Hydrocarbons is the first chapter where Organic Chemistry turns into reactions. Every named reaction, every product question and every mechanism in Class 12 builds on what you learn here. This hydrocarbons NEET PYQ analysis shows which reactions and concepts NTA repeats, so you can study the chapter in the order that scores.
For how mechanisms build across the syllabus, see our organic mechanisms preparation guide. This article focuses on what past papers actually ask.
Want to test yourself first? Solve the Hydrocarbons PYQs for free, then use this guide to fix the gaps.
Table of Contents
Why Hydrocarbons Matters
NEET usually asks two to three questions from Hydrocarbons. The chapter also supplies the reactions reused in Haloalkanes, Alcohols and Aldehydes, so weak preparation here costs marks across the whole Organic section.
The reasoning behind every product, such as carbocation stability and electron effects, comes from our GOC PYQ patterns article, so revise it first. Keep the Hydrocarbons class 11 notes open for the full theory, and see the chapter’s share in the NEET 2027 chapter weightage breakdown.
Hydrocarbons NEET PYQ Analysis: The Repeated Question Types
When you sort a decade of NEET chemistry PYQs from this chapter, five question types cover almost everything.
| Question type | Frequency | What it tests |
|---|---|---|
| Alkene addition reactions | Most frequent | Markovnikov’s rule, peroxide effect, ozonolysis |
| Aromaticity and electrophilic substitution | Very frequent | Hückel’s rule, directing groups, Friedel–Crafts |
| Alkyne reactions | Very frequent | Acidity, Lindlar and Birch-type reduction, hydration |
| Preparation of alkanes | Regular | Wurtz, Kolbe, decarboxylation |
| Conformations and isomerism | Regular | Newman projections, geometrical isomers |
For a quick warm-up before the full set, try these five quick hydrocarbons practice questions. For a compact list of reagents, keep our named reagents revision list handy.
Alkanes: Preparation and Conformations
- Wurtz reaction: 2R–X + 2Na → R–R (dry ether). It suits symmetrical alkanes. With two different alkyl halides, you get a mixture.
- Kolbe’s electrolysis: sodium salts of carboxylic acids give alkanes at the anode.
- Decarboxylation: sodium salt + soda lime gives an alkane with one carbon less.
- Conformations of ethane: staggered is more stable than eclipsed. The energy difference is only about 12.5 kJ/mol, so the forms interconvert freely at room temperature.
Alkenes: Addition Reactions That Repeat
| Reaction | Rule or product |
|---|---|
| HX addition | Markovnikov’s rule: H goes to the carbon with more hydrogens |
| HBr with peroxide | Anti-Markovnikov (peroxide effect), only with HBr |
| Ozonolysis | Breaks C=C into two carbonyl compounds |
| Cold dilute KMnO₄ (Baeyer’s test) | Gives a diol; purple colour disappears |
| Br₂ in CCl₄ | Red-brown colour disappears; a test for unsaturation |
Markovnikov’s rule follows from carbocation stability: the more stable carbocation forms first. The peroxide effect works only with HBr because the free radical steps are favourable only for HBr, not HCl or HI.
Geometrical (cis–trans) isomerism needs two different groups on each double-bonded carbon. The cis form usually has the higher dipole moment and boiling point; the trans form often has the higher melting point.
Alkynes: Acidity and Reduction
- Terminal alkynes are weakly acidic because the sp-hybridised carbon holds the negative charge well. Acidity order: HC≡CH > H₂C=CH₂ > H₃C–CH₃.
- Terminal alkynes react with sodium or ammoniacal AgNO₃ (Tollens’ reagent); internal alkynes do not.
- Lindlar’s catalyst reduces an alkyne to a cis-alkene. Sodium in liquid ammonia gives a trans-alkene.
- Hydration with dilute H₂SO₄ and HgSO₄ gives a ketone, except ethyne, which gives acetaldehyde.
Aromatic Hydrocarbons
Hückel’s rule: a compound is aromatic if it is cyclic, planar, fully conjugated and has (4n + 2) π electrons. Benzene (6), the cyclopentadienyl anion (6) and the tropylium cation (6) are aromatic. Cyclobutadiene (4) and cyclooctatetraene are not.
Benzene prefers electrophilic substitution, not addition, because substitution keeps the aromatic ring intact.
| Substituent | Effect on ring | Directs to |
|---|---|---|
| –OH, –NH₂, –OCH₃, –CH₃ | Activating | Ortho and para |
| Halogens (–Cl, –Br) | Deactivating | Ortho and para |
| –NO₂, –CHO, –COOH, –CN | Deactivating | Meta |
Halogens are the classic trap: they deactivate the ring but still direct ortho and para.
Worked PYQ-Pattern Questions
Q1. What are the products of ozonolysis of but-2-ene?
Answer: Two molecules of acetaldehyde (ethanal). The C=C breaks into two identical CH₃CHO units.
Q2. Propene reacts with HBr in the presence of a peroxide. What is the main product?
Answer: 1-Bromopropane. Peroxide reverses the addition to anti-Markovnikov.
Q3. But-2-yne is reduced with sodium in liquid ammonia. What forms?
Answer: trans-But-2-ene. Lindlar’s catalyst would give the cis form.
Q4. Which of these is aromatic? (a) Cyclobutadiene (b) Cyclopentadienyl anion (c) Cyclooctatetraene (d) Cyclopentadiene
Answer: (b). It is cyclic, planar and has 6 π electrons.
Q5. Which group is meta-directing? (a) –OH (b) –CH₃ (c) –Cl (d) –NO₂
Answer: (d) –NO₂. It withdraws electrons by resonance and deactivates the ortho and para positions most.
Common Traps in This Chapter
- Applying the peroxide effect to HCl or HI
- Calling halogens meta-directing because they deactivate the ring
- Mixing up Lindlar (cis) and Na/liquid NH₃ (trans) reductions
- Forgetting that hydration of ethyne gives an aldehyde, not a ketone
- Counting π electrons wrongly for charged rings
Practise Every Hydrocarbons PYQ
This hydrocarbons NEET PYQ analysis gives you the reactions and rules. Practice makes them automatic. Every previous year question from this chapter is available chapter-wise, free:
👉 Solve all Hydrocarbons NEET PYQs
For every product question, draw the intermediate (carbocation or radical) before choosing an option. That single step prevents most wrong answers.
Conclusion
Hydrocarbons is where Organic Chemistry becomes predictable. Use this hydrocarbons NEET PYQ analysis to master alkene additions, alkyne reductions, aromaticity and directing effects. Then practise until every product follows from a reason you can state in one line.
❓ FAQ
Q: How many questions come from Hydrocarbons in NEET? A: Usually two to three questions, and its reactions are reused in several Class 12 Organic chapters.
Q: What is the most important topic in Hydrocarbons? A: Alkene addition reactions, including Markovnikov’s rule and the peroxide effect, are asked most, followed by aromaticity and directing effects.
Q: Why does the peroxide effect work only with HBr? A: With HBr, both free radical chain steps release energy. With HCl and HI, one step is unfavourable, so the radical route does not work.
Q: How do I know if a compound is aromatic? A: Check that it is cyclic, planar, fully conjugated and has 4n + 2 π electrons, for example 2, 6 or 10.
Q: How should I revise Hydrocarbons for NEET? A: Use a hydrocarbons NEET PYQ analysis to list the repeated reactions, make one reaction table, and solve past questions chapter-wise.
Q: Where can I practise Hydrocarbons NEET PYQs for free? A: You can solve every previous year question from this chapter for free using the practice link in this article.