Hydrocarbons: Complete Guide & Formula Sheet with Free PDF Download (JEE & NEET)
Hydrocarbons — Competishun
Hydrocarbons: Complete Guide & Formula Sheet with Free PDF Download (JEE & NEET)
Hydrocarbons is one of the most fundamental and scoring chapters in organic chemistry for JEE and NEET. It carries a weightage of 2-3 questions every year, with 2-3 questions appearing annually. This chapter covers the chemistry of compounds made of only carbon and hydrogen, which form the backbone of all organic chemistry.
This chapter deals with four main classes of hydrocarbons: Alkanes (saturated, single bonds), Alkenes (unsaturated, double bonds), Alkynes (unsaturated, triple bonds), and Aromatic Hydrocarbons (benzene and its derivatives). Each class has its own characteristic properties, reactions, and mechanisms. The chapter also covers important named reactions like the Wurtz reaction, Markovnikov addition, and Friedel-Crafts reactions.
This page gives you the complete guide to Hydrocarbons with all concepts explained in depth. You will find clear definitions, nomenclature, preparation methods, reaction mechanisms, and practice questions. Download the free PDF below and keep it handy for quick revision before your JEE Main, JEE Advanced, or NEET exam.
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Download Free PDFWhat are Hydrocarbons?
Hydrocarbons are classified into two main categories: Aliphatic and Aromatic hydrocarbons.[reference:1]
Aliphatic Hydrocarbons
Open-chain or cyclic compounds. Includes Alkanes (single bonds), Alkenes (double bonds), and Alkynes (triple bonds).[reference:2]
Aromatic Hydrocarbons
Cyclic compounds containing at least one benzene ring. Benzene (C₆H₆) is the simplest aromatic hydrocarbon.[reference:3]
Glossary of Hydrocarbons Terms — Complete A to Z
Before diving deep into each topic, let's understand the key terminology used in this chapter:
| Term | Definition |
|---|---|
| Alkane | A saturated hydrocarbon with only single carbon-carbon bonds (C-C). General formula: CnH2n+2.[reference:4][reference:5] |
| Alkene | An unsaturated hydrocarbon containing at least one carbon-carbon double bond (C=C). General formula: CnH2n.[reference:6] |
| Alkyne | An unsaturated hydrocarbon containing at least one carbon-carbon triple bond (C≡C). General formula: CnH2n-2.[reference:7] |
| Aromatic Hydrocarbon | A cyclic hydrocarbon containing a benzene ring with alternating double bonds and special stability.[reference:8] |
| Wurtz Reaction | A method for preparing symmetrical alkanes by reacting alkyl halides with sodium in dry ether.[reference:9][reference:10] |
| Markovnikov Rule | When HX adds to an unsymmetrical alkene, H attaches to the carbon with more hydrogens, X to the carbon with fewer hydrogens. |
| Friedel-Crafts Reaction | An electrophilic aromatic substitution reaction where an alkyl or acyl group is introduced onto an aromatic ring. |
| Mastering these terms is essential for understanding the chemistry of hydrocarbons. | |
Alkanes — Saturated Hydrocarbons (CnH2n+2)
Nomenclature of Alkanes
- IUPAC Naming: The parent chain is the longest continuous carbon chain. The suffix "-ane" is used. Substituents are named as prefixes with locants.[reference:13]
- Common Names: First four alkanes: Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈), Butane (C₄H₁₀). Higher alkanes use Greek prefixes.
Preparation of Alkanes
| Method | Reaction | Key Points |
|---|---|---|
| Wurtz Reaction | 2R-X + 2Na (dry ether) → R-R + 2NaX | Symmetrical alkanes only. Works with primary halides.[reference:14][reference:15] |
| Sabatier-Senderens Reduction | Unsaturated hydrocarbon + H₂ (Ni, 200°C) → Alkane | Hydrogenation of alkenes/alkynes.[reference:16][reference:17] |
| Reduction of Alkyl Halides | R-X + 2[H] → R-H + HX | Using LiAlH₄ or Zn/HCl.[reference:18] |
| Decarboxylation | R-COONa + NaOH (sodalime, heat) → R-H + Na₂CO₃ | Removes CO₂ to give alkane.[reference:19] |
| Kolbe's Electrolysis | R-COONa (electrolysis) → R-R + 2CO₂ + 2Na | Gives symmetrical alkanes.[reference:20] |
| The Wurtz reaction is one of the most important methods for preparing alkanes and is frequently tested in JEE and NEET.[reference:21] | ||
Physical Properties of Alkanes
- State: C₁-C₄ are gases, C₅-C₁₇ are liquids, and C₁₈+ are solids.[reference:22]
- Boiling Point: Increases with molecular weight. Branched alkanes have lower boiling points than straight-chain isomers.
- Solubility: Insoluble in water (non-polar), soluble in organic solvents.
- Reactivity: Alkanes are relatively inert due to strong C-C and C-H sigma bonds.[reference:23]
Chemical Reactions of Alkanes
| Reaction | Reagent | Product |
|---|---|---|
| Combustion | O₂ (heat) | CO₂ + H₂O + heat |
| Halogenation (Free Radical Substitution) | X₂ (hv or heat) | R-X + HX |
| Nitration | HNO₃ (vapour phase, 400°C) | R-NO₂ + H₂O |
| Sulphonation | H₂SO₄ (fuming, heat) | R-SO₃H + H₂O |
| Aromatisation | Cr₂O₃/Al₂O₃ (heat) | Aromatic hydrocarbons |
| Halogenation of alkanes follows a free radical mechanism. The order of reactivity is F₂ > Cl₂ > Br₂ > I₂.[reference:24] | ||
Alkenes — Unsaturated Hydrocarbons with C=C (CnH2n)
Structure and Bonding in Alkenes
The carbon atoms in the double bond are sp² hybridised, with bond angle 120°. The double bond consists of one sigma (σ) bond and one pi (π) bond. The π bond is weaker and more reactive.[reference:28]
Nomenclature of Alkenes
- IUPAC Naming: The parent chain contains the double bond. The suffix "-ene" is used. The position of the double bond is indicated by the lowest locant.[reference:29]
- Examples: Ethene (CH₂=CH₂), Propene (CH₂=CH-CH₃), But-1-ene (CH₂=CH-CH₂-CH₃).
Preparation of Alkenes
| Method | Reaction | Key Points |
|---|---|---|
| Dehydration of Alcohols | R-CH₂-CH₂OH (H⁺, heat) → R-CH=CH₂ + H₂O | Follows Saytzeff rule (more substituted alkene major).[reference:30] |
| Dehydrohalogenation of Alkyl Halides | R-CH₂-CH₂-X (alc. KOH) → R-CH=CH₂ + HX | β-Elimination reaction.[reference:31] |
| Partial Hydrogenation of Alkynes | R-C≡C-R' + H₂ (Lindlar's catalyst) → R-CH=CH-R' | Gives cis-alkenes.[reference:32] |
| Dehydration of alcohols and dehydrohalogenation of alkyl halides are the most common methods for preparing alkenes.[reference:33] | ||
Physical Properties of Alkenes
- State: C₂-C₄ are gases, C₅-C₁₇ are liquids, and C₁₈+ are solids.
- Boiling Point: Similar to alkanes but slightly lower.
- Solubility: Practically insoluble in water, soluble in organic solvents.[reference:34]
Chemical Reactions of Alkenes
Alkenes undergo electrophilic addition reactions due to the presence of loosely held π electrons.[reference:35]
| Reaction | Reagent | Product |
|---|---|---|
| Hydrogenation | H₂ (Ni/Pt/Pd) | Alkane |
| Halogenation | X₂ (CCl₄) | Vicinal dihalide |
| Hydrohalogenation | HX | Alkyl halide (Markovnikov addition) |
| Hydration | H₂O (H⁺) | Alcohol (Markovnikov addition) |
| Hydroboration-Oxidation | BH₃, then H₂O₂/OH⁻ | Anti-Markovnikov alcohol |
| Ozonolysis | O₃, then Zn/H₂O | Carbonyl compounds |
| Polymerisation | Catalyst | Polymer |
| The Markovnikov rule applies to the addition of HX and H₂O to unsymmetrical alkenes.[reference:36] | ||
Alkynes — Unsaturated Hydrocarbons with C≡C (CnH2n-2)
Structure and Bonding in Alkynes
The carbon atoms in the triple bond are sp hybridised, with bond angle 180°. The triple bond consists of one sigma (σ) bond and two pi (π) bonds.[reference:39]
Nomenclature of Alkynes
- IUPAC Naming: The parent chain contains the triple bond. The suffix "-yne" is used. The position of the triple bond is indicated by the lowest locant.[reference:40]
- Examples: Ethyne (HC≡CH), Propyne (HC≡C-CH₃), But-1-yne (HC≡C-CH₂-CH₃).
Preparation of Alkynes
| Method | Reaction | Key Points |
|---|---|---|
| Dehydrohalogenation of Vicinal Dihalides | R-CHX-CHX-R' + alc. KOH → R-C≡C-R' + 2HX | Double elimination. |
| From Calcium Carbide | CaC₂ + H₂O → HC≡CH + Ca(OH)₂ | Preparation of ethyne. |
| The dehydrohalogenation of vicinal dihalides is the most common method for preparing alkynes.[reference:41] | ||
Physical Properties of Alkynes
- State: C₂-C₄ are gases, higher are liquids.
- Boiling Point: Slightly higher than alkenes and alkanes.[reference:42]
- Solubility: Insoluble in water, soluble in organic solvents.
Acidity of Terminal Alkynes
Terminal alkynes (HC≡C-R) are weakly acidic due to the sp hybridisation of the carbon bearing the hydrogen. The acidity order is: HC≡CH > CH₂=CH₂ > CH₃-CH₃.[reference:43][reference:44]
Chemical Reactions of Alkynes
| Reaction | Reagent | Product |
|---|---|---|
| Hydrogenation | H₂ (Ni/Pt/Pd) | Alkane |
| Partial Hydrogenation | H₂ (Lindlar's catalyst) | cis-Alkene |
| Halogenation | X₂ | Halogenated alkene/alkane |
| Hydration | H₂O (H⁺, Hg²⁺) | Ketone (Markovnikov addition) |
| Metal Acetylides | NaNH₂, AgNO₃, CuCl | Metal acetylides (precipitates) |
| Terminal alkynes react with ammoniacal silver nitrate (AgNO₃/NH₄OH) to give white precipitate (silver acetylide) and with ammoniacal cuprous chloride (CuCl/NH₄OH) to give red precipitate (copper acetylide). | ||
Aromatic Hydrocarbons — Benzene and Its Derivatives
Structure of Benzene
Benzene is a six-membered ring with alternating double bonds. It has a planar structure with bond angle 120°. The π electrons are delocalised over the entire ring, giving it exceptional stability (aromaticity).[reference:48]
Nomenclature of Aromatic Compounds
- Monosubstituted Benzene: Phenol (C₆H₅OH), Toluene (C₆H₅CH₃), Aniline (C₆H₅NH₂).
- Disubstituted Benzene: o-, m-, p- (ortho, meta, para).[reference:49]
Preparation of Benzene
- From Alkynes: 3HC≡CH (red hot tube) → C₆H₆
- From Alkanes: C₆H₁₂ (catalyst, heat) → C₆H₆ + 3H₂
Physical Properties of Benzene
- State: Colourless liquid with a characteristic smell.
- Boiling Point: 80°C.
- Solubility: Insoluble in water, soluble in organic solvents.
- Stability: Benzene is unusually stable due to delocalisation of π electrons.[reference:50]
Chemical Reactions of Benzene
Benzene undergoes electrophilic substitution reactions rather than addition reactions, due to the stability of the aromatic ring.[reference:51]
| Reaction | Reagent | Product |
|---|---|---|
| Halogenation | X₂ (Lewis acid catalyst) | Halobenzene (C₆H₅X)[reference:52][reference:53] |
| Nitration | Conc. HNO₃ + conc. H₂SO₄ (323-333 K) | Nitrobenzene (C₆H₅NO₂)[reference:54] |
| Sulphonation | Fuming H₂SO₄ | Benzenesulphonic acid (C₆H₅SO₃H)[reference:55] |
| Friedel-Crafts Alkylation | R-X + AlCl₃ | Alkylbenzene (C₆H₅-R)[reference:56][reference:57] |
| Friedel-Crafts Acylation | RCOCl + AlCl₃ | Ketone (C₆H₅-COR)[reference:58][reference:59] |
| Friedel-Crafts alkylation and acylation are important reactions for introducing alkyl and acyl groups onto the benzene ring.[reference:60] | ||
Practice Questions — From JEE and NEET
| Question | Answer |
|---|---|
| Q1: What is the product of the Wurtz reaction between bromoethane and sodium in dry ether? | Butane (CH₃-CH₂-CH₂-CH₃). |
| Q2: Which alkane has the highest boiling point: n-pentane, isopentane, or neopentane? | n-Pentane (straight chain has maximum surface area). |
| Q3: What is the major product of the addition of HBr to propene? | 2-Bromopropane (Markovnikov addition). |
| Q4: What is the product of the ozonolysis of ethene? | Formaldehyde (HCHO). |
| Q5: Which compound gives a white precipitate with ammoniacal AgNO₃: 1-butyne or 2-butyne? | 1-Butyne (terminal alkyne). |
| Q6: What is the major product of the nitration of benzene? | Nitrobenzene (C₆H₅NO₂). |
| Q7: What is the IUPAC name of CH₃-CH=CH-CH₃? | But-2-ene. |
| Q8: What is the product of the Friedel-Crafts alkylation of benzene with chloromethane? | Toluene (C₆H₅CH₃). |
| Practise these types of questions to become comfortable with applying hydrocarbons concepts in exam scenarios. | |
All Hydrocarbons Formulas at a Glance
| Formula/Concept | What It Means |
|---|---|
| CnH2n+2 | General formula for alkanes (saturated)[reference:62] |
| CnH2n | General formula for alkenes (one double bond)[reference:63] |
| CnH2n-2 | General formula for alkynes (one triple bond)[reference:64] |
| C6H6 | Benzene (aromatic hydrocarbon)[reference:65] |
| 2R-X + 2Na → R-R + 2NaX | Wurtz reaction (symmetrical alkanes)[reference:66] |
| R-COOH + NaOH (sodalime) → R-H + Na2CO3 | Decarboxylation[reference:67] |
| C2H4 + H2 → C2H6 | Hydrogenation of alkene |
| C2H2 + 2H2 → C2H6 | Hydrogenation of alkyne |
| C6H6 + Cl2 (AlCl3) → C6H5Cl + HCl | Halogenation of benzene[reference:68] |
| C6H6 + HNO3 (H2SO4) → C6H5NO2 + H2O | Nitration of benzene[reference:69] |
| Memorise these formulas for Hydrocarbons. They are the key to scoring full marks in this chapter. | |
Common Mistakes in Hydrocarbons
- Confusing alkanes, alkenes, and alkynes: Alkanes have only single bonds (CnH2n+2), alkenes have double bonds (CnH2n), and alkynes have triple bonds (CnH2n-2).[reference:70][reference:71]
- Misapplying the Markovnikov rule: The H atom of HX attaches to the carbon with more H atoms. The X attaches to the carbon with fewer H atoms.[reference:72]
- Forgetting the anti-Markovnikov addition: In the presence of peroxide, HBr adds anti-Markovnikov (Kharasch effect).
- Confusing aromatic substitution and addition: Benzene undergoes electrophilic substitution, not addition, due to aromatic stability.[reference:73]
- Forgetting the conditions for Friedel-Crafts reactions: Requires a Lewis acid catalyst (AlCl₃, FeCl₃).[reference:74]
- Not checking for terminal alkyne acidity: Only terminal alkynes (HC≡C-R) are acidic and give metal acetylide precipitates.[reference:75]
Why Hydrocarbons Matters for JEE and NEET
- High weightage: Hydrocarbons appears in 2-3 questions in every JEE Main, JEE Advanced, and NEET chemistry paper.[reference:76]
- Foundation for organic chemistry: Understanding hydrocarbons is essential for understanding all of organic chemistry, including alcohols, aldehydes, ketones, and carboxylic acids.
- Direct scoring: Many questions are direct, especially on nomenclature, Wurtz reaction, Markovnikov rule, and Friedel-Crafts reactions.[reference:77]
- Conceptual clarity: This chapter rewards students who understand the mechanisms rather than just memorizing reactions.
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Frequently Asked Questions — Hydrocarbons
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