Identifying Alcohols
Alcohols
Alcohols are organic compounds that are amongst the most recognizable and familiar, as they have wide-ranging applications and uses in everyd…
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 SN1 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.
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 SN1 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.
View the full transcript and gain access to JoVE Lab Manual videos
Q1: What is the difference between primary, secondary, and tertiary alcohols?
Primary alcohols have a hydroxyl group connected to a carbon with one alkyl group. Secondary alcohols have two alkyl groups attached to the hydroxyl-bearing carbon. Tertiary alcohols have three alkyl groups bonded to that carbon. These structural differences determine how each alcohol type reacts in identification tests.
Q2: How does the ferric chloride test distinguish phenols from aliphatic alcohols?
Ferric chloride solution is initially red-orange. When added to phenol, an aromatic alcohol, it forms a purple iron-three-phenol complex due to coordination changes at the iron center. Aliphatic alcohols do not react with ferric chloride, so the solution remains red-orange, allowing clear differentiation between aromatic and aliphatic types.
Q3: What color change indicates a positive Jones test result?
The Jones test uses chromium trioxide in sulfuric acid, which appears bright reddish-orange due to Cr(VI) oxidation state. When primary or secondary alcohols are oxidized, chromium is reduced to Cr(III), forming green complexes. The solution changes from orange to dark green, indicating a positive result for primary and secondary alcohols.
Q4: Why do tertiary alcohols not react with Jones' reagent?
Tertiary alcohols are resistant to oxidation because the carbon bearing the hydroxyl group is bonded to three alkyl groups, preventing the oxidation mechanism from occurring. Since Jones' reagent cannot oxidize tertiary alcohols, no color change happens and the solution remains orange, distinguishing them from primary and secondary alcohols.
Q5: How does the Lucas test differentiate between primary, secondary, and tertiary alcohols?
Lucas' reagent forms a carbocation intermediate through an SN1 nucleophilic substitution reaction. Tertiary alcohols form stable carbocations and react immediately, producing a cloudy oily layer. Secondary alcohols react more slowly over seconds to minutes. Primary alcohols show no reaction at room temperature unless heated, allowing classification based on reaction rate and visible cloudiness.
Q6: What is the role of zinc chloride in the Lucas test?
Zinc chloride coordinates to the hydroxyl oxygen of the alcohol, generating an excellent leaving group. This coordination facilitates the formation of a carbocation, which is then attacked by a chlorine ion to form an alkyl chloride. The resulting alkyl chloride is insoluble in water, creating the characteristic cloudy appearance that indicates a positive test result.
Q7: What is the structural difference between aliphatic and aromatic alcohols?
Aliphatic alcohols are derived from hydrocarbons and do not contain a benzene ring, such as methanol and ethanol. Aromatic alcohols have a hydroxyl group connected directly to a benzene ring carbon. The simplest aromatic alcohol is phenol. This structural difference affects their chemical reactivity and allows identification through specific tests like the ferric chloride test.