Apr 19, 2012 - DNT) can be used for the production of toluene diisocyanate (TDI) and ... many attempts to improve the yield of 2,4-DNT and the ratio of 2,4-:2 ...
Highly Regioselective Process for the Production of Important Industrial Intermediates in High Yields Mohammad Hayal Alotaibi*,a, Keith Smithb and Gamal El-Hitic a
Petrochemical Research Institute, King Abdulaziz City for Science and Technology, P.O. Box 6086, Riyadh 11442, Kingdom of Saudi Arabia. KACST b School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK c Department of Optometry, College of Applied Medical Sciences, King Saud University, P.O. Box 10219, Riyadh 11433, Saudi Arabia Various nitration systems comprising nitric acid, acid anhydride and zeolite Hβ in the absence of solvent are described. Direct double nitration of toluene over a nitric acid, propanoic anhydride and zeolite H system has been developed to give 2,4-dinitrotoluene in 98% yield, with a 2,4:2,6-dinitrotoluene ratio of over 120. This system also nitrates activated mono-substituted benzenes (anisole and phenetole) and moderately activated mono-substituted benzenes (ethylbenzene and propylbenzene) to give mainly 2,4-dinito derivatives. The zeolite can be recovered, regenerated and reused to give almost the same yield as that given when fresh zeolite is used.
Introduction
Dinitration of other mono-substituted benzenes
Polyurethane is widely used as an intermediate for production of varnish, wheels, furniture and has many other applications. 2,4-Dinitrotoluene (2,4DNT) can be used for the production of toluene diisocyanate (TDI) and toluenediamine that can be used to produce polyurethane. There have been many attempts to improve the yield of 2,4-DNT and the ratio of 2,4-:2,6-DNT isomers (Scheme 1: R = Me). Dinitration of toluene with mixed acids (HNO3 and H2SO4) produces 2,4- and 2,6-dinitritoluenes in a ratio of only 4:1. The yield and selectivity have been improved by the use of solid catalysts. For example, the 2,4-:2,6-dinitritoluene ratio can be improved to 9:1 (in 85% yield) when dinitration is carried out over claycop, using nitric acid and acetic anhydride, but the method requires a large excess of nitric acid and the use of carbon tetrachloride as a solvent. Smith et al. successfully produced 2,4DNT in 96% yield with a 2,4-:2,6-DNT ratio of 70:1 by simple direct dinitration of toluene over a HNO3/acetic anhydride/trifluoroacetic anhydride/Hβ system in two steps. However, trifluoroacetic anhydride (TFAA) is toxic, volatile and expensive. Therefore, there is a need for a selective synthesis involving cheaper and less toxic materials.1-3
Several mono-substituted benzenes, including ones that are activated, moderately activated and moderately deactivated were nitrated under the conditions optimized for toluene. High yields were obtained in the case of activated and moderately activated substrates but the reaction was slow in the case of halobenzenes.
Table: Nitration of mono-substituted benzenes Entry
Results and Discussion Dinitration of toluene using different types of acidic anhydrides A range of acid anhydrides was used to produce the corresponding acyl nitrate in situ. Chloroacetic anhydride and propionic anhydride were found to be the most effective anhydrides in double nitration of toluene.4,5
(RCO)2O
+
HNO3
RC(O)ONO2
+
R
Yield (%)
Mass balance
2-N
3-N
4-N
2,4-DN
2,6-DN
(%)
1
OMe
trace
trace
trace
97
1
98
2
OEt
trace
trace
trace
96
1
97
0.4
0.8
98
0.8
100
17
73
trace
97
3
Me
4
Et
7
trace
5
Pr
3
trace
7
89
trace
99
6
F
7
trace
85
5
97
7
Cl
2
92
2
2
98
8
Br
2
92
2
2
98
RCO2H
Conclusion Dinitration of toluene using propionic anhydride
Nitration of toluene using nitric acid/propionic anhydride/zeolite Hβ gave 2,4-DNT in 98% yield with a 2,4-:2,6- ratio over 100:1. This is easily the most selective quantitative double nitration of toluene ever recorded. Propionic anhydride is more attractive than trifluoroacetic anhydride in terms of selectivity, volatility, toxicity, and cost. Since propionic acid can be recovered, and the zeolite can be reused several times, the method could be attractive for larger scale syntheses. It also works well for dinitration of activated substrates.
The optimized conditions produced a 98% yield of 2,4-DNT with a 2,4-:2,6DNT ratio of 100:1.
Efficiency of recycled zeolite in dinitration of toluene The results showed that under the standard conditions there was only a slight decrease (to 94%) in the yield of 2,4-DNT even after using the same zeolite seven times, and the selectivity was virtually the same.
R
R
R
R
R
NO2 HNO 3/Ac2O
R NO2
+
+
Hß
NO2
O 2N +
+
NO2 NO2 2-N
3-N
4-N
NO 2 2,4-DN
2,6-DN
R = OMe, OEt, Me, Et, Pr, F, Cl, Br
Acknowledgement We thank the Saudi Ministry of Interior, King Abdulaziz City for Science and Technology and Cardiff University for financial support.
References 1. G.A. Olah, R. Malhotra, S.C. Narang, Nitration: Methods and Mechanisms, VCH, New York, 1989. 2. R. Taylor, Electrophilic Aromatic Substitution, Wiley, Chichester, 1990. 3. K. Smith, T. Gibbins, R.W. Millar, R.P. Claridge, J. Chem. Soc. Perkin Trans. 1 (2000) 2753. 4.K. Smith, G. El-Hiti and M. Alotaibi, Methods for the nitration of aromatic compounds, PCT/GB2011/051985; 13th October 2011, PCT Int. Appl., 2012, WO2012/049513 A1, published April 19 2012 5. K. Smith, M. H. Alotaibi and G. A. El-Hiti, Highly regioselectivedinitration of toluene over zeolite Hβ, J. Catal., 297 (2013) 244–247.
