5.3 Hydrocarbons: Alkanes, Alkenes, Alkynes & Aromatic Compounds

5.3 Hydrocarbons: Alkanes, Alkenes, Alkynes & Aromatic Compounds

Hydrocarbons are organic compounds composed exclusively of carbon and hydrogen atoms. They form the foundational framework of organic chemistry and are broadly classified into aliphatic (alkanes, alkenes, alkynes, and alicyclic) and aromatic compounds (benzene and its derivatives). Thorough knowledge of hybridization, geometric structure, reaction mechanisms, Markovnikov's rule, and aromatic substitution is essential for the AMC initial test.

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Hybridization & Structural Comparison of Hydrocarbons

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Alkanes: Synthesis & Free-Radical Halogenation

Alkanes are saturated hydrocarbons containing single carbon-carbon $\sigma$-bonds. Because of their non-polar character and high $\sigma$-bond strength, they exhibit low chemical reactivity (hence termed paraffins — little affinity).

Methods of Preparation

1. Sabatier-Senderens Reaction (Catalytic Hydrogenation): $CH_2=CH_2 + H_2 \xrightarrow[200-300^\circ\text{C}]{Ni} CH_3-CH_3$
2. Wurtz Reaction: Coupling of alkyl halides using metallic sodium in dry ether to yield symmetrical alkanes with an even number of carbon atoms: $2 R-X + 2 Na \xrightarrow{\text{Dry Ether}} R-R + 2 NaX$
3. Kolbe's Electrolytic Method: Electrolysis of concentrated aqueous solution of sodium/potassium salts of fatty acids: $2 CH_3COOK + 2 H_2O \xrightarrow{\text{Electrolysis}} CH_3-CH_3 \text{ (at Anode)} + 2 CO_2 + H_2 \text{ (at Cathode)} + 2 KOH$
4. Reduction of Alkyl Halides: Using $Zn / HCl$ or $LiAlH_4$.

Reaction Mechanism: Free-Radical Halogenation of Methane

Halogenation of alkanes (chlorination or bromination) occurs via a free-radical chain mechanism under ultraviolet (UV) light or elevated temperature ($250-400^\circ\text{C}$):

1. Chain Initiation: Homolytic cleavage of halogen molecules forming free radicals: $Cl-Cl \xrightarrow{h\nu \text{ (UV)}} 2 Cl^\bullet$
2. Chain Propagation: $Cl^\bullet + CH_4 \rightarrow \text{H-}Cl + {}^\bullet CH_3$ ${}^\bullet CH_3 + Cl_2 \rightarrow CH_3Cl + Cl^\bullet$
3. Chain Termination: Combination of any two radicals to stop the chain: $Cl^\bullet + Cl^\bullet \rightarrow Cl_2 \quad | \quad {}^\bullet CH_3 + Cl^\bullet \rightarrow CH_3Cl \quad | \quad {}^\bullet CH_3 + {}^\bullet CH_3 \rightarrow CH_3-CH_3$

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Alkenes: Synthesis, Markovnikov's Rule & Ozonolysis

Alkenes are unsaturated hydrocarbons containing a carbon-carbon double bond (one strong $\sigma$ bond and one weaker $\pi$ bond formed by sideways overlap of unhybridized $p$-orbitals).

Preparation Methods

1. Dehydration of Alcohols: Heating with concentrated $H_2SO_4$ at $170^\circ\text{C}$ or over hot $Al_2O_3$ ($350^\circ\text{C}$). Ease of dehydration follows: $3^\circ > 2^\circ > 1^\circ$ alcohols.
2. Dehydrohalogenation of Alkyl Halides: Boiling with alcoholic $KOH$: $CH_3-CH_2-Br + \text{Alc. } KOH \rightarrow CH_2=CH_2 + KBr + H_2O$
3. Dehalogenation of Vicinal Dihalides: Heating with zinc dust in anhydrous methanol.

Electrophilic Addition & Markovnikov's Rule

When an unsymmetrical reagent ($H-X$) adds to an unsymmetrical alkene, the positive part (hydrogen) adds to the carbon atom of the double bond that carries the greater number of hydrogen atoms.

$\text{Mechanistic Basis: Addition proceeds via the formation of the more stable carbocation intermediate } (3^\circ > 2^\circ > 1^\circ > {}^+CH_3).$

$CH_3-CH=CH_2 + HBr \rightarrow CH_3-CH(Br)-CH_3 \quad (2\text{-bromopropane, Major Product})$

Peroxide Effect (Anti-Markovnikov Addition / Kharasch Effect)

In the presence of organic peroxides ($R-O-O-R$), addition of $HBr$ (only) to unsymmetrical alkenes follows an anti-Markovnikov pathway via a free-radical mechanism, adding $Br^\bullet$ to the less substituted carbon atom to yield $1\text{-bromopropane}$. ($HCl$ and $HI$ do not show the peroxide effect).

Chemical Identification Tests

1. Baeyer's Test: Discharge of the pink/purple color of cold alkaline $1%\text{ } KMnO_4$ solution forming vicinal glycols ($CH_2OH-CH_2OH$). Confirms unsaturation.
2. Bromine Water Test: Decolorization of reddish-brown $Br_2$ in $CCl_4$ without evolution of $HBr$ gas.
3. Ozonolysis: Reaction with ozone ($O_3$) followed by zinc/water reduction ($Zn / H_2O$) cleaves the $C=C$ double bond to yield aldehydes and/or ketones, revealing the precise location of double bonds.

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Alkynes & Acidity of Terminal Alkynes

Alkynes contain a $C\equiv C$ triple bond ($1\text{ } \sigma$ bond and $2\text{ } \pi$ bonds). Carbon atoms are $sp$-hybridized with $50%$ $s$-character.

Acidity of Terminal Alkynes

Terminal alkynes ($R-C\equiv C-H$) possess weakly acidic hydrogen atoms because the high $s$-character ($50%$) of the $sp$-hybridized carbon atom pulls electron density strongly toward the carbon nucleus, polarizing the $C-H$ bond.

• Reaction with Sodium Metal: $2 R-C\equiv C-H + 2 Na \rightarrow 2 R-C\equiv C^- Na^+ + H_{2(g)}$
• Ammoniacal Silver Nitrate Test (Tollens' Reagent): Formation of a white precipitate of silver acetylide: $H-C\equiv C-H + 2 [Ag(NH_3)_2]NO_3 \rightarrow Ag-C\equiv C-Ag \downarrow \text{(White)} + 2 NH_4NO_3 + 2 NH_3$
• Ammoniacal Cuprous Chloride Test: Formation of a red/brown precipitate of copper acetylide: $H-C\equiv C-H + Cu_2Cl_2 + 2 NH_4OH \rightarrow Cu-C\equiv C-Cu \downarrow \text{(Red)} + 2 NH_4Cl + 2 H_2O$

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Aromatic Hydrocarbons: Benzene & Electrophilic Substitution

Benzene ($C_6H_6$) is a planar, regular hexagonal ring with carbon-carbon bond lengths of $1.39\text{ \AA}$, intermediate between a single ($1.54\text{ \AA}$) and double bond ($1.34\text{ \AA}$).

Aromaticity & Huckel's Rule

For a compound to be classified as aromatic, it must satisfy four criteria:

1. Cyclic geometry.
2. Planar structure ($sp^2$-hybridized atoms in ring).
3. Complete Conjugation (continuous overlapping $p$-orbitals).
4. Huckel's $(4n+2) \pi$-Electron Rule: Must contain $(4n+2) \pi$-electrons, where $n = 0, 1, 2, 3...$ (For benzene, $n=1 \implies 6 \pi$-electrons).

Resonance Energy of Benzene

The theoretical heat of hydrogenation of hypothetical 1,3,5-cyclohexatriene is $-358.5\text{ kJ mol}^{-1}$ ($3 \times -119.5\text{ kJ mol}^{-1}$). Experimental hydrogenation of benzene is only $-208\text{ kJ mol}^{-1}$. The difference of $150.5\text{ kJ mol}^{-1}$ represents the Resonance Energy (Stabilization Energy) of benzene.

Electrophilic Aromatic Substitution (EAS) Mechanism

Benzene resists addition reactions to preserve its resonance stabilization energy. Instead, it undergoes Electrophilic Aromatic Substitution (EAS) via a three-step mechanism:

  Step 1: Electrophile Generation (E+)
  Step 2: Attack on Benzene Ring ──► Formation of Arenium Ion (Wheland Intermediate)
  Step 3: Loss of Proton (H+) ──► Restoration of Aromatic Ring Stability

Directing Effects of Substituents on Benzene Ring

• Ortho/Para-Directors (Activating Groups): Electron-donating groups increase electron density at the ortho and para positions relative to meta. (e.g., $-OH, -NH_2, -OCH_3, -R, -NR_2$). Exception: Halogens ($-F, -Cl, -Br, -I$) are deactivating due to strong inductive ($-I$) effects but are ortho/para-directing due to resonance electron donation ($+M$).
• Meta-Directors (Deactivating Groups): Strongly electron-withdrawing groups decrease electron density overall, especially at ortho/para positions, directing incoming electrophiles to the meta position (e.g., $-NO_2, -COOH, -CHO, -CN, -SO_3H, -COR$).