Beilstein Test

The Beilstein test confirms the presence of a halogen in solution, although it does not distinguish between chlorine, bromine, or iodine. A copper wire is dipped into the halogen-containing solution and thrust into a flame. The copper oxide on the wire reacts with the organic halide to produce a copper-halide compound that gives a blue-green color to the flame.

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Procedure: In the fume hood, clean a looped copper wire by thrusting it into the tip of the blue cone of a Bunsen burner flame until it glows (Figure 6.46a). Be sure to "burn off" any residual liquid on the wire (make sure any green flames from previous tests are gone before you begin).

Allow the copper to cool to room temperature, then dip it into a test tube containing 5-10 drops of your sample, coating it as much as possible (Figure 6.46b). If the sample is a solid, adhere some of the solid to the copper wire by first wetting the wire with distilled water then touching it to the solid.

Immediately plunge the wire with sample into the blue cone of the flame. A positive result is a green flame, although it might be short-lived and faint (it may be easier to see if the fume hood light is turned off). A negative result is the absence of this green color (Figure 6.46c+d).

The Benedict"s test can verify the presence of reducing carbohydrates: compounds that have hemiacetals in their structures and are therefore in equilibrium with the free carbonyl form (aldehyde or \(\alpha\)-hydroxyketone). The carbonyl forms are oxidized by the \(\ce{Cu^{2+}}\) in the Benedict"s reagent (which complexes with citrate ions to prevent the precipitation of \(\ce{Cu(OH)_2}\) and \(\ce{CuCO_3}\)). An insoluble \(\ce{Cu_2O}\) is the inorganic product of this reaction, which usually has a red-brown color (Figure 6.47). Carbohydrates with only acetal linkages are non-reducing sugars and give a negative result with this test.

Procedure: Dissolve \(10\)-\(30 \: \text{mg}\) of solid or 3 drops liquid sample in a minimal amount of water \(\left( 0.5 \: \text{mL} \right)\) in a small test tube (\(13\) x \(100 \: \text{mm}\)). Add \(2 \: \text{mL}\) of Benedict"s reagent.\(^9\) Warm the blue solution in a boiling water bath for 2 minutes (Figure 6.48a). A positive result is the formation of a reddish-brown solution or precipitate after some time, while a negative result is retention of the blue color (Figure 6.48c+d).

Conjugated aldehydes are unreactive in the Benedict"s test, and the author found many non-conjugated aldehydes to also be unreactive. Formation of colloids seem to prevent the formation of the red precipitate (Figure 6.49 shows the appearance of propionaldehyde in the hot water bath, forming a cloudy colloid).

The reaction may only work for compounds that are water soluble (like carbohydrates), as the reaction seems to initiate at the surface (Figure 6.50), and the author found aldehydes that formed an insoluble layer on the surface to be unreactive.

The Benedict"s test is related to the Fehling"s test, which uses different ligands on the copper oxidizing species. The Fehling"s reagent uses a \(\ce{Cu^{2+}}\) ion complexed with two tartrate ions.


Bicarbonate Test

Carboxylic acids and sulfonic acids can react with sodium bicarbonate \(\left( \ce{NaHCO_3} \right)\) to produce carbon dioxide and water (Figure 6.51). Other mainstream functional groups (most phenols and alcohols) are not acidic enough to produce a gas with bicarbonate.

A solution of 2,4-dinitrophenylhydrazine (2,4-DNPH) in ethanol is a test for aldehydes or ketones (Figure 6.59). Most aldehydes or ketones will react with the orange reagent to give a red, orange, or yellow precipitate. Esters and other carbonyl compounds are generally not reactive enough to give a positive result for this test.

The color of the precipitate may give evidence for the amount of conjugation present in the original carbonyl: an orange precipitate forms for non-conjugated carbonyls (Figure 6.60c shows the result for 2-butanone), and a red precipitate forms for conjugated carbonyls (Figure 6.60d shows the result for cinnamaldehyde).

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