Identifying Alcohols

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.