Question

Describe a chemical procedure to separate a mixture of benzyl alcohol and \(a\)-cresol and to recover each in pure form.

Short answer

Answer: The mixture can be separated using a liquid-liquid extraction technique based on the difference in acidity between benzyl alcohol and α-cresol. By adding sodium hydroxide to the dissolved mixture, α-cresol reacts to form sodium α-cresolate in the aqueous layer, while benzyl alcohol remains in the organic layer. The layers are then separated, and each compound is recovered in pure form by evaporating the solvent.

Step-by-step solution

Understand the properties of benzyl alcohol and α-cresol

Benzyl alcohol is a clear, colorless liquid with a weak aromatic odor. It is a primary alcohol, meaning it has only one hydroxyl group (-OH) attached to the aromatic benzene ring. α-cresol, on the other hand, is a phenol, which means it has a hydroxyl group attached to a benzene ring and a methyl group (-CH3) at the ortho position (adjacent to the hydroxyl group). Due to the presence of the hydroxyl group in both compounds, they can form hydrogen bonds. However, phenols generally have higher acidity than primary alcohols.

Select an appropriate separation technique

Exploiting the difference in acidity between benzyl alcohol and α-cresol, we can use a liquid-liquid extraction technique to separate the compounds. This technique is based on the partitioning of a solute between two immiscible phases, usually water and an organic solvent.

Carry out the liquid-liquid extraction

Before starting the extraction, we should choose an organic solvent that is immiscible with water and dissolves both benzyl alcohol and α-cresol; for example, diethyl ether or ethyl acetate. Follow these steps for the liquid-liquid extraction procedure: 1. Dissolve the mixture in the chosen organic solvent. 2. Add an aqueous solution of sodium hydroxide (NaOH) to the dissolved mixture. Because of the difference in acidity, α-cresol will react with NaOH to form a soluble salt (sodium α-cresolate) in the aqueous layer, while benzyl alcohol will remain dissolved in the organic layer. 3. Separate the organic and aqueous layers by using a separatory funnel. Collect the organic layer containing the benzyl alcohol in a flask, and the aqueous layer containing the sodium α-cresolate in a separate flask. Note: This step may need to be repeated several times to ensure complete separation of the compounds.

Recover benzyl alcohol from the organic layer

To recover benzyl alcohol in pure form, follow these steps: 1. Dry the organic layer by adding an anhydrous inorganic salt, such as magnesium sulfate (MgSO4) or sodium sulfate (Na2SO4), and mix it well. This step removes any remaining traces of water in the organic layer. 2. Filter the drying agent from the organic layer using a glass funnel with filter paper. 3. Evaporate the solvent using a rotary evaporator under reduced pressure to obtain pure benzyl alcohol.

Recover α-cresol from the aqueous layer

To recover α-cresol in pure form, follow these steps: 1. Acidify the aqueous layer containing sodium α-cresolate by adding dilute hydrochloric acid (HCl) dropwise with constant stirring while monitoring the pH. The addition of HCl will convert the sodium α-cresolate back to α-cresol. 2. Once α-cresol is formed, it will separate from the aqueous layer as it is insoluble in water. 3. Extract the newly formed α-cresol using an organic solvent (e.g., diethyl ether or ethyl acetate) in a separatory funnel. 4. Dry, filter and evaporate the organic solvent, as described in Step 4, to obtain pure α-cresol.