Picrates:
Because tertiary amines are incapable of forming sustituted amides or ureas, the usual derivative is some type of salt Picrates, derivatives of picric acid (2,4,6-trinitophenol), are relatively easy to prepare.
A solution of 0.5 g of the amine in 10 mL of 95% ethanol is prepared. If all of the amine does not dissolve, filter the mixture.
The amine solution is added to 10 mL of a saturated solution of picric acid in 95% ethanol, which is contained in a 50-mL Erlenmeyer flask.
A boiling chip is added and the solution is heated to boiling on a steam bath.
Remove the flask from the heat source and allow the solution to cool slowly. The yellow picrate should precipitate.
Filter the crude product and crystallize it from 95% ethanol.
Picric acid combines with amines to yield molecular compounds (picrates), which usually possess characteristic melting points. Most picrates have the composition 1 mol amine : 1 mol picric acid. The picrates of the amines, particularly of the more basic ones, are generally more stable than the molecular complexes formed between picric acid and the hydrocarbons.
If the amine is soluble in water, mix it with a slight excess (about 25 per cent.) of a saturated solution of picric acid in water (the solubility in cold water is about 1 per cent.). If the amine is insoluble in water, dissolve it by the addition of 2-3 drops of dilute hydrochloric acid (1:1) for each 2-3 ml. of water, then add a slight excess of the reagent. If a heavy precipitate does not form immediately after the addition of the picric acid solution, allow the mixture to stand for some time and then shake vigorously. Filter off the precipitated picrate and recrystallize it from boiling water, alcohol or dilute alcohol, boiling 10 per cent. acetic acid, chloroform or, best, benzeneThe method described by Vogel may be a better procedure for heat sensitive amines, since the only heating step is after formation of the picrate and even then, by use of MeOH, temperatures could remain relatively low. Keep in mind that while he describes benzene as the a good choice during recrystalization, it has since been noted to be carcinogenic and is therefore probably not the best choice. The formation of a picrate does not in any way identify a compound as a tertiary amine, since they also form with hydrocarbons, aromatic hydrocarbons, some halogen derivatives and aromatic ethers. Picrates are simply a way of creating a derivative with a characteristic melting point, which is often used in melting point determinations by combination with a known compound (mixed melting points, M.m.p.), watching for depression of melting points.
Because tertiary amines are incapable of forming sustituted amides or ureas, the usual derivative is some type of salt Picrates, derivatives of picric acid (2,4,6-trinitophenol), are relatively easy to prepare.
A solution of 0.5 g of the amine in 10 mL of 95% ethanol is prepared. If all of the amine does not dissolve, filter the mixture.
The amine solution is added to 10 mL of a saturated solution of picric acid in 95% ethanol, which is contained in a 50-mL Erlenmeyer flask.
A boiling chip is added and the solution is heated to boiling on a steam bath.
Remove the flask from the heat source and allow the solution to cool slowly. The yellow picrate should precipitate.
Filter the crude product and crystallize it from 95% ethanol.
Picric acid combines with amines to yield molecular compounds (picrates), which usually possess characteristic melting points. Most picrates have the composition 1 mol amine : 1 mol picric acid. The picrates of the amines, particularly of the more basic ones, are generally more stable than the molecular complexes formed between picric acid and the hydrocarbons.
If the amine is soluble in water, mix it with a slight excess (about 25 per cent.) of a saturated solution of picric acid in water (the solubility in cold water is about 1 per cent.). If the amine is insoluble in water, dissolve it by the addition of 2-3 drops of dilute hydrochloric acid (1:1) for each 2-3 ml. of water, then add a slight excess of the reagent. If a heavy precipitate does not form immediately after the addition of the picric acid solution, allow the mixture to stand for some time and then shake vigorously. Filter off the precipitated picrate and recrystallize it from boiling water, alcohol or dilute alcohol, boiling 10 per cent. acetic acid, chloroform or, best, benzeneThe method described by Vogel may be a better procedure for heat sensitive amines, since the only heating step is after formation of the picrate and even then, by use of MeOH, temperatures could remain relatively low. Keep in mind that while he describes benzene as the a good choice during recrystalization, it has since been noted to be carcinogenic and is therefore probably not the best choice. The formation of a picrate does not in any way identify a compound as a tertiary amine, since they also form with hydrocarbons, aromatic hydrocarbons, some halogen derivatives and aromatic ethers. Picrates are simply a way of creating a derivative with a characteristic melting point, which is often used in melting point determinations by combination with a known compound (mixed melting points, M.m.p.), watching for depression of melting points.
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