Aromatic Nitration in Liquid Nitrogen Tetroxide Promoted by Metal

Chapter 4

Aromatic Nitration in Liquid Nitrogen Tetroxide Promoted by Metal Acetylacetonates Ripudaman Malhotra and David S. Ross

Downloaded by MIT on May 20, 2013 | http://pubs.acs.org Publication Date: April 24, 1996 | doi: 10.1021/bk-1996-0623.ch004

SRI International, 333 Ravenswood Avenue, Menlo Park, CA 94025

Aromatics such as benzene and toluene are not nitrated by liquid N2O4 at 0°C to any significant extent. However, simultaneous passage of N O and O2 through a solution of benzene in liquid N2O4 leads to nitration of benzene giving mono-, di-, and even trinitrobenzenes. Prompted by the possibility that one-electron oxidation of arenes followed by their reaction with NO2 might lead to these nitrations, we examined the reaction of benzene and toluene in liquid N2O4 with various transition metal oxidants. Because most simple salts such as acetates and nitrates are too ionic, they could not be dissolved in liquid N2O4. However, inner complexes, in which the ionic and coordinating valencies are simultaneously satisfied, are soluble in this medium, and we were able to conduct a study with a range of acetylacetonate complexes. In concert with our hypothesis, nitration was readily effected by the oxidizing acetylacetonates of Fe(III), Ce(IV), Co(III), Μn (III) and Cu(II). However, nitrations were also effected by the nonoxidizing acetylacetonates of Fe(II) and Li.

By itself, N2O4 is sufficiently reactive to effect nitration of activated aromatic substrates such as phenol and anisole (7) as well as of polycyclic arenes such as pyrene and perylene (2). On the other hand, simple arenes such as benzene and toluene are not nitrated by liquid N2O4 to any significant extent. However, N2O4 can be activated by several means to effect the nitration of benzene, toluene, and other unactivated arenes. Suzuki and coworkers have reported on the use of ozone and N2O4 to nitrate benzene and toluene (3). We have previously shown that simultaneous passage of NO and O2 through a solution of benzene in liquid N2O4 leads to nitration of benzene giving mono-, di-, and even trinitrobenzenes (4) In this paper we describe the results of nitrations promoted by metal acetylacetonates in liquid N2O4.

0097-6156/96/0623-0031$15.00/0 © 1996 American Chemical Society

In Nitration; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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32

Our study with metal acetylacetonates was a direct offshoot of our investigations with the NO/O2/N2O4 system and therefore it would be useful to review some of those results first. Figure 1 is a schematic of the apparatus used in that study. The reactor is designed to admit controlled flows of NO and O2, as well as to provide for sampling during the course of a run. Typically 20-40 mL of the nitrogen tetroxide is introduced into the cooled 100-mL reactor. The tetroxide recovered from the storage tank is generally dark green due to the presence of lower oxides of nitrogen. It is stirred under an atmosphere of O2 until the liquid turns a straw-yellow color. Figure 2 illustrates what happens when benzene (or toluene) is stirred in liquid N2O4 at 0°C. In a control run, when only N2 was bubbled through the solution, no conversion to nitrobenzene was observed over a period of several hours. Passage of O2 through the solution results in a small degree of conversion. However, when NO and O2 were simultaneously passed, substantial quantities of nitrobenzene (or nitrotoluene) along with dinitrobenzenes (or dinitrotoluenes) were formed. Curiously enough, nitrobenzene itself does not form dinitrobenzene in this system. Thus, the dinitrobenzene forms either directly from benzene or from an intermediate on the path to nitrobenzene. Moreover, the isomer ratio of the dinitrobenzenes was very different from that observed in conventional nitration systems. The extent of O- and P-dinitration is much larger in the NO/O2/N2O4 system: the o:m:p ratio was 13:56:31 in marked contrast to 6:92:2 in mixed acids. A speculative mechanism for this reaction is depicted in the scheme below. The initial reaction of N O and O2 produces an unsymmetrical NO3 species, which in the presence of excess NO2 radicals leads to the symmetrical NO3. NO3 is recognized to be a very strong one-electron oxidant and could react with the arene to give the radical cation, which in turn could react with NO2 to give the nitrocyclohexadienyl cation, the Wheland intermediate. Alternatively, as shown in the scheme, N2O4 could add to the radical cation and ultimately lead to polynitrated products. Results and Discussion Prompted by the possibility that one-electron oxidation of arenes followed by their reaction with NO2 might lead to the observed nitrations, we examined the reaction of benzene and toluene in liquid N2O4 with various transition metal oxidants. Because most simple salts such as acetates and nitrates are very ionic, they could not be dissolved in liquid N2O4, which has a very low dielectric constant of 2.42. However, inner complexes, in which the ionic and coordinating valencies are simultaneously satisfied, are soluble in this medium, and we were able to conduct a study with a range of acetylacetonate complexes. Fast and Slow Modes of Nitration. We studied the effect of acetylacetonates of Co(III) and Fe(III) on the nitration of benzene and toluene in liquid N2O4. These runs were conducted by dissolving about 10 mmol of the acetylacetonate



Figure 1. Apparatus for nitration of aromatic hydrocarbons in liquid N2O4 with NO/O2.


34


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Time (min) Figure 2. Nitration of benzene and toluene in liquid N2O4 at 0°C. Fifty mmol aromatic substrate was used in each run. • benzene, N2 (0.22 mmol/min) benzene, O2 (0.32 mmol/min) benzene, 02 (0.30 mmol/min), NO (0.17 mmol/min) • benzene, O2 (0.49 mmol/min), NO (0.32 mmol/min) toluene, O2 (0.62 mmol/min), NO (0.40 mmol/min) 0

α

A


4. MALHOTRA & ROSS

Aromatic Nitration in Liquid Nitrogen Tetroxide

NO + 0 N0

3

2

NO3

—

NO3- + ΑτΗ+·

+ ArH


Reaction ofarene radical cation with NO2 and N2O4 then provides an array of products: Η ArH+" + N 0

N0

é

2

2

ΑΓΝ02

• N 0 2

ΑΓΗ+· + N 0 2

4

NOz

—

4

>

N0 N02

N0

2

2

1-

NOo

loss of HONO's yields

NO, Ar Η

\ HNO3 + A r N 0

2

Ar(N0 ) 's 2

2

Scheme 1. One-electron oxidation mechanism for nitrations in liquid N 0 2


4

35

NITRATION


36

(acac) in 20 mL of N2O4.* Following the addition of about 50 mmol of the arene, production of nitroarenes was observed to proceed over several hours. Subsequently we observed that if the arene is first dissolved in the liquid N2O4, addition of the metal acac results in a rapid nitration, and the reaction is essentially complete in about 15 min. We refer to the former and the latter procedures as the "slow" and "fast" modes. In the "slow" mode, we did not observe any dinitration. In the "fast" mode, some dinitration (-5%) was observed, which was completely suppressed in the presence of a cosolvent such as nitromethane. A significant result was the absence of any phenolic byproduct (< 0.4%, the detection limit). The results of metal acac promoted nitrations are summarized in Table 1. The progress of reaction in the "fast" mode for several cases is shown in Figure 3. Rapid nitration ensued upon addition of the metal acac to the solution of benzene in liquid N2O4 at 0°C and then ceased in about 15 min. although a large excess of benzene was still present. In concert with our hypothesis, nitration was readily effected by the oxidizing acetylacetonates of Fe(III), Ce(IV), Co(III), Μη (ΠΙ) and Cu(II). However, nitrations were also effected by the nonoxidizing acetylacetonates of Fe(II) and Li(I), and this result raises serious questions about our premise that the nitration proceeds through oxidation of the arene by the metal acac. The reaction appears to be stoichiometric and not catalytic, although the yield of nitration per mole of acac consumed varies with the metal. The stoichiometry is around 1 for Cu (II), Ce (IV), Li(I), and Fe(II); between 1 and 2 for Co (III) and Fe(III); and less than 1 for Μη(ΙΠ).

Table 1. Nitration of Benzene with Various Metal Acetylacetonates in Liquid Nitrogen Tetroxide at 0°C DNB Isomer Distribution * Nitrobenzene (o:m:v) %DNB M(acac)/mmole (mmole) — 2.1 Mn(acac)3/2.0 1.3 1.2 >1.4 0.9:85.3:13.8 Cu(acac)23.8 8.5:80.4:11.1 1.4 Ce(acac)4/1.54 1.6 465 min 25°C. • Toluene, 60.0 mmol; Fe-adduct, 14.9 mmol; N2O4,5.0 mL; nitromethane, 20.0 mL; Temp.: 0-120 min 0°C, > 120 min 25°C. ^ Toluene, 60.0 mmol; Fe(acac)3,14.9 mmol; N2O4 30.0 mL; nitromethane, 20.0 mL; Temp., 25°C. 0

Nitrations Promoted by β-Dicarbonyls. We examined the ability of a variety of β-dicarbonyl compounds to effect nitrations in liquid N2O4. Included in this study were compounds such as 2,4-pentanedione, ethyl acetoacetate, and dimedone. We also tested compounds like acetic anhydride and succinic anhydride, which are not β-dicarbonyls in the common parlance, but which do have carbonyls β to each other. As controls, we studied the nitrating ability of comparable amounts of nitric acid and acetic acid in liquid N2O4. The yield data are summarized in Table 2, and the kinetic data for some of the runs are shown in Figure 5. Of the various compounds investigated, three stand out as being extremely potent: 2,4-pentanedione, ethyl acetoacetate, and acetic anhydride. These three compounds promote nitrations at rates at least two orders of magnitude greater than by 100% HNO3 in this medium. Curiously enough, dimedone and succinic anhydride, which are structurally related to 2,4pentanedione and acetic anhydride respectively, were not very effective. The


39

40

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Table 2. Nitration of toluene in nitrogen tetroxide at 0°C promoted by various reagents Reagent name

Amount (mmol)

Structure

Time (min)

ΜΝΪ Yield (mmol)

MNT Isomer ratio

Background

20

101

0.54

53:4:43

Background

5

109

0.06

62:5:34

2


N2O4 (mLV

Acetic anhydride *

0

0

7.34

10

66

7.3

59:2:39

2,4-Pentanedione

0

0

14.3

5

104

7.0

61:4:35

12.0

5

102

6.1

61:3:36

15.0

5

75

0.06

62:3:35

14.4

5

149

0.06

63:3:34

14.3

5

108

0.24

69:7:24

14.3

5

108

0.24

60:5:35

14.3

5

145

0.18

62:5:33

13.9

5

89

0.24

18:8:74

1

Ethyl acetoacetate

0

0

—^

^OC H 2

5

100% Nitric acid 1 Acetic acid

OH

c

,0

CH C; 3

OH d

Succinic anhydride Diethyl oxalate

0^°ν^;0 H CA

,0(^Η

5

0

0

Dimedone

Tetranitromethane

5

X C(N02)

4

a- Nitromethane Nitromethane yield -10 Conducted at 25°C Did not dissolve completely

b c

d


4. MALHOTRA & ROSS


τ

Aromatic Nitration in Liquid Nitrogen Tetroxide41 1

1

1

1

1

1

1

Γ

Time (min) Figure 5. Nitration of toluene in N2O4 with various β-dicarbonyl compounds. • Acetic anhydride, 25°C ± 2,4-Pentadione, 0°C • Ethyl acetoacetate, 0°C Dimedone, 0°C Nitric acid, 0°C D

0

difference may be due to the fact that each of the effective agents can adopt a conformation in which the carbonyl groups are parallel to each other, while dimedone and succinic anhydride are cyclic and cannot adopt such an orientation. Also worth noting is the ineffectiveness of acetic acid, particularly in view of the extreme potency of acetic anhydride. When added to liquid N2O4, 2,4-pentanedione, ethyl acetoacetate, and dimedone underwent a rapid reaction and they could not be recovered from the solution. We expected the formation of enol nitrates, or acyl nitrates, but were unable to characterize the decomposition products by G C or GC/MS techniques.


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42


Summary

II

We have shown that a variety of metal acetoacetonates promote nitrations of arenes such as benzene and toluene in liquid N2O4. The nitration appears to be stoichiometric, albeit not 1:1. The effectiveness of metal acac's does not depend on the oxidation state of the metal, and the nitration is not likely to be effected via one-electron oxidation of the arene. The rates of nitration, and to some extent the product distribution, depend upon the order of addition of the reagent. We also showed that the nitrato complexes of several metals are effective in promoting nitrations in liquid N2O4, and that these agents act as catalysts for the nitration reaction. The mechanism by which metal acac's promote nitration remains a mystery, and our efforts at uncovering it were rewarded only with even more surprising findings that simple organics such as 2,4-pentanedione and acetic anhydride bring about nitrations in liquid N2O4 at rates many orders of magnitude faster than that by nitric acid! Acknowledgment We gratefully acknowledge financial support by A i r Products and Chemicals Inc. and able assistance in the laboratory by Mr. Robert Johnson. Literature Cited 1. Underwood, G. R.; Silverman, R. S.; Vanderwalde, A. J. Chem. Soc., Perkin 1973, 1177. 2. (a) Radner, F. Acta. Chem. Scand. Series B, 1983, 37, 65. (b) Eberson, L.; Radner, F. Acta. Chem. Scand. Series B, 1985, 39, 343. 3. Suzuki, H.; Murashima, T.; Shimizu, K., and Tsukumoto, K. J. C. S. Chem. Comm. 1991, 1049. 4. Blucher, W. and Ross, D. S. 174th National ACS, Chicago, Ill., "Studies in Aromatic and Amine Nitration," Final Report, U.S. Army Research Office, Contract No. DAAG29-76-C0040, 1979. 5. (a) Addison, C. C. Chem. Rev. 1980, 80, 21. (b) Addison, C. C., Boorman, P. M., and Logan, N. J. Chem. Soc. 1965, 4978. RECEIVED October 24, 1995


Aromatic Nitration in Liquid Nitrogen Tetroxide Promoted by Metal

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