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The Knorr Synthesis Of Pyrroles Essay

1011 words - 4 pages

The Knorr Synthesis of Pyrroles

A mixture of ethyl acetoacetate 1 (7.52 g, 7.5 mL) in glacial acetic acid (18 mL) was prepared and cooled to 5oC while stirring. Sodium nitrate solution (2.1 g in 5 mL deionised water) was added at such a rate that the temperature of reaction did not exceed 7oC. Once this addition was complete the mixture was stirred with cooling for 10 minutes before being allowed to warm to room temperature.
Zinc dust (4.0 g) was added to the reaction mixture in small portions so that the mixture refluxed gently, the reaction mixture was then heated to reflux for one hour.
The mixture was then allows to cool and the excess zinc to settle. While still hot the solution was decanted into stirred cold water (200 mL) precipitating the crude product. The zinc residue was washed with warm glacial acetic acid (2 x 5 mL) and this was added to the aqueous solution.
This suspension was allowed to stand for 10 minutes, before being filtered, washed (water 2 x 10 mL) and air dried. The crude product was recrystallised from ethanol to yield a yellow solid 3. 26% yield (1.83 g); Elemental analysis for C12H17NO4: C: 54.918%, H: 6.704%, N: 5.122% (Theoretical: C: 60.24%, H: 7.16%, N: 5.85%, Loss:26.75%); Mr 239.2 g mol-1; ν/cm-1: 1259.48 (N-C), 1667.41 (C=C), 1688.41 (C=O), 2976.58 (C-H), 3265.02 (N-H); 1H-NMR (400 MHz, CDCl3), δ/ppm: 1.24 and 1.38 (dt, 3H, C-CH3, C12 and C15), 2.51 and 2.56 (ds, 6H, 2 x =C-CH3, C6 and C7), 4.31 (m, 4H, 2 x -CH2-, C11 and C14), 5.3 (s, 1H, N5-H); 13C-{1H}-NMR (400 MHz, CDCl3), δ/cm-1:11.98 (C6 and C7), 14.43 (C12 and C15), 59.50 and 60.29 (C11 and C14), 133.5 (C3), 117.5 (C1), 131.3 (C2), 138.6 (C4), 165.47 (C8 and C9) .

Results and Discussion
Product 3 was formed in a low yield of 27% this is probably due to the loss during purification of the product but may also be due to the short amount of time the reaction was allowed to proceed for. To improve on this the reaction should be left longer before quenching and more care should be taken when transferring and during recrystallisation of the crude product.
The overall scheme (scheme 1) and mechanism (scheme 2) of the reaction are shown below:

Scheme 1 The reaction scheme taking place in the experiment outlined above – synthesis of 3.

Scheme 2 The reaction mechanism for the addition of 1 and 2 to give product 3.
The 1H-NMR spectrum of product 3 (fig. 2) shows obvious solvent impurity peaks at 3.73 ppm due to enthanol1 and 2.17 due to acetone1. However, if these are ignored the spectrum shows characteristic peaks of the product; the first of which are at 1.24 (3H) and 1.38 (3H) ppm and are due to the two methyl groups on the ester groups (C12 and C15, fig. 1)2, these partially cover the other small peaks due to ethanol at 1.25 and 1.32 ppm1. The next peaks due to the product are at large singlets at 2.51 (3H) and 2.56 (3H) ppm which correspond to the two...

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