Learn Qualitative Analysis of Organic Compounds with Easy Methods

Qualitative analysis of organic compounds shows us how to find out which elements are present in an organic compound. These could be carbon, hydrogen, nitrogen, sulphur, or halogens (such as chlorine and bromine). This type of analysis in organic chemistry helps us understand what a compound is composed of. It is essential to know this before we begin to dive into reactions or formulas.

Just remember

Qualitative analysis is all about detection. It's not about measuring amounts like quantitative analysis. In this chapter, 

Qualitative analysis is essential in organic chemistry, and it is also important to learn it for these reasons.

• This is a fundamental concept in organic chemistry. It helps you analyse the presence of different elements that exist in organic compounds.
• Qualitative analysis of organic compounds in advanced chemistry at bachelor's and postgraduate levels helps students learn about reaction mechanisms, synthesis, and how structures are determined.  You can also refer to the IISER Chemistry Syllabus for reference, as it requires knowledge of this topic. 

By now, you are thinking of how to perform qualitative analysis of organic compounds. You have to be clear with the standard tests we are outlining below..

1. Finding Carbon and Hydrogen

Let's say we have to look for carbon and hydrogen.

What we do is heat an organic compound with dry copper oxide $\left(\mathrm{C}\mathrm{u}\mathrm{O}\right)$ in a test tube. Once we do that, here's what we can see. 

• Carbon turns into carbon dioxide $\left(C{O}_2\right)$ . If you pass this gas through lime water $\left(\mathrm{C}\mathrm{a}{\left(\mathrm{O}\mathrm{H}\right)}_2\right)$ , it turns milky. The reason is that there is calcium carbonate formation $\left(CaC{O}_3\right)$ : ${\mathrm{C}\mathrm{O}}_2+\mathrm{C}\mathrm{a}{\left(\mathrm{O}\mathrm{H}\right)}_2\mathrm{}->{\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}$
• Hydrogen turns into water $\left(H_{2}O\right)$ . You'll see water droplets on the cooler part of the test tube. To confirm, this water turns white anhydrous copper sulphate $\left(CuS{O}_4\right)$ blue: ${\mathrm{C}\mathrm{u}\mathrm{S}\mathrm{O}}_4+5{\mathrm{H}}_2\mathrm{O}->{\mathrm{C}\mathrm{u}\mathrm{S}\mathrm{O}}_4\cdot 5{\mathrm{H}}_2\mathrm{O}$

This test shows that carbon and hydrogen are present. It's also true for almost all organic compounds.

2. Finding Nitrogen, Sulfur, and Halogens (Lassaigne's Test)

Lassaigne's test is a well-known method to detect nitrogen, sulphur, and halogens. Here are the steps.

First, we mix the organic compound with sodium metal and heat it in a small tube (called fusion). This makes a sodium fusion extract, which we use for the following tests:

a) Test for Nitrogen: If the compound has nitrogen, it forms sodium cyanide $\left(NaCN\right)$ during fusion.

To test, add sodium hydroxide $\left(NaOH\right)$ , ferrous sulfate $\left(FeS{O}_4\right)$ , and a few drops of ferric chloride $\left(FeC{l}_3\right)$ to the extract.

After that, you can add hydrochloric acid $\left(HCl\right)$ .

If you see a Prussian blue colour, nitrogen is present.

This happens because of a compound called ferric ferrocyanide: $\begin{array}{c}F{e}^{2+}+6C{N}^{-}->[Fe{\left(CN\right)}_6{]}^{4-}\\ 3[Fe{\left(CN\right)}_6{]}^{4-}+4F{e}^{3+}->F{e}_4{\left[Fe{\left(CN\right)}_6\right]}^3\end{array}$

b) Test for Sulphur: If sulphur is there, it forms sodium sulphide $\left(N{a}_{2}S\right)$ .

We can check in two ways:

• Add lead acetate $\left(\mathrm{P}\mathrm{b}{\left({\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\right)}_2\right)$ to the extract. A black precipitate of lead sulphide $\left(PbS\right)$ means sulphur is present: ${\mathrm{N}\mathrm{a}}_2\mathrm{S}+\mathrm{P}\mathrm{b}{\left({\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\right)}_2\mathrm{}\mathrm{}\mathrm{}->\mathrm{P}\mathrm{b}\mathrm{S}+2{\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{N}\mathrm{a}$
• Add sodium nitroprusside $\left(N{a}_2\right[Fe{\left(CN\right)}_{5}NO\left]\right)$ . A violet colour confirms sulphur: $N{a}_{2}S+N{a}_2\left[Fe{\left(CN\right)}_{5}NO\right]->N{a}_4\left[Fe{\left(CN\right)}_{5}NOS\right]$

c) Test for Halogens: Halogens (like chlorine, bromine, or iodine) form sodium halides (NaX, where X is $\mathrm{C}\mathrm{l},\mathrm{B}\mathrm{r}$ , or I )). To test, add nitric acid $\left(HN{O}_3\right)$ to the extract, then silver nitrate $\left(AgN{O}_3\right)$ . Look for these precipitates:

• White precipitate $\left(\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l}\right)$ , dissolves in ammoniameans chlorine.
• Pale yellow precipitate $\left(\mathrm{A}\mathrm{g}\mathrm{B}\mathrm{r}\right)$ , partially dissolves in ammoniameans bromine.
• Yellow precipitate (AgI), doesnt dissolve in ammoniameans iodine. $\mathrm{N}\mathrm{a}\mathrm{X}+{\mathrm{A}\mathrm{g}\mathrm{N}\mathrm{O}}_3->\mathrm{A}\mathrm{g}\mathrm{X}+{\mathrm{N}\mathrm{a}\mathrm{N}\mathrm{O}}_3$

If both nitrogen and sulfur are present, fusion makes sodium thiocyanate (NaSCN). Add FeCl 3 , and youll see a blood-red color: $NaSCN+FeC{l}_3\mathrm{}\mathrm{}\mathrm{}\mathrm{}->Fe{\left(SCN\right)}_3+NaCl$

 

3. Finding Oxygen

 

There is no direct test for oxygen in qualitative analysis of organic compounds.

We guess it's there if the compound has oxygen-containing groups. That could include -OH (in alcohols) or -COOH (in acids). 

Here are two questions on qualitative analysis that you should get started with. 

1. Question: A compound gives a Prussian blue color after sodium fusion, $FeS{O}_4$ , and $FeC{l}_3$ . Which element is there?

Answer: Prussian blue means nitrogen is present. It forms NaCN, which makes ferric ferrocyanide.

Tip: Prussian blue is the key sign for nitrogen!

 

2. Question: A compound gives a yellow precipitate with $AgN{O}_3$ after sodium fusion, and it doesnt dissolve in ammonia. Which element?

Answer: Yellow precipitate that doesnt dissolve in ammonia is AgI, so iodine is present.

Here are some points to remember so that you don't get confused before your exams. 

• Carbon and hydrogen: Heat with CuO , look for $C{O}_2$ (milky lime water) and $H_{2}O$ (blue $CuS{O}_4$ ).
• Lassaignes test: Nitrogen (Prussian blue), sulphur (black precipitate or violet colour), halogens (white/yellow precipitates with $AgN{O}_3$ ).
• Oxygen: No direct test, guess from functional groups.
• Add $HN{O}_3$ before halogen test to avoid interference.

• If nitrogen and sulphur are both there, you'll get a blood-red colour, not Prussian blue. 

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  If nitrogen is present, the compound might be an amine. Halogens might mean an alkyl halide. This is where your concept of functional groups should be clear.