Quick Revision Notes: Hydrocarbons

Class 11 Chemistry (MP Board)

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1. Introduction & Classification

Hydrocarbons are organic compounds containing only Carbon ($C$) and Hydrogen ($H$).

• Classification:
  1. Aliphatic (Open Chain):
     • Saturated: Alkanes (General Formula: $C_n{H}_{2n+2}$) - Single bonds only.
     • Unsaturated:
       • Alkenes ($C_n{H}_{2n}$) - Contains at least one double bond ($C=C$).
       • Alkynes ($C_n{H}_{2n-2}$) - Contains at least one triple bond ($C \equiv C$).
  2. Aromatic (Closed Chain): Compounds containing benzene rings or related systems (e.g., Benzene, Toluene).

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2. Alkanes (Paraffins)

Alkanes are saturated open-chain hydrocarbons with $sp^3$ hybridization and tetrahedral geometry (bond angle $109.5^\circ$).

Key Preparation Methods:

• Wurtz Reaction: Alkyl halides react with sodium metal in dry ether to give higher alkanes with an even number of carbon atoms.
  • 2 R-X + 2 Na -> R-R + 2 NaX
• Decarboxylation: Heating sodium salt of carboxylic acid with sodalime ($NaOH + CaO$).
  • R-COONa + NaOH ->[CaO / \Delta] R-H + Na_2CO_3
• Hydrogenation of Alkenes/Alkynes: Addition of hydrogen in the presence of catalyst ($Pt, Pd,$ or $Ni$).
  • R-CH=CH-R + H_2 ->[Pt/Pd/Ni] R-CH_2-CH_2-R

Chemical Properties:

• Halogenation: Free radical substitution reaction in the presence of sunlight ($UV$ light).
  • Order of reactivity: $3^\circ > 2^\circ > 1^\circ$
• Combustion: Complete combustion produces carbon dioxide and water with the release of large amount of heat.
  • C_n{H}_{2n+2} + [(3n+1)/2] O_2 -> n CO_2 + (n+1) H_2O

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3. Alkenes (Olefins)

Alkenes are unsaturated hydrocarbons containing a carbon-carbon double bond. They have $sp^2$ hybridization with a bond angle of $120^\circ$.

Key Preparation Methods:

• Dehydration of Alcohols: Heating alcohols with concentrated $H_2SO_4$.
  • R-CH_2-CH_2-OH ->[Conc. H_2SO_4 / \Delta] R-CH=CH_2 + H_2O
• Dehydrohalogenation of Alkyl Halides: Treatment of alkyl halide with alcoholic $KOH$ (Elimination / $\beta$-elimination reaction following Saytzeff's Rule).

Chemical Properties (Electrophilic Addition Reactions):

• Markovnikov's Rule: During the addition of an unsymmetrical reagent ($H-X$) to an unsymmetrical alkene, the negative part of the reagent gets attached to the carbon atom having lesser number of hydrogen atoms.
• Anti-Markovnikov's Rule (Peroxide Effect): In the presence of organic peroxide, addition of $HBr$ to unsymmetrical alkenes takes place contrary to Markovnikov's rule. (Note: Only applicable to HBr, not HCl or HI).
• Ozonolysis: Reaction with ozone followed by hydrolysis with $Zn/H_2O$ to cleave the double bond and form aldehydes/ketones.

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4. Alkynes (Acetylenes)

Alkynes are unsaturated hydrocarbons containing a carbon-carbon triple bond. They have $sp$ hybridization with a linear geometry (bond angle $180^\circ$).

Key Preparation Methods:

• From Calcium Carbide: Hydrolysis of calcium carbide.
  • CaC_2 + 2 H_2O -> Ca(OH)_2 + C_2H_2 (Acetylene)
• Dehydrohalogenation of Vicinal Dihalides: Treatment with alcoholic $KOH$ followed by sodamide ($NaNH_2$).

Chemical Properties:

• Acidic Character: Terminal alkynes are acidic in nature because the $sp$ hybridized carbon holds the electrons tightly. They react with strong bases like sodamide or ammoniacal silver nitrate/cuprous chloride solutions.
• Addition Reactions: Undergo electrophilic addition similar to alkenes, but take up two moles of reagents (e.g., hydrogenation, halogenation).

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5. Aromatic Hydrocarbons (Arenes)

Aromatic hydrocarbons contain benzene rings. They obey Huckel's Rule of Aromaticity:

• Must be cyclic.
• Must be planar.
• Complete delocalization of $\pi$-electrons in the ring.
• Must contain $(4n + 2) \pi$-electrons (where $n = 0, 1, 2, 3...$).

Electrophilic Aromatic Substitution Reactions of Benzene:

1. Nitration: Reaction with conc. $HNO_3$ and conc. $H_2SO_4$ at $313-323 K$.
   • Electrophile: Nitronium ion ($NO_2^+$) $\rightarrow$ Forms Nitrobenzene.
2. Halogenation: Reaction with $Cl_2$ or $Br_2$ in the presence of anhydrous $FeCl_3$ or $AlCl_3$.
   • Electrophile: Halonium ion ($Cl^+$ or $Br^+$) $\rightarrow$ Forms Chlorobenzene/Bromobenzene.
3. Sulphonation: Reaction with fuming sulphuric acid ($H_2SO_4 + SO_3$ or oleum).
   • Electrophile: Sulphur trioxide ($SO_3$) $\rightarrow$ Forms Benzenesulphonic acid.
4. Friedel-Crafts Alkylation: Reaction with alkyl halide in the presence of anhydrous $AlCl_3$.
   • Electrophile: Carbocation ($R^+$) $\rightarrow$ Forms Alkylbenzene (Toluene).
5. Friedel-Crafts Acylation: Reaction with acyl halide ($R-COCl$) or acid anhydride in the presence of anhydrous $AlCl_3$.
   • Electrophile: Acylium ion ($R-CO^+$) $\rightarrow$ Forms Acylbenzene (Acetophenone).

Directive Influence of Functional Groups in Monosubstituted Benzene:

• Ortho and Para Directing (Activating groups): $-OH, -OCH_3, -NH_2, -CH_3, -R$ etc. (Exception: Halogens are deactivate but $o,p$-directing).
• Meta Directing (Deactivating groups): $-NO_2, -CN, -CHO, -COOH, -SO_3H$ etc.