JEE Main Chemistry Question (2026) — Solution

Step-by-step solution

The general formula of an alkane is C_n H_ 2n+2 . The balanced chemical equation for the complete combustion of an alkane is: C_n H_ 2n+2 + ( 3n+1 2 ) O_2 n CO_2 + (n+1) H_2O Given that 8 moles of oxygen are required for complete combustion: 3n+1 2 = 8 3n + 1 = 16 n = 5 The molecular formula of the alkane (Y) is C_5H_ 12 . Since (Y) yields only one monochlorinated product (Z) upon reaction with Cl_2/h , all the hydrogen atoms in the alkane must be equivalent. Among the isomers of C_5H_ 12 (n-pentane, isopentane, and neopentane), only 2,2-dimethylpropane (neopentane) has all equivalent hydrogen atoms. The structure of neopentane is C(CH_3)_4. In this structure, the central carbon is quaternary, and the four surrounding methyl carbons are primary carbon atoms. Therefore, the total number of primary carbon atoms in (Y) is 4. Answer: 4