Question

Which of the following compounds gives blood red colouration when its Lassaigne's extract is treated with alkali and ferric chloride?
A.Thiourea
B.Diphenyl Sulphide
C.Phenyl hydrazine
D.Benzamide

Answer 1

Hint: The Lassaigne's extract is prepared for carrying out the test on the organic compounds in order to detect the content of some specific elements.
Each element or combination of elements give different colours and are characterised by those colours.

Complete step-by-step answer: Lassaigne’s Test is a kind of test which is done on a compound in order to detect the content of some specific elements, namely Sulphur, Bromine, nitrogen, Iodine. This test is carried out by following two simple steps. The first step involves the preparation of the extract of sodium fusion, or also called sodium fusion extract which is the full form of SFE. In the next step detection of these said elements is done using this extract.
Now, we will be discussing the preparation of the extract and then we will relate that information with the information provided in the question.
The preparation of the SFE involves a small quantity of the organic substance which we would be testing, and this organic substance is fused with a small amount of sodium metal inside an instrument called a fusion tube. Now this tube is heated till it becomes red hot, and then it is plunged into pure distilled water and the substance inside it is boiled for some minutes and then again it is cooled and then it goes through the process of filtration.
The filtrate which is obtained, is termed as the sodium fusion extract or the Lassaigne’s extract. It is generally alkaline in nature. If the extract is somehow, not alkaline, a few drops of sodium hydroxide may be added to make it alkaline.
The next step consists of detection of the elements using the extract. If an organic compound contains both the elements sulphur as well as nitrogen, it will give a blood red coloured product.
Now, in the given options only the first option which is Thiourea contains carbon, nitrogen as well as sulphur. So, it will give a red coloured final product.

Hence, the correct answer would be A, which is Thiourea.

Note: Lassaigne’s Test is a kind of test which is done on a compound in order to detect the content of a number of specific elements, which are Sulphur, Bromine, nitrogen, Iodine.
If a compound contains only nitrogen, it will give an end product of Prussian blue colour. Similarly, sulphur containing compounds give deep violet colour.