Comprehensive Notes on Hydrocarbons: Alkanes, Alkenes, and Alkynes

Preparation of Alkanes via Grignard Reagent

• General Formula: Grignard reagents are organometallic compounds with the general formula $RMgX$.

• Reaction Mechanism: $R-MgX$ reacts with compounds containing acidic hydrogen ($A-H$) to produce an alkane ($R-H$).

  • Equation: $R-MgX + A-H \rightarrow R-H + Mg(X)A$

• Specific Examples:

  • $CH_3-CH_2MgBr + HC\equiv CH \rightarrow CH_3-CH_3 + Mg(Br)C\equiv CH$

  • $CH_3-CH_2MgBr + H_2O \rightarrow CH_3-CH_3 + Mg(Br)OH$

  • $PhMgBr + H-C \equiv C-H \rightarrow Ph-H + Mg(Br)C \equiv CH$

  • Reaction with poly-functional acidic compounds: $CH_3MgBr$ reacts with a complex molecule containing three acidic hydrogens (one $\text{-COOH}$ and two $\text{-OH}$ groups) to produce 3 moles of methane ($3\,CH_4$).

Hydrogenation and Isomerism in Alkanes

• Catalysts for Reduction:

  • Heterogeneous Catalysts: $H_2, Pt, \Delta$; $H_2, Pd, \Delta$; $H_2, Ni, \Delta$; $H_2, PtO_2$ (Adam's catalyst); Raney $Ni$ (Alloy of $Al$ and $Ni$).

  • Homogeneous Catalysts: Wilkinson's catalyst ($H_2[RhCl(PPh_3)_3]$); $H_2, Ir, \Delta$.

• Hydrogenation Principles:

  • Full reduction of alkenes: $CH_2=CH_2 \xrightarrow{H_2, catalyst} CH_3-CH_3$.

  • Full reduction of alkynes: $CH \equiv CH \xrightarrow{2H_2, catalyst} CH_3-CH_3$.

  • Stereochemistry (Syn Addition): Hydrogenation is a syn addition process where metal adsorbs hydrogen on its surface and both hydrogens add from the same side via a radical mechanism.

  • Erythreo vs. Threo:

    • $Cis \text{ alkene} + Syn \text{ addition} \rightarrow Erythreo \text{ isomer}$.

    • $Trans \text{ alkene} + Syn \text{ addition} \rightarrow Threo \text{ isomer}$.

  • Stereochemical Examples (with Deuterium $D_2$):

    • $Cis-2-butene + D_2 \xrightarrow{Ni} Meso$

    • $Trans-2-butene + D_2 \xrightarrow{Ni} Racemic \text{ mixture}$

• Rate of Hydrogenation:

  • $\text{Rate of Hydrogenation} \propto \text{Heat of Hydrogenation} \propto \frac{1}{\text{Stability of alkene/alkyne}}$.

  • Comparison of rates: Cyclic alkenes and those with fewer substituents generally have higher rates (e.g., cyclohexene is faster than tetrasubstituted alkenes).

  • In systems like anthracene (4 rings), reduction occurs in the middle ring because the remaining rings stay aromatic, ensuring maximum reactivity.

Wurtz Reaction

• General Reaction: $R-X + 2Na + R-X \xrightarrow{\text{dry ether}} R-R + 2NaX$.

  • Limitation: Methane ($CH_4$) cannot be prepared by this method.

• Mechanism:

  • $2Na \rightarrow 2Na^+ + 2e^-$

  • $2R-X + 2e^- \rightarrow 2R^. + 2X^-$

  • $R^. + R^. \rightarrow R-R$

• Key Details:

  • Mainly used for symmetrical alkanes.

  • Dry Ether Importance: Sodium reacts with water in aqueous media. In the presence of water, sodium reacts with $R-X$ to give substitution products instead of the coupled alkane.

  • Side Reactions: $3^\circ$ alkyl halides give disproportionation products as the major product.

  • If different alkyl halides are used ($R-X$ and $R'-X$), a mixture of three products is formed ($R-R, R'-R', R-R'$).

Decarboxylation Reactions

• Sodalime Decarboxylation:

  • Reagent: $\text{NaOH} + \text{CaO}$ (Sodalime) + Heat ($\Delta$).

  • Equation: $RCOOH \xrightarrow{NaOH+CaO, \Delta} R-H + Na_2CO_3$.

  • Mechanism: Involves the formation of a carbanion intermediate ($R^-$, which is the Rate Determining Step or RDS).

  • Rate: $\text{Rate} \propto \text{Stability of carbanion}$.

  • Reactivity Order: Molecules with electron-withdrawing groups (EWG) that stabilize the carbanion react faster (e.g., $NO_2$ or $CN$ substituted phenylacetic acids).

• Decarboxylation on Heating (Thermal):

  • Occurs in $\beta$-keto acids ($EWG-CH_2-COOH$ where EWG is $\text{-C=O}, \text{-CHO, -CN}$, etc.).

  • Mechanism: Proceed via a 6-membered cyclic transition state; no discrete intermediate is formed.

Kolbe's Electrolysis

• General Reaction: Electrolysis of sodium or potassium salts of fatty acids.

  • $2R-COOK + 2H_2O \rightarrow R-R \text{ (Anode)} + 2CO_2 \text{ (Anode)} + H_2 \text{ (Cathode)} + 2KOH \text{ (Cathode)}$.

• Step-by-step Process:

  • Anode (Oxidation):

    1. $2R-COO^- \rightarrow 2R-COO^. + 2e^-$

    2. $2R-COO^. \rightarrow 2R^. + 2CO_2$

    3. $R^. + R^. \rightarrow R-R$

  • Cathode (Reduction):

    1. $2H_2O + 2e^- \rightarrow 2OH^- + H_2$

    2. $2K^+ + 2OH^- \rightarrow 2KOH$

Miscellaneous Reduction Methods

• Frankland Reaction: Uses Zinc instead of Sodium: $R-X + Zn + R-X \xrightarrow{\text{dry ether}} R-R + ZnX_2$.

• Di-imide Reduction ($N_2H_4, H_2O_2, h\nu$ or $N_2H_2, \Delta$):

  • Provides syn addition of hydrogen.

  • $Cis \text{ alkene} + Syn \text{ addition} \rightarrow Erythreo$.

  • $Trans \text{ alkene} + Syn \text{ addition} \rightarrow Threo$.

• Reduction by $HI$ and Red Phosphorus ($P$):

  • Strong reducing agent that reduces almost all functional groups (alkanes, alcohols, aldehydes, ketones, carboxylic acids, esters) to alkanes.

  • Specific examples:

    • Alcohol: $R-OH \xrightarrow{HI, Red P} R-I \rightarrow R-H$

    • Aldehyde: $CH_3CHO \xrightarrow{HI, Red P} CH_3-CH_3$

    • Carboxylic Acid: $RCOOH \xrightarrow{HI, Red P} R-CH_3$

  • $n$-hexane can be prepared from glucose or long chains via this method.

• From Carbides:

  1. $Al_4C_3 + 12H_2O \rightarrow 3CH_4 + 4Al(OH)_3$

  2. $CaC_2 + 2H_2O \rightarrow C_2H_2 (acetylene) + Ca(OH)_2$

  3. $Mg_2C_3 + 4H_2O \rightarrow 2Mg(OH)_2 + CH_3-C\equiv CH$

  4. $Be_2C + 4H_2O \rightarrow 2Be(OH)_2 + CH_4$

Corey House Reaction and Gillman Reagent

• Corey House Synthesis:

  • Steps to produce Gillman reagent: $R-X + 2Li \rightarrow RLi + LiX$, then $2RLi + CuI \rightarrow [R_2Cu]Li \text{ (Gillman's Reagent)} + LiI$.

  • Reaction: $R_2CuLi + R'-X \rightarrow R-R' + RCu + LiX$.

• Raney Nickel Desulphurization: Removes sulfur from thioacetals/thioketals to produce alkanes using Raney $Ni$.

Reduction of Alkyl Halides

• $LiAlH_4$ (LAH): Reduces $1^\circ$ and $2^\circ$ alkyl halides to alkanes. $3^\circ$ alkyl halides undergo $E_2$ elimination to form alkenes.

• $NaBH_4$: Reduces $2^\circ$ and $3^\circ$ alkyl halides to alkanes.

• Organotin Hydrides ($(Ph)_3SnH$ or $(Bu)_3SnH$): Reduces $1^\circ, 2^\circ, 3^\circ$ alkyl halides to alkanes.

Chemical Properties: Free Radical Substitution

• Halogenation ($Cl_2, h\nu$):

  • Successive chlorination: $CH_4 \rightarrow CH_3Cl \rightarrow CH_2Cl_2 \rightarrow CHCl_3 \rightarrow CCl_4$.

  • Requirement: An $sp^3$ hybridized carbon with at least one $C-H$ bond.

  • Stability of Radicals: \text{Benzylic/Allylic} > 3^\circ > 2^\circ > 1^\circ > CH_3^..

  • Relative Reactivity of Halogens: F_2 > Cl_2 > Br_2 > I_2.

    • Chlorination is highly exothermic/explosive.

    • Iodination is reversible; oxidizing agents like $HIO_3, HNO_3, HgO$ are used to make it irreversible.

• Regioselectivity (Chlorination vs. Bromination):

  • Chlorination ($1^\circ:2^\circ:3^\circ$): Relative reactivity is $1:3.8:5$.

  • Bromination ($1^\circ:2^\circ:3^\circ$): Relative reactivity is $1:82:1600$. Bromination is highly selective for $3^\circ$ carbons.

• Insertion Reaction: Reaction with diazomethane ($CH_2N_2, \Delta$) inserts a methylene group ($:CH_2$) into $C-H$ bonds.

Advanced Oxidation and Other Reactions

• Isomerization: $\text{n-alkane} \xrightarrow{\text{Anhydrous } AlCl_3 + HCl} \text{iso-alkane}$.

• Aromatization: Alkanes with 6 or more carbons ($n$-hexane onwards) form aromatic compounds when passed over oxides of $V, Cr, Mo$ at high temperatures ($V_2O_5, Cr_2O_3, Mo_2O_3, \Delta$).

• Oxidation (NCERT):

  • $CH_4 + O_2 \xrightarrow{Cu/523K/100atm} CH_3OH$

  • $CH_4 + O_2 \xrightarrow{Mo_2O_3, \Delta} HCHO + H_2O$

  • $3^\circ$ hydrogens can be oxidized by $KMnO_4, \Delta$ to alcohols.

• Nitration and Sulfonation:

  • Nitration: High temperature (400–450°C) vapor phase reaction. $R-H + HNO_3 \rightarrow RNO_2 + H_2O$.

  • Sulfonation: Alkanes with 6 or more carbons react with fuming sulphuric acid to give sulfonic acids.

Electrophilic Addition to Alkenes (Classical Carbocation)

• Reaction with $HX$:

  • Mechanism: Formation of classical carbocation (RDS) followed by rearrangement for stability.

  • Markovnikov's Rule: In unsymmetrical alkenes, hydrogen adds to the carbon with more hydrogen atoms.

• Hydration:

  • $Dil. H_2SO_4$: Proceed via carbocation and Markovnikov's addition.

  • Dimerization: Alkenes can dimerize in the presence of concentrated acids to form larger branched alkenes.

• Prins Reaction: Reaction of alkenes with formaldehyde ($HCHO$) in acidic medium to form diols or cyclic ethers.

Electrophilic Addition (Non-Classical Carbocation)

• Halogenation ($X_2, CCl_4$):

  • Proceeds via a cyclic halonium ion (no rearrangement).

  • Anti-addition stereochemistry ($Cis + Anti \rightarrow Threo$; $Trans + Anti \rightarrow Erythreo$).

  • Unsaturation Test: Decolorization of bromine water (reddish brown) or chlorine water (greenish yellow).

• Halohydrin Formation: Reaction with $X_2 / H_2O$ gives halohydrins where $OH$ and $X$ add in anti fashion.

• Oxymercuration-Demercuration ($OM-DM$):

  • Reagents: 1. $Hg(OAc)_2, H_2O/THF$; 2. $NaBH_4, OH^-$

  • Result: Markovnikov addition of water without rearrangement. Hydrogen comes from $NaBH_4$, $OH$ comes from $H_2O$.

• Hydroboration-Oxidation ($HBO$):

  • Reagents: 1. $BH_3/THF$; 2. $H_2O_2, OH^-$

  • Result: Anti-Markovnikov addition of water. Syn addition stereochemistry. Hydrogen comes from $BH_3$, $OH$ comes from $H_2O_2$.

Ozonolysis

• Reductive Ozonolysis ($O_3, Zn/H_2O$ or $Me_2S$): Cleaves double/triple bonds to form aldehydes or ketones.

• Oxidative Ozonolysis ($O_3, H_2O_2$ or $Ag_2O$): Aldehydes formed are further oxidized to carboxylic acids ($HCOOH \rightarrow H_2O + CO_2$).

Oxidation with Permanganate and Osmium

• Syn-Hydroxylation:

  • Baeyer's Reagent: Cold alkaline $1\%\,KMnO_4$ (purple to colorless).

  • $OsO_4 / Na_2SO_3$.

• Anti-Hydroxylation: Peracids ($MCPBA$, Peracetic acid) followed by $H_3O^+$.

• Strong Oxidation: Hot $KMnO_4, H^+$ or $K_2Cr_2O_7$ cleaves the molecule into ketones and carboxylic acids. Terminal carbons are oxidized to $CO_2 + H_2O$.

Free Radical Addition and Alkyne Specifics

• Kharasch Peroxide Effect: Anti-Markovnikov addition of $HBr$ in the presence of peroxides. Not shown by $HCl, HF, HI$ due to endothermic steps.

• Acidity of Terminal Alkynes: React with metals ($Na, NaNH_2$), Tollens reagent ($AgNO_3, NH_4OH$ - white ppt), and ammoniacal cuprous chloride ($Cu_2Cl_2, NH_4OH$ - red ppt).

• Wittig Reaction: Alkenes formed from carbonyls and phosphorus ylides ($Ph_3P=CH_2$).

• Simmon-Smith Reaction: Reaction with $CH_2I_2 / Zn-Cu$ to form cyclopropanes via carbene insertion.

Questions & Discussion

• Q: How many alkanes can be prepared by Wurtz reaction?

  • A: Symmetrical alkanes are the main products. Symmetrical ones like butane are preferred over unsymmetrical ones.

• Q: Which compound shows decarboxylation on heating?

  • A: $\beta$-keto acids, malonic acids, and acids with electron-withdrawing groups at the $\beta$ position show this through a 6-membered cyclic transition state.

• Q: What is the rate of sodalime decarboxylation for different isomers?

  • A: Reactivity depends on carbanion stability. Groups like $\text{-NO}_2$ increase the rate significantly.

• Q: Does rearrangement occur in Hydroboration-Oxidation?

  • A: No, the reaction proceeds via a concerted four-centered transition state ($4MTCS$).