Hydrocarbons (Class 11)

Cheatsheet Content

### Introduction to Hydrocarbons - **Definition:** Organic compounds composed solely of hydrogen and carbon atoms. - **Classification:** 1. **Open-chain (Acyclic) Hydrocarbons:** * **Saturated:** Alkanes (single bonds, C-C) * **Unsaturated:** Alkenes (at least one C=C double bond), Alkynes (at least one C≡C triple bond) 2. **Cyclic Hydrocarbons:** * **Alicyclic:** Cycloalkanes, Cycloalkenes (resemble open-chain but in a ring) * **Aromatic:** Benzene and its derivatives (special stability due to delocalized $\pi$-electrons) ### Alkanes - **General Formula:** $C_nH_{2n+2}$ - **Structure:** Tetrahedral geometry around each carbon, $sp^3$ hybridization. - **Nomenclature:** Suffix "-ane". Example: Methane ($CH_4$), Ethane ($CH_3CH_3$). - **Isomerism:** Chain isomerism (e.g., n-butane and isobutane). - **Physical Properties:** * Non-polar, insoluble in water, soluble in organic solvents. * Boiling point increases with molecular mass ($n$-alkanes > branched alkanes). - **Preparation:** 1. **From Unsaturated Hydrocarbons:** Hydrogenation of alkenes/alkynes ($H_2/Ni, Pd, Pt$). 2. **Wurtz Reaction:** $2RX + 2Na \xrightarrow{\text{dry ether}} R-R + 2NaX$ (for symmetrical alkanes). 3. **Decarboxylation of Carboxylic Acids:** $RCOONa + NaOH \xrightarrow{CaO, \Delta} RH + Na_2CO_3$. 4. **Kolbe's Electrolytic Method:** $2RCOO^-Na^+ + 2H_2O \xrightarrow{\text{electrolysis}} R-R + 2CO_2 + H_2 + 2NaOH$. - **Chemical Properties:** 1. **Halogenation (Free radical mechanism):** $CH_4 + Cl_2 \xrightarrow{hv} CH_3Cl + HCl$. 2. **Combustion:** $CH_4 + 2O_2 \xrightarrow{} CO_2 + 2H_2O + \text{heat}$. 3. **Controlled Oxidation:** e.g., $2CH_4 + O_2 \xrightarrow{Cu/573K/100 atm} 2CH_3OH$. 4. **Isomerization:** $n$-alkane $\xrightarrow{AlCl_3/HCl} \text{branched alkane}$. 5. **Aromatization:** $n$-hexane $\xrightarrow{Cr_2O_3/Al_2O_3/773K/10-20 atm} \text{Benzene}$. 6. **Pyrolysis/Cracking:** Higher alkanes $\xrightarrow{\Delta} \text{lower alkanes + alkenes}$. ### Alkenes - **General Formula:** $C_nH_{2n}$ - **Structure:** Planar geometry around $C=C$, $sp^2$ hybridization. Bond angle $\approx 120^\circ$. - **Nomenclature:** Suffix "-ene". Example: Ethene ($CH_2=CH_2$). - **Isomerism:** * Chain and position isomerism. * **Geometrical (cis-trans) isomerism:** Due to restricted rotation around $C=C$ bond. Requires different groups on each carbon of the double bond. - **Preparation:** 1. **From Alkynes:** $RC \equiv CR' + H_2 \xrightarrow{Pd/BaSO_4 (\text{Lindlar's catalyst})} \text{cis-alkene}$. $RC \equiv CR' + H_2 \xrightarrow{Na/\text{liquid }NH_3} \text{trans-alkene}$. 2. **From Alkyl Halides (Dehydrohalogenation):** $RCH_2CH_2X \xrightarrow{alc. KOH, \Delta} RCH=CH_2 + HX$. Follows Saytzeff's rule (more substituted alkene is major product). 3. **From Alcohols (Dehydration):** $RCH_2CH_2OH \xrightarrow{conc. H_2SO_4, \Delta} RCH=CH_2 + H_2O$. 4. **From Vicinal Dihalides (Dehalogenation):** $RCHBr-CHBrR' + Zn \xrightarrow{\Delta} RCH=CHR' + ZnBr_2$. - **Chemical Properties (Electrophilic Addition):** 1. **Addition of $H_2$ (Hydrogenation):** $CH_2=CH_2 + H_2 \xrightarrow{Ni/Pt/Pd} CH_3-CH_3$. 2. **Addition of Halogen ($X_2$):** $CH_2=CH_2 + Br_2 \xrightarrow{CCl_4} BrCH_2-CH_2Br$ (test for unsaturation). 3. **Addition of Hydrogen Halides ($HX$):** Follows Markovnikov's rule (H adds to carbon with more H atoms, X to carbon with fewer H atoms). * Anti-Markovnikov's addition (Peroxide effect) for $HBr$ only (in presence of peroxides). 4. **Addition of Water (Hydration):** $CH_2=CH_2 + H_2O \xrightarrow{H^+} CH_3-CH_2OH$. 5. **Ozonolysis:** $R_2C=CR_2 \xrightarrow{O_3} \text{ozonide} \xrightarrow{Zn/H_2O} \text{aldehydes/ketones}$. 6. **Oxidation:** * **Baeyer's Reagent (cold, dilute, alkaline $KMnO_4$):** Forms vicinal diols. * **Acidified $KMnO_4$:** Cleavage of double bond, forms carboxylic acids/ketones. 7. **Polymerization:** $nCH_2=CH_2 \xrightarrow{\text{high temp, pressure, catalyst}} (-CH_2-CH_2-)_n$ (Polyethylene). ### Alkynes - **General Formula:** $C_nH_{2n-2}$ - **Structure:** Linear geometry around $C \equiv C$, $sp$ hybridization. Bond angle $180^\circ$. - **Nomenclature:** Suffix "-yne". Example: Ethyne ($HC \equiv CH$). - **Acidity of Terminal Alkynes:** Terminal alkynes have acidic hydrogen atoms due to $sp$ hybridized carbon (high electronegativity). React with strong bases/metals. * $HC \equiv CH + Na \xrightarrow{} HC \equiv C^-Na^+ + 1/2 H_2$. - **Preparation:** 1. **From Calcium Carbide:** $CaC_2 + 2H_2O \xrightarrow{} HC \equiv CH + Ca(OH)_2$. 2. **From Vicinal Dihalides (Dehydrohalogenation):** $RCHX-CHXR' \xrightarrow{alc. KOH} RCH=CXR' \xrightarrow{NaNH_2} RC \equiv CR'$. - **Chemical Properties (Electrophilic Addition, but less reactive than alkenes):** 1. **Addition of $H_2$:** $RC \equiv CR' + H_2 \xrightarrow{Pd/BaSO_4} \text{cis-alkene} \xrightarrow{H_2} \text{alkane}$. 2. **Addition of Halogen ($X_2$):** Forms tetrahaloalkane. 3. **Addition of Hydrogen Halides ($HX$):** Follows Markovnikov's rule. $HC \equiv CH + HX \xrightarrow{} CH_2=CHX \xrightarrow{HX} CH_3-CHX_2$. 4. **Addition of Water (Hydration):** $HC \equiv CH + H_2O \xrightarrow{HgSO_4/H_2SO_4} [CH_2=CHOH] \xrightarrow{\text{tautomerization}} CH_3CHO$. 5. **Polymerization:** * **Linear polymerization:** $nHC \equiv CH \xrightarrow{\text{CuCl}} (-CH=CH-CH=CH-)_n$ (Polyacetylene). * **Cyclic polymerization:** $3HC \equiv CH \xrightarrow{\text{red hot iron tube, 873K}} \text{Benzene}$. ### Aromatic Hydrocarbons (Benzene) - **Structure of Benzene ($C_6H_6$):** * Cyclic, planar molecule. * All C-C bond lengths are equal (intermediate between single and double bond). * Each carbon is $sp^2$ hybridized. * Delocalized $\pi$-electron cloud above and below the ring (resonance stability). * **Hückel's Rule:** (4n+2)$\pi$ electrons (where n=0, 1, 2...). Benzene has 6 $\pi$ electrons (n=1). - **Nomenclature:** Benzene, Toluene (methylbenzene), Phenol (hydroxybenzene), Aniline (aminobenzene). * Disubstituted benzene: Ortho (1,2), Meta (1,3), Para (1,4). - **Preparation of Benzene:** 1. **Cyclic Polymerization of Ethyne:** $3HC \equiv CH \xrightarrow{\text{red hot iron tube, 873K}} \text{Benzene}$. 2. **Decarboxylation of Benzoic Acid:** $C_6H_5COONa + NaOH \xrightarrow{CaO, \Delta} C_6H_6 + Na_2CO_3$. 3. **Reduction of Phenol:** $C_6H_5OH + Zn \xrightarrow{\Delta} C_6H_6 + ZnO$. - **Chemical Properties (Electrophilic Substitution Reactions):** * Due to stability of $\pi$-electron cloud, benzene undergoes substitution rather than addition. 1. **Nitration:** Benzene $+ HNO_3 \xrightarrow{conc. H_2SO_4} \text{Nitrobenzene} + H_2O$. 2. **Halogenation:** Benzene $+ Cl_2 \xrightarrow{FeCl_3} \text{Chlorobenzene} + HCl$. 3. **Sulfonation:** Benzene $+ conc. H_2SO_4 \xrightarrow{\Delta} \text{Benzenesulphonic acid} + H_2O$. 4. **Friedel-Crafts Alkylation:** Benzene $+ R-X \xrightarrow{anhyd. AlCl_3} \text{Alkylbenzene} + HX$. 5. **Friedel-Crafts Acylation:** Benzene $+ RCOCl \xrightarrow{anhyd. AlCl_3} \text{Acylbenzene} + HCl$. - **Directive Influence of Substituents in Monosubstituted Benzene:** * **Ortho-para directing groups (and activating):** $-CH_3, -OH, -NH_2, -OCH_3, -X$ (halogens are deactivating but o,p-directing). * **Meta directing groups (and deactivating):** $-NO_2, -CN, -CHO, -COOH, -SO_3H$.