Hydrocarbons

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Hydrocarbons — Alkanes, Alkenes, Alkynes, Aromatics

Hydrocarbons contain only carbon and hydrogen. They are classified as saturated (C-C single bonds only, sp³), unsaturated (C=C or C≡C), and aromatic (contains benzene ring or similar aromatic system).

Alkanes (CₙH₂ₙ₊₂): Saturated acyclic hydrocarbons. General formula for straight chain: CH₃(CH₂)ₙCH₃.

Nomenclature:

• Methane (C₁), Ethane (C₂), Propane (C₃), Butane (C₄), Pentane (C₅), Hexane (C₆), Heptane (C₇), Octane (C₈)
• For branched alkanes: longest chain → number → name substituents
• Isomers: Butane has 2 isomers (n-butane, isobutane); Pentane has 3 isomers

Physical Properties:

• Boiling point increases with MW (more C atoms = higher BP)
• Straight chain > branched (more surface area for London dispersion forces)
• All alkanes are non-polar, insoluble in water
• Density less than water (all float on water)

Alkenes (CₙH₂ₙ): Unsaturated hydrocarbons with at least one C=C double bond.

Nomenclature:

• Double bond takes priority over alkyl substituents
• Number the chain to give double bond lowest possible number
• Example: CH₂=CH-CH₂-CH₃ = Propene; CH₃-CH=CH-CH₃ = 2-Butene
• E/Z (trans/cis) notation for geometrical isomers when each double bond carbon has two different groups

Alkynes (CₙH₂ₙ₋₂): Unsaturated hydrocarbons with at least one C≡C triple bond.

Nomenclature:

• CH≡CH (ethyne), CH₃-C≡CH (propyne), CH₃-C≡C-CH₃ (2-butyne)
• Acidic hydrogen: Terminal alkynes (R-C≡CH) have acidic hydrogen (pKa ~25) Reaction with NaNH₂: R-C≡CH + NaNH₂ → R-C≡CNa + NH₃

Aromatic Hydrocarbons (Arenes):

• Simplest: Benzene (C₆H₆) — planar, hexagonal ring with delocalized π-electrons
• Huckel’s rule: Aromaticity requires (4n+2) π electrons (n = 0, 1, 2…)
• Benzene has 6 π electrons (n=1, so 4(1)+2 = 6 ✓)

⚡ Exam tip: For E/Z nomenclature — if the two higher priority groups on each carbon are on the same side, it’s Z (zusammen = together). If opposite sides, it’s E (entgegen = opposite). Priority determined by Cahn-Ingold-Prelog rules (higher atomic number first).

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Hydrocarbons — Chemistry Study Guide

Hybridization and Geometry:

Hydrocarbon	Hybridization	Bond Angles	Geometry
Alkane (sp³ C)	sp³	109.5°	Tetrahedral
Alkene (sp² C)	sp²	120°	Trigonal planar
Alkyne (sp C)	sp	180°	Linear
Benzene	sp²	120°	Planar hexagonal

Bond Lengths:

• C-C (alkane): 154 pm
• C=C (alkene): 134 pm
• C≡C (alkyne): 120 pm
• C-C (benzene): 139 pm (intermediate due to resonance)

Alkane Reactions:

1. Halogenation: CH₄ + Cl₂ → CH₃Cl + HCl (UV light required; photochemical reaction) CH₄ + 2Cl₂ → CH₂Cl₂ + 2HCl (sequential halogenation occurs) Mechanism: Free radical substitution (homolytic bond cleavage) Propagation: ·CH₃ + Cl₂ → CH₃Cl + Cl· Termination: ·CH₃ + ·Cl → CH₃Cl

2. Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O (complete combustion) 2CH₄ + 3O₂ → 2CO + 4H₂O (incomplete combustion, limited O₂)

3. Isomerization: n-Butane → Isobutane (with AlCl₃/HCl catalyst)

4. Aromatization (Catalytic reforming): n-Hexane → Benzene + 4H₂ (Pt/Reforming at 500°C)

5. Pyrolysis (Cracking): C₁₀H₂₂ → C₅H₁₀ + C₅H₁₂ (thermal cracking at high T)

Alkene Reactions:

1. Addition Reactions:

   a) Hydrogenation: CH₂=CH₂ + H₂ → CH₃-CH₃ (Pt/Pd/Ni catalyst, syn addition)

   b) Halogenation: CH₂=CH₂ + Br₂ → CH₂Br-CH₂Br (anti addition in inert solvents) Test for unsaturation: Br₂ in CCl₄ (decolorizes) or KMnO₄ (decolorizes)

   c) Hydrohalogenation (Markovnikov addition): CH₃-CH=CH₂ + HBr → CH₃-CHBr-CH₃ (H adds to less substituted C) Reason: More stable carbocation intermediate is formed If HBr + peroxide (anti-Markovnikov): CH₃-CH₂-CH₂Br (Kharasch effect)

   d) Hydration: CH₂=CH₂ + H₂O → CH₃-CH₂OH (H₃PO₄ catalyst, direct hydration) CH₃-CH=CH₂ + H₂O → CH₃-CH(OH)-CH₃ (indirect via H₂SO₄)

   e) Ozonolysis: CH₂=CH-CH=CH₂ + 2O₃ → 2HCHO + (COOH)₂ (glyoxal) Ozonide intermediate → reductive cleavage with Zn/H₂O or Zn/CH₃COOH → carbonyls

2. Oxidation:

   a) Combustion: Same as alkanes

   b) Cold KMnO₄ (Baeyer test): R-CH=CH₂ + [O] + H₂O → R-CH(OH)-CH₂OH (diol, dihydroxylation)

   c) Hot KMnO₄ (oxidative cleavage): R-CH=CH-R’ + KMnO₄ (hot) → R-COOH + R’-COOH

   d) Ozonolysis: R-CH=CH-R’ + O₃ → R-CHO + R’-CHO (with Zn/H₂O)

   e) Catalytic oxidation: CH₂=CH₂ + ½O₂ → (CH₃CHO) → CH₃COOH (Wacker process)

3. Polymerization: n(CH₂=CH₂) → (–CH₂-CH₂–)ₙ (polyethylene, Ziegler-Natta catalyst) n(CH₃-CH=CH₂) → (–CH(CH₃)-CH₂–)ₙ (polypropylene)

4. Addition of Carbenes: :CH₂ (singlet carbene) adds to alkenes to form cyclopropanes CH₂=N₂ → :CH₂ + N₂ (photolysis or thermolysis)

Alkyne Reactions:

1. Acidic Character of Terminal Alkynes:

   • pKa of acetylene ~25; R-C≡CH can form acetylides
   • NaNH₂: R-C≡CH + NaNH₂ → R-C≡CNa + NH₃ (sodium acetylide)
   • AgNO₃/NH₄Cl: R-C≡CH + AgNO₃ → R-C≡CAg↓ (white precipitate) + NH₄Cl
   • Cu₂Cl₂/NH₄Cl: R-C≡CH + Cu₂Cl₂ → R-C≡CCu↓ (red-brown precipitate) + NH₄Cl

2. Reduction:

   • Lindlar’s catalyst (Pd/CaCO₃ poisoned with quinoline): alkyne → cis-alkene
   • Na/NH₃ (liquid): alkyne → trans-alkene (dissolving metal reduction) Mechanism: Na donates electron to triple bond, forming radical anion; protonation gives trans-alkene.

3. Hydroboration-Oxidation: R-C≡CH + R₂BH → R-CH=CH-BR₂; then H₂O₂/NaOH → R-CH₂-CHO (aldehyde from terminal alkyne, anti-Markovnikov)

4. Hydration (Kucherov Rule): R-C≡CH + H₂O + Hg²⁺/H₂SO₄ → R-CO-CH₃ (methyl ketone) Tautomerization of enol to ketone occurs.

5. Oxidative Cleavage: R-C≡C-R’ + KMnO₄ (hot) → R-COOH + R’-COOH

Aromatic Substitution:

Benzene undergoes electrophilic aromatic substitution (EAS).

General Mechanism:

1. Lewis acid catalyst generates electrophile (E⁺)
2. π electrons of benzene attack E⁺ → forms arenium ion (sigma complex)
3. Deprotonation by base restores aromaticity

Reactions:

1. Nitration: C₆H₆ + HNO₃ (conc.) → C₆H₅NO₂ + H₂O (H₂SO₄ as catalyst) E⁺ = NO₂⁺ (nitronium ion)

2. Halogenation: C₆H₆ + Cl₂ → C₆H₅Cl + HCl (FeCl₃ or AlCl₃ catalyst) C₆H₆ + Br₂ → C₆H₅Br + HBr (FeBr₃ catalyst) E⁺ = Cl⁺ or Br⁺

3. Sulfonation: C₆H₆ + H₂SO₄ → C₆H₅SO₃H + H₂O (reversible, uses fuming H₂SO₄) E⁺ = SO₃H⁺

4. Friedel-Crafts Alkylation: C₆H₆ + R-Cl → C₆H₅R + HCl (AlCl₃ catalyst) Problem: polyalkylation, carbocation rearrangement possible E⁺ = R⁺ (carbocation from alkyl halide)

5. Friedel-Crafts Acylation: C₆H₆ + R-COCl → C₆H₅-CO-R + HCl (AlCl₃ catalyst) E⁺ = R-C≡O⁺ (acylium ion) Advantage: no polyacylation (deactivating ketone slows further substitution)

Activating and Deactivating Groups:

Activating (+): Ortho/para directors

• –OH, –NH₂, –OCH₃, –CH₃ (alkyl)
• Reason: electron-donating groups stabilize the arenium ion intermediate

Deactivating (–): Meta directors

• –NO₂, –CN, –COOH, –SO₃H, –CHO
• Reason: electron-withdrawing groups destabilize the arenium ion

Halogens (–X): Deactivating but ortho/para directing (anomaly due to resonance donation)

⚡ Exam tip: When benzene has two substituents, the position of the third substitution depends on the directing effects. If both are ortho/para directors, the stronger activator wins. If one is meta and one is ortho/para, the ortho/para director wins.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Hydrocarbons — Comprehensive Chemistry Notes

Mechanism of Electrophilic Aromatic Substitution — Detailed:

Step 1: Generation of electrophile

• For nitration: HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻

Step 2: Attack on benzene ring

• The π electrons form a bond with E⁺, creating an arenium ion with positive charge delocalized over the ring
• This intermediate is not aromatic and is relatively unstable

Step 3: Deprotonation

• Loss of H⁺ from the ring restores aromaticity; the catalyst is regenerated

Energy diagram: The formation of the arenium ion is the rate-determining step ( RDS). Activating groups lower the activation energy for this step; deactivating groups raise it.

Arenium Ion Stabilization:

Ortho/para directors stabilize the intermediate through resonance:

• –OH on benzene: when electrophile attacks ortho, the positive charge can be delocalized onto the oxygen
• This makes ortho/para products predominant

Hyperconjugation: Alkyl groups (–CH₃) donate electron density via hyperconjugation — the C-H bonds adjacent to the ring can donate electron density into the ring, stabilizing the intermediate.

Benzene vs Cyclobutadiene vs Cyclooctatetraene:

• Cyclobutadiene (4 π electrons): Anti-aromatic (4n = 4, n=1 → 4n, not 4n+2), highly unstable
• Benzene (6 π electrons): Aromatic (4n+2 = 6, n=1), very stable
• Cyclooctatetraene (8 π electrons): Not aromatic, exists as tub-shaped molecule with alternating single and double bonds; undergoes addition reactions (8n = 8, not 4n+2)

Non-Benzenoid Aromatic Compounds:

• Cyclopentadienyl anion (C₅H₅⁻): 6 π electrons (4n+2, n=1), aromatic
• Tropylium cation (C₇H₇⁺): 6 π electrons, aromatic
• Cyclopropenyl cation: 2 π electrons (4n+2, n=0), aromatic

Hückel’s Rule Proof: For a planar, monocyclic, conjugated system with (4n+2) π electrons: n = 0 → 2 π electrons (aromatic: cyclopropenyl cation) n = 1 → 6 π electrons (aromatic: benzene, cyclopentadienyl anion, tropylium) n = 2 → 10 π electrons (e.g., naphthalene, though naphthalene is polycyclic)

Addition vs Substitution in Aromatics: Benzene undergoes substitution rather than addition because:

1. Substitution maintains aromaticity (very stable)
2. Addition would require breaking the delocalized π system
3. The ΔG for substitution is more favorable due to aromatic stabilization energy (~36 kcal/mol for benzene)

Birge-Van Vleck rules for π-electron count: Count all π electrons in the cyclic conjugated system. If the number is 2, 6, 10, 14… (4n+2), it’s potentially aromatic.

Combustion Enthalpies and Resonance Energy:

The resonance energy of benzene = Experimental heat of combustion − Theoretical heat of combustion (if it were cyclohexatriene)

• Experimental: 3267 kJ/mol (for 3 double bonds)
• Theoretical for 1,3,5-cyclohexatriene: ~3600 kJ/mol (higher)
• Resonance energy = ~330 kJ/mol

Clar’s Rule for Polycyclic Aromatic Hydrocarbons: In alternant PAHs, the Kekulé structure with the maximum number of disjoint aromatic sextets is the most important resonance contributor.

Cumulated Systems — Allenes and Cumulenes:

• Allene (CH₂=C=CH₂): Central carbon is sp-hybridized, terminal are sp². The two π bonds are perpendicular. Not aromatic, but cumulene with 4n+2 π electrons would be aromatic.
• Cumulenes (polyynes): R-(C=C)ₙ-R with cumulated double bonds

Cycloalkanes:

General formula: CₙH₂ₙ (same as alkenes, isomerism with alkenes)

• Cyclopropane (C₃H₆): angle strain 49.5°, highly reactive, banana bonds
• Cyclobutane (C₄H₈): angle strain ~25°, puckered ring
• Cyclopentane (C₅H₁₀): angle strain ~5°, envelope conformation
• Cyclohexane (C₆H₁₂): no angle strain, chair conformation preferred
• Cyclohexane chair: 12 C-H bonds — 6 axial, 6 equatorial

Conformational Analysis of Cyclohexane:

Chair conformation:

• Axial bonds: perpendicular to ring, alternate up/down
• Equatorial bonds: roughly in plane of ring, outward
• In methylcyclohexane: equatorial conformation is more stable (gauche effect; methyl prefers equatorial)

Bredt’s Rule: Cannot have a bridgehead double bond in a small ring (n < 8) because the double bond cannot achieve the required geometry without excessive strain.

** Woodward-Hoffmann Rules (Pericyclic Reactions):**

For electrocyclic reactions:

• Thermal ring closure: 4n electrons → conrotatory; 4n+2 electrons → disrotatory
• Photochemical ring closure: opposite

For [2+2] cycloadditions:

• Thermal: suprafacial-suprafacial allowed for two π electrons; 4n electrons requires photochemical conditions
• Woodward-Hoffmann: Conservation of orbital symmetry

Isomerism in Alkenes:

Geometrical isomers (E/Z):

• Conditions: Each double bond carbon must have two different groups
• If one carbon has identical groups (e.g., CH₂=), E/Z not applicable
• Priority: CIP rules — compare atomic numbers of directly attached atoms

Example: 2-butene

• cis-2-butene: CH₃ and H on same side
• trans-2-butene: CH₃ and H on opposite sides
• trans is more stable (less steric strain between two CH₃ groups)

Optical Isomerism in Hydrocarbons:

Allenes with different substituents on each end are chiral (no plane of symmetry, no center of symmetry):

• R-C≡C-R’ with R ≠ R’ and each C has two different substituents → chiral

Also, substituted cumulenes (buta-1,2,3-triene, etc.) can be chiral if substituents are different.

Chiral alkanes: 3-methylhexane is chiral (has a chiral center at C-3)

• 2,3-dimethylbutane: meso form possible

Catalytic Reforming — Octane Number:

Octane number measures fuel quality:

• n-Heptane: Octane number 0 (knocks badly)
• Isooctane (2,2,4-trimethylpentane): Octane number 100 (no knock)
• Branched alkanes have higher octane numbers than straight-chain
• Aromatics have high octane numbers
• Tetraethyl lead was used as anti-knock additive (now banned for environmental reasons)

Cracking — Thermal vs Catalytic:

Thermal cracking: Free radical mechanism at high T (700-900°C) Catalytic cracking (FCC): Protonic acid catalysis (zeolites) at lower T (500-600°C)

• Produces more branched alkanes and aromatics
• Zeolites have shape selectivity

Sandmeyer Reaction (from Haloalkanes to Aromatics):

Ar-NH₂ + HBF₄ → Ar-N₂⁺BF₄⁻ → (gentle heat) → Ar-F + N₂ + BF₃ Ar-NH₂ + CuCl → Ar-Cl + N₂ (chlorobenzene from aniline) Ar-NH₂ + CuBr → Ar-Br + N₂

This is not a hydrocarbon reaction per se but is important in converting aromatic nitro compounds to other substituted aromatics.

Diels-Alder Reaction:

[4+2] cycloaddition between a diene and a dienophile.

• Diene must be in s-cis conformation
• Stereochemistry is retained (cis-dienophile gives cis-substituted cyclohexene)
• Suprafacial-suprafacial, thermally allowed
• Example: Butadiene + Ethylene → Cyclohexene

⚡ Exam tip: When asked about aromaticity of a given ring system, check:

1. Is it planar (or nearly planar)?
2. Is it cyclic?
3. Is it conjugated (every atom in ring has p-orbital)?
4. Does it have 4n+2 π electrons?

⚡ Exam tip: In JEE, questions often test whether students know that benzene with a –NO₂ group (strongly deactivating) will direct incoming electrophiles to meta position, while benzene with –OH (activating) will direct to ortho/para.

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