Question

Fractional distillation under atmospheric pressure would most likely be used to separate which of the following compounds? a. Methylene chloride (boiling point of \(40^{\circ} \mathrm{C}\) ) and water (boiling point of \(\left.10^{\circ} \mathrm{C}\right)\) b. Ethyl acetate (boiling point of \(77^{\circ} \mathrm{C}\) ) and ethanol (boiling point of \(\left.80^{\circ} \mathrm{C}\right)\) c. Aniline (boiling point of \(184^{\circ} \mathrm{C}\) ) and benzyl alcohol (boiling point of \(\left.205^{\circ} \mathrm{C}\right)\) d. Aniline (boiling point of \(184^{\circ} \mathrm{C}\) ) and water (boiling point of \(\left.100^{\circ} \mathrm{C}\right)\)

Short answer

Aniline (boiling point 184°C) and water (boiling point 100°C) - choice d.

Step-by-step solution

Understand Fractional Distillation

Fractional distillation is a technique used to separate compounds with different boiling points. It is more effective when the boiling points of the substances are not too close to each other.

Identify the compound pairs

Assess the given compound pairs: a. Methylene chloride (boiling point 40°C) and water (boiling point 100°C) b. Ethyl acetate (boiling point 77°C) and ethanol (boiling point 80°C) c. Aniline (boiling point 184°C) and benzyl alcohol (boiling point 205°C) d. Aniline (boiling point 184°C) and water (boiling point 100°C)

Examine boiling point differences

Calculate the differences in boiling points of each pair:a. 60°C difference (100°C - 40°C)b. 3°C difference (80°C - 77°C)c. 21°C difference (205°C - 184°C)d. 84°C difference (184°C - 100°C)

Analyze suitability for fractional distillation

Select the pair that has a moderate boiling point difference. Too small a difference is hard to separate using fractional distillation. Between the pairs:a. 60°C - Suitableb. 3°C - Difficult to separatec. 21°C - Possible but requires precise controld. 84°C - Suitabled. has the largest suitable difference for effective fractional distillation under atmospheric pressure.

Key concepts

Boiling Points

Boiling points are crucial when it comes to separating compounds in organic chemistry. The boiling point of a substance is the temperature at which it changes from a liquid to a gas. Different substances have different boiling points due to the variations in their molecular structures and bonding.
When we talk about fractional distillation, boiling points become a key factor. This technique relies heavily on the differences in boiling points of the compounds to be separated. The greater the difference, the easier it usually is to separate the substances. For example:

• Methylene chloride has a boiling point of 40°C.
• Water has a boiling point of 100°C.
• Ethyl acetate has a boiling point of 77°C.
• Ethanol has a boiling point of 80°C.

Knowing these values helps to decide which substances are suitable for separation using fractional distillation.

Distillation Techniques

Distillation is a process used to separate components of a mixture based on their boiling points. There are various distillation techniques, but the main two are simple distillation and fractional distillation.
In simple distillation, the mixture is heated until the compound with the lower boiling point evaporates. The vapor is then condensed back into a liquid in a different container. This method is efficient when separating compounds with significantly different boiling points.
Fractional distillation, on the other hand, is used when the boiling points of the compounds are closer together. It involves a column (called a fractionating column) filled with materials that provide a large surface area. This setup allows multiple vaporizations and condensations to occur as the mixture travels up the column. As a result, compounds with slightly different boiling points can be separated effectively.
During fractional distillation, the compound with the lower boiling point reaches the top of the column first and is collected. The compound with the higher boiling point will condense lower in the column and can be collected later. This method is especially useful in separating complex mixtures in organic chemistry.

Separation of Compounds

The process of separating compounds is a fundamental aspect of organic chemistry. When a mixture contains two or more substances with different boiling points, fractional distillation can be used effectively.
To decide whether fractional distillation is suitable, consider the differences in boiling points:

• If the boiling points are too close (e.g., a 3°C difference), it becomes difficult to separate them efficiently.
• If the boiling points have a moderate difference (e.g., a 60°C or 84°C difference), fractional distillation works well.
• Too large a difference is usually handled by simple distillation.

Let's look at our original exercise:

• Methylene chloride (40°C) and water (100°C) - 60°C difference: Suitable.
• Ethyl acetate (77°C) and ethanol (80°C) - 3°C difference: Difficult.
• Aniline (184°C) and benzyl alcohol (205°C) - 21°C difference: Possible but requires precision.
• Aniline (184°C) and water (100°C) - 84°C difference: Suitable.

The goal of fractional distillation is to obtain pure substances by exploiting their boiling point differences. Among the pairs given, aniline and water, which have an 84°C difference, are the best candidate for fractional distillation due to the significant boiling point difference.