HYDROCARBONS
NCERT Masterclass & NEET Study Module
1. Alkanes (Paraffins)
Alkanes are saturated open-chain hydrocarbons containing only carbon-carbon single bonds. General formula: $C_n H_{2n+2}$. They are highly unreactive under normal conditions (hence 'paraffins').
- Wurtz Reaction: Used to prepare symmetrical alkanes with an even number of carbon atoms. Methane CANNOT be prepared by this method.
$2 R-X + 2Na \xrightarrow{\text{Dry Ether}} R-R + 2NaX$ - Decarboxylation: Sodium salts of carboxylic acids heated with soda lime ($NaOH + CaO$) yield alkanes containing ONE carbon less than the carboxylic acid.
$CH_3COONa + NaOH \xrightarrow{CaO, \Delta} CH_4 + Na_2CO_3$ - Kolbe's Electrolytic Method: Aqueous solution of sodium/potassium salt of carboxylic acid on electrolysis gives alkane at the ANODE.
- Conformations of Ethane: The 'Staggered' conformation is most stable (minimum torsional strain). The 'Eclipsed' conformation is least stable.
2. Alkenes (Olefins)
Alkenes contain at least one double bond ($C=C$). General formula: $C_n H_{2n}$. Due to the restricted rotation around the double bond, they show Geometrical (cis-trans) Isomerism.
- From Alkynes: Reduction with Lindlar's catalyst ($Pd/CaCO_3$ + quinoline) gives cis-alkene. Reduction with Sodium in liquid ammonia (Birch reduction) gives trans-alkene.
- Dehydrohalogenation ($\beta$-elimination): Alkyl halides reacted with Alcoholic KOH give alkenes according to Saytzeff's rule (highly substituted alkene is the major product).
Markovnikov's Rule: During electrophilic addition to an unsymmetrical alkene, the negative part of the addendum goes to the carbon atom containing fewer hydrogen atoms.
Anti-Markovnikov's Rule (Peroxide/Kharasch Effect): Occurs ONLY with $HBr$ in the presence of peroxides. The addition happens via a free radical mechanism.
Ozonolysis: Alkenes react with ozone ($O_3$) followed by cleavage with $Zn/H_2O$ to form aldehydes and/or ketones. Highly useful for locating the exact position of the double bond.
3. Alkynes
Contain at least one triple bond ($C \equiv C$). General formula: $C_n H_{2n-2}$.
- Acidic Character: Terminal alkynes (like ethyne, propyne) are slightly acidic because the $sp$ hybridized carbon is highly electronegative (50% s-character) and easily releases the attached $H^+$. They react with Na metal and ammoniacal silver nitrate (Tollens' reagent).
- Hydration: Alkynes react with water in the presence of heavy metal catalysts ($Hg^{2+} / H_2SO_4$). Ethyne gives Acetaldehyde, while all other alkynes give Ketones.
- Cyclic Polymerization: Passing ethyne gas through a red-hot iron tube at 873K magically trimerizes it to form Benzene ($C_6H_6$).
4. Aromatic Hydrocarbons (Benzene)
Aromatic compounds have exceptional stability due to the delocalization of $\pi$-electrons. Benzene is the parent aromatic compound.
5. Reactions of Benzene & Directive Influence
Because Benzene has a stable $\pi$-electron cloud, it resists addition reactions and prefers Electrophilic Substitution Reactions (e.g., Nitration, Halogenation, Friedel-Crafts alkylation and acylation).
- Ortho/Para Directing (Activating groups): $-OH, -NH_2, -OCH_3, -CH_3$. They push electrons into the ring (+R effect), increasing density at ortho and para positions. Exception: Halogens ($-Cl$) are deactivating but still ortho/para directing.
- Meta Directing (Deactivating groups): $-NO_2, -CN, -CHO, -COOH$. They pull electrons out of the ring (-R effect), making ortho/para positions highly electron deficient, so the electrophile is forced to attack the meta position.
Carcinogenicity: Polynuclear hydrocarbons containing more than two fused benzene rings (like 1,2-benzanthracene) are highly toxic and carcinogenic (cancer-causing).
← Back to NEET Resource Hub🚀 NEET HYDROCARBONS MEGA QUIZ (100 MCQ)
Solve the 5 parts below to master Alkanes, Alkenes, Alkynes, and Aromaticity.
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