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Chemistry Question 64 – JEE-MAIN 2026

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Identify the correct IUPAC name of hydrocarbon (x) containing three primary carbon atoms and with molar mass 72 g mol−1.

Use the given molar mass to find the general formula of the hydrocarbon.

Step 1: Determine the Molecular Formula✦ Active

The molar mass of the hydrocarbon is 72 g mol−1. For an alkane with the general formula CnH2n+2, the molar mass is 12n+(2n+2)=14n+2. Setting this equal to the given molar mass:

14n+2=72 ⟹14n=70 ⟹n=5

Thus, the molecular formula of the hydrocarbon is C5H12.

Step 2: Evaluate Options based on Molecular Formula and Primary Carbons○ Expand

We will now examine each option to check if it has the molecular formula C5H12 and contains exactly three primary carbon atoms.

1. **1,1-Dimethylcyclopropane:** This is a cycloalkane with 5 carbons. Its general formula is CnH2n, so for n=5, it's C5H10. Its molar mass is 5×12+10×1=70 g mol−1. This does not match 72 g mol−1.

2. **2,2-Dimethylpropane (Neopentane):** Structure is C(CH3)4. Its molecular formula is C5H12. It has four CH3 groups, meaning it has 4 primary carbon atoms. This does not match the requirement of three primary carbons.

3. **2-Methylbutane (Isopentane):** Structure is CH3−CH(CH3)−CH2−CH3. Its molecular formula is C5H12. It has three CH3 groups (at C1, C4, and the methyl branch at C2), meaning it has 3 primary carbon atoms. This matches both criteria.

4. **n-pentane:** Structure is CH3−CH2−CH2−CH2−CH3. Its molecular formula is C5H12. It has two CH3 groups (at C1 and C5), meaning it has 2 primary carbon atoms. This does not match the requirement of three primary carbons.

💡 Teacher's Secret Hint

Remember that primary carbons are those bonded to only one other carbon atom.

Step 3: Identify the Correct Option○ Expand

Based on the analysis, only 2-Methylbutane has the correct molecular formula (C5H12) and contains three primary carbon atoms.

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