Hydrocarbons

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Hydrocarbons — Key Facts for MDCAT

Hydrocarbons contain only carbon and hydrogen atoms. They are classified into:

Alkanes (saturated, C$n$H${2n+2}$):

• Single bonds only (sp³ hybridisation)
• Methane CH$_4$, Ethane C$_2$H$_6$, Propane C$_3$H$_8$, Butane C$4$H${10}$
• General formula: C$n$H${2n+2}$
• Combustion: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$
• Substitution reactions with halogens (UV light): $CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl$

Alkenes (unsaturated, C$n$H${2n}$):

• At least one C=C double bond (sp² hybridisation)
• Ethene C$_2$H$_4$, Propene C$_3$H$_6$
• Addition reactions: H$_2$ (hydrogenation), Br$_2$ (bromination test for unsaturation), HX (Markovnikov’s rule)
• Markovnikov’s Rule: H adds to carbon with more H atoms; X adds to carbon with fewer H atoms
• Polymerisation: n(CH$_2$=CH$_2$) → (–CH$_2$–CH$_2$–)$_n$ (polythene)

Alkynes (C$n$H${2n-2}$):

• At least one C≡C triple bond (sp hybridisation)
• Ethyne (acetylene) C$_2$H$_2$: HC≡CH
• Addition reactions: can undergo two successive additions
• Acidity of alkynes: Terminal alkynes (HC≡C–) are weakly acidic; they react with NaNH$_2$ or AgNO$_3$ to form acetylides

Aromatic Hydrocarbons (Arenes):

• Benzene C$_6$H$_6$: planar hexagonal ring with delocalised π electrons
• Hückel’s rule: (4n+2) π electrons where n = 0, 1, 2… (for benzene: 6 π electrons, n=1 ✓)
• Electrophilic substitution reactions: nitration, halogenation, sulfonation, Friedel-Crafts alkylation

⚡ Exam tip: MDCAT frequently tests the addition vs substitution distinction — alkanes undergo substitution (free radical mechanism, UV light), while alkenes/alkynes undergo addition (electrophilic addition). For alkene addition with HBr, use Markovnikov’s rule. The bromine test (brown Br$_2$ water decolourises with alkenes) is a key test for unsaturation.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students who want genuine understanding.

Hydrocarbons — Complete Study Guide

Hybridisation Summary:

Hybridisation	Geometry	Bond Angle	Bonds
sp³	Tetrahedral	109.5°	4 single bonds
sp²	Trigonal planar	120°	3 bonds (1 double)
sp	Linear	180°	2 bonds (1 triple or 2 double)

Alkane Reactions:

• Halogenation: CH$_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl$ (free radical mechanism: initiation → propagation → termination)
• Combustion: Complete: $C_nH_{2n+2} + \frac{3n+1}{2}O_2 \rightarrow nCO_2 + (n+1)H_2O$
• Isomerism: Butane has 2 isomers (n-butane and isobutane); pentane has 3 isomers

Alkene Reactions:

• Hydrogenation: $CH_2=CH_2 + H_2 \xrightarrow{Pd} CH_3–CH_3$
• Halogenation: $CH_2=CH_2 + Br_2 \rightarrow CH_2Br–CH_2Br$ (1,2-dibromoethane)
• Hydrohalogenation: $CH_3CH=CH_2 + HBr \rightarrow CH_3CH(Br)CH_3$ (Markovnikov, major product)
• Hydration: $CH_2=CH_2 + H_2O \xrightarrow{H^+} CH_3CH_2OH$ (ethanol, acid-catalysed)
• Ozonlysis: Ozonolysis with Zn/H$_2$O gives carbonyl compounds; determines position of double bond

Alkyne Reactions:

• Reduction: H$_2$/Pd (Lindlar’s catalyst) → cis-alkene; Na/NH$_3$ → trans-alkene
• Hydration (Kucherov rule): Hg$^{2+}$, H$_2$SO$_4$ → ketone (Markovnikov addition, tautomerisation)
• Acidity: Terminal alkynes react with Na metal → acetylide anion: $HC≡CH + Na \rightarrow Na^+C≡CH^- + \frac{1}{2}H_2$

Benzene — Aromatic Character:

1. Planar ring with delocalised electrons above and below the plane
2. All C–C bonds are equivalent (intermediate between single and double)
3. Undergoes electrophilic substitution (not addition) due to aromatic stability
4. Delocalisation energy (resonance energy) ≈ 150 kJ/mol — gives benzene its stability
5. Nitration: $C_6H_6 + HNO_3 \xrightarrow{H_2SO_4} C_6H_5NO_2 + H_2O$
6. Halogenation: $C_6H_6 + Cl_2 \xrightarrow{FeCl_3} C_6H_5Cl + HCl$

⚡ Common student mistakes: Writing Markovnikov addition in the wrong direction for unsymmetrical alkenes. Confusing the conditions for alkane halogenation (UV light, free radical) with alkene addition. For benzene, thinking it undergoes addition reactions like typical alkenes — it doesn’t, due to aromaticity.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Hydrocarbons — Advanced Notes

Free Radical Mechanism for Alkane Halogenation:

1. Initiation: $Cl_2 \xrightarrow{h\nu} 2Cl^\cdot$ (homolytic fission)
2. Propagation: $Cl^\cdot + CH_4 \rightarrow \cdot CH_3 + HCl$; $\cdot CH_3 + Cl_2 \rightarrow CH_3Cl + Cl^\cdot$
3. Termination: $\cdot CH_3 + \cdot Cl \rightarrow CH_3Cl$; $2\cdot CH_3 \rightarrow C_2H_6$

Multiple substitution products possible (mono-, di-, tri-chloromethane).

Electrophilic Addition Mechanism for Alkenes: Step 1: The electrophile ($E^+$) attacks the π bond → forms a carbocation intermediate (more substituted carbocation is more stable: tertiary > secondary > primary) Step 2: The nucleophile ($Nu^-$) attacks the carbocation → neutral product

Stability of carbocations: benzyl > allyl > tertiary > secondary > primary > methyl

Markovnikov’s Rule Explained: In $CH_3CH=CH_2 + HBr$:

• H adds to C-1 (has 2 H atoms) → forms $CH_3\overset{+}{CH}CH_3$ (secondary carbocation)
• Br adds to C-2 → product: $CH_3CH(Br)CH_3$ (2-bromopropane)

Anti-Markovnikov Addition: HBr addition in presence of peroxides (R-O-O-R) causes anti-Markovnikov addition via free radical mechanism. Example: $CH_3CH=CH_2 + HBr \xrightarrow{peroxide} CH_3CH_2CH_2Br$

Cyclic Hydrocarbons:

• Cycloalkanes: C$n$H${2n}$, ring strain in small rings (cyclopropane has bond angle 60° vs normal 109.5°, so it undergoes ring-opening reactions easily)
• Cyclohexene: Unsaturated cycloalkane

Isomerism in Hydrocarbons:

• Structural isomerism: Different connectivity (chain, position, functional group)
• Geometric isomerism (cis-trans): In alkenes with different substituents on each carbon of the double bond; cis = same groups on same side, trans = opposite sides
• Conformational isomerism: Different spatial arrangements due to rotation about single bonds (e.g., staggered vs eclipsed conformations of ethane)

Petroleum Processing:

• Cracking: Breaking larger alkanes into smaller ones (thermal: high T; catalytic: lower T, zeolite catalyst)
• Reforming: Converting n-alkanes to branched alkanes and aromatic compounds
• Octane rating: Isooctane (2,2,4-trimethylpentane) = 100; n-heptane = 0. Higher octane = better anti-knock properties.

MDCAT Question Patterns: MDCAT Pakistan questions on hydrocarbons frequently test: (1) naming reactions and their mechanisms, (2) Markovnikov’s rule application, (3) distinguishing alkanes, alkenes, alkynes by chemical tests (Br$_2$ test, Baeyer’s test — alkaline KMnO$_4$), (4) aromaticity and Hückel’s rule, (5) polymer identification (polythene from ethene, PVC from vinyl chloride, polystyrene from styrene). Average 2–3 questions per paper.

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