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Lab: Chemistry

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Identifying Alcohols

Identifying Alcohols


Source: Lara Al Hariri and Ahmed Basabrain at the University of Massachusetts Amherst, MA, USA

  1. Ferric Chloride Test for Phenols

    In this lab, you will identify an unknown alcohol using the ferric chloride test, the Jones test, and the Lucas test. You'll test known alcohols alongside the unknown alcohol as examples of positive and negative results for each test.

    The four known alcohols are 1-butanol, a primary alcohol, 2-butanol, a secondary alcohol, 2-methyl-2-propanol, a tertiary alcohol, and phenol. The four possible unknown alcohols are 1-propanol, 2-propanol, 2-methyl-2-butanol, and para-chlorophenol.

    You'll start with the ferric chloride test for the presence of phenols. Iron(III) chloride, or ferric chloride, forms a brown complex when it's dissolved in water. If you add a phenol to this solution, a purple iron(III)-phenol complex will form instead. This effect is not observed with non-aromatic or aliphatic alcohols.

    • Put on a lab coat, safety glasses, and gloves before you begin. Note: The alcohols that you will test are toxic and flammable, and the reagents that you will use are toxic and corrosive, so you will perform this lab in a fume hood.
    • Now, get your assigned unknown alcohol from your instructor and bring it to your fume hood. Label a 250-mL beaker ‘aqueous metal waste’.
    • Next, put five small test tubes in a rack and label them ‘1-butanol’, ‘2-butanol’, ‘2-methyl-2-propanol’, ‘phenol’, and ‘unknown’. Bring the rack to the analytical balance to obtain phenol.
    • If your unknown is a solid, bring it with you. Weigh about 0.2 g of phenol and pour it into the corresponding tube. Do the same for your unknown if it is a solid and then return to your hood.
    • Now label three small vials ‘1-butanol’, ‘2-butanol’, and ‘2-methyl-2-propanol’.
    • Bring the vials to the solvent hood and use clean Pasteur pipettes to fill each vial with about 1 mL of the matching alcohol. Note: Do not reuse pipettes.
    • Next, obtain at least four Pasteur pipettes and a 250-mL beaker. Get more pipettes whenever you need them, and don't put used pipettes back in the beaker.
    • In your fume hood, pipette 3 – 4 drops of 1-butanol, 2-butanol, and 2-methyl-2-propanol into the corresponding tubes.
    • To the phenol tube, add several drops of deionized water and shake to mix. This will provide better visual results for the ferric test.
    • If your unknown alcohol is a liquid, use a clean pipette to place 3 – 4 drops of it in the unknown test tube.
    • Now, obtain the bottle of iron(III) chloride solution and add 2 – 3 drops to each test tube.
    • Stir each mixture well with a glass rod. Use deionized water to rinse off residual alcohol before mixing the contents of the next tube. Make sure that the phenol dissolves completely along with the unknown if it is a solid.
    • Wait 5 min before comparing your unknown to the known solutions. Note: If your unknown solution is purple, like the phenol solution, then your unknown is a phenol derivative. If it looks like the other three tubes, it is not a phenol derivative. Record the results of the test in your lab notebook.

      Table 1: Identifying Unknown Alcohol

      Alcohol Ferric Chloride Test Jones Test Lucas Test
      Click Here to download Table 1
    • The next tests don't work with phenols, so you'll only test aliphatic alcohols for the rest of the lab. If your unknown is a phenol derivative, skip the instructions for the unknown and test only the three known aliphatic alcohols.
  2. The Jones Test for Aliphatic Primary and Secondary Alcohols

    In this section, you'll perform the Jones test for primary and secondary alcohols. Jones' reagent is made with chromium trioxide and sulfuric acid in water, which forms chromic acid (H2CrO4) in situ. This powerful reagent oxidizes secondary alcohols to ketones, primary alcohols to aldehydes, which after forming an aldehyde hydrate, are further reduced to carboxylic acids.

    The oxidation state of chromium is the key to this test. Chromium is in the +6 oxidation state in the Jones' reagent. The Cr(VI) complexes in the reagent give it its bright reddish, orange color.

    When Cr(VI) oxidizes an alcohol, chromium is reduced to the +3 oxidation state. First, the alcohol and chromic acid form a chromate ester. Then, a base (H2O) cleaves the C-H bond of the alcohol, forming the carbonyl group while reducing Cr(VI) to Cr(IV). Because the carbon of the alcohol undergoes a 2-electron oxidation, and Cr(VI) a 2-electron reduction, this step is a reduction-oxidation step. Cr(IV) participates in further oxidation steps and is eventually reduced to Cr(III). Cr(III) is often present as hexaaquachromium(III) ions — [Cr(H2O)6]3+ — and Cr(III) complexes, where H2O molecules are replaced by one or more sulphate ions, [Cr(H2O)5(SO4)]+. These complexes give Cr(III) the characteristic green color. So, when you mix Jones' reagent with a primary or secondary alcohol in acetone, the orange solution will turn green.

    Jones oxidation of alcohols doesn't work with tertiary alcohols because the -OH group is already bonded to three carbon atoms and cannot form an extra C-O bond. Thus, you won't see a color change if you combine Jones' reagent with a tertiary alcohol because chromium isn't reduced.

    • Label four clean test tubes ‘1-butanol’, ‘2-butanol’, ‘2-methyl-2-propanol’, and ‘unknown’ and place 10 drops of acetone in each. Acetone is the preferred solvent for Jones oxidation.
    • Then, use clean pipettes to add 3 – 4 drops of the known aliphatic alcohols and your unknown alcohol to the corresponding tubes.
    • Stir each solution well with the glass rod and rinse the rod with water after mixing each one.
    • Now, obtain the bottle of Jones' reagent and carefully add 2 – 3 drops to each tube. The reagent is strongly acidic and corrosive, so be careful with it and tightly cap the bottle when you're done. Then, mix each solution well, cleaning the glass rod after each one.
    • Let the solutions react for 10 min, and then compare your unknown to the knowns. Note: If the unknown solution turned green, like 1-butanol and 2-butanol, the test is positive for a primary or secondary alcohol. If it stayed orange, like 2-methyl-2-propanol, the test is negative.
    • Record the results in your lab notebook.
  3. The Lucas Test for Classifying Small Saturated Alcohols

    The Jones test doesn't distinguish between primary and secondary alcohols, so you'll use the Lucas test to identify which is which.

    Lucas' reagent, which is a mixture of zinc chloride and hydrochloric acid, converts secondary and tertiary alcohols to chloroalkanes at room temperature. Chloroalkanes are nearly insoluble in water, so a positive result appears as the mixture separates into a cloudy chloroalkane-containing layer over a clear layer.

    The rate-determining step of the reaction involves converting the alcohol to a carbocation, so the speed of the reaction depends on how stable the carbocation is.

    Tertiary carbocations are very stable, so tertiary alcohols give a positive reaction almost immediately. Secondary carbocations are less stable, so secondary alcohols give a positive result after a few seconds to a few minutes. Primary carbocations are too unstable for this reaction, so primary alcohols give a negative result.

    • Now, label for clean test tubes as ‘1-butanol’, ‘2-butanol’, ‘2-methyl-2-propanol’, and ‘unknown’.
    • Obtain the bottle of Lucas' reagent and use a graduated cylinder to measure 2 mL of the reagent for each tube.
    • Next, add 2 drops of 2-methyl-2-propanol to the appropriate tube. Immediately, stir the mixture for a few seconds and place it in the rack. The mixture will promptly separate into a clear lower layer and a cloudy upper layer.
    • Thoroughly rinse the glass rod and record the immediate positive result in your lab notebook.
    • Mix the remaining alcohols with Lucas' reagent in the same way, using a fresh pipette for each alcohol and rinsing the glass rod each time. Note: Don't add too much alcohol or jostle the tubes because that can slow down the reaction.
    • Watch carefully after mixing the unknown alcohol with Lucas' reagent to ensure that you can distinguish an immediate positive result from a positive result after a brief delay.
    • Check on the tubes every few minutes and record positive results as you see them. Note approximately how long it took for delayed positive results to appear.
    • Once the tubes have sat for at least 15 min, you can assume that the results are final. Record the results for 1-butanol, 2-butanol, and for your unknown if applicable.
    • Now, let's clean up. Your instructor will collect the Cr(VI) solutions, so leave any tubes of orange solution from the Jones test in your hood.
    • Empty the other test tubes into the waste beaker and rinse them with deionized water. Pour the waste into the aqueous metal waste container using a funnel and rinse the beaker with deionized water.
    • Now, wash your glassware as usual and dispose of used pipettes in the glass waste. Lastly, clean your hood and throw out any trash in the lab trash container.
  4. Results

    Identify your unknown alcohol. The four possibilities are 1-propanol, 2-propanol, 2-methyl-2-butanol, and para-chlorophenol.

    • Consider the results of the ferric chloride test first. If the mixture with your unknown alcohol turned purple, like phenol did, your alcohol must be para-chlorophenol. If you saw a negative result, your unknown is one of the three aliphatic alcohols.
    • Look at the results of the Jones test. If the mixture with the unknown alcohol turned green, the unknown could be primary or secondary. If the mixture with your unknown alcohol stayed orange, your unknown alcohol is tertiary. Therefore, a negative Jones result means that your alcohol must be 2-methyl-2-butanol.
    • Lastly, look at the results of the Lucas test. If you had a negative result in the Jones test, you should also have seen an immediate positive result in the Lucas test. This confirms that your alcohol was tertiary. If your unknown had a positive result after a short delay, then your unknown alcohol is secondary, and therefore, must be 2-propanol. Finally, if your unknown had a negative result in the Lucas test, your alcohol is primary, and therefore, must be 1-propanol.

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