What does a positive ferric chloride test indicate about an alcohol?

A positive ferric chloride test produces a purple iron(III)-phenol complex, indicating the presence of a phenol. Iron(III) chloride forms a brown complex in water, but when phenol is added, the solution turns purple. This color change does not occur with non-aromatic or aliphatic alcohols, making it a reliable indicator of phenolic compounds.

Why does the Jones test produce different color changes for primary and secondary alcohols?

The Jones test uses chromic acid, which oxidizes primary and secondary alcohols. Chromium in Jones' reagent exists in the +6 oxidation state, giving it a bright reddish-orange color. When Cr(VI) oxidizes an alcohol, chromium is reduced to the +3 oxidation state, forming green Cr(III) complexes. This color change from orange to green indicates successful oxidation of primary or secondary alcohols.

How does the Lucas test distinguish between primary, secondary, and tertiary alcohols?

The Lucas test relies on carbocation stability. Tertiary alcohols form stable carbocations and react immediately, producing a cloudy layer. Secondary alcohols form less stable carbocations and react within seconds to minutes. Primary alcohols form unstable carbocations and do not react. The reaction rate directly correlates with carbocation stability, allowing classification by observation time.

Why does the Jones test fail to produce a color change with tertiary alcohols?

Tertiary alcohols lack a hydrogen atom bonded to the carbon bearing the hydroxyl group, preventing oxidation. In the Jones test mechanism, chromium must form a chromate ester and cleave a C-H bond to create a carbonyl group. Since tertiary alcohols cannot form this extra C-O bond, chromium is not reduced, and the orange color persists rather than turning green.

What is the chemical basis for the color change in Jones' reagent during alcohol oxidation?

Jones' reagent contains chromium in the +6 oxidation state, which gives it a bright reddish-orange color. During oxidation, Cr(VI) is reduced to Cr(III), which forms hexaaquachromium(III) ions and complexes with sulfate ions. These Cr(III) complexes are characteristically green, so the solution changes from orange to green when primary or secondary alcohols are oxidized.

How does the Lucas test mechanism depend on carbocation formation?

Lucas' reagent converts alcohols to chloroalkanes through a carbocation intermediate. The rate-determining step involves forming a carbocation from the alcohol. Tertiary carbocations are highly stable and form rapidly, causing immediate cloudiness. Secondary carbocations are less stable, producing delayed cloudiness. Primary carbocations are too unstable to form, resulting in no reaction and no visible change.

What sequence of tests should you use to identify an unknown alcohol?

Start with the ferric chloride test to determine if the alcohol is a phenol derivative. If negative, use the Jones test to distinguish between aliphatic alcohols and tertiary alcohols based on color change. Finally, apply the Lucas test to classify primary and secondary alcohols by reaction speed. This three-step approach systematically narrows down the unknown's identity.

Identifying Alcohols

Concepts Instructor Prep Student Protocol

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Alcohols are a class of organic molecules possessing at least one hydroxyl functional group connected to a carbon atom. Methanol and ethanol are two familiar alcohols. Both are aliphatic alcohols, meaning that they are derived from a hydrocarbon and do not contain a benzene group.

There are three classifications of aliphatic alcohols. The first is a primary alcohol, where the hydroxyl group is connected to a carbon atom that is connected to one carbon-based, or alkyl, group. In a secondary alcohol, the hydroxyl group is connected to a carbon atom that has two alkyl groups. Finally, in a tertiary alcohol, the hydroxyl group is connected to a carbon atom with three alkyl groups.

Alcohols can also be aromatic, in which a hydroxyl group is connected directly to the carbon of a benzene ring. The simplest form of an aromatic alcohol is phenol. Compounds that contain phenol are known as phenol derivatives or phenols.

Certain reactions undergone by alcohols can be used to distinguish them based on visible differences in the reaction’s outcome. One such reaction is the ferric chloride test, which tests for the presence of phenols. When ferric chloride is added to a phenol, a purple iron-three-phenol complex forms. This is not observed when it is added to an aliphatic alcohol.

Another useful reaction is the Jones Test, which is used to identify primary and secondary aliphatic alcohols. Jones’ reagent, a mix of sulfuric acid and chromium trioxide in water, is a strong oxidizing agent. It reacts with primary alcohols to form aldehydes — which then form carboxylic acids— and it reacts with secondary alcohols to form ketones. The addition of the Jones reagent to primary and secondary alcohols causes the solution to change colors from orange to dark green. Tertiary alcohols do not react with Jones’ reagent because they are resistant to oxidation.

Finally, we can distinguish aliphatic alcohols with the Lucas test. Lucas’ reagent, which is a mixture of zinc chloride and hydrochloric acid, reacts with secondary and tertiary alcohols through an S_N1 nucleophilic substitution reaction. The zinc chloride coordinates to the hydroxyl oxygen to generate an excellent leaving group. Once eliminated, a positively charged carbocation remains. Then, a chlorine ion attacks the carbocation to form an alkyl chloride, which is insoluble in water and appears cloudy.

Tertiary alcohols react immediately with Lucas’ reagent to form an oily layer at room temperature. Secondary alcohols give a positive result after a few seconds to a few minutes. Primary alcohols give a negative result unless they are heated.

In this lab, you will use the ferric chloride test, Jones test, and Lucas test to identify an unknown alcohol.

Print Procedure Steps

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

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
  1-butanol
  2-butanol
  2-methyl-2-propanol
  Phenol
  Unknown
  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.
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.
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.
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.