How much of JEE Main comes from Hydrocarbons?

JEEGenie estimates about 3% of JEE Main marks come from Hydrocarbons, based on recent papers. The share varies from year to year.

Which formulas matter most in Hydrocarbons?

The Hydrocarbons sheet on this page has 22 formula entries, starting with Alkanes: General Formula & Nomenclature, Alkanes: Combustion Enthalpy, Alkanes: Free Radical Halogenation, Alkenes: General Formula & Isomerism, Alkenes: Addition Reactions (Markovnikov's Rule). Each one comes with when to use it and the trap to avoid.

What topics does Hydrocarbons cover for JEE?

Hydrocarbons covers Alkanes, Alkenes, Alkynes, Aromatic Hydrocarbons, Reaction Mechanisms, Preparation and Properties.

How should I revise Hydrocarbons before the exam?

Learn each formula together with its condition of use, read the trap listed beside it, then practise questions and review every mistake. The practice link on this page opens Hydrocarbons questions on JEEGenie.

How many previous-year questions are there on Hydrocarbons?

JEEGenie has 387 previous-year questions on Hydrocarbons, and this page shows a few with worked explanations.

Is practice on Hydrocarbons free on JEEGenie?

Sign-up is free and includes a 7-day Pro trial. After the trial, free accounts keep a limited daily quota.

Hydrocarbons for JEE Main & Advanced — formulas, what is asked, and traps

JEEGenie estimates about 3% of JEE Main marks come from Hydrocarbons. This page covers 22 formula entries, each with when to use it, what JEE asks and the traps. A common slip: Confusing aliphatic with aromatic; aliphatic includes open-chain and cyclic non-aromatic compounds.

Key points to know

Hydrocarbons. Organic compounds composed exclusively of carbon and hydrogen atoms, classified into aliphatic (alkanes, alkenes, alkynes) and aromatic (arenes) based on structure and bonding.
Alkanes: General Formula & Nomenclature. Alkanes have general formula $C_{n} H_{2 n + 2}$ ; IUPAC names use suffix '-ane' with longest carbon chain as parent, numbering to give lowest locants to substituents.
Alkenes: General Formula & Isomerism. Alkenes have general formula $C_{n} H_{2 n}$ ; exhibit structural (position, chain) and geometrical (cis-trans) isomerism due to restricted rotation around C=C.
Alkynes: General Formula & Acidity. Alkynes have general formula $C_{n} H_{2 n - 2}$ ; terminal alkynes (R-C≡C-H) are weakly acidic (pKa ~25) due to sp hybridization, can form metal acetylides.
Aromatic Hydrocarbons (Arenes). Compounds containing benzene-like rings that follow Hückel's rule (4n+2 π electrons) for aromaticity; planar, cyclic, conjugated system.
Benzene: Structure & Stability. Benzene has a planar hexagonal structure with delocalized π electrons (resonance hybrid); unusually stable due to resonance energy (~152 kJ/mol).
Friedel-Crafts Reactions. Alkylation (using alkyl halide + AlCl₃) and acylation (using acyl halide + AlCl₃) introduce alkyl or acyl groups; acylation avoids carbocation rearrangements.
Alkynes: Preparation from Vicinal Dihalides. Vicinal dihalides undergo double dehydrohalogenation with strong base (e.g., alcoholic KOH, then NaNH₂) to give alkynes.
Alkanes: Conformations (Newman Projections). Ethane has staggered (more stable, dihedral angle 60°) and eclipsed (less stable, 0°) conformations; butane has anti (most stable), gauche, and eclipsed conformers.

Formula sheet

Name	Formula	When to use
Alkanes: General Formula & Nomenclature	$C_{n} H_{2 n + 2}$	When naming or writing formulas for saturated hydrocarbons.
Alkanes: Combustion Enthalpy	$Δ H_{c} = n Δ H_{f} (C O_{2}) + (n + 1) Δ H_{f} (H_{2} O) - Δ H_{f} (alkane)$ (negative for combustion)	When calculating heat released or balancing combustion reactions.
Alkanes: Free Radical Halogenation	$R - H + X_{2} \overset{h ν}{\to} R - X + H X$	When predicting products of halogenation or relative rates.
Alkenes: General Formula & Isomerism	$C_{n} H_{2 n}$	When analyzing alkene structure or predicting isomers.
Alkenes: Addition Reactions (Markovnikov's Rule)	$R C H = C H_{2} + H X \to R C H X - C H_{3}$ (Markovnikov)	When predicting major product of HX addition to alkenes.
Alkenes: Ozonolysis	$R_{2} C = C R_{2} + O_{3} \to R_{2} C = O + R_{2} C = O$ (after reduction)	When determining alkene structure from ozonolysis products.
Alkynes: General Formula & Acidity	$R - C \equiv C - H + N a N H_{2} \to R - C \equiv C^{-} N a^{+} + N H_{3}$	When identifying acidic hydrogens or preparing alkynides.
Alkynes: Addition Reactions (Hydration)	$R - C \equiv C H + H_{2} O \overset{H g^{2 +} / H^{+}}{\to} R - C O - C H_{3}$	When predicting product of alkyne hydration.
Alkynes: Reduction to Alkenes/Alkanes	$R - C \equiv C - R + H_{2} \overset{L i n d l a r}{\to} c i s - R C H = C H R$	When selectively reducing alkynes to alkenes or alkanes.
Aromatic Hydrocarbons (Arenes)	$C_{6} H_{6}$ (benzene)	When determining aromaticity of a compound.
Benzene: Structure & Stability	Resonance structures: $\leftrightarrow$	When explaining benzene's reactivity or stability.
Electrophilic Aromatic Substitution (EAS) Mechanism	$C_{6} H_{6} + E^{+} \to C_{6} H_{5} E + H^{+}$	When predicting products of EAS reactions (nitration, halogenation, sulfonation, Friedel-Crafts).
Directing Effects in EAS	Ortho/para directors: $+ I$ or $+ M$ groups; Meta directors: $- I$ or $- M$ groups	When predicting major product in disubstituted benzene reactions.
Friedel-Crafts Reactions	$C_{6} H_{6} + R C l \overset{A l C l_{3}}{\to} C_{6} H_{5} R + H C l$	When synthesizing alkylbenzenes or aryl ketones.
Alkanes: Preparation from Alkyl Halides (Wurtz Reaction)	$2 R - X + 2 N a \overset{d r y e t h e r}{\to} R - R + 2 N a X$	When preparing symmetrical alkanes from alkyl halides.
Alkenes: Preparation by Dehydrohalogenation	$R - C H_{2} - C H_{2} X + K O H (a l c .) \to R - C H = C H_{2} + K X + H_{2} O$	When preparing alkenes from alkyl halides.
Alkynes: Preparation from Vicinal Dihalides	$R - C H X - C H_{2} X \overset{2 K O H (a l c .)}{\to} R - C \equiv C H + 2 K X + 2 H_{2} O$	When synthesizing alkynes from dihalides.
Benzene: Nitration	$C_{6} H_{6} + H N O_{3} \overset{H_{2} S O_{4}}{\to} C_{6} H_{5} N O_{2} + H_{2} O$	When introducing nitro group on benzene.
Benzene: Sulfonation	$C_{6} H_{6} + H_{2} S O_{4} \to C_{6} H_{5} S O_{3} H + H_{2} O$	When introducing sulfonic acid group or as a blocking group in synthesis.
Alkanes: Conformations (Newman Projections)	Staggered: $Δ H \approx 12 kJ/mol$ more stable than eclipsed for ethane	When analyzing stability or energy of alkane conformations.
Alkenes: Stability (Heat of Hydrogenation)	Stability $\propto 1 / ∣ Δ H_{h y d r o g e n a t i o n} ∣$ (more stable alkene has the smaller magnitude of $Δ H_{h y d r o g e n a t i o n}$ , i.e. less heat released)	When comparing alkene stability.
Alkynes: Ozonolysis	$R - C \equiv C - R^{'} + O_{3} \to R C O O H + R^{'} C O O H$	When determining alkyne structure from ozonolysis products.

Open the Hydrocarbons formula page

What JEE asks

Hydrocarbons. JEE often asks to classify given compounds or predict properties based on saturation.
Alkanes: General Formula & Nomenclature. JEE tests naming of branched alkanes with multiple substituents, often with common names like isobutane.
Alkanes: Combustion Enthalpy. JEE may ask for enthalpy change or compare heats of combustion for different alkanes.
Alkanes: Free Radical Halogenation. JEE asks to predict major product based on radical stability or calculate relative yields.
Alkenes: General Formula & Isomerism. JEE tests identification of cis-trans isomers and stability (trans usually more stable).
Alkenes: Addition Reactions (Markovnikov's Rule). JEE often gives unsymmetrical alkene and asks product with HBr with/without peroxide.
Alkenes: Ozonolysis. JEE gives ozonolysis products and asks to deduce original alkene structure.
Alkynes: General Formula & Acidity. JEE tests reaction of terminal alkynes with NaNH₂ or AgNO₃/NH₃ to form precipitates.
Alkynes: Addition Reactions (Hydration). JEE asks to distinguish between terminal and internal alkynes via hydration products.
Alkynes: Reduction to Alkenes/Alkanes. JEE tests stereochemistry of reduction products, often with alkynes having different substituents.
Aromatic Hydrocarbons (Arenes). JEE gives heterocycles or polycyclic compounds to test aromaticity.
Benzene: Structure & Stability. JEE asks about bond lengths, resonance energy, or why benzene undergoes substitution not addition.
Electrophilic Aromatic Substitution (EAS) Mechanism. JEE tests mechanism, intermediates, and effect of substituents on rate and orientation.
Directing Effects in EAS. JEE gives a substituted benzene and asks product of nitration or halogenation.
Friedel-Crafts Reactions. JEE tests limitations (e.g., deactivating groups prevent reaction) and rearrangements.
Alkanes: Preparation from Alkyl Halides (Wurtz Reaction). JEE asks to predict product or identify side products.
Alkenes: Preparation by Dehydrohalogenation. JEE tests Saytzeff vs Hofmann elimination with different bases.
Alkynes: Preparation from Vicinal Dihalides. JEE may ask stepwise mechanism or product after partial elimination.
Benzene: Nitration. JEE tests mechanism, role of H₂SO₄, and effect of substituents on further nitration.
Benzene: Sulfonation. JEE tests reversibility and use in directing effects.
Alkanes: Conformations (Newman Projections). JEE asks to draw Newman projections or compare stability of conformers.
Alkenes: Stability (Heat of Hydrogenation). JEE gives heats of hydrogenation and asks to rank stability or identify alkene.
Alkynes: Ozonolysis. JEE may ask to deduce alkyne from products or differentiate from alkene.

Common traps

Confusing aliphatic with aromatic; aliphatic includes open-chain and cyclic non-aromatic compounds. Hydrocarbons
Forgetting to include all carbon atoms in the longest chain when naming branched alkanes. Alkanes: General Formula & Nomenclature
Forgetting to balance oxygen correctly; coefficient depends on n. Alkanes: Combustion Enthalpy
Assuming all H atoms are equally reactive; tertiary H is more reactive due to stability of radical. Alkanes: Free Radical Halogenation
Forgetting that alkenes with same substituents on each carbon show cis-trans isomerism only if each carbon has two different groups. Alkenes: General Formula & Isomerism
Applying Markovnikov's rule to symmetrical alkenes where both carbons have same number of H. Alkenes: Addition Reactions (Markovnikov's Rule)
Confusing reductive vs oxidative workup products; reductive gives aldehydes, oxidative gives acids. Alkenes: Ozonolysis
Assuming all alkynes are acidic; only terminal alkynes (with H on sp carbon) are acidic. Alkynes: General Formula & Acidity
Forgetting that hydration of terminal alkyne gives a methyl ketone, not an aldehyde. Alkynes: Addition Reactions (Hydration)
Assuming Lindlar's catalyst gives trans-alkene; it gives cis-alkene. Alkynes: Reduction to Alkenes/Alkanes
Assuming all cyclic compounds with alternating double bonds are aromatic; must satisfy Hückel's rule. Aromatic Hydrocarbons (Arenes)
Assuming benzene has alternating single and double bonds (Kekulé structure); it is a resonance hybrid. Benzene: Structure & Stability
Forgetting that the σ-complex is non-aromatic and less stable; rate-determining step is usually formation of σ-complex. Electrophilic Aromatic Substitution (EAS) Mechanism
Assuming all deactivating groups are meta-directors; halogens are deactivating but ortho/para-directing due to +M effect. Directing Effects in EAS

Topics in Hydrocarbons

Alkanes
Alkenes
Alkynes
Aromatic Hydrocarbons
Reaction Mechanisms
Preparation and Properties

Previous-year questions on Hydrocarbons

JEEGenie has 387 previous-year questions on Hydrocarbons. A few worked examples follow.

Year	Questions
2026	20
2025	19
2024	21
2023	23
2022	19
2021	30
2020	13
2019	26
2018	7
2017	6
2016	9
2015	11
2014	11
2013	5
2012	7
2011	8
2010	6
2008	4
2007	4
2006	6
2005	10
2004	6
2003	3
2002	9
2001	9
2000	8
1999	8
1998	10
1997	8
1996	5
1995	2
1994	4
1993	7
1992	3
1991	3
1990	1
1989	2
1988	2
1987	3
1986	3
1985	6
1984	4
1983	6
1982	3
1981	5
1980	1
1979	1

Question 1 hard

Consider the isomers of hydrocarbon with molecular formula

C_{5} H_{10}

. These isomers do not decolourise

K M n O_{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 ________.

Answer: 14

Explanation. 1.

C_{5} H_{10}

has one degree of unsaturation. Since the isomers do not decolorize

K M n O_{4}

, they are **cycloalkanes**, not alkenes. 2. The possible cycloalkane isomers are: Cyclopentane, methylcyclobutane, ethylcyclopropane, 1,1-dimethylcyclopropane, and 1,2-dimethylcyclopropane. 3. Count the number of non-equivalent hydrogen positions (which give distinct monochloro structural isomers) for each: - **Cyclopentane**: all 10 H are equivalent

\to

**1** product. - **Methylcyclobutane**: Ring positions: C-1 (bearing CH

_{3}

) and C-2/C-4 are equivalent, C-3 is unique

\to

3 ring positions. Methyl group: 1 position. Total = **4** products. - **Ethylcyclopropane**: Ring: C-1 (bearing ethyl), C-2/C-3 equivalent

\to

2 ring positions. Ethyl group: CH

_{2}

and CH

_{3}

are distinct

\to

2 positions. Total = **4** products. - **1,1-Dimethylcyclopropane**: Ring: C-2/C-3 equivalent

\to

1 position. Two equivalent methyl groups

\to

1 position. Total = **2** products. - **1,2-Dimethylcyclopropane**: Ring: C-1/C-2 equivalent, C-3 unique

\to

2 positions. Two equivalent methyl groups

\to

1 position. Total = **3** products. 4. Sum the distinct monochloro products:

1 + 4 + 4 + 2 + 3 = 14

14

From NTA/IIT papers: JEE Main 2026 · 2 Apr Shift 1

Question 2 medium

Number of bromo derivatives obtained on treating ethane with excess of

{B r}_{2}

in diffused sunlight is ___________

Answer: 9

Explanation. In diffused sunlight, excess

B r_{2}

brominates ethane by a free-radical chain, so every degree of substitution from one to six Br atoms is formed. Count the structurally distinct products, using the fact that the two carbons of ethane are equivalent. -

C_{2} H_{5} B r

: bromoethane — 1 -

C_{2} H_{4} B r_{2}

: 1,1-dibromoethane and 1,2-dibromoethane — 2 -

C_{2} H_{3} B r_{3}

: 1,1,1- and 1,1,2-tribromoethane — 2 -

C_{2} H_{2} B r_{4}

: 1,1,1,2- and 1,1,2,2-tetrabromoethane — 2 -

C_{2} H B r_{5}

: pentabromoethane (only one H remains, so one structure) — 1 -

C_{2} B r_{6}

: hexabromoethane — 1 The patterns pair up by symmetry: replacing

k

H atoms is equivalent to leaving

6 - k

H atoms, so the counts for di/tetra and mono/penta match. Total number of bromo derivatives:

1 + 2 + 2 + 2 + 1 + 1 = 9

Answer: 9

From NTA/IIT papers: JEE Main 2023 · 6 Apr Shift 1

Frequently asked questions

How much of JEE Main comes from Hydrocarbons?: JEEGenie estimates about 3% of JEE Main marks come from Hydrocarbons, based on recent papers. The share varies from year to year.
Which formulas matter most in Hydrocarbons?: The Hydrocarbons sheet on this page has 22 formula entries, starting with Alkanes: General Formula & Nomenclature, Alkanes: Combustion Enthalpy, Alkanes: Free Radical Halogenation, Alkenes: General Formula & Isomerism, Alkenes: Addition Reactions (Markovnikov's Rule). Each one comes with when to use it and the trap to avoid.
What topics does Hydrocarbons cover for JEE?: Hydrocarbons covers Alkanes, Alkenes, Alkynes, Aromatic Hydrocarbons, Reaction Mechanisms, Preparation and Properties.
How should I revise Hydrocarbons before the exam?: Learn each formula together with its condition of use, read the trap listed beside it, then practise questions and review every mistake. The practice link on this page opens Hydrocarbons questions on JEEGenie.
How many previous-year questions are there on Hydrocarbons?: JEEGenie has 387 previous-year questions on Hydrocarbons, and this page shows a few with worked explanations.
Is practice on Hydrocarbons free on JEEGenie?: Sign-up is free and includes a 7-day Pro trial. After the trial, free accounts keep a limited daily quota.