrogen migration (4c) leading to the M-55 fragment observed in the spectra of (I) and (II).
1-
.,b 1 5
-YC3H,
B
iir-C-0
0
I F i g u r e 3. F r a g m e n t a t i o n p a t t e r n for Ar-C-NH-t-BU ( H i t a c h i I P e r k i n - E l m e r RMS-4. 70eV):m e t a s t a b l e t r a n r i t i o n s d e n o t e d by M *
hlet reoresents the nrotons ortho to the methoxv moun.' while ;he downfield doublet corresponds to the Grot& ortho to the amide moup. . . The shift of HE in (II) relative to HHin (Ill)is 1.38 6. The splitting of the aryl protons of (II) and (JD) may he explained hy considering that in each instance JAB-^^^^^ = 10 Hz hut that JAR.^^^^ = 0 HZ. This being the case, the natterns ao~roximateAB svstemss and the characteristic k3 pair of doublets is ohservkd (3d).
It should he noted that the metastable data lends no support for this type of pathway. However, M* for the transformation of M-15 into M-55 is present in the spectra of (I) and (11). A mechanism consistent with these findings is
Mass Spectra
The mass spectra (HitachiJPerkin-Elmer RMS-4, 70 eV) of (I), (II), and (III) may he explained by the fragmentation pattern shown in Figure 3 (10). In each instance, a strong molecular ion (M t ) is observed (3e), and theemmost abundant fragment in each spectrum is M-72 (Ar-C==O). Each spectrum also exhibits a strong M-15 (-CHs.) peak. The metastable transitions M* (3f) are defined as M* = MzZ/M1, where MI is the precursor ion and M2 is the product ion. The presence of M* is positive evidence that, during the fragmentation process, M, gives rise to the fragment Mz. Theverified M* values (10) are as follows M Mz M* (1)
177 162 162
-
162 105 122
148.27 68.06 91.88
R= H. NO,
Acknowledgment
Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support in portions of this work. Literature Cited
The appearance of a fragment a t M-56 in the spectrum of (ID)as contrasted to the presence of fragments a t M-55 in both (I) and (11) is quite startling. The process leading to the M-56 fragment in (In) may he considered to he a McLafferty rearrangement (4b) in which the p-methoxy group affords increased electron density a t the electron deficient oxygen. This process
(11 a1 Silverstpin. R. M.. and Barder, G. C.. J. CHEM. EDUC.. 39. 546 119621: hi Silversicin. R.M.J. CHEM. EDUC.. 46.794 (19681. (2) a1 Bellamy. L. J.. "The Infrared Sppctra of Complex Moleculer." 2nd Ed.. John Wiley & Sona. Inc.. New York. 1958. Chap23; hl p. 138: cl p. 205: dl p. 18. (31 a1 Silverstein. R. M.. and Baslor. G. C.. "Spectrometric ldentificstion af Orqanic Compoundi." 2nd Ed.. John Wiley & Sons. I n c , New York. 1967. Chap % hl p. 11% c i p l l t d l p. 127: el p. 32: 0 p. M. (1) a1 McLafieny, F. W. ''Interpretation of Mass Spectra." W. A. Benjamin. Inc.. NewYork. 1967.Chaps34.andS: b l p . 123;el D. 137. 161 Rubattom. G.M.. andPiehardo. J. L.. Synth. Commun.. 3. 18S119731. 161 Glikman~.G.. Torck. B.. Hellin. M.. and Courremant. F.. Bull. S o c Chim fr. 1376 119661. 171 Laccy. R. N . . J Chem Soc.. 1633. (19601. (81 Fuwn. N., Jo~ien.M . ~ L . ,and Sheiton. E. M.. J A m e i C h r m Soc. 7L 2526 ,,." , a u...,. ,
7 See, for example, the spectrum of p-methoxy benzoic acid in reference (111, Volume 1, nmr spectrum number 196. 8Although the splitting is actually not first-order (AB), but rather more complex, (3dA we feel that, for identification purposes, the simplified explanation is valid.
618
/ Journalof Chemical Education
191 a1 Jaekman. L. M.. "AgplVafions of Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry." Pergamon Prerr. New York. 1959, p. 123: bl p. 50, c) p.
( I l l Hizh Resolution NMR E , Vol. 1. 1962. Vol.2. 196%
