Qualitative Analysis of Organic Compounds

Edited By Shivani Poonia | Updated on Oct 19, 2024 12:43 PM IST

Imagine one day you will be able to unlock the secrets of organic compounds by doing a few simple tests—that is the power of qualitative analysis. It is an inseparable tool in the sphere of chemistry that shows a way for the identification of elements and functional groups of organic substances. Be it a student approaching this amazing sphere of organic chemistry or a professional researcher trying to understand the secrets of complicated molecules, qualitative analysis will help you.

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This Story also Contains

Fundamentals of Qualitative Analysis
Bases of Qualitative Analysis
Applications and Relevance
Some Solved Examples
Conclusion

Qualitative Analysis of Organic Compounds

In the paper, a deep dive into the world of qualitative analysis of organic compounds is made together with all its major concepts, various features, and practical applications. By the end, you will have fully grasped this powerful analytical technique and how it shapes our understanding of the organic world around us. Qualitative analysis defines which elements or functional groups are contained in an organic compound. In contrast to quantitative analysis, which indicates how much of each component is present, it seeks an answer to "What is present?" rather than "How much is present?"

Fundamentals of Qualitative Analysis

The most prevalent factors in an organic compound include carbon, hydrogen, oxygen, nitrogen, sulfur, and halogens. Qualitative analysis therefore comprises a series of tests for detecting the above-mentioned elements and identification of the different functional groups present in the compound.

One of the significant stages of qualitative analysis is carbon and hydrogen detection. This is normally attained through Liebig's combustion method, where the compound to be analyzed is heated up in the presence of copper II oxide CuO. Carbon in the compound is oxidized into carbon dioxide gas CO₂which is identified through its passage through limewater, Ca(OH)₂ since it turns milky. Hydrogen, on the other hand, is turned to water, H₂O, identifiable by its reaction with anhydrous copper II sulfate CuSO₄

Other elements such as nitrogen, sulfur, and halogens require special tests. The nitrogen is tested through Lassaigne's test by fusing it with sodium metal and the formed sodium cyanide is tested by its reaction with ferrous sulfate and ferric chloride to form Prussian blue.

Carbon and hydrogen are detected by heating the compound with copper(II) oxide. Carbon present in the compound is oxidized to carbon dioxide (tested with lime water, which develops turbidity) and hydrogen to water (tested with anhydrous copper sulfate, which turns blue).

\begin{aligned}
& \mathrm{C}+2 \mathrm{CuO} \xrightarrow{\Delta} 2 \mathrm{Cu}+\mathrm{CO}_2 \\
& 2 \mathrm{H}+\mathrm{CuO} \xrightarrow{\Delta} \mathrm{Cu}+\mathrm{H}_2 \mathrm{O} \\
& \mathrm{CO}_2+\mathrm{Ca}(\mathrm{OH})_2 \longrightarrow \mathrm{CaCO}_3 \downarrow+\mathrm{H}_2 \mathrm{O} \\
& 5 \mathrm{H}_2 \mathrm{O}+\mathrm{CuSO}_4 \longrightarrow \mathrm{CuSO}_4 \cdot 5 \mathrm{H}_2 \mathrm{O}
\end{aligned}

Bases of Qualitative Analysis

These will be dealt with along with their techniques and procedures of qualitative analysis. Some of the important features are explained herein:

1. Solubility tests:

These are carried out to examine the nature of solubility of the compound in water, dilute acids, and dilute bases. The nature of this solubility behavior helps in inferring the basic nature of the compound and roughly classifying it.

2. Functional group analysis:

The determination of what functional groups are contained in an organic compound is very important in establishing its reactivity, and hence the probable uses. Special tests are conducted for the identification of certain groups such as alcohols, aldehydes, ketones, carboxylic acids, and amines.

3. Derivative preparation:

An organic compound is sometimes identified by the preparation of its derivative, which may be some certain ester or 2,4-dinitrophenylhydrazone. Its melting point is matched against the known values to establish the identity of the compound.

4. Chromatographic techniques:

Thin-layer and column chromatography afford routes to the separation and purification of organic compounds. From TLC, useful information about polarity and identification can be derived through the retardation factor, Rf, that one obtains from TLC.

5. Spectroscopic methods:

Infrared, nuclear magnetic resonance, and mass spectroscopy are some of the sophisticated techniques used to obtain sufficient structural information about organic compounds. All these techniques provide specific fingerprints for the various functional groups which go a long way in the identification of unknown compounds.

Applications and Relevance

The qualitative analysis of organic compounds finds applications in several fields, including:

1. Pharmaceutical industry: Qualitative analysis is central to pharmaceutical drug development and quality control. Examples may be in the identification of active ingredients, detection of impurities, and assessment of purity and safety in relation to formulations of drugs.

2. Forensic science: This is part of forensic science in the identification and characterization of unknown materials from scenes of crime. They include, for instance, drugs, explosives, and poisons.

3. Environmental monitoring: Qualitative analysis is applied in the detection and identification of organic pollutants either in air, water, or soil samples. This information is very vital in assessing its impact on the environment and developing strategies for remediation.

4. Food and beverage industries: Qualitative analysis is done to assure quality and safety by detecting of adulterants, preservatives, and contaminants in food and beverages.

5. Academic research: Qualitative analysis in organic chemistry assumes a critical role in the identification of newly synthesized compounds, studies of natural products, and respective derivatives in the structure elucidation of these compounds.

Some Solved Examples

Example 1:
Question:
In the detection of carbon in an organic compound, which compound is the organic substance heated with?

1. Copper (II) Oxide
2. Iron (III) Oxide
3. Copper (I) Oxide
4. None of these

Solution:
Carbon atoms present in an organic compound are detected by heating the compound with copper(II) oxide. During this process, carbon is oxidized to CO(2) while CuOs reduced to metallic copper.

$[\mathrm{C +2 CuO \longrightarrow CO_2 + 2 Cu}]$

Hence, the answer is option (1).

Example 2:
Question:

Which one of the following formulas is used for the calculation of the presence of percentage Hydrogen in an organic compound?

1)$\frac{12 \times \text { mass of } \mathrm{CO}_2 \times 100}{44 \times \text { mass of O.C }}$
2) $\frac{2 \times \text { mass of } \mathrm{H}_2 \mathrm{O} \times 100}{18 \times \text { mass of } \mathrm{O} . \mathrm{C}}$
3)$\frac{1.4 \times \mathrm{N} \times \mathrm{V}}{\text { mass of } \mathrm{O} . \mathrm{C}}$
4)$
\frac{32 \times \text { mass of } \mathrm{BaSO}_4 \times 100}{233 \times \text { mass of } \mathrm{O} . \mathrm{C}}
$
Solution:

As we have learned,

Detection of Hydrogen

The hydrogen atom present in an organic compound is detected by heating the organic compound with CuO wherein all the hydrogen present in the compound is converted into H₂O

In Liebig's method, the percentage of hydrogen in an organic compound can be computed by the formula

$\% \mathrm{H}=\frac{2 \times \text { mass of } \mathrm{H}_2 \mathrm{O} \times 100}{18 \times \text { mass of } \mathrm{O} . \mathrm{C}}$

Hence, the answer is the option (2).

Example 3:
Question:

The gas released during the detection of carbon in an organic compound is (X) and this gas turns lime water milky by forming the precipitate (Y). The gas (X) and the precipitate (Y) respectively are :

1) $\mathrm{CO}_2$ and $\mathrm{Ca}(\mathrm{OH})_2$
2) CO and $\mathrm{CaCO}_3$
3) $\mathrm{CO}_2$ and $\mathrm{CaCO}_3$
4) CO and $\mathrm{Ca}(\mathrm{OH})_2$

Solution:

As we have learned,

Carbon present in the compound is oxidized to CO₂ which is tested by Lime water.

When CO₂ is passed through lime water Ca(OH)₂ , it turns milky due to the formation of CaCO₃

$\mathrm{CO}_2+\mathrm{Ca}(\mathrm{OH})_2 \longrightarrow \mathrm{CaCO}_3+2 \mathrm{H}_2 \mathrm{O}$

Hence, the answer is the option (3).

Hence, the answer is option (3).

Example 4:
Question:

During the detection of hydrogen in an organic compound, blue colour appears due to the presence of

1) $\mathrm{CaCO}_3$
2) $\mathrm{Ca}(\mathrm{OH})_2$
3) $\mathrm{Cu}\left(\mathrm{NO}_3\right)_2$
4) $\mathrm{CuSO}_4 \cdot 5 \mathrm{H}_2 \mathrm{O}$

Solution:

As we have learned,

Hydrogen present in the compound is converted to water vapour which is tested by anhydrous Copper Sulphate. Anhydrous CuSO₄ is colorless and turns blue due to the formation of a complex with water molecules.

$\mathrm{CuSO}_4$ (anhyd.) $+5 \mathrm{H}_2 \mathrm{O} \longrightarrow \mathrm{CuSO}_4 \cdot 5 \mathrm{H}_2 \mathrm{O}$

Hence, the answer is the option (4)

Conclusion

It is therefore an important kind of analytical technique in the world of chemistry, cutting across such areas of development as pharmaceuticals, environmental monitoring, and forensic investigations. In this regard, knowledge and mastery of qualitative analytical techniques give the key to the chemist to unlock mysteries of organic compounds and unravel mysteries of complex molecules in efforts toward advancing science and technology.