Hydrocarbons revision notes

1. Key Concepts & Definitions

Hydrocarbons

Compounds of carbon and hydrogen only, playing a key role as fuels (LPG, CNG, LNG) and in the manufacture of polymers.

Classification

Saturated: Contain only carbon-carbon single bonds (Alkanes). Unsaturated: Contain at least one multiple bond (Alkenes, Alkynes). Aromatic: Special cyclic compounds, classified as benzenoids (containing a benzene ring) and non-benzenoids.

Conformations

Different spatial arrangements of atoms resulting from free (but slightly hindered) rotation about a C–C single bond (e.g., eclipsed, staggered, and skew conformations in ethane).

Dihedral/Torsional Angle

The angle of rotation about the C–C single bond in conformations.

Torsional Strain

The weak repulsive interaction between adjacent bonds that hinders completely free rotation around single bonds.

Geometrical Isomerism

Stereoisomerism arising due to restricted rotation about a carbon-carbon double bond, yielding cis (identical groups on the same side) and trans (identical groups on opposite sides) forms.

Aromatization (Reforming)

Conversion of aliphatic n-alkanes (≥6 carbons) to aromatic hydrocarbons using heat and catalysts.

Pyrolysis (Cracking)

Decomposition of higher alkanes into smaller fragments (lower alkanes, alkenes) by the application of heat.

Sigma Complex (Arenium Ion)

The intermediate formed during Electrophilic Aromatic Substitution (EAS) where one carbon becomes sp3sp^3sp3 hybridized, causing a temporary loss of aromaticity.

Polyacetylene

Formed by the linear polymerization of ethyne; its thin films conduct electricity and are used as cheap, lightweight electrodes in batteries.

Carcinogenicity

Cancer-producing property found in polynuclear hydrocarbons (containing more than two fused benzene rings) formed from incomplete combustion.

2. Important Rules, Laws & Principles

• Markovnikov's Rule: In the addition of an unsymmetrical reagent to an unsymmetrical alkene, the negative part of the addendum attaches to the carbon atom possessing the lesser number of hydrogen atoms.
• Anti-Markovnikov Rule (Kharash or Peroxide Effect): In the presence of a peroxide, the addition of HBr to unsymmetrical alkenes takes place contrary to Markovnikov's rule. This proceeds via a free radical chain mechanism.
• Hückel Rule of Aromaticity: For a ring system to be aromatic, it must satisfy:
  1. Planarity.
  2. Complete delocalisation of the $\pi$ electrons in the ring.
  3. Presence of $(4n + 2) \pi$ electrons, where $n = 0, 1, 2, ...$.
• Directive Influence of Substituents: The functional group already attached to a benzene ring determines the position of the incoming electrophile:
  • Activating (o/p-directing): Increase electron density at o- and p- positions via resonance (e.g., –OH, –NH₂, –NHR, –NHCOCH₃, –OCH₃, –CH₃, –C₂H₅).
  • Deactivating (m-directing): Withdraw electron density, leaving the m- position relatively more electron-rich (e.g., –NO₂, –CN, –CHO, –COR, –COOH, –COOR, –SO₃H).
  • Deactivating (o/p-directing): Halogens (–F, –Cl, –Br, –I) deactivate due to strong –I effect but direct o/p due to +R effect.

3. Reactions & Mechanisms

• Preparation of Alkanes:
  • Hydrogenation: Alkenes/Alkynes + $H_2$ / (Pt/Pd/Ni) $\rightarrow$ Alkane.
  • From Alkyl Halides: RX + Zn/dil. HCl $\rightarrow$ RH.
  • Wurtz Reaction: 2RX + 2Na (dry ether) $\rightarrow$ R–R. Creates higher symmetrical alkanes.
  • Decarboxylation: RCOONa + Soda lime (NaOH+CaO) $\xrightarrow{\Delta}$ RH (one carbon less).
  • Kolbe's Electrolysis: Aqueous RCOONa electrolyzed $\rightarrow$ R–R at anode (via free radical mechanism).
• Reactions of Alkanes:
  • Halogenation (Free Radical): $CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + CH_2Cl_2 + CHCl_3 + CCl_4$. Mechanism involves Initiation, Propagation, and Termination.
  • Reaction with Steam: $CH_4 + H_2O \xrightarrow{Ni, 1273 K} CO + 3H_2$ (Used for dihydrogen gas prep).
  • Controlled Oxidation:
    • $CH_4 + O_2 \xrightarrow{Cu / 523K / 100 atm} CH_3OH$.
    • $CH_4 + O_2 \xrightarrow{Mo_2O_3, \Delta} HCHO$.
    • $2CH_3CH_3 + 3O_2 \xrightarrow{(CH_3COO)_2Mn, \Delta} 2CH_3COOH$.
  • Isomerisation: n-Alkane $\xrightarrow{Anhy. AlCl_3 / HCl}$ Branched alkane.
  • Aromatization: Hexane $\xrightarrow{Cr_2O_3/V_2O_5/Mo_2O_3, 773K, 10-20 atm}$ Benzene.
• Preparation of Alkenes:
  • From Alkynes: Alkyne + $H_2$ / Lindlar's catalyst (Pd/C poisoned with S or quinoline) $\rightarrow$ cis-alkene. Alkyne + Na / liquid $NH_3$ $\rightarrow$ trans-alkene.
  • Dehydrohalogenation ($\beta$-elimination): RX + alcoholic KOH $\xrightarrow{\Delta}$ Alkene.
  • Dehalogenation: Vicinal dihalide + Zn $\rightarrow$ Alkene + $ZnX_2$.
  • Acidic Dehydration: Alcohol $\xrightarrow{Conc. H_2SO_4, \Delta}$ Alkene.
• Reactions of Alkenes:
  • Addition of Halogens: Alkene + $Br_2$/$CCl_4$ $\rightarrow$ Vicinal dibromide (Discharges reddish-orange color). Involves a cyclic halonium ion.
  • Addition of Sulphuric Acid: Reacts with cold conc. $H_2SO_4$ via Markovnikov addition to form alkyl hydrogen sulphates.
  • Oxidation: Alkene + Cold dil. aqueous $KMnO_4$ (Baeyer's Reagent) $\rightarrow$ Vicinal glycol.
  • Ozonolysis: Alkene + $O_3 \rightarrow$ Ozonide $\xrightarrow{Zn/H_2O}$ Aldehydes/Ketones.
  • Polymerisation: Ethene/Propene $\xrightarrow{\text{High Temp/Pressure, Catalyst}}$ Polythene/Polypropene.
• Preparation of Alkynes:
  • Industrial: $CaC_2 + 2H_2O \rightarrow Ca(OH)_2 + C_2H_2$.
  • From Vicinal Dihalides: Vicinal dihalide + alc. KOH $\rightarrow$ Alkenyl halide $\xrightarrow{NaNH_2}$ Alkyne.
• Reactions of Alkynes:
  • Acidic character: Ethyne + Na $\rightarrow$ Sodium ethynide + $H_2$.
  • Addition of HX: Adds two molecules of HX to form gem-dihalides.
  • Hydration: Alkyne + $H_2O \xrightarrow{HgSO_4 / dil. H_2SO_4, 333K}$ Carbonyl compounds.
  • Cyclic Polymerization: Ethyne $\xrightarrow{\text{Red hot iron tube, 873K}}$ Benzene.
• Preparation of Benzene:
  • From Phenol: Phenol + Zn dust $\xrightarrow{\Delta}$ Benzene + ZnO.
  • Decarboxylation: Sodium benzoate + Soda lime (NaOH/CaO) $\xrightarrow{\Delta}$ Benzene + $Na_2CO_3$.
• Reactions of Arenes (Electrophilic Substitution - SE):
  • Nitration: Benzene + $HNO_3 + H_2SO_4 \rightarrow$ Nitrobenzene (Electrophile: $NO_2^+$).
  • Halogenation: Benzene + $X_2 \xrightarrow{Anhy. AlCl_3/FeCl_3}$ Halobenzene (Electrophile: $Cl^+$).
  • Sulphonation: Benzene + Fuming $H_2SO_4$ (Oleum) $\rightarrow$ Benzene sulphonic acid.
  • Friedel-Crafts Alkylation/Acylation: Benzene + RX / RCOCl $\xrightarrow{Anhy. AlCl_3}$ Alkylbenzene / Acylbenzene.
• Addition Reactions of Benzene:
  • Hydrogenation: Benzene + $3H_2 \xrightarrow{Ni, \text{High Temp/Pressure}}$ Cyclohexane.
  • Halogenation: Benzene + $3Cl_2 \xrightarrow{UV \text{ light}}$ Benzene hexachloride ($C_6H_6Cl_6$, BHC, or Gammaxane).

4. Formulae & Equations

• General Formulas:
  • Alkanes: $C_nH_{2n+2}$
  • Alkyl groups: $C_nH_{2n+1}$
  • Alkenes: $C_nH_{2n}$
  • Alkynes: $C_nH_{2n-2}$
• Combustion Equations:
  • Alkanes: $C_nH_{2n+2} + (\frac{3n+1}{2}) O_2 \rightarrow nCO_2 + (n+1) H_2O$.
  • General Hydrocarbons: $C_xH_y + (x + \frac{y}{4}) O_2 \rightarrow x CO_2 + \frac{y}{2} H_2O$.
• Bond Enthalpies:
  • C–C single bond: $348 \text{ kJ mol}^{-1}$.
  • C=C double bond: $681 \text{ kJ mol}^{-1}$.
  • C$\equiv$C triple bond: $823 \text{ kJ mol}^{-1}$.
• Bond Lengths:
  • C–C single bond (alkanes): $154 \text{ pm}$.
  • C=C double bond (alkenes): $133 \text{ pm}$.
  • C$\equiv$C triple bond (alkynes): $120 \text{ pm}$.
  • C–C bond in Benzene: $139 \text{ pm}$ (Intermediate between single and double bonds).

5. Trends & Comparisons

• Boiling Point:
  • Increases with increasing molecular mass (van der Waals forces increase with surface area).
  • Decreases with branching (molecule becomes spherical, lowering surface area).
• Melting Point & Dipole Moment of Alkenes: trans-isomers generally have a higher melting point and a lower dipole moment (often zero if symmetrical) than cis-isomers.
• Acidic Character: Ethyne > Ethene > Ethane (sp > sp² > sp³ character).
• Rate of Free Radical Halogenation: $F_2 > Cl_2 > Br_2 > I_2$.
• Rate of Dehydrohalogenation: $I > Br > Cl$ (for halogens) and $3^\circ > 2^\circ > 1^\circ$ (for alkyl groups).
• Reactivity of Hydrogen Halides to Alkenes: $HI > HBr > HCl$.
• Rotational Energy Barrier in Ethane: The energy difference between staggered and eclipsed conformations is exactly $12.5 \text{ kJ mol}^{-1}$.

7. ⚠️ EXCEPTIONS & ANOMALIES

• Alkyl Fluorides Exception: While alkyl halides react with zinc and dilute HCl to yield alkanes, alkyl fluorides are an exception and cannot be used in this reduction method.
• Methane Preparation Constraints: Methane is an anomaly as it cannot be synthesized using Kolbe’s electrolytic method or the Wurtz reaction.
• Wurtz Reaction Limitation: Suited only for preparing symmetrical alkanes; odd number of carbons yields a mixture.
• Halogenation Extremes (Alkanes): Fluorination is too violent to be controlled. Iodination is unusually slow and reversible, requiring an oxidizing agent like $HIO_3$ or $HNO_3$ to consume the HI byproduct.
• Oxidation of Alkanes: Alkanes are completely inert to oxidizing agents except for those with a tertiary ($3^\circ$) hydrogen atom, which oxidize to alcohols by $KMnO_4$.
• Isolation of Conformational Isomers: Ethane conformations cannot be isolated because the energy barrier is only $12.5\text{ kJ mol}^{-1}$, easily overcome at room temperature.
• Methene Instability: The theoretical first member of the alkene series, methene ($CH_2$), has a very short life, making ethene the functionally first stable member.
• Iodine Addition to Alkenes: Unlike chlorine and bromine, iodine does not show addition reactions with alkenes under normal conditions.
• The Peroxide Effect Anomaly: The Anti-Markovnikov addition of HX in the presence of peroxides happens only with $HBr$. It fails completely with $HCl$ and $HI$.
• Alkyne Hydration Requirement: Alkynes are completely unreactive with water. They uniquely require mercuric sulphate ($HgSO_4$) and dilute sulphuric acid to force hydration into carbonyl compounds.
• Selective Acidity of Alkynes: Not all hydrogen atoms in alkynes are acidic—only those attached to the $sp$ hybridized carbon atoms.
• Conductivity of Polyacetylene: Unlike typical organic polymers which are insulators, thin films of polyacetylene conduct electricity.
• Benzene's Addition Reluctance: Despite high unsaturation, benzene refuses typical electrophilic addition reactions under normal conditions, requiring vigorous conditions to break the ring.
• Temporary Loss of Aromaticity: During electrophilic aromatic substitution, the intermediate arenium ion temporarily loses aromatic character because the attacked carbon changes to $sp^3$ hybridization.
• Friedel-Crafts Alkylation Anomaly: Primary straight-chain halides (like 1-chloropropane) do not yield primary alkylbenzenes. They rearrange to form more stable secondary carbocations, yielding products like isopropylbenzene.
• Halogens as Anomalous Directing Groups: Halogens are the only functional groups that are deactivating (due to strong –I effect) yet act as ortho/para directing groups (due to +R effect).

8. Previous Year JEE Topics from This Chapter

• Ozonolysis Products: Reverse engineering an unknown alkene from its carbonyl products.
• Carbocation Rearrangements: Markovnikov addition to alkenes and Friedel-Crafts Alkylation involving 1,2-hydride or 1,2-methyl shifts.
• Aromaticity: Applying Hückel's Rule to classify rings.
• EAS Directing Effects: Predicting major product position with multiple activating/deactivating substituents.
• Terminal Alkyne Acidity: Reactions of 1-alkynes to distinguish terminal from internal alkynes.
• Conformational Isomerism: Ranking the stability of staggered vs eclipsed conformers based on torsional strain.

6. Memory Aids & Patterns (Top JEE Traps)

• [JEE TIP] Trap 1 - The Wurtz Symmetry Limitation:

  • Misconception: The Wurtz reaction is an efficient method for synthesizing any desired alkane, regardless of the carbon chain length.
  • Correct Understanding: The Wurtz reaction is highly preferred only for preparing symmetrical alkanes with an even number of carbon atoms. Attempting to couple two different alkyl halides to create an odd-carbon alkane generates a complex mixture of three distinct alkanes that are nearly impossible to separate due to similar boiling points.

• [JEE TIP] Trap 2 - The Friedel-Crafts Carbocation Shift:

  • Misconception: Friedel-Crafts alkylation attaches the alkyl group directly to the benzene ring via the specific carbon atom that originally held the halogen.
  • Correct Understanding: The reaction proceeds through a free carbocation intermediate, which will undergo hydride or methyl shifts to maximize its thermodynamic stability before attacking the ring. For example, reacting benzene with 1-chloropropane yields isopropylbenzene (cumene) as the major product, not $n$-propylbenzene.

• [JEE TIP] Trap 3 - Peroxide Effect Halogen Monopolization:

  • Misconception: The anti-Markovnikov peroxide effect (Kharasch effect) alters the addition mechanism for all hydrogen halides ($\text{HX}$).
  • Correct Understanding: The free-radical mechanism applies strictly to $\text{HBr}$. It fails with $\text{HCl}$ because the $\text{H}-\text{Cl}$ bond is thermodynamically too strong to break homolytically. It fails with $\text{HI}$ because, while the $\text{H}-\text{I}$ bond is weak, the resulting iodine radicals preferentially combine with each other to form $\text{I}_2$ rather than attacking the alkene.

• [JEE TIP] Trap 4 - The Halogen Directing Split:

  • Misconception: Electrophilic aromatic substitution directing effects always run parallel to activation properties; all deactivating groups must direct meta.
  • Correct Understanding: Halogens are a unique anomaly. Due to their intense electronegativity ($-\text{I}$ effect), they withdraw electron density and are strongly deactivating. However, because they possess lone pairs capable of delocalization ($+\text{R}$ effect), they uniquely act as ortho/para-directing groups.

• [JEE TIP] Trap 5 - Selective Alkyne Acidity Barrier:

  • Misconception: All hydrogen atoms in an alkyne molecule possess enhanced acidity due to the nearby triple bond.
  • Correct Understanding: Acidity is strictly restricted to hydrogen atoms attached directly to the triply bonded, $sp$-hybridized carbon atoms. Therefore, only terminal alkynes exhibit weak acidity and react with strong bases like $\text{NaNH}_2$ or Tollenis reagent. Internal alkynes completely lack acidic hydrogens.

• [JEE TIP] Trap 6 - The Stereospecific Alkyne Reduction Fork:

  • Misconception: Reducing an alkyne to an alkene yields a random equilibrium mixture of geometric cis and trans isomers.
  • Correct Understanding: The stereochemical outcome depends entirely on the chosen reducing agent. Hydrogenation using Lindlar's catalyst ($\text{H}_2/\text{Pd}/\text{BaSO}_4$ poisoned with quinoline) performs syn-addition to yield a cis-alkene. Conversely, reduction using Birch conditions ($\text{Na}$ or $\text{Li}$ in liquid $\text{NH}_3$) undergoes a radical-anion mechanism to yield a trans-alkene.

• [JEE TIP] Trap 7 - Spherical Branching vs. Boiling Point:

  • Misconception: Isomeric alkanes with more branching have higher boiling points due to a more compact, dense molecular structure.
  • Correct Understanding: Highly branched alkanes have lower boiling points than their straight-chain isomers. Increased branching forces the molecule to adopt a spherical geometry, which drastically minimizes its outer surface area. This reduction in surface area weakens the intermolecular Van der Waals forces, lowering the energy required to vaporize the liquid.

• [JEE TIP] Trap 8 - The Tertiary Alkane Oxidation Exception:

  • Misconception: Alkanes are completely inert and can never be oxidized by chemical oxidizing agents like aqueous $\text{KMnO}_4$.
  • Correct Understanding: While ordinary linear and secondary alkanes resist oxidation, any alkane possessing a tertiary ($3^\circ$) hydrogen atom is an exception. The weak $3^\circ$ $\text{C}-\text{H}$ bond is selectively targeted and oxidized by $\text{KMnO}_4$ to synthesize a tertiary alcohol.

• [JEE TIP] Trap 9 - The Methane Synthetic Bottleneck:

  • Misconception: Methane ($\text{CH}_4$), being the simplest alkane, can easily be prepared using primary hydrocarbon coupling reactions.
  • Correct Understanding: Methane cannot be synthesized via the Wurtz reaction or Kolbe's electrolytic method. Both synthetic pathways rely fundamentally on the homolytic coupling of two distinct alkyl radicals, meaning the absolute smallest possible hydrocarbon product they can generate is ethane ($\text{CH}_3-\text{CH}_3$).

• [JEE TIP] Trap 10 - Conformational Isolation Kinetic Barrier:

  • Misconception: Staggered and eclipsed conformational isomers of ethane can be physically isolated at exceptionally low room temperatures.
  • Correct Understanding: Conformational isomers cannot be isolated at room temperature. The energy barrier separating the staggered and eclipsed forms is only about $12.5\text{ kJ/mol}$, which is easily and continuously overcome by the ambient thermal kinetic energy of the molecules, resulting in rapid, continuous rotation.

• [JEE TIP] Trap 11 - The Ozonolysis Carbon-Carbon Cleavage:

  • Misconception: Determining reductive ozonolysis products requires drafting the complex, unstable cyclic ozonide ring intermediate every time.
  • Correct Understanding: Use the direct structural shortcut: conceptually slice the reactive $\text{C}=\text{C}$ double bond completely in half. Affix a carbonyl oxygen atom ($\text{=O}$) directly to each of the two newly exposed carbon ends to instantly reveal the resulting aldehydes or ketones.

• [JEE TIP] Trap 12 - Benzene Aromatic Stability Defense:

  • Misconception: Benzene reacts via addition reactions just as readily as open-chain conjugated dienes or simple alkenes do.
  • Correct Understanding: Benzene strictly favors electrophilic aromatic substitution over addition. Substitution allows the ring to preserve its highly stable $(4n+2)\pi$ Huckel aromatic resonance. Forcing benzene into an addition reaction requires destroying this aromaticity, which demands aggressive, high-energy conditions like intense UV radiation or extreme heat and pressure.