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.
At the end of this lab, students should know...
An alcohol is an organic compound possessing a hydroxyl functional group connected to an alkyl or aryl group (ROH).
Primary aliphatic alcohols are those with one alkyl group connected to the carbon bonded to the OH group. Secondary aliphatic alcohols are those with a hydroxyl group connected to a carbon that has two alkyl groups. Tertiary aliphatic alcohols are those where the hydroxyl group is connected to a carbon that has three alkyl groups. Aromatic alcohols are alcohols that have a hydroxyl group connected to the carbon of a ring structure such as benzene.
The Jones Test is used to distinguish between different aliphatic alcohols. The addition of Jones’ reagent to primary or secondary alcohols causes a color change from red-orange to a dark green precipitate based on the reduction of chromium(VI) to chromium(III). Tertiary aliphatic alcohols do not react.
The Lucas Test uses the reaction rate to distinguish between the three types of aliphatic alcohols. Tertiary alcohols react immediately, forming a secondary phase in the reaction mixture. Secondary alcohols react more slowly, forming layers in solution over the course of several minutes. Primary alcohols do not react unless heat is added, forming a cloudy solution.
The ferric chloride test is used to test for the presence of aromatic compounds. Ferric chloride reacts with the aromatic alcohol to form a purple solution due to the displacement of the chloride anions with the aromatic rings. Ferric chloride will not react with any aliphatic alcohol.
Source: Lara Al Hariri at the University of Massachusetts Amherst, MA, USA
Here, we show the laboratory preparation for 10 students working in pairs, with some excess. Please adjust quantities as needed.
| 13Small test tubes |
| 1Test tube rack |
| 3Small vials with caps |
| 150-mL beaker |
| 1250-mL beaker |
| 15-mL graduated cylinder |
| 1Glass stirring rod |
| 1Roll of labels or lab tape |
| 1Labeling pen/marker |
| Pipette bulbs |
Videos from this collection:
Now Playing
Chemistry
0 Views
Chemistry
0 Views
Chemistry
0 Views
Chemistry
0 Views
Chemistry
0 Views
Chemistry
0 Views
Chemistry
0 Views
Chemistry
0 Views
Chemistry
0 Views
Chemistry
0 Views
Chemistry
0 Views
Chemistry
0 Views
Chemistry
0 Views
Chemistry
0 Views
Chemistry
0 Views
See More