Home › Chemistry Class 11 › Hydrocarbons
Hydrocarbons
Chemistry · Class 11NCERT pp. 295–3262 sets × 25 questions
Key facts and formulas for Hydrocarbons, three worked sample questions, and two full practice sets. Every question has a hint, a reason for every option and a step-by-step solution, with a calculator or animation where one helps. English and Hinglish.
Formula sheet for this chapter
| Idea | Rule or fact |
|---|
| Sigma bonds in an alkane CₙH₂ₙ₊₂ | C–C bonds (n − 1) + C–H bonds (2n + 2) = 3n + 1 |
| Moles and gas volume | n = mass ÷ molar mass; 1 mol of gas = 22.4 L at STP |
| Wurtz reaction | 2 R–X + 2 Na → R–R + 2 NaX; product has twice the carbons of R |
| Decarboxylation | R–COONa + NaOH (soda lime) → R–H + Na₂CO₃; loses one carbon |
| Markovnikov and peroxide effect | HX adds H to carbon with more H; with peroxide HBr adds the reverse way |
| Saytzeff rule | Elimination gives the more substituted (more stable) alkene as major product |
| Ozonolysis and KMnO₄ | C=C is cut; Zn/H₂O gives aldehydes or ketones, hot KMnO₄ gives acids |
| Acidity of hydrocarbons | sp (50% s) > sp² (33%) > sp³ (25%): HC≡CH > H₂C=CH₂ > CH₃CH₃ |
| Aromaticity | Planar, cyclic, conjugated with (4n + 2) π electrons (Huckel rule) |
| Directing effects in benzene | –CH₃, –OCH₃ are o/p directing activators; –NO₂ is a meta director and deactivator |
Three sample questions
Q1. The number of sigma bonds in propane is
- 10
- 8
- 11
- 9
Hint and solution
Hint: Draw propane, C₃H₈, and count every single bond. Each single bond is one sigma bond.
- CH₃–CH₂–CH₃ : 3 C, 8 H — Write the structure of propane. It has three carbons and eight hydrogens, all joined by single bonds.
- C–C bonds = 2 — Three carbons in a chain are joined by two C–C bonds.
- C–H bonds = 3 + 2 + 3 = 8 — Count the hydrogens on each carbon. Each C–H bond is one sigma bond, so there are eight.
- Total σ = 2 + 8 = 10 — Add them: 2 + 8 = 10 sigma bonds. Propane has no pi bond. Option A.
Answer: A. Two C–C bonds and eight C–H bonds make 10 sigma bonds.
Q2. The IUPAC name of CH₃CH(CH₃)CH₂CH(CH₃)CH₂CH₃ is
- 3,5-dimethylhexane
- 2-ethyl-4-methylpentane
- 2,4-dimethylhexane
- 2,4-dimethylheptane
Hint and solution
Hint: First find the longest continuous carbon chain, then number from the end that gives the branches the lowest numbers.
- Longest chain: C–C–C–C–C–C — Trace the longest chain. It runs CH₃, CH, CH₂, CH, CH₂, CH₃, which is six carbons, so the parent is hexane.
- Two CH₃ branches — Two methyl groups hang off the second and fourth carbons of the chain.
- Lowest locants: 2,4 not 3,5 — Number from the left end. Branches get 2 and 4. From the right they would get 3 and 5, which is higher.
- Name = 2,4-dimethylhexane — Join branch positions, prefix and parent. The name is 2,4-dimethylhexane. Option C.
Answer: C. The longest chain has six carbons. Methyl groups are on C2 and C4.
Q3. Wurtz reaction of 1-bromopropane with sodium in dry ether gives mainly
- butane
- hexane
- pentane
- propane
Hint and solution
Hint: In the Wurtz reaction two alkyl groups from two molecules join together. Count the carbons of the new chain.
- 2 R–X + 2 Na → R–R + 2 NaX — The Wurtz reaction joins two alkyl groups with sodium in dry ether.
- R = CH₃CH₂CH₂– (3 C) — The alkyl group of 1-bromopropane is propyl, which has three carbons.
- R–R = C₃ + C₃ = C₆ — Two propyl groups join, so the product has 3 + 3 = 6 carbons.
- Product: CH₃(CH₂)₄CH₃ — The straight chain with six carbons is hexane. Option B.
Answer: B. The alkane has twice the carbons of the alkyl group: C₃ + C₃ = C₆.
How to use this chapter
Read the NCERT pages first, then attempt Set 1 without the hints. Note every wrong answer in a mistake list, read the reason for each option, and retry Set 2 two days later. A topic is solid when you can say why each wrong option is wrong.