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6.4D: Individual Tests

  • Page ID
    95742
  • 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.

    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).

    Beilstein Test: Negative result with hexanes is a flame that's not green, positive result with 1-chlorobutane is a green flame
    Figure 6.46: a) Cleaning copper wire in a flame, b) Dipping copper wire into the reagent, c) Negative result (hexanes), d) Positive result (1-chlorobutane).

    Benedict's Test

    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.

    Figure 6.47: Structure of hemiacetals and acetals, along with the reaction of a hemiacetal with the Benedict's reagent.

    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).

    Benedict's test: Negative result is blue solution, positive result is reddish-brown solution
    Figure 6.48: a) Heating the Benedict's solution in a boiling water bath, b) Benedict's test results: left tube is sucrose (negative), right tube is glucose (positive), c) Negative result, d) Positive result.

    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).

    Figure 6.49: Reaction of propionaldehyde in the Benedict's test: a) In the beginning, colloid formation on the surface, b) After time. Notice orange solid on the test tube rim, indicating a reaction is possible when exposed to the atmosphere.

    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.

    Benedict's reagent reacting with glucose: solution transitions from blue to red color
    Figure 6.50: Time-lapse reaction progress of the Benedict's reagent reacting with glucose.

    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.

    Figure 6.51: Reaction of carboxylic and sulfonic acids with bicarbonate ion.

    Procedure: Add \(2 \: \text{mL}\) of \(5\% \: \ce{NaHCO_3} \left( aq \right)\) into a test tube and add 5 drops or \(50 \: \text{mg}\) of your sample. Mix the solution by agitating the test tube. A positive test for carboxylic acids is the formation of bubbles or frothing (Figure 6.52).

    Bicarbonate Test: negative result is no bubbles, positive result is bubbling or frothing
    Figure 6.52: a) Negative result (acetone), b) Positive result (lactic acid), c) Positive result (octanoic acid).

    Bromine Test

    A solution of bromine in \(\ce{CH_2Cl_2}\) is a test for unsaturation (alkenes and alkynes) and in some cases the ability to be oxidized (aldehydes). The bromine solution is orange and upon reaction the solution turns colorless due to the consumption of bromine. Bromine reacts with alkenes and alkynes through addition reactions and with aldehydes through oxidation (Figure 6.53). It gives no reaction with aromatics, making this a good test to distinguish alkenes from aromatics.

    Figure 6.53: Reaction of an alkene, alkyne, and aldehyde with bromine.

    Procedure: Dissolve 4 drops or \(50 \: \text{mg}\) of sample in \(1 \: \text{mL}\) of dichloromethane \(\left( \ce{CH_2Cl_2} \right)\) or 1,2-dimethoxyethane. Add 2 drops of the orange \(5\% \: \ce{Br_2}\) in \(\ce{CH_2Cl_2}\) solution to the test tube and observe. A positive result is the immediate disappearance of the orange color to produce a clear or slightly yellow solution (Figure 6.54). A negative result is the retention of the orange color. An aldehyde may require a small amount of time to decolorize the solution and produce a positive result (approximately 1 min, Figure 6.55) and conjugated aldehydes are unreactive (Figure 6.55).

    Bromine Test: negative result is orange solution, positive result is clear solution
    Figure 6.54: a) Addition of the bromine solution (orange liquid) to a test tube, b) Bromine results: hexanes (left tube), 1-hexene (right tube), c) Negative result, d) Positive result.
    Figure 6.55: Bromine results for isobutyraldehyde (left tube) and benzaldehyde, a conjugated aldehyde (right tube): a) Immediately after addition of the bromine reagent, b) After 1 minute.

    Chromic Acid (Jones) Test

    A solution of \(\ce{CrO_3}\) in \(\ce{H_2SO_4}\) is a test for polar functional groups that can be oxidized, which includes aldehydes, primary alcohols, and secondary alcohols (Figure 6.57). Tertiary alcohols give a negative result with this test (Figure 6.56). The orange \(\ce{Cr^{6+}}\) reagent converts to a blue-green \(\ce{Cr^{3+}}\) species, which often precipitates in acetone.

    Chromic Acid (Jones) Test: Negative result is orange solution, positive result is blue-green solution
    Figure 6.56: Negative (a) and positive (b) results for the chromic acid test.
    Figure 6.57: Reaction of a primary alcohol, secondary alcohol, and aldehyde with the chromic acid reagent.

    Procedure: Place \(1 \: \text{mL}\) of acetone in a small test tube (\(13\) x \(100 \: \text{mm}\)) and add 2 drops or \(20 \: \text{mg}\) of your sample. While wearing gloves, add 2 drops of the orange chromic acid reagent\(^{10}\) (safety note: the reagent is highly toxic!) and mix by agitating. A positive result is a blue-green color or dark precipitate, while a negative result is a yellow-orange solution or precipitate with no dark-colored precipitate (Figure 6.58).

    Water works better than acetone to rinse chromium reagents into the waste beaker, although some time needs to be allowed for dissolution of the \(\ce{Cr^{3+}}\) species.

    Figure 6.58: a) Chromic acid results (left to right): t-butanol (tertiary, negative), 1-propanol (primary, positive), 2-propanol (secondary, positive), benzaldehyde (positive), b) Immediate reaction of benzaldehyde, c) Benzaldehyde reaction after 1 minute.

    2,4-DNPH (Brady's) Test

    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).

    Figure 6.59: Reaction of an aldehyde and ketone with 2,4-DNPH.

    Procedure: Add 3 drops of sample to a small test tube (\(13\) x \(100 \: \text{mm}\)), or dissolve \(10 \: \text{mg}\) of solid sample in a minimal amount of ethanol in the test tube. While wearing gloves, add about \(1 \: \text{mL}\) of the orange 2,4-DNPH reagent\(^{11}\) (safety note: the reagent is highly toxic!) and mix the test tube by agitating.

    A positive result is the immediate formation of a large amount of brightly colored precipitate (red, orange, or yellow). A negative result is the absence of this precipitate and a transparent yellow-orange solution (Figure 6.60).

    2,4-DNPH (Brady's) Test: Negative result is clear yellow solution, positive result is red or orange precipitate
    Figure 6.60: a) 2,4-DNPH results for (left to right) ethyl acetate (negative), 2-butanone (positive), benzaldehyde (positive), cinnamaldehyde (positive), b) Negative result, c) Positive result from a non-conjugated carbonyl, d) Positive result from a conjugated carbonyl.

    Ferric Hydroxamate Test

    The ferric hydroxamate procedure is a probe for the ester functional group. Esters heated with hydroxylamine produce hydroxamic acids, which form intense, colored complexes (often dark maroon) with \(\ce{Fe^{3+}}\). A possible structure of these complexes is shown in Figure 6.61. This test is related to the phenol test, and as in that test, compounds with high enolic character can give a colored complex with \(\ce{Fe^{3+}}\). Therefore, a preliminary test is performed to see if the carbonyl compound being tested produces enough enol to form a colored complex with \(\ce{Fe^{3+}}\), which would lead to a false positive result.

    Figure 6.61: Reaction of ester with hydroxamic acid.

    Procedure: Perform a preliminary test to be sure that this test will not give a false positive. Add the following to a small test tube (\(13\) x \(100 \: \text{mm}\)): \(1 \: \text{mL}\) ethanol, 2 drops or \(20 \: \text{mg}\) of your sample, \(1 \: \text{mL}\) of \(1 \: \text{M} \: \ce{HCl} \left( aq \right)\), and 2 drops of \(5\% \: \ce{FeCl_3} \left( aq \right)\) solution. If the solution is clear or yellow (the color of the \(\ce{FeCl_3}\), Figure 6.62a), this test will work and not produce a false positive (continue on). If a definite color other than yellow appears, this test will not work for your sample, as it forms a colored complex with \(\ce{Fe^{3+}}\) even without hydroxylamine.

    Into a clean medium sized test tube (\(18\) x \(150 \: \text{mm}\)), add \(1 \: \text{mL}\) of \(0.5 \: \text{M}\) aqueous hydroxylamine hydrochloride \(\left( \ce{NH_2OH} \cdot \ce{HCl} \right)\), \(0.5 \: \text{mL}\) of \(6 \: \text{M} \: \ce{NaOH} \left( aq \right)\), and 5 drops or \(50 \: \text{mg}\) of sample. Heat the mixture in a boiling water bath for about 3 minutes (the volume will reduce by about half, Figure 6.62b).

    Quickly cool the solution by immersing it in a tap water bath, then add \(2 \: \text{mL}\) of \(1 \: \text{M} \: \ce{HCl} \left( aq \right)\). If the solution becomes cloudy, add enough ethanol to clarify it. Then add 6-10 drops of a yellow \(5\% \: \ce{FeCl_3} \left( aq \right)\) solution. Vigorously mix the tube.

    A positive result is a deep burgundy, umber, or magenta color (red/brown) while a negative result is any other color (Figure 6.62c+d). Note: use water to rinse out the test tubes,and if a red result won't easily clean up, add a few drops of \(6 \: \text{M} \: \ce{HCl}\).

    Ferric Hydroxamate Test: negative result is dark orange solution, positive result is deep maroon or burgundy solution
    Figure 6.62: a) Preliminary procedure to be sure the test will not give a false positive, b) Heating the solution, c) Negative result (2-pentanone), d) Positive result (ethyl acetate).

    Iodoform Test

    A solution of iodine \(\left( \ce{I_2} \right)\) and iodide \(\left( \ce{I^-} \right)\) in \(\ce{NaOH}\) can be used to test for methyl ketones or secondary alcohols adjacent to a methyl group. This is a very specific test that will give a positive result (formation of a canary yellow precipitate) only for compounds with the structure \(\ce{RCH(OH)CH_3}\) or \(\ce{RC=OCH_3}\) (Figure 6.63). It does not work for all alcohols or ketones, and does not work well for water-insoluble compounds.

    Figure 6.63: Reaction of secondary alcohol or methyl ketone with the iodoform reagent.

    Procedure: Add 10 drops sample to a small test tube (\(13\) x \(100 \: \text{mm}\)) or \(0.10 \: \text{g}\) dissolved in the minimal amount of 1,2-dimethoxyethane followed by \(1 \: \text{mL}\) of \(10\% \: \ce{NaOH} \left( aq \right)\). Next add 10 drops of the dark brown iodoform reagent\(^{12}\) (\(\ce{I_2}/\ce{KI}\) solution) and vigorously mix the test tube by agitating.

    A positive result is a cloudy yellow solution, or a yellow precipitate. A negative result is a clear, yellow, or orange solution with no precipitate (Figure 6.64).

    If the sample is not water soluble, a small organic layer separate from the solution may be seen (it will likely be on top). This layer may become dark yellow or brown from dissolving the iodine. Vigorously mix the tube to encourage a reaction, but if the darkened organic layer remains and no precipitate forms, this is still a negative result (Figure 6.64d).

    Note: a false positive result may occur if the test tube was cleaned with acetone before use, and residual acetone remained in the tube.

    Iodoform Test: Negative result is clear solution, positive result is solution with yellow precipitate
    Figure 6.64: a) Iodoform results (left to right): 1-propanol, 2-propanol, acetone, 1-octanol (b) Negative result, c) Positive result, d) Negative result when the sample is insoluble in the reagent.

    Lucas Test

    The Lucas reagent (concentrated \(\ce{HCl}\) and \(\ce{ZnCl_2}\)) is a test for some alcohols. Alcohols can react through an \(S_\text{N}1\) mechanism to produce alkyl halides that are insoluble in the aqueous solution and appear as a white precipitate or cloudiness. The test cannot be used for water-insoluble alcohols (generally > 5 carbon atoms), as they may produce a cloudiness or second layer regardless if any reaction occurred or not.

    \[2^\text{o} \: \text{or} \: 3^\text{o} \: \ce{ROH} + \ce{HCl}/\ce{ZnCl_2} \rightarrow \ce{RCl} \left( s \right)\]

    As the mechanism is \(S_\text{N}1\), a tertiary alcohol should react immediately, a secondary alcohol react more slowly (perhaps in 5 minutes if at all) and primary alcohols often don't react at all. Benzylic alcohols \(\left( \ce{Ph-C-OH} \right)\), allylic alcohols \(\left( \ce{C=C-C-OH} \right)\) and propargylic alcohols \(\left( \ce{C \equiv C-C-OH} \right)\) often give immediate results just like tertiary alcohols.

    Procedure: Place \(2 \: \text{mL}\) of the Lucas reagent\(^{13}\) (safety note: the reagent is highly acidic and corrosive!) into a small test tube (\(13\) x \(100 \: \text{mm}\)). Add 10 drops of sample, and mix by agitating the test tube.

    A positive result is a white cloudiness within 5 minutes or a new organic layer \(\left( \ce{RCl} \right)\) formation on the top.\(^{14}\) A negative result is the absence of any cloudiness or only one layer (Figure 6.65).

    Lucas Test: Negative result is clear solution, positive result is solution with cloudy white precipitate
    Figure 6.65: a) Lucas test results (left to right): 1-propanol (primary, negative), 2-propanol (secondary, negative), t-butanol (tertiary, positive), benzyl alcohol (benzylic, positive), b) Negative result, c) Positive result.

    Permanganate (Baeyer) Test

    A potassium permanganate \(\left( \ce{KMnO_4} \right)\) solution is a test for unsaturation (alkenes and alkynes) or functional groups that can be oxidized (aldehydes and some alcohols, Figure 6.66). The permanganate ion \(\left( \ce{MnO_4^-} \right)\) is a deep purple color, and upon reduction converts to a brown precipitate \(\left( \ce{MnO_2} \right)\). Permanganate cannot react with aromatics, so is a good test to discern between alkenes and aromatics. A positive reaction with alcohols is not always dependable (a negative result is seen with benzyl alcohols in Figure 6.67).

    Figure 6.66: Reaction of an alkene and aldehyde with permanganate ion.

    Procedure: Dissolve 4 drops or \(40 \: \text{mg}\) of sample in \(1 \: \text{mL}\) of ethanol (or 1,2-dimethoxyethane) in a small test tube (\(13\) x \(100 \: \text{mm}\)). While wearing gloves, add 3 drops of the deep purple \(1\% \: \ce{KMnO_4} \left( aq \right)\) solution to the test tube (safety note: reagent is corrosive and will stain skin brown!). Mix the test tube with agitation, and allow it to sit for 1 minute. A positive result is the appearance of a brown color or precipitate. A negative result is a deep purple with no precipitate (unreacted \(\ce{KMnO_4}\), Figure 6.67).

    Permanganate (Baeyer) Test: Negative result is deep purple solution, positive result is brown precipitate
    Figure 6.67: a) Baeyer test results for (left to right) ethyl benzene (negative), 1-hexene (positive), isobutyraldehyde (positive), benzyl alcohol (mostly negative), b) Negative result, c+d) Positive results.

    pH Test

    Carboxylic acids and sulfonic acids produce acidic aqueous solutions (Figure 6.68a), which can be confirmed by turning blue litmus paper pink. The paper changes color (Figure 6.68c) as the indicator molecules react in the lowered pH and form a structure that has a different color.

    pH Test: negative result is no color change in blue litmus paper, positive result is pink color change
    Figure 6.68: a) Reaction of carboxylic acid with water to produce a slightly acidic solution, b) Results of a negative pH test, c) Results of a positive pH test.

    Procedure: Dissolve 3 drops or \(30 \: \text{mg}\) of sample in \(1 \: \text{mL}\) of water. Dip a glass stirring rod into the solution and touch the rod to blue litmus paper. A positive result is a pink or red color on the litmus paper (Figure 6.68c). If the sample doesn't dissolve in water, instead dissolve the same amount of unknown in \(1 \: \text{mL}\) of ethanol. Add enough water to make the solution barely cloudy. Then add a few drops of ethanol to turn the solution clear again, and test with the litmus paper.

    Phenol Test

    A ferric chloride solution is a test for phenols, as they form intensely colored complexes with \(\ce{Fe^{3+}}\) (often dark blue). The actual structure of these complexes is debated,\(^{15}\) but may be of the general form in Figure 6.69. Some carbonyl compounds with high enol content can give false positives with this test.

    Figure 6.69: Proposed structure of color complex of \(\ce{Fe^{3+}}\) with phenol.

    Procedure: Place \(1 \: \text{mL}\) water in a small test tube (\(13\) x \(100 \: \text{mm}\)) along with either 3 drops or \(30 \: \text{mg}\) of sample. Add 3 drops of the yellow \(5\% \: \ce{FeCl_3} \left( aq \right)\) solution, and mix by agitating.

    A positive result is an intense blue, purple, red, or green color while a negative result is a yellow color (the original color of the \(\ce{FeCl_3}\) solution, Figure 6.70).

    Phenol Test: Negative result is yellow, positive result is blue or red
    Figure 6.70: a) Phenol test results for (left to right) lactic acid (negative), p-cresol (positive), pyrogallol (positive), b) Negative result, c) Positive result (typical color), d) Positive result (less common color, by pyrogallol, a bidentate ligand).

    Silver Nitrate Test

    A dilute solution of silver nitrate in ethanol is a test for some alkyl halides. Silver has a high affinity for halogens (forms strong \(\ce{AgX}\) ionic bonds), and so encourages an \(S_\text{N}1\) mechanism. For this reason, tertiary alkyl halides react faster than secondary alkyl halides (which may or may not react, even with heating), and primary alkyl halides or aromatic halides give no reaction. Benzylic \(\left( \ce{PhCH_2X} \right)\) and allylic \(\left( \ce{CH_2=CHCH_2X} \right)\) alkyl halides will also give a fast reaction. A positive test result is the formation of the insoluble \(\ce{AgX}\) (Figure 6.71). \(\ce{AgCl}\) and \(\ce{AgBr}\) are white solids, while \(\ce{AgI}\) is a yellow solid.

    Figure 6.71: Reaction of alkyl halides with the silver nitrate solution.

    Procedure: In a small test tube (\(13\) x \(100 \: \text{mm}\)), add \(2 \: \text{mL}\) of \(1\% \: \ce{AgNO_3}\) in ethanol solution. Add 4 drops of liquid sample or \(40 \: \text{mg}\) fo solid dissolved in the minimal amount of ethanol. Mix the test tube by agitating. Some compounds will have an initial insolubility when first mixed, but the solid often dissolves with swirling. A positive result is a sustaining white or yellow cloudiness. If cloudiness does not occur within 5 minutes, heat the tube in a \(100^\text{o} \text{C}\) water bath for 1 minute (Figure 6.72b). Absence of cloudiness even at \(100^\text{o} \text{C}\) is a negative result (Figures 6.72+6.73).

    Figure 6.72: a) Silver nitrate results at room temperature for (left to right) 1-chlorobutane (primary, negative), 2-chlorobutane (secondary, negative), 2-chloro-2-methylpropane (tertiary, positive), b) Boiling water bath, c) Results after boiling water, middle tube is faintly cloudy (secondary, positive).

    For reactions that produce an intense precipitate, the solution may also turn blue litmus paper pink (Figure 6.73c+d). An analysis of the reaction mechanism can explain the source of this acidity.

    Silver Nitrate Test: Negative result is clear solution, positive result is precipitate
    Figure 6.73: a) Negative result, b) Positive result, c) Litmus test on a negative \(\ce{AgNO_3}\) test, d) Litmus test (acidic) on a positive \(\ce{AgNO_3}\) test.

    Sodium Iodide (Finkelstein) Test

    A solution of sodium iodide in acetone is a test for some alkyl chlorides and bromides. The mechanism is largely \(S_\text{N}2\), so primary alkyl halides react faster than secondary alkyl halides, and tertiary alkyl halides generally give no reaction. The reaction is driven by the precipitation of the \(\ce{NaCl}\) or \(\ce{NaBr}\) in the acetone solvent. Therefore, a positive test result is the appearance of a white cloudiness (\(\ce{NaX}\) solid).

    \[\begin{array}{ccccccccc} \ce{CH_3CH_2X} & + & \ce{NaI} \: \text{(acetone)} & \rightarrow & \ce{CH_3CH_2I} & + & \ce{NaX} \left( s \right) & & \left( \ce{X} = \ce{Cl}, \ce{Br} \right) \\ & & & & & & \text{white solid} & & \end{array}\]

    Procedure: In a small test tube (\(13\) x \(100 \: \text{mm}\)), add \(2 \: \text{mL}\) of \(15\% \: \ce{NaI}\) in acetone solution.\(^{16}\) Add 4 drops of liquid sample or \(40 \: \text{mg}\) of solid dissolved in the minimal amount of ethanol. Mix the test tube by agitating.

    A positive result is a sustaining white cloudiness. If cloudiness does not occur within 5 minutes, heat the tube in a \(50^\text{o} \text{C}\) water bath for 1 minute. Absence of cloudiness even at \(50^\text{o} \text{C}\) is a negative reaction (Figures 6.74+6.75).

    Figure 6.74: Sodium iodide results at room temperature for (left to right), 1-chlorobutane (primary, positive), 2-chlorobutane (secondary, negative), 2-chloro-2-methylpropane (tertiary, negative), b) Heating in a warm water bath, using a thermometer to monitor the temperature, c) Results after the warm water bath, resulting in the clouding of the middle tube (secondary, positive).
    Sodium Iodide (Finkelstein) Test: Negative result is clear solution, positive result is precipitate
    Figure 6.75: a) Positive result (primary alkyl halide), b) Positive result after heating in a \(50^\text{o} \text{C}\) water bath (secondary), c) Negative result (tertiary).

    Tollens Test

    The Tollens reagent \(\left( \ce{Ag(NH_3)_2^+} \right)\) is a mild oxidizing agent that can oxidize aldehydes, but not alcohols or other carbonyl compounds. A positive test result is the formation of elemental silver (Figure 6.76), which precipitates out as a "silver mirror" on the test tube, or as a black colloidal precipitate.

    Figure 6.76: Reaction of an aldehyde with the Tollens reagent.

    Procedure: While wearing gloves, mix \(1 \: \text{mL}\) of \(5\% \: \ce{AgNO_3} \left( aq \right)\) (safety note: toxic!) with \(1 \: \text{mL}\) of \(10\% \: \ce{NaOH} \left( aq \right)\) in a medium sized test tube (\(18\) x \(150 \: \text{mm}\)). A dark precipitate of silver oxide will form (Figure 6.77b). Add dropwise enough \(10\% \: \ce{NH_4OH} \left( aq \right)\) to just dissolve the precipitate (note some time should be allowed between additions). This solution is now the Tollens reagent \(\ce{Ag(NH_3)_2^+}\) (Figure 6.77c).

    Dissolve 3 drops or \(30 \: \text{mg}\) of sample in a few drops of diethyl ether (omit solvent if compound is water soluble). Add this solution to the \(2\)-\(3 \: \text{mL}\) of previously prepared Tollens reagent. Mix the test tubes by agitating. A positive result is a silver mirror on the edges of the test tube, or formation of a black precipitate. A negative result is a clear solution (Figures 6.77d+6.78).

    Clean-up: The reagent may form a very explosive substance (silver fulminate) over time, so the test should be immediately cleaned up. Acidify the solution with \(5\% \: \ce{HCl} \left( aq \right)\), then dispose in a waste beaker. A silver mirror can be removed from the glassware by adding a small amount of \(6 \: \text{M} \: \ce{HNO_3} \left( aq \right)\).

    Figure 6.77: a) Silver nitrate solution b) \(\ce{Ag_2O}\) precipitate formed after addition of \(\ce{NaOH}\), c) Clarified solution after the addition of ammonium hydroxide, d) Tollens test results for (left to right) 2-pentanone (negative), isobutyraldehyde, benzaldehyde (positive).
    Tollens Test: Negative result is clear solution, positive result is precipitate
    Figure 6.78: a) Negative result, b+c) Positive results.

    \(^9\)The Benedict's reagent is prepared as follows, as published by the Flinn Scientific catalog: \(173 \: \text{g}\) of hydrated sodium citrate and \(100 \: \text{g}\) of anhydrous sodium carbonate is added to \(800 \: \text{mL}\) of distilled water with heating. The mixture is filtered, then combined with a solution of \(17.3 \: \text{g}\) copper(II) sulfate pentahydrate dissolved in \(100 \: \text{mL}\) distilled water. The combined solutions are diluted to \(1 \: \text{L}\).

    \(^{10}\)The chromic acid reagent is prepared as follows: \(25.0 \: \text{g}\) of chromium(VI) oxide is added to \(25 \: \text{mL}\) concentrated sulfuric acid, which is then added in portions to \(75 \: \text{mL}\) of water. The reagent has a very long shelf life (10+ years).

    \(^{11}\)Preparation of the 2,4-DNPH reagent, as published in B. Ruekberg, J. Chem. Ed., 2005, 82(9), p. A1310, is as follows: To a dry \(125 \: \text{mL}\) Erlenmeyer flask is added \(3 \: \text{g}\) 2,4-dinitrophenylhydrazine, \(20 \: \text{mL}\) water and \(70 \: \text{mL}\) of \(95\%\) ethanol. The solution is cooled in an ice bath with stirring, and when at \(10^\text{o} \text{C}\), \(15 \: \text{mL}\) of concentrated sulfuric acid is added slowly in portions. If the temperature exceeds \(20^\text{o} \text{C}\) during the addition, the solution should be allowed to cool to \(10^\text{o} \text{C}\) before continuing. The solution is then warmed to \(60^\text{o} \text{C}\) with stirring, and if solids remain, they are filtered. Finally, the solution is cooled.

    \(^{12}\)Preparation of the iodoform reagent is as follows: \(10 \: \text{g} \: \ce{KI}\) and \(5 \: \text{g} \: \ce{I_2}\) is dissolved in \(100 \: \text{mL}\) water.

    \(^{13}\)Preparation of the Lucas reagent is as follows: \(160 \: \text{g}\) of fresh anhydrous \(\ce{ZnCl_2}\) is dissolved in \(100 \: \text{mL}\) of cold concentrated \(\ce{HCl}\).

    \(^{14}\)Although chlorinated organics are typically denser than water, the Lucas reagent has a high quantity of solute, and chlorinated compounds tend to be less dense than the reagent.

    \(^{15}\)See Nature, 24 June 1950, 165, 1012.

    \(^{16}\)This solution often has a yellow tin to it.

    Contributor

    • Lisa Nichols (Butte Community College). Organic Chemistry Laboratory Techniques is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. Complete text is available online.