StemCET Logo

Chemistry Question 72 – JEE-MAIN 2026

← Back
Consider the isomers of hydrocarbon with molecular formula C5H10. These isomers do not decolourise KMnO4 solution. These isomers are subjected to chlorination with chlorine in presence of light to give monochloro compounds. The total number of monochloro compounds (structural isomers only) formed is _______.

Calculate the Degree of Unsaturation (DBE) for C5H10 and use the KMnO4 test result to identify the class of hydrocarbons.

Step 1: Identify the parent hydrocarbons✦ Active

The molecular formula C5H10 has a Degree of Unsaturation (DBE) of 1. The condition "do not decolourise KMnO4 solution" indicates that the hydrocarbons are saturated or cycloalkanes, as alkenes and alkynes would decolorize KMnO4. Given DBE=1, the hydrocarbons must be cycloalkanes.

The structural isomers of C5H10 that are cycloalkanes are:

1. Cyclopentane 2. Methylcyclobutane 3. 1,1-Dimethylcyclopropane 4. 1,2-Dimethylcyclopropane 5. Ethylcyclopropane

Step 2: Determine unique hydrogen environments for each isomer○ Expand

Free radical chlorination replaces one hydrogen atom with a chlorine atom. Each unique set of equivalent hydrogen atoms in a molecule will lead to a distinct structural monochloro isomer.

- For Cyclopentane: All 10 H atoms are equivalent. (1 unique H environment) - For Methylcyclobutane: There are 4 unique sets of H atoms (methyl CH3, ring CH, ring CH2 adjacent to CH, ring CH2 opposite to CH). (4 unique H environments) - For 1,1-Dimethylcyclopropane: There are 2 unique sets of H atoms (two equivalent methyl CH3 groups, two equivalent ring CH2 groups). (2 unique H environments) - For 1,2-Dimethylcyclopropane: There are 3 unique sets of H atoms (two equivalent methyl CH3 groups, two equivalent ring CH groups, one ring CH2 group). (3 unique H environments) - For Ethylcyclopropane: There are 4 unique sets of H atoms (ethyl CH2, ethyl CH3, ring CH attached to ethyl, two equivalent ring CH2 groups). (4 unique H environments)

💡 Teacher's Secret Hint

Remember to consider symmetry carefully to identify equivalent hydrogen atoms. Stereoisomers are not counted for the final monochloro products.

Step 3: Calculate the total number of monochloro compounds○ Expand

Summing the number of unique hydrogen environments for each structural isomer:

Total=(Cyclopentane: 1)+(Methylcyclobutane: 4)+(1,1-Dimethylcyclopropane: 2)+(1,2-Dimethylcyclopropane: 3)+(Ethylcyclopropane: 4)
Total=1+4+2+3+4=14

Therefore, a total of 14 structural monochloro compounds are formed.

✦ STEM Console utilizes AI models to generate step-by-step explanations and math clues. AI can make mistakes.