Hydrocarbons – Complete NCERT Revision

 

Hydrocarbons – Complete NCERT Revision (Reactions + Mechanisms)

Hydrocarbons are organic compounds containing only carbon and hydrogen. This chapter is heavily tested in JEE Main via reactions and in Advanced via mechanisms & exceptions.

1. Alkanes (Paraffins)

General formula: $C_nH_{2n+2}$

Hybridisation: $sp^3$ (tetrahedral, $109.5^\circ$)

Preparation of Alkanes

• Wurtz Reaction:
  $2R–X + 2Na \rightarrow R–R + 2NaX$
  Limitation: Mixture with different alkyl halides

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• Kolbe Electrolysis:
  $2RCOO^- \rightarrow R–R + 2CO_2 + 2e^-$

1. At the Anode: Oxidation

The process begins at the anode, where the carboxylate ion is oxidized. It loses an electron to form an unstable carboxylate radical, which then loses carbon dioxide to form an alkyl radical.

2. At the Cathode: Reduction

Simultaneously, at the cathode, water molecules are reduced. They gain electrons to form hydroxide ions and hydrogen radicals. The hydrogen radicals then combine to form hydrogen gas.

3. Product Formation and Overall Reaction

Finally, the alkyl radicals formed at the anode combine to create the final alkane product. The overall reaction shows the electrolysis of the carboxylate salt to produce an alkane, carbon dioxide, hydrogen gas, and sodium hydroxide.

• Decarboxylation:
  $RCOONa + NaOH \xrightarrow{CaO} RH + Na_2CO_3$

Reactions of Alkanes

1. Free Radical Substitution (Halogenation)

Overall: $CH_4 + Cl_2 \xrightarrow{hv} CH_3Cl + HCl$

Mechanism:

• Initiation: $Cl_2 \xrightarrow{hv} 2Cl^\bullet$
• Propagation:
  $Cl^\bullet + CH_4 \rightarrow CH_3^\bullet + HCl$
  $CH_3^\bullet + Cl_2 \rightarrow CH_3Cl + Cl^\bullet$
• Termination: Radical recombination

JEE Trap: $3^\circ > 2^\circ > 1^\circ$ H abstraction stability

2. Combustion

$C_nH_{2n+2} + \dfrac{3n+1}{2}O_2 \rightarrow nCO_2 + (n+1)H_2O$

3. Isomerisation

n-alkane → branched alkane (AlCl₃ / HCl)

2. Alkenes (Olefins)

General formula: $C_nH_{2n}$

Hybridisation: $sp^2$ (planar)

Preparation

• Dehydration of alcohol:
  $RCH_2CH_2OH \xrightarrow{conc.H_2SO_4} Alkene$
• Dehydrohalogenation:
  $RCH_2CH_2X \xrightarrow{alc.KOH} Alkene$

Reactions of Alkenes

1. Electrophilic Addition (Key Concept)

Mechanism steps:

• π bond attacks electrophile
• Carbocation formation (rate-determining)
• Nucleophile attack

2. Markovnikov Addition

$CH_3–CH=CH_2 + HBr \rightarrow CH_3–CHBr–CH_3$

Rule: H adds to carbon with more hydrogens

3. Anti-Markovnikov (Peroxide Effect)

$CH_3–CH=CH_2 + HBr \xrightarrow{ROOR} CH_3–CH_2–CH_2Br$

Only for HBr

4. Ozonolysis

$RCH=CHR \xrightarrow{O_3/Zn} Aldehydes/Ketones$

5. Oxidation (Baeyer Test)

Cold dilute $KMnO_4$ → Glycol (purple → colorless)

3. Alkynes

General formula: $C_nH_{2n-2}$

Hybridisation: $sp$ (linear)

Preparation

Double dehydrohalogenation using alc.KOH then NaNH₂

Reactions

1. Acidic Nature (Terminal Alkynes)

$RC \equiv CH + NaNH_2 \rightarrow RC \equiv C^- Na^+$

2. Addition Reactions

• H₂/Pd → Alkane
• H₂/Lindlar → Cis-alkene
• Na/NH₃ → Trans-alkene

3. Ozonolysis

Gives diketones / acids

4. Aromatic Hydrocarbons (Benzene)

Aromaticity Conditions:

• Cyclic
• Planar
• Conjugated
• Hückel Rule: $(4n+2)\pi$ electrons

Electrophilic Substitution Reactions

1. Nitration

$C_6H_6 + HNO_3 \xrightarrow{H_2SO_4} C_6H_5NO_2$

2. Sulphonation

$C_6H_6 + SO_3 \xrightarrow{fuming} C_6H_5SO_3H$

3. Halogenation

$C_6H_6 + Cl_2 \xrightarrow{FeCl_3} C_6H_5Cl$

4. Friedel–Crafts Alkylation

$C_6H_6 + RCl \xrightarrow{AlCl_3} Alkylbenzene$

Exception: Fails for –NO₂, –SO₃H substituted benzene

5. Friedel–Crafts Acylation

$C_6H_6 + RCOCl \xrightarrow{AlCl_3} Acylbenzene$

High-Yield Exam Pointers

• Mechanism > reaction in Advanced
• Peroxide effect only with HBr
• Alkanes → free radical, Alkenes → electrophilic
• Benzene resists addition due to aromatic stability

✔ NCERT-aligned
✔ JEE Main + Advanced ready
✔ Mechanism-focused revision