Question

Describe exactly how you would go about separating a mixture of the three water-insoluble liquids, aniline (b. p. \(\left.184^{\circ}\right), \mathrm{n}\) -butylbenzene (b. p. \(183^{\circ}\) ), and n-valeric acid (b. p. \(187^{\circ}\) ), recovering each compound pure and in essentially quantitative yield. Do the same for a mixture of the three water-insoluble solids, \(p\) -toluidine, o-bromobenzoic acid, and \(\mathrm{p}\) -nitroanisole.

Short answer

To separate the mixture of water-insoluble liquids, perform the following steps: 1. Add a strong acid to the mixture to form water-soluble salts, then separate n-butylbenzene through decantation. 2. Neutralize anilinium salt with a strong base to recover aniline. 3. Neutralize valeric acid salt with a diluted base to recover n-valeric acid. 4. Purify the aniline, n-butylbenzene, and n-valeric acid by washing, then remove solvent by evaporation. To separate the mixture of water-insoluble solids: 5. Perform recrystallization using a suitable solvent to separate p-toluidine from the mixture. 6. Add a basic aqueous solution to separate o-bromobenzoic acid from p-nitroanisole. 7. Recover o-bromobenzoic acid from its sodium salt by adding an acid. Wash and dry the obtained solids to ensure purity.

Step-by-step solution

Separating the Liquid Mixture by Chemical Reaction

Add a strong acid (e.g., hydrochloric acid) to the liquid mixture. Aniline and n-valeric acid will react with the acid, forming water-soluble salts (anilinium chloride and potassium valerate respectively), while n-butylbenzene will remain unreacted and can be easily separated through decantation.

Neutralizing Anilinium Salt

To recover aniline from the anilinium salt, add a strong base (e.g., sodium hydroxide) to the aqueous solution until the pH becomes slightly basic. In this condition, the anilinium salt will dissociate, releasing aniline as a free base. Aniline can then be extracted using an organic solvent (e.g., diethyl ether), which will be separated using a separatory funnel.

Neutralizing Valeric Acid Salt

Repeat the process of neutralization, but this time, use a more diluted base solution to prevent complete neutralization. N-valeric acid will be released and can easily be extracted using an organic solvent.

Purification and recovery of liquids

Purify the aniline, n-butylbenzene, and n-valeric acid by washing the organic phase with water several times. The washing process will remove any remaining impurities. Finally, remove the solvent by evaporation or distillation, recovering each liquid in its pure form. Now, let's work on separating the mixture of water-insoluble solids:

Separating the Solid Mixture by Recrystallization

To separate the solid mixture, we need to find a suitable solvent that has different solubility for each compound. This step may require a trial and error approach to achieve a suitable solvent. For instance, p-toluidine could be soluble in hot ethanol, while p-nitroanisole and o-bromobenzoic acid are not. Thus, we can separate p-toluidine from the mixture by hot filtration, followed by cooling down the ethanol solution to recrystallize p-toluidine.

Separating the Remaining Solids by Solubility

Since o-bromobenzoic acid is an acidic compound, it can dissolve in a basic aqueous solution, forming its salt. So, add an aqueous solution of sodium hydroxide to the solid mixture. The o-bromobenzoic acid will dissolve forming sodium o-bromobenzoate, leaving p-nitroanisole behind as an insoluble solid. Filter the mixture to separate p-nitroanisole.

Recovery of o-bromobenzoic acid

Finally, to recover o-bromobenzoic acid from its sodium salt, add an acid (e.g., hydrochloric acid) to the aqueous solution of sodium o-bromobenzoate. The o-bromobenzoic acid will precipitate and can be separated by filtration. Wash and dry the obtained solids to ensure their purity.

Key concepts

Recrystallization Technique

Recrystallization is a commonly used method in chemistry to purify solid compounds. It involves dissolving a solid material in a solvent at a high temperature and then allowing the solution to cool. As the solution cools, the solubility of the solid decreases, and it begins to form crystals.

The key to successful recrystallization is to find a solvent that will dissolve the desired compound when hot but not when cold, while the impurities remain dissolved or do not dissolve at all. Performing a hot filtration can remove undissolved solids, and as the filtrate cools, the pure compound crystallizes out. Carefully cooling the solution can lead to the formation of large, pure crystals. After crystallization, the solid is collected by filtration.

To improve the effectiveness of recrystallization, make sure to:

• Choose an appropriate solvent
• Control temperature changes to allow slow crystal growth
• Use charcoal if necessary to remove colored impurities
• Employ gravity or vacuum filtration to collect crystals

It's a delicate balance of conditions that leads to successful recrystallization and a pure product.

Chemical Extraction

Chemical extraction is a separation process used to isolate a particular compound from a mixture. It leverages differences in the compounds' solubility in two immiscible liquids—typically, an aqueous solution and an organic solvent.

In the academic exercise mentioned, aniline and n-valeric acid are extracted from the organic phase into the aqueous phase by forming water-soluble salts after reacting with an acid. Subsequently, chemical extraction is used to separate aniline from its salt form by adding a base to form a free base aniline, which is then extracted into an organic solvent.

Some tips to ensure an effective chemical extraction include:

• Ensure thorough mixing of the two phases to maximize contact
• Choose proper solvents with high selectivity for the target compound
• Be patient during the separation to allow complete partitioning
• Repeat extractions if necessary for higher purity

This method is widely used because it's efficient and can produce highly pure compounds if optimized correctly.

Acid-Base Neutralization

Acid-base neutralization is a reaction where an acid reacts with a base to form water and a salt. This reaction is pivotal in various chemical separation processes since it can convert compounds into water-soluble or water-insoluble forms, thus facilitating separation.

In the context of the given problem, neutralization is used twice: first, to convert aniline into aniline salt, making it water-soluble and, second, to recover o-bromobenzoic acid from its sodium salt form by adding an acid to precipitate the pure acid.

When executing an acid-base neutralization for separation purposes, it's essential to:

• Monitor the pH of the solution
• Ensure the correct stoichiometric amounts of acid or base
• Be aware of the heat released during the reaction
• Choose the right solvent for the resulting substances

Understanding the behaviors of different acids and bases during neutralization allows chemists to design processes that precisely isolate and recover desired compounds.