1. Chapter at a Glance
Hydrocarbons contain only carbon and hydrogen. They are grouped as alkanes (single bonds), alkenes (C=C), alkynes (C≡C) and aromatic hydrocarbons (benzene and its relatives). This chapter covers their preparation, physical properties, conformations, and characteristic reactions: free-radical substitution in alkanes, electrophilic addition to alkenes and alkynes, and electrophilic substitution in benzene. NEET usually asks one or two questions, especially on Markovnikov/anti-Markovnikov addition, ozonolysis, acidity of alkynes and directing effects in benzene.
- Named preparations: Wurtz, Kolbe, decarboxylation, Clemmensen/Wolff–Kishner.
- Conformations of ethane (staggered vs eclipsed).
- Markovnikov's rule and the peroxide (Kharasch) effect.
- Ozonolysis products; acidity of terminal alkynes.
- Aromaticity (Hückel rule) and ortho/para vs meta directing groups.
2. Alkanes (CₙH₂ₙ₊₂)
- sp³ carbons, C–C 154 pm; called paraffins because they are unreactive.
- Preparation: hydrogenation of alkenes/alkynes (Ni, Pt or Pd); Wurtz reaction 2RX + 2Na → R–R (dry ether, gives even-carbon alkanes; not suitable for methane); decarboxylation RCOONa + NaOH (CaO) → R–H (one carbon fewer); Kolbe's electrolysis of RCOONa gives R–R at the anode.
- Boiling point rises with molar mass; branching lowers it (smaller surface area): n-pentane > isopentane > neopentane.
- Halogenation by a free-radical chain mechanism (initiation, propagation, termination) in light: CH₄ + Cl₂ → CH₃Cl + HCl, then further substitution. Reactivity F₂ > Cl₂ > Br₂ > I₂; hydrogen replaced 3° > 2° > 1°.
- Combustion releases a lot of heat; controlled oxidation, isomerisation, aromatisation (n-hexane → benzene over Cr₂O₃/V₂O₅/Mo₂O₃) and pyrolysis (cracking) are industrially useful.
- Conformations of ethane arise from rotation about the C–C bond. The staggered form is more stable than the eclipsed form by about 12.5 kJ mol⁻¹ (torsional strain). They cannot be separated at room temperature.
3. Alkenes (CₙH₂ₙ)
- sp² carbons; C=C (134 pm) = one σ + one π bond. Restricted rotation leads to cis–trans isomerism; cis isomers are usually more polar and boil higher, trans isomers pack better and melt higher.
- Preparation: partial hydrogenation of alkynes (Lindlar's catalyst gives cis; Na/liquid NH₃ gives trans); dehydrohalogenation of alkyl halides with alcoholic KOH; dehydration of alcohols with conc. H₂SO₄; dehalogenation of vicinal dihalides with Zn.
- Electrophilic addition: H₂, X₂, HX, H₂O (acid-catalysed).
- Bromine water is decolourised — a test for unsaturation.
- Markovnikov's rule: in adding HX to an unsymmetrical alkene, H goes to the carbon that already has more H atoms (via the more stable carbocation). CH₃CH=CH₂ + HBr → CH₃CHBrCH₃.
- Anti-Markovnikov (peroxide or Kharasch effect): with HBr in the presence of peroxide, addition goes by a free-radical path: CH₃CH=CH₂ → CH₃CH₂CH₂Br. This works only for HBr, not HCl or HI.
- Ozonolysis: O₃ then Zn/H₂O cleaves C=C into two carbonyl compounds. It is used to locate the double bond.
- Cold dilute alkaline KMnO₄ (Baeyer's reagent) gives a diol and loses its purple colour; hot acidic KMnO₄ cleaves the double bond.
- Polymerisation: ethene gives polythene.
4. Alkynes (CₙH₂ₙ₋₂)
- sp carbons, linear; C≡C (120 pm) = one σ + two π bonds.
- Preparation: ethyne from calcium carbide and water, CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂; double dehydrohalogenation of vicinal dihalides.
- Acidity: the terminal H of an alkyne is weakly acidic because the sp carbon (50% s-character) holds the electron pair of the anion firmly. HC≡CH reacts with Na or NaNH₂ to give sodium acetylide; terminal alkynes give precipitates with ammoniacal AgNO₃ (white) and Cu₂Cl₂ (red). Order: HC≡CH > CH₂=CH₂ > CH₃CH₃.
- Addition: of H₂, X₂, HX (Markovnikov, twice) and water.
- Hydration with dilute H₂SO₄/HgSO₄ at 333 K: ethyne gives ethanal (acetaldehyde); other alkynes give ketones.
- Cyclic polymerisation: three ethyne molecules passed through a red-hot iron tube (873 K) give benzene.
5. Aromatic Hydrocarbons (Arenes)
- Benzene C₆H₆: planar hexagon, all C–C bonds equal (139 pm), six delocalised π electrons (Kekulé structures are resonance forms).
- Hückel's rule: a compound is aromatic if it is cyclic, planar, fully conjugated and has (4n + 2) π electrons (n = 0, 1, 2…). Benzene (6), naphthalene (10), cyclopentadienyl anion (6) and tropylium cation (6) are aromatic.
- Preparation: cyclic polymerisation of ethyne; decarboxylation of sodium benzoate with soda lime; reduction of phenol with zinc dust.
- Benzene prefers electrophilic substitution (keeps its aromatic ring) over addition.
| Reaction | Reagents | Electrophile | Product |
|---|---|---|---|
| Nitration | Conc. HNO₃ + conc. H₂SO₄ | NO₂⁺ | Nitrobenzene |
| Halogenation | Cl₂ with anhydrous FeCl₃ | Cl⁺ | Chlorobenzene |
| Sulphonation | Fuming H₂SO₄ | SO₃ | Benzenesulphonic acid |
| Friedel–Crafts alkylation | RCl + anhydrous AlCl₃ | R⁺ | Alkylbenzene |
| Friedel–Crafts acylation | RCOCl + anhydrous AlCl₃ | RCO⁺ | Aryl ketone |
Under UV light benzene can also add Cl₂ to form benzene hexachloride (BHC, C₆H₆Cl₆), and it adds H₂ over Ni to give cyclohexane.
6. Directive Influence of Substituents
| Group type | Examples | Directs to | Effect on rate |
|---|---|---|---|
| Activating, ortho/para | –OH, –NH₂, –OCH₃, –CH₃, –NHCOCH₃ | o and p | Faster than benzene |
| Deactivating, ortho/para | –Cl, –Br, –I | o and p | Slower (−I beats +R) |
| Deactivating, meta | –NO₂, –CN, –CHO, –COOH, –SO₃H, –NR₃⁺ | m | Much slower |
Benzene and many polycyclic arenes are toxic and some are carcinogenic (benzo[a]pyrene in tobacco smoke).
7. Formula and Reaction Master Table
| Name / type | Reaction |
|---|---|
| Wurtz | 2RX + 2Na → R–R + 2NaX (dry ether) |
| Decarboxylation | RCOONa + NaOH → RH + Na₂CO₃ (CaO, heat) |
| Kolbe's electrolysis | 2RCOONa + 2H₂O → R–R + 2CO₂ + H₂ + 2NaOH |
| Markovnikov addition | CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ |
| Peroxide effect | CH₃CH=CH₂ + HBr (ROOR) → CH₃CH₂CH₂Br |
| Ozonolysis | R₂C=CR′₂ + O₃, Zn/H₂O → R₂C=O + O=CR′₂ |
| Hydration of ethyne | HC≡CH + H₂O (HgSO₄/H₂SO₄) → CH₃CHO |
| Calcium carbide | CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂ |
| Hückel's rule | (4n + 2) π electrons ⇒ aromatic |
| General formulas | alkane CₙH₂ₙ₊₂; alkene CₙH₂ₙ; alkyne CₙH₂ₙ₋₂ |
8. Master Summary Table
| Class | Typical reaction | NEET Note |
|---|---|---|
| Alkanes | Free-radical substitution | Staggered ethane more stable |
| Alkenes | Electrophilic addition | Markovnikov; peroxide only with HBr |
| Alkynes | Addition; acidic terminal H | Ethyne + water → ethanal |
| Benzene | Electrophilic substitution | Needs Lewis acid catalyst |
| Aromaticity | (4n + 2) π e⁻ | Planar, cyclic, conjugated |
| Directing groups | o/p or m | Halogens: o/p but deactivating |
| Ozonolysis | Cleaves C=C | Locates double bond |
9. Mind Map
- Wurtz, Kolbe
- Halogenation
- Conformations
- Markovnikov
- Peroxide effect
- Ozonolysis
- Acidic H
- Hydration
- Benzene from ethyne
- Hückel
- EAS reactions
- Directing groups
10. Tips and Tricks
- To work backwards from ozonolysis products, join the two C=O carbons with a double bond.
- Wurtz with two different halides gives a mixture of three alkanes — not a good method for odd-carbon alkanes.
- Decarboxylation removes one carbon; Kolbe doubles the alkyl group.
- Lindlar → cis alkene; Na/liquid NH₃ → trans alkene.
- Applying the peroxide effect to HCl or HI.
- Calling halogens meta-directing because they deactivate the ring.
- Thinking internal alkynes give a precipitate with ammoniacal AgNO₃.
- Expecting methane from the Wurtz reaction.
11. Important One-liners
- Neopentane has the lowest boiling point among the pentane isomers.
- The staggered conformation of ethane is more stable than the eclipsed one.
- Bromine water is decolourised by alkenes and alkynes.
- Hydration of ethyne gives ethanal; hydration of propyne gives propanone.
- Ethyne is more acidic than ethene and ethane.
- All C–C bonds in benzene are equal in length.
- –OH and –NH₂ are strongly activating and ortho/para-directing; –NO₂ is meta-directing.
12. NEET-pattern Questions with Detailed Answers
These questions are original and based on concepts that NEET UG asks again and again.
The main product of propene with HBr in the absence of peroxide is:
- 1-Bromopropane
- 2-Bromopropane
- 1,2-Dibromopropane
- Propane
Show solution
By Markovnikov's rule, H⁺ adds to the terminal CH₂. This forms the more stable secondary carbocation, and Br⁻ then attaches to C-2. 1-Bromopropane forms only in the presence of peroxide.
Answer: (B)
Which compound is aromatic?
- Cyclobutadiene
- Cyclooctatetraene
- Benzene
- Cyclohexene
Show solution
Benzene is planar, cyclic and fully conjugated with 6 π electrons, which fits 4n + 2 for n = 1. Cyclobutadiene has 4 and cyclooctatetraene has 8 π electrons (4n, not aromatic). Cyclohexene is not fully conjugated.
Answer: (C)
Ozonolysis of an alkene gives propanone and ethanal. The alkene is:
- But-1-ene
- 2-Methylbut-2-ene
- Pent-2-ene
- 2-Methylpropene
Show solution
Joining (CH₃)₂C=O and O=CHCH₃ through a C=C gives (CH₃)₂C=CHCH₃, which is 2-methylbut-2-ene. 2-Methylpropene would give propanone and methanal, and pent-2-ene would give ethanal and propanal.
Answer: (B)
Which gives a white precipitate with ammoniacal silver nitrate?
- But-2-yne
- But-1-yne
- But-1-ene
- Butane
Show solution
Only terminal alkynes have the acidic ≡C–H that forms a silver acetylide. But-2-yne is an internal alkyne and has no such hydrogen. Alkenes and alkanes do not react.
Answer: (B)
The electrophile in the nitration of benzene is:
- NO₂⁻
- NO₂⁺
- NO⁺
- HNO₃
Show solution
Concentrated H₂SO₄ protonates HNO₃, which then loses water to give the nitronium ion NO₂⁺. This ion attacks the ring. NO₂⁻ is a nucleophile, not an electrophile.
Answer: (B)
Which group is ortho/para-directing but deactivating?
- –OH
- –CH₃
- –Cl
- –NO₂
Show solution
The −I effect of Cl lowers the overall reactivity of the ring. Its +R effect still places electron density at the ortho and para positions. –OH and –CH₃ are activating, and –NO₂ is meta-directing.
Answer: (C)
Sodium propanoate is electrolysed (Kolbe's method). The hydrocarbon formed at the anode is:
- Ethane
- Propane
- Butane
- Hexane
Show solution
The propanoate ion loses CO₂ to form an ethyl radical, and two ethyl radicals join to give butane. Decarboxylation with soda lime would give ethane instead.
Answer: (C)
The order of acidity of ethane, ethene and ethyne is:
- Ethane > ethene > ethyne
- Ethyne > ethene > ethane
- Ethene > ethyne > ethane
- All are equal
Show solution
Acidity rises with the s-character of the carbon holding the H: sp (50%) > sp² (33%) > sp³ (25%). A carbon with more s-character holds the resulting negative charge more tightly, stabilising the anion.
Answer: (B)
13. Quick Revision Summary
- Alkanes: Wurtz, decarboxylation, Kolbe; free-radical halogenation; staggered > eclipsed.
- Alkenes: Markovnikov; peroxide effect only with HBr; ozonolysis; Baeyer test.
- Alkynes: acidic terminal H; HgSO₄ hydration; Lindlar (cis) vs Na/NH₃ (trans).
- Benzene: (4n + 2) rule; nitration (NO₂⁺), Friedel–Crafts (AlCl₃).
- o/p: –OH, –NH₂, –CH₃, –X; m: –NO₂, –CN, –CHO, –COOH.