Introduction
2-Nitropropene is a chemical that can be used in the synthesis of Phenyl-2-Propanone (P2P) through a Friedel-Crafts type alkylation of benzene. The reaction also works on other aromatic compounds, such as 1,2-methylenedioxybenzene.
2-Nitropropene is most conveniently produced by first doing a Knoevenagel condensation between nitroethane and formaldehyde, to form 2-nitro-1-propanol (some 2-nitro-2-methyl-1,3-propanediol is also invariably formed as a by-product, as a result of another formaldehyde molecule condensing with the formed 2-nitro-1-propanol). A russian article6 claims that the optimal conditions for 2-nitro-1-propanol preparation is to use EtNO2 and paraformaldehyde with 10M KOH as catalyst, use of elevated temperature and reaction time 2-3 min, but I have no details.
The 2-nitro-1-propanol must then be dehydrated to form the desired 2-Nitropropene, and this can be done either by heating it with a dehydrating agent like phtalic anhydride, or by a 2-step procedure, where the alcohol group is first esterifyed with acetic anhydride, and then by heating the resulting acetate with a base to form 2-nitropropene. The reason for the esterification is that acetate is a much better leaving group than hydroxide in the base-catalyzed elimination reaction. The yields are similar with either reaction.
Experimental
2-Nitropropanol1
75.1g Nitroethane, 0.3g calcium hydroxide and 80g 40% formaldehyde solution was dissolved in 75ml ethanol with stirring and was allowed to stand for 48h at room temperature. Distillation at 100-105°C/13 mmHg (85-86°C/6 mmHg, 99°C/10 mmHg) gave 48g 2-nitropropanol (46%) and 14.3g of 2-nitro-2-methyl- 1,3-propanediol, the latter remained as a crystalline residue in the distillation flask after distillation of the 2-nitropropanol.
2-Nitropropyl acetate2
105 grams of 2-nitropropanol (1 mol) was placed in a 250ml two-necked RB flask equipped with a reflux condenser and an addition funnel, and 110g of acetic anhydride (1.078 mol) was added dropwise with good stirring. The solution was then refluxed for 30 minutes, cooled and poured into 200ml cold water, the organic phase separated and washed with 100ml water, and then vacuum distilled (bp ca 100°C/10 mmHg, yield 90%+). The acetate is a clear water-white liquid with a faint slightly mustardlike odor, almost insoluble in H2O.
2-Nitropropene3
Sodium carbonate (0.25 mol, 14.5g) is added to a solution of 2-nitropropyl acetate (0.5 mol, 73.5g) dissolved in 50ml benzene and refluxed in a RB flask for six hours. The flask is cooled and the product decanted from the solids. The residue (consisting of sodium acetate) is extracted with 50 ml benzene, dissolved in 100ml water, and the solution extracted with 3x25ml benzene. The pooled organic extracts are dried over anhydrous MgSO4, and the mixture fractionally distilled to afford 2-nitropropene (bp 57°C at 100 mmHg).
2-Nitropropene from 2-Nitropropanol4
Caution! This procedure should be carried out in a hood since 2-nitropropene is a powerful lachrymator. Nitroolefins have a tendency to undergo "fume-offs" (which can be like explosions) near the end of a distillation, particularly if air is let in on a hot distillation residue (from a vacuum distillation).
A 250 mL, one-necked, round-bottomed flask equipped with a magnetic stirring bar is charged with 96.5 g (0.65 mol) of phthalic anhydride (Note 1) and 52.5g (0.50 mol) of 2-nitro-1-propanol (Note 2). A 10-cm vacuum-insulated Vigreux column, a stillhead fitted with a thermometer, a condenser, a 50-mL, round-bottomed receiving flask, and a water aspirator are installed in due order, and the reaction vessel is placed in an oil bath and evacuated to 110 mm (Note 3). The bath temperature is raised to 150°C and maintained for 30 min while the phthalic anhydride melts to give a homogeneous solution. The receiving flask is immersed in an ice bath, stirring is started, and the bath temperature is raised to 180°C. As the reaction mixture darkens, green-colored 2-nitropropene is gradually distilled off with water, bp 50-65°C (110 mm). The bath temperature is held at 180-185°C until the distillation ceases (ca 1 h). The distillate is transferred into a 50-mL separatory funnel, and the lower layer is separated from water (Note 4) and dried over anhydrous magnesium sulfate. Redistillation under reduced pressure through a 10-cm vacuum-insulated Vigreux column (Note 5) gives 25.0-31.4 g (57-72%) of 2-nitropropene, which is collected in an ice-cooled receiving flask as a transparent green liquid, bp 56-57°C (86 mm), nD20 1.4348 [lit.7 bp 58°C (90 mm), nD19 1.4292, d20 1.0492] (Note 5). The distilling flask is cooled to room temperature before the vacuum is released.
Notes:
- Commercial phthalic anhydride, was used without further purification.
- The checkers purchased 2-nitro-1-propanol (ca. 98% purity) from EGA-Aldrich and used it without further purification. The submitters prepared this reagent from nitroethane and formalin according to the procedure of Feuer5 yield 70-75%, bp 79-80°C (5 mm).
- Lower pressure may cause a loss of the product because of its volatility.
- If the layers do not separate well, a small amount of MgSO4 should be added.
- It is important that the bath temperature be kept as low as possible to avoid fume-off decompositions. In the checked procedure the bath temperature was never allowed to exceed 80°C. Toward the end of the distillation the pressure was reduced to ~60 mmHg to achieve complete distillation. Although pure 2-nitropropene may be stored in a freezer as a low-melting solid for several weeks, it is recommended to prepare it immediately before use since it tends to polymerize and to darken slowly on storage. 2-Nitropropene polymerizes readily in the presence of a trace of alkali.
The above procedure tested by Scooby Doo:
1 mole of 2-Nitro-1-Propanol and 1.35 mole of phthalic anhydride were placed in a distillation app. A vacuum of 80 mmHg was applied (and kept there during the whole reaction)and the flask was heated. Everything melted a 130-140°C, and tiny bubbles became evident at 150-160°C. The reaction began and heating was adjusted to control the rate. It stayed around 162°C and then dropped down to 155°C. More heat was applied and the reaction stopped when it reached 180°C.
The receiving flask contained water and a ketone yellow colored oil. The vac was released with N2 being drawn in. Don't let any air into the flask otherwise the 2-Nitropropene breaks down extremely fast as well a being very volatile, the receiving flask was in an ice bath for the entire reaction. If air does get to it a orange vapour is emitted form it which as the same hurt factor as Bromine. The oil will rapidly turn green and will break down into tar in a few days. The oil however stores very well in the freezer under N2, for months.
In the ref they went and distilled straight away which Batman tried but was pretty silly since the water causes problems. A better approach is to dry with molecular sieves (which worked really well) then distill.
Other 2-Nitropropene references: JACS 98, 4679 (1976), Synthesis 407 (1980)