Learning Objective: To study how to determine by qualitative analysis the physical properties of the state, nature, odor, color, solubility, and physical constants of an unknown organic compound.
Skill Level - Intermediate
Prerequisites:
Chapter: Physical Properties
Sub-topic: Preliminary Qualitative Investigation of an unknown compound
Author's Note: The most exciting part of science is the investigation, where we put on a detective hat to explore the unknown. Broadly, the two common ways we do that are - qualitatively and quantitatively. In qualitative, we figure out what we see, hear, and smell without putting any numeric value to the observation, whereas in quantitative, the observations are assigned a number. For example, the yellow color of a liquid is a qualitative analysis, whereas its 0.9 g/cm3 density is a quantitative analysis.
In this section, we have covered preliminary investigations that typically accompany the qualitative analysis of an organic compound. We have limited the experiments to cover the physical property measurement techniques like the testing for water solubility, flame test, nature, odor, and color of popular organic compounds and classified them according to their probable functional groups. These experiments do not in any way change the identity of the compound but reveal only its nature. Since, evaluating the identity of an organic compound based on only its physical properties is rarely sufficient. Still, we have tried to explain how qualitative physical property measurements are the first step toward understanding an unknown compound's nature and identity.
Preliminary Qualitative Analysis of some Organic Compounds
Intensive Physical Property Measurements
The first preliminary investigation of an unknown organic compound employs the sense of sight and smell. As you know, lab practices strictly prohibit tasting and touching organic compounds. So, we look for whether the unknown compound is solid or liquid, its color, whether it has an odor, and how it burns.
If solid, whether it has a crystalline or amorphous appearance. And when liquid, is it colorless, yellowish, beige, or has any other shade? Some organic compounds belonging to a specific class have a peculiar characteristic smell. For example, most esters smell fruity, and thiol (-SH) containing functional groups smell like rotten eggs. So, visual and olfactory investigation becomes the first step in solving the case of an unknown compound.
Similarly, certain assumptions are made based on molecular weights. It is observed that lower hydrocarbon compounds containing up to 4 carbons are gaseous, mid-sized ones from 5 to 17 carbons are liquid and higher hydrocarbons with 18 or more carbons are solid at room temperature.
Additionally, the compound’s nature is tested using the litmus strips. When the unknown sample is mixed with water and alcohol and litmus paper is dipped, the change in color of the litmus would reveal its nature, whether acidic, basic, or neutral.
Such simple experiments are .....
Our focus will be on organic chemistry analytical practices commonly employed in an undergraduate/graduate chemistry lab.
Test | Observation | Inference |
|---|---|---|
Nature | i) Solid | Carbohydrate, acid, phenol, amine, higher hydrocarbon may be present. |
ii) Liquid | Alcohol, ketone, aldehyde, ester, phenol, amines, may be present. |
Determination of Physical Constants
Several factors like molecular weight, intermolecular forces, and the presence of functional groups influence the physical constants of melting and boiling point. Generally, smaller molecular-weight compounds have lower physical constants than larger molecular-weight compounds due to stronger London dispersion forces. The molecules with polar functional groups or functional groups capable of showing hydrogen bonding exhibit elevated melting and boiling points than those without such groups.
Therefore, physical constant measurements of melting and boiling point can provide a rough estimate....
