A Novel Synthesis of Pendimethalin
3,4-Dimethylbromobenzene (8)
251
In a 250 ml four-necked flask with a mechanical stirrer, an addition funnel, a condenser
and a thermometer was placed o-xylene (1) (106 g, 114 ml, 1.0 mol), clean iron filings
(1.2 g) and iodine (0.1 g). The mixture was stirred and cooled in an ice-salt mixture.
Bromine (160 g, 1.0 mol) was added dropwise at -5-0◦ over a period of 3 h. After all
the bromine had been added, the reaction mixture was allowed to stand overnight with
stirring. It was poured into water and washed successively with a 100 ml portion of water,
a 100 ml portion of 20% sodium hydroxide solution and a 100 ml portion of water. The
organic layer was separated from the water and dried over magnesium sulfate. The crude
product was purified by fractional distillation under vacuum to give 167.4 g (91% yield)
of 3,4-dimethylbromobenzene (8) as a colorless liquid, bp. 104–105◦C/76 mm Hg, lit.6
214–215◦C/760 mm Hg. MS (m/e): 186, 184, 105, 89, 77.
3,4-Dimethylanisole (9)
In a polytetrafluoroethylene-lined autoclave reactor with a stirrer was charged with com-
pound 8 (184 g, 1.0 mol), sodium methoxide (59.4 g 1.1 mol), methanol (250 ml), and
cuprous bromide (1.8 g) as a catalyst. The mixture was stirred and then heated to 125–130◦C.
After 4 h, the reaction mixture was cooled to room temperature. Then the mixture was fil-
tered by gravity to remove solids. The filtrate was washed with 100 ml of water and the
organic layer was separated and dried over magnesium sulfate to give the product [192.0 g,
96% yield, 98% purity (GC)], as a colorless liquid, bp. 198–201◦C, lit.13 202◦. MS (m/e):
136, 121, 105, 91, 77.
3,4-Dimethyl-2,6-dinitroanisole (10)
To a solution of compound 9 (136 g, 1.0 mol) in 1,2-dichloroethane (200 ml) at 20–25◦C,
was added dropwise 95% fuming nitric acid (73 g, 1.1 mol) at the same temperature over
2 h. After stirring for 3 h at the same temperature, the mixture was allowed to separate
into two phases. The organic phase contained approximately 1,2-dichloroethane (200 ml)
and the mono-nitro derivative of compound 9 (172 g). Then it was heated to 55–60◦C and
95% fuming nitric acid (132.6 g, 2.0 mol) in concentrated sulfuric acid (265 g) was added
dropwise at the same temperature over 2 h. After stirring for an additional 3 h at the same
temperature, the mixture was allowed to separate into two phases. The organic phase was
washed successively with water, a 20% sodium hydroxide solution and water. The organic
layer was separated from the water and dried over magnesium sulfate. The solvent was
removed under reduced pressure. The solid residue was crystallized from ethyl alcohol to
give a pale yellow solid [214.7g, 90% yield, 95% purity (GC)], mp. 67–69◦C, lit.3 67–68◦C.
MS (m/e): 226, 121, 105, 91, 77.
N-(1-Ethylpropyl)-3,4-dimethyl-2,6-dinitroaniline (6)
A mixture of compound 10 (226 g, 1.0 mol), 1-ethylpropylamine (130.5 g, 1.5 mol) and
calcium chloride (5.1 g) as a catalyst was stirred at 50◦C. The progress of the reaction was
monitored with GC/MS. After completion of the reaction, excess of 1-ethylpropylamine