Hydrocarbons JEE Main previous year questions with solutions

5 solved JEE Main questions on Hydrocarbons, free to read — no sign-in needed. The full chapter has 242 questions; sign in to attempt the remaining 237 in the exam simulator.

1. Q1JEE Main 2026 (02 Apr, Shift 1)Preparation and Reactions of cycloalkanes and cycloalkenes
   Consider the isomers of hydrocarbon with molecular formula $C_5H_{10}$. These isomers do not decolourise $KMnO_4$ 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 _______.
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   Answer: 14

   The hydrocarbon with molecular formula $C_5H_{10}$ has a degree of unsaturation of $1$. Since its isomers do not decolourise $KMnO_4$ solution, they do not contain any carbon-carbon double bonds. Thus, they must be cycloalkanes. The possible structural isomers of cyclic $C_5H_{10}$ and the number of their monochloro structural isomers are as follows: 1. Cyclopentane: All carbon atoms are equivalent. Chlorination gives $1$ monochloro product (chlorocyclopentane). 2. Methylcyclobutane: There are $4$ different types of hydrogen atoms (on the methyl group, on $C_1$, on $C_2/C_4$, and on $C_3$). Chlorination gives $4$ monochloro products. 3. Ethylcyclopropane: There are $4$ different types of hydrogen atoms (on the $CH_3$ of ethyl, on the $CH_2$ of ethyl, on $C_1$ of the ring, and on $C_2/C_3$ of the ring). Chlorination gives $4$ monochloro products. 4. 1,1-Dimethylcyclopropane: There are $2$ different types of hydrogen atoms (on the two equivalent methyl groups, and on the two equivalent $CH_2$ groups of the ring). Chlorination gives $2$ monochloro products. 5. 1,2-Dimethylcyclopropane: There are $3$ different types of hydrogen atoms (on the two equivalent methyl groups, on the two equivalent $CH$ groups of the ring, and on the $CH_2$ group of the ring). Chlorination gives $3$ monochloro products. Since each of these cycloalkanes has a different carbon skeleton, their monochloro derivatives will all be distinct structural isomers. Total number of monochloro compounds (structural isomers only) = $1 + 4 + 4 + 2 + 3 = 14$. Answer: $14$
2. Q2JEE Main 2025 (03 Apr, Shift 1)Reactions of alkanes
   0.5 g of an organic compound on combustion gave 1.46 g of $\mathrm{CO}_2$ and 0.9 g of $\mathrm{H}_2 \mathrm{O}$. The percentage of carbon in the compound is _______. (Nearest integer) [Given : Molar mass (in $\mathrm{g} \mathrm{mol}^{-1}$) $\mathrm{C}: 12, \mathrm{H}: 1$, O: 16]
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   Answer: 80

   Organic $\rightarrow \mathrm{CO}_2$ Compound Applying POAC on ' C ' (mole) of ' C ' in compound $=\mathrm{n}_{\mathrm{CO}_2} \times 1$ So mass of ' C ' in compound $=\frac{1.46}{44} \times 12$ So, $\%$ of ' C ' in compound $=\frac{1.46}{44} \times \frac{12}{0.5} \times 100$ $=79.63$
3. Q3JEE Main 2024 (31 Jan, Shift 1)Properties and Preparation of Alkanes
   Number of alkanes obtained on electrolysis of a mixture of ${CH}_{3}COONa$ and ${C}_{2}{H}_{5}COONa$ is_____.
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   Answer: 3

   An aqueous solution of sodium or potassium salt of carboxylic acid is electrolysed in this reaction, resulting in the dissociation of the salt into a carboxylate ion and sodium or potassium ions. The total number of different alkanes are formed when the following mixture is subjected to electrolysis: ${CH}_{3}{COONa}_{\left(aq\right)}and{C}_{2}{H}_{5}{COONa}_{\left(aq\right)}$ In this ethane, butane and propane are formed. ${CH}_{3}COONa\rightarrow C^{∙}{H}_{3}$ ${C}_{2}{H}_{5}COONa\rightarrow {C^{∙}}_{2}{H}_{5}$ $2{C^{∙}}_{2}{H}_{5}\rightarrow {CH}_{3}-{CH}_{2}-{CH}_{2}-{CH}_{3}$ $2C{H^{∙}}_{3}\rightarrow {CH}_{3}-{CH}_{3}$ $C^{∙}{H}_{3}+{C^{∙}}_{2}{H}_{5}\rightarrow {CH}_{3}-{CH}_{2}-{CH}_{3}$ So, the total number of alkanes formed here will be $3$
4. Q4JEE Main 2023 (24 Jan, Shift 2)Preparation and Properties of Benzene
   Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R. Assertion A : Benzene is more stable than hypothetical cyclohexatriene. Reason R : The delocalized $\pi$ electron cloud is attracted more strongly by nuclei of carbon atoms. In the light of the above statements, choose the correct answer from the options given below:
   1. A.A is true but R is false.
   2. B.A is false but R is true.
   3. C.Both A and R are correct and R is the correct explanation of A.
   4. D.Both A and R are correct but R is NOT the correct explanation of A.
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   Answer: (C)

   The delocalised $\pi -$electron cloud is attracted more strongly by the nuclei of the carbon atoms than the electron cloud localised between two carbon atoms. Therefore, presence of delocalised $\pi -$electrons in benzene makes it more stable than the hypothetical cyclohexatriene.
5. Q5JEE Main 2022 (24 Jun, Shift 2)Properties and Preparation of Alkenes
   Given below are two statements. Statement I: The presence of weaker $\pi$-bonds make alkenes less stable than alkanes Statement II: The strength of the double bond is greater than that of carbon-carbon single bond. In the light of the above statements, choose the correct answer from the options : given below.
   1. A.Both Statement I and Statement II are correct.
   2. B.Both Statement I and Statement II are incorrect.
   3. C.Statement I is correct but Statement II is incorrect.
   4. D.Statement I is incorrect but Statement II is correct.
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   Answer: (A)

   As sigma bond is formed along the internuclear axis it is stronger than pi-bond, and due to the presence of pi-bonds alkenes will be more reactive towards electrophilic reagents.Hence they are less stable than alkanes. The strength of carbon-carbon double bond is greater than single bond as bond strength is directly proportional to bond order. Hence, both the statements are correct.