Question

Extraction is an effective method for separating compounds which can be treated with an acid or base and made to differ in: A. boiling point. B. molecular weight. C. water solubility. D. optical activity.

Short answer

C. water solubility.

Step-by-step solution

Understand the Concept

Extraction is a separation technique used to isolate compounds based on their differing properties. Typically, the property that most effectively distinguishes compounds in an extraction process is their differential solubility in water.

Identify Compounds' Property Affected by Acid or Base

When compounds are treated with an acid or a base, a common property that changes is their solubility in water. Acids and bases can convert neutral compounds into ionic compounds, making them more soluble in water.

Evaluate Given Options

Consider how each property (boiling point, molecular weight, water solubility, optical activity) could change when a compound is treated with an acid or base. Only water solubility will typically change significantly.

Select the Correct Answer

After evaluating the options, the property that enables effective separation via extraction because it changes with the addition of an acid or base is water solubility. Thus, the correct answer is C.

Key concepts

Acid-Base Extraction

Acid-base extraction is a common technique in organic chemistry used to separate acidic, basic, and neutral compounds from mixtures. This method exploits the acid and base properties of the compounds and their ability to form more water-soluble salts. For example, an acidic compound can react with a base to form its conjugate base, which is generally more soluble in water. Conversely, a basic compound reacts with an acid to form its conjugate acid, also increasing its solubility in water. This process makes use of the concept that acids and bases react to form charged species (ions), which are typically more soluble in polar solvents like water. The mixture to be extracted is often dissolved in an organic solvent that does not mix with water, allowing the separation of compounds based on their differential solubility in water versus the organic layer.

Solubility

Solubility refers to the ability of a substance (the solute) to dissolve in a solvent to form a homogeneous solution. In the context of acid-base extraction, solubility plays a pivotal role. When an acid or base is added to a mixture, it can convert neutral molecules into more polar or ionic forms, which are more soluble in water. For instance, converting benzoic acid (a non-polar molecule) into its sodium salt (a polar molecule) by adding a base like sodium hydroxide greatly increases its solubility in water. Conversely, the same method can be applied to basic compounds by adding an acid. It's key to know that solubility in different solvents can be manipulated by adjusting the pH of the environment, allowing for selective extraction of compounds. Thus, understanding the solubility principles helps in predicting and carrying out successful extractions.

Separation Techniques

Separation techniques in organic chemistry are methods used to isolate, purify, and identify compounds in a mixture. One such technique is extraction, specifically acid-base extraction, which separates compounds based on their solubility differences upon altering pH levels. The process typically involves mixing the compound mixture with an immiscible organic solvent, where the compounds show different stabilities in organic vs. aqueous phases. After shaking and allowing the layers to separate, compounds preferentially soluble in water move to the aqueous layer, while those more soluble in the organic solvent remain. By repeating this process in a stepwise manner, and perhaps alternating the pH to favor different solubilities at each step, an effective separation can be achieved. This method is particularly useful for separating a complex mixture into its component fractions, each purified for further use or analysis. The clarity and precision of separation techniques are what make them indispensable in organic synthesis and analysis.