Distinguish between the following groups of compounds by using a simple chemical method.
(1) CH_3CH=CHCOOH and CH_3CH_2CH_2COOH
(2) Benzoic acid、 benzyl alcohol phenol
(1) CH_3CH=CHCOOH is an α,β-unsaturated carboxylic acid and CH_3CH_2CH_2COOH is a saturated carboxylic acid. The main difference is the presence of a C=C bond in the former. You can differentiate them by bromine water test. CH_3CH=CHCOOH will decolorize bromine water due to the addition reaction with C=C bond, turning bromine water from brown to colourless. CH_3CH_2CH_2COOH will not react with bromine water hence the brown colour remains unchanged.
(2) Benzoic acid (a carboxylic acid), benzyl alcohol (an alcohol) and phenol (a phenol, a stronger acid than alcohol but weaker than carboxylic acid). A simple method to distinguish these three compounds is by using the sodium bicarbonate test. Only benzoic acid will effervesce (Carbon dioxide gas is produced) when reacted with sodium bicarbonate due to the acid-base reaction, whereas benzyl alcohol and phenol will not react. Phenol can be distinguished from benzyl alcohol by using ferric chloride test. Phenol will form a purple complex with FeCl_3, whereas benzyl alcohol will not show any visible reaction.
To distinguish between the compounds in each group, we can use a simple chemical method such as a solubility test and/or a pH test. Here's how you can differentiate between each group:
(1) CH₃CH=CHCOOH and CH₃CH₂CH₂COOH:
- Solubility test: Both compounds are carboxylic acids and are soluble in water due to the presence of -COOH group. Hence, they will dissolve in water.
- pH test: Take a small amount of each compound and add a few drops of water to form a solution. Then, use a pH indicator (such as litmus paper) to test the pH of the solutions. CH₃CH=CHCOOH (acrylic acid) has a lower pKa value compared to CH₃CH₂CH₂COOH (butyric acid). As a result, acrylic acid will have a lower pH (more acidic) than butyric acid.
(2) Benzoic acid, benzyl alcohol, and phenol:
- Solubility test: Benzoic acid and phenol are soluble in water due to the presence of -OH group and can form hydrogen bonds with water molecules. However, benzyl alcohol is not as soluble in water due to the longer hydrocarbon chain. It may partially dissolve, but not completely.
- pH test: Phenol is slightly acidic and can partially ionize in water to form phenoxide ions. Therefore, it will have a slightly lower pH compared to neutral water. In contrast, both benzoic acid and benzyl alcohol are acidic compounds. Benzoic acid is stronger, and it will have a lower pH compared to benzyl alcohol.
By conducting these simple tests, you can differentiate between the compounds in each group based on their solubility and pH properties.