Problem 70 - A Complete Resource Book in Chemistry for JEE Main

Question

The identity of two different solid organic compounds having the same melting point can be best ascertained by determining their (a) dipole moment (b) solubility (c) melting point of their \(1: 1\) mixture (d) refractive indices

Step-by-Step Solution

Answer

(c) Melting point of their 1:1 mixture is the best method for identification.

Step 1: Understanding the Problem

We have two different solid organic compounds with the same melting point, and we must find the best method to ascertain their identities by distinguishing between them. The options given are dipole moment, solubility, melting point of their 1:1 mixture, and refractive indices.

Step 2: Identifying Similarities and Differences

Since both compounds have the same melting point, measuring the melting point alone will not distinguish between them. Other properties like dipole moment, solubility, and refractive index could potentially differ based on the specific structure and molecular makeup of the compounds.

Step 3: Analyzing the 1:1 Mixture

The melting point of a 1:1 mixture is a key technique known as a mixed melting point test. If two compounds are identical, the melting point of their mixture will remain the same. If they are different, the mixture will show a depressed and broader melting point range.

Step 4: Choosing the Best Method

Among the given options, determining the melting point of their 1:1 mixture is the standard and most direct method to distinguish two different solid organic compounds with the same melting point. This method leverages the principle of melting point depression seen in mixtures.

Key Concepts

Melting Point Analysis

Melting point analysis is a fundamental technique in organic chemistry used to characterize solid organic compounds. This technique involves determining the temperature at which a solid turns into a liquid. The melting point of a pure compound is a sharp, specific temperature, often used to verify the purity of a compound. Impurities usually lead to a depression and broadening of the melting point range. By comparing the experimental melting point with known values, chemists can gain insights into the identity and purity of a compound. Melting point analysis is one of the simplest methods to ascertain the quality and authenticity of a compound in laboratory settings.

Mixed Melting Point Test

A mixed melting point test is a clever method used to identify two different compounds that have the same pure melting point. To perform this test, equal parts of the two compounds are mixed and their melting point is measured.

• If the compounds are identical, the mixture will have a sharp melting point identical to the pure substances.
• If the compounds are different, the melting point of the mixture will be lower and will cover a broader temperature range.

This approach leverages the principle of melting point depression, a phenomenon seen when different substances are mixed, causing a disruption in the orderly structure needed for a sharp melting point. This method is widely used for distinguishing close analogs in organic chemistry.

Organic Compounds Identification

Identifying organic compounds is crucial in organic chemistry for both research and industry applications. Various techniques are used to determine the identity of compounds based on their physical and chemical properties. Apart from melting point analysis, chemists may use

• infrared spectroscopy to assess functional groups,
• nuclear magnetic resonance (NMR) spectroscopy for molecular structure,
• mass spectrometry for molecular weight and composition.

These methods allow scientists to build a comprehensive understanding of a compound's identity. The mixed melting point test is particularly useful when dealing with compounds that have similar physical appearances and melting points, as it provides a straightforward way to differentiate them.

Physical Properties of Compounds

The physical properties of organic compounds—such as melting point, boiling point, solubility, refractive index, and molecular weight—play a vital role in their identification and characterization. These properties arise from the molecular structure and interactions between molecules, influencing how compounds behave in different environments.

• The boiling point and melting point provide clues about intermolecular forces.
• The solubility helps to understand interactions with different solvents.
• Refractive index measures how light passes through a substance, which can aid in distinguishing between compounds.

Through the study and analysis of these physical properties, chemists can deduce significant information about a compound's structure and uses, further aiding in research and development.