Problem 100 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Which of the following oil is obtained from benzene after fractional distillation of coal tar? [BHU 2004] (a) Light oil (b) Heavy oil (c) Middle oil (d) Anthracene oil

Step-by-Step Solution

Answer

Light oil is the correct answer.

Step 1: Understanding Coal Tar Distillation

Coal tar is a by-product obtained in the production of coke and coal gas from coal. It undergoes fractional distillation, a process of separation by heating, to obtain different fractions based on their boiling points.

Step 2: Identify Benzene Fraction

Benzene is a light, volatile component found in the fraction known as 'Light oil'. This fraction is distilled at lower boiling points compared to other heavier fractions.

Step 3: Determine the Correct Option

Among the options given, 'Light oil' corresponds to the fraction where benzene is primarily obtained due to its lower boiling point.

Key Concepts

Benzene Fraction

Benzene is a key component of coal tar and is mainly found in a specific fraction during its distillation process. The distillation of coal tar, which is a by-product from the production of coke and coal gas, allows us to separate chemicals based on their boiling points. Among these fractions, benzene is an important light and volatile compound. It belongs to the 'Light oil' fraction due to its lower boiling point compared to heavier oils.

This process of extraction is called fractional distillation, where different chemicals are separated in a sequence depending on their boiling temperatures. The 'Benzene Fraction' occurs early in this sequence as it evaporates at a lower temperature.

In summary, understanding where benzene fits in the distillation process is crucial. Since it is obtained primarily in the 'Light oil' fraction, it is separated early and forms a critical part of many industrial applications.

Fractional Distillation

Fractional distillation is a technique used to separate mixtures into different components. Specifically, it is valuable in the petrochemical industry for refining coal tar. In this method, components of a mixture are boiled and separated based on their distinct boiling points.

The process involves heating the coal tar in a distillation apparatus. As the temperature rises, different fractions or mixtures begin to boil and turn into vapor at various levels. These vapors are then condensed back into liquids at different stages of the distillation column. The key here is that each fraction boils at a specific temperature range, allowing separation.

For coal tar, the lighter fractions, like benzene, distill out first at lower temperatures, while heavier oils separate at higher temperatures. This method is critical not only for isolating benzene but also for obtaining a range of products used in chemical manufacturing, dyes, and pharmaceuticals.

Light Oil

Light oil is a fraction obtained from coal tar distillation and is characterized by its lower boiling point. This fraction is where benzene, among other compounds, is typically found. Light oil plays an essential role in the industrial sector due to its versatility.

Key components of light oil include benzene, toluene, and xylene, all of which have industrial significance. These substances are foundational in producing other chemicals, materials, and are used as solvents.

The reason benzene and other compounds are grouped into the 'Light oil' fraction is their volatility. This means they evaporate quickly at lower temperatures compared to 'Middle oil' or 'Heavy oil'. Understanding these properties helps in effectively separating and utilizing coal tar fractions for various applications in everyday products.