}]]}]}]}] 30:T5f72,

### Introduction to Hydrocarbons Hydrocarbons are organic compounds composed solely of carbon and hydrogen atoms. They are fundamental to organic chemistry and serve as primary energy sources (fuels) and raw materials for many industrial applications. #### Importance - **Energy Sources:** LPG, CNG, LNG, petrol, diesel, kerosene, coal gas. - **Industrial Applications:** Manufacture of polymers (polythene, polypropene), solvents for paints, starting materials for dyes and drugs. ### Classification of Hydrocarbons Hydrocarbons are classified based on the types of carbon-carbon bonds present: - **Saturated Hydrocarbons (Alkanes):** Contain only carbon-carbon single bonds. - **Alkanes:** Open chain structures (e.g., methane, ethane). General formula: $C_nH_{2n+2}$. - **Cycloalkanes:** Closed chain (ring) structures. - **Unsaturated Hydrocarbons:** Contain carbon-carbon multiple bonds. - **Alkenes:** Contain at least one carbon-carbon double bond. General formula: $C_nH_{2n}$. - **Alkynes:** Contain at least one carbon-carbon triple bond. General formula: $C_nH_{2n-2}$. - **Aromatic Hydrocarbons:** Special type of cyclic compounds characterized by aromaticity (e.g., benzene). ### Alkanes Saturated open-chain hydrocarbons with C-C single bonds. #### Structure - **Methane ($CH_4$):** Tetrahedral structure, carbon at center, four hydrogens at corners. H-C-H bond angle $109.5^\circ$. - **Alkanes:** C-C bond length $154 \text{ pm}$, C-H bond length $112 \text{ pm}$. C-C and C-H $\sigma$ bonds formed by $sp^3$ hybrid orbitals of carbon and $1s$ orbitals of hydrogen. #### Nomenclature and Isomerism - **Nomenclature:** Follows IUPAC system (refer to Unit 8 for detailed rules). - **Isomerism:** - **Butane ($C_4H_{10}$):** Exhibits two structural isomers (n-butane, isobutane). - **Pentane ($C_5H_{12}$):** Exhibits three structural isomers (n-pentane, isopentane, neopentane). - **Chain Isomers:** Differ in the chain of carbon atoms. - **Carbon Atom Classification:** - **Primary ($1^\circ$):** Attached to no other or only one carbon atom. - **Secondary ($2^\circ$):** Attached to two carbon atoms. - **Tertiary ($3^\circ$):** Attached to three carbon atoms. - **Quaternary ($4^\circ$):** Attached to four carbon atoms. #### Preparation of Alkanes ##### 1. From Unsaturated Hydrocarbons (Hydrogenation) - Alkenes and alkynes add dihydrogen gas in the presence of catalysts (Pt, Pd, or Ni) to form alkanes. - **Example:** $$\text{CH}_2=\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Pt/Pd/Ni}} \text{CH}_3-\text{CH}_3$$ (Ethene to Ethane) $$\text{CH}_3-\text{C}\equiv\text{C}-\text{H} + 2\text{H}_2 \xrightarrow{\text{Pt/Pd/Ni}} \text{CH}_3-\text{CH}_2-\text{CH}_3$$ (Propyne to Propane) ##### 2. From Alkyl Halides - **Reduction:** Alkyl halides (except fluorides) react with Zn and dilute HCl to give alkanes. $$\text{CH}_3-\text{Cl} + \text{H}_2 \xrightarrow{\text{Zn, H}^+} \text{CH}_4 + \text{HCl}$$ (Chloromethane to Methane) - **Wurtz Reaction:** Alkyl halides react with sodium metal in dry ether to form higher alkanes with an even number of carbon atoms. $$2\text{CH}_3-\text{Br} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{CH}_3-\text{CH}_3 + 2\text{NaBr}$$ (Bromomethane to Ethane) ##### 3. From Carboxylic Acids - **Decarboxylation:** Sodium salts of carboxylic acids are heated with soda lime (NaOH + CaO) to yield alkanes with one carbon atom less. $$\text{CH}_3\text{COO}^-\text{Na}^+ + \text{NaOH} \xrightarrow{\Delta, \text{CaO}} \text{CH}_4 + \text{Na}_2\text{CO}_3$$ (Sodium ethanoate to Methane) - **Kolbe's Electrolytic Method:** Electrolysis of aqueous solutions of sodium or potassium salts