Beilstein Test

The Beilstein demo 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 lớn "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 lớn cool to lớn room temperature, then dip it into a thử nghiệm 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 khổng lồ the copper wire by first wetting the wire with distilled water then touching it khổng lồ the solid.

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

The Benedict"s test can verify the presence of reducing carbohydrates: compounds that have hemiacetals in their structures & are therefore in equilibrium with the không tính tiền carbonyl size (aldehyde or (alpha)-hydroxyketone). The carbonyl forms are oxidized by the (ceCu^2+) in the Benedict"s reagent (which complexes with citrate ions lớn prevent the precipitation of (ceCu(OH)_2) & (ceCuCO_3)). An insoluble (ceCu_2O) is the inorganic product of this reaction, which usually has a red-brown màu sắc (Figure 6.47). Carbohydrates with only acetal linkages are non-reducing sugars và give a negative result with this test.

Procedure: Dissolve (10)-(30 : extmg) of solid or 3 drops liquid sample in a minimal amount of water (left( 0.5 : extmL ight)) in a small chạy thử tube ((13) x (100 : extmm)). địa chỉ (2 : extmL) 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 màu xanh (Figure 6.48c+d).

Conjugated aldehydes are unreactive in the Benedict"s test, and the author found many non-conjugated aldehydes lớn also be unreactive. Formation of colloids seem to lớn 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 lớn initiate at the surface (Figure 6.50), & the tác giả found aldehydes that formed an insoluble layer on the surface to lớn be unreactive.

The Benedict"s kiểm tra is related lớn the Fehling"s test, which uses different ligands on the copper oxidizing species. The Fehling"s reagent uses a (ceCu^2+) ion complexed with two tartrate ions.


Bicarbonate Test

Carboxylic acids and sulfonic acids can react with sodium bicarbonate (left( ceNaHCO_3 ight)) to produce carbon dioxide and water (Figure 6.51). Other mainstream functional groups (most phenols & alcohols) are not acidic enough to produce a gas with bicarbonate.

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

The màu sắc 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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