Comprehensive Study Guide: Hydrocarbons - Alkanes, Alkenes, Alkynes, and Arenes

Introduction to Hydrocarbons and Structural Classification

Hydrocarbons are organic chemical compounds composed exclusively of carbon ($C$) and hydrogen ($H$) atoms. They form the fundamental structural backbone of organic chemistry and are broadly classified into four major families:

1. Alkanes: Saturated open-chain hydrocarbons containing only carbon-carbon single bonds.

2. Alkenes: Unsaturated hydrocarbons containing at least one carbon-carbon double bond ($>C=C<$).

3. Alkynes: Unsaturated hydrocarbons containing at least one carbon-carbon triple bond ($-C \equiv C-$).

4. Arenes (Aromatic Hydrocarbons): Cyclic, conjugated planar systems satisfying Hückel's criteria of aromatic stabilization (exemplified by benzene, $C_6H_6$).

Alkanes: Structure, Bonding, and Physical Parameters

Alkanes are saturated acyclic hydrocarbons wherein every carbon atom adopts tetrahedral geometry.

• General Formula (G.F.): $C_n H_{2n+2}$

  • When $n = 1$: $CH_4$ (Methane)

  • When $n = 2$: $C_2H_6 \implies H_3C-CH_3$ (Ethane)

• Carbon-Carbon ($C-C$) Single Bond Length: $1.54\,\text{\AA}$

• Carbon-Carbon ($C-C$) Single Bond Energy: $100\,\text{kcal\,mol}^{-1}$

• Hybridisation: Every carbon atom in an alkane is $sp^3$ hybridised with four equivalent $sp^3$ hybrid orbitals directed toward the corners of a regular tetrahedron (bond angle approximately $109.5^\circ$).

Preparation Methods of Alkanes

Wurtz Reaction

Alkyl halides react with metallic sodium in dry ether ($\text{Et}_2\text{O}$) solvent to yield symmetrical higher alkanes containing twice the number of carbon atoms originally present in the alkyl group.

\n2R-X + 2Na \xrightarrow{\text{dry Ether}} R-R + 2NaX\n

Specific Symmetrical Couplings

• Synthesis of Ethane:   \n  CH_3-Cl + 2Na + Cl-CH_3 \xrightarrow{\text{dry } \text{Et}_2\text{O}} H_3C-CH_3 + 2NaCl\n  

• Synthesis of n-Butane:   \n  2CH_3-CH_2-Cl + 2Na \xrightarrow{\text{dry Ether}} CH_3-CH_2-CH_2-CH_3 + 2NaCl\n  

• Intermolecular Coupling of Cycloalkyl Halides:   Bromocyclohexane on treatment with sodium in dry ether couples to form bicyclohexyl:   \n  2C_6H_{11}-Br + 2Na \xrightarrow{\text{dry } \text{Et}_2\text{O}} C_6H_{11}-C_6H_{11} + 2NaBr\n  

Free Radical Mechanism of the Wurtz Reaction

The Wurtz reaction predominantly proceeds via a single-electron transfer (SET) free radical pathway:

1. Electron Transfer from Sodium:    \n   2Na \rightarrow 2Na^+ + 2e^-\n   

2. Alkyl Radical Generation:    \n   2R-X + 2e^- \rightarrow 2\dot{R} + 2X^-\n       \n   2H_3C-Cl + 2e^- \rightarrow 2\dot{C}H_3 + 2Cl^-\n   

3. Radical Dimerization:    \n   \dot{C}H_3 + \dot{C}H_3 \rightarrow H_3C-CH_3\n   

Intramolecular Wurtz Reaction

When two halogen atoms are located on non-adjacent carbons of the same molecule, treatment with sodium in dry ether results in intramolecular ring closure:

\nCl-CH_2-CH_2-CH_2-Cl + 2Na \xrightarrow{\text{dry } \text{Et}_2\text{O}} \text{Cyclopropane } (\Delta) + 2NaCl\n

Catalytic Hydrogenation of Alkenes and Alkynes

Unsaturated hydrocarbons undergo catalytic addition of molecular hydrogen in the presence of finely divided transition metals (Raney nickel $Ni$, palladium $Pd$, or platinum $Pt$) with heating (Sabatier-Senderens reaction):

• From Alkenes:   \n  C_n H_{2n} + H_2 \xrightarrow{Ni/Pd/Pt, \text{ heat}} C_n H_{2n+2}\n     \n  H_2C=CH_2 + H_2 \xrightarrow{Ni, \Delta} H_3C-CH_3\n     \n  CH_3-CH=CH_2 + H_2 \xrightarrow{Ni, \Delta} CH_3-CH_2-CH_3\n  

• From Alkynes:   \n  C_n H_{2n-2} + 2H_2 \xrightarrow{Ni/Pd/Pt, \text{ heat}} C_n H_{2n+2}\n     \n  HC \equiv CH + 2H_2 \xrightarrow{Ni, \Delta} H_3C-CH_3\n  

Kolbe's Electrolysis

Electrolysis of concentrated aqueous solutions of sodium or potassium salts of carboxylic acids yields symmetrical alkanes at the anode accompanied by decarboxylation:

\n2R-COONa + 2H_2O \xrightarrow{\text{Electrolysis}} R-R + 2CO_2 + 2NaOH + H_2\n

• At the Anode (Oxidation): Alkane ($R-R$) and carbon dioxide ($CO_2$) gas are liberated.

• At the Cathode (Reduction): Sodium hydroxide ($NaOH$) and hydrogen gas ($H_2$) are produced.

Electrolytic Mechanism

1. Ionization:    \n   2R-COONa \rightleftharpoons 2R-COO^- + 2Na^+\n   

2. Anode Process:    \n   2R-COO^- \xrightarrow{-2e^-} 2R-COO^\bullet\n       \n   2R-COO^\bullet \rightarrow 2\dot{R} + 2CO_2\n       \n   \dot{R} + \dot{R} \rightarrow R-R\n   

3. Cathode Process:    \n   2H_2O + 2e^- \rightarrow 2OH^- + H_2\n       \n   2Na^+ + 2OH^- \rightarrow 2NaOH\n   

Decarboxylation of Carboxylic Acid Salts

Heating sodium salts of carboxylic acids with soda lime (a solid mixture of sodium hydroxide, $NaOH$, and calcium oxide, $CaO$, in approximately a $3:1$ ratio) results in the loss of carbon dioxide as sodium carbonate, yielding an alkane containing one less carbon atom than the parent carboxylate:

\n2R-COONa + NaOH \xrightarrow{CaO, \Delta} R-H + Na_2CO_3\n \nCH_3-COONa + NaOH \xrightarrow{CaO, \Delta} CH_4 + Na_2CO_3\n

Chemical Properties and Reactions of Alkanes

Alkanes are paraffinic and chemically inert under ambient conditions. They possess non-polar strong $\sigma$ bonds and do not react with conventional oxidizing agents, reducing agents, strong mineral acids, or strong aqueous bases. At elevated temperatures or under ultraviolet ($h\nu$) radiation, they undergo substitution and oxidative transformations.

Free Radical Halogenation

Alkanes react with halogens ($X_2$) upon exposure to heat or light ($h\nu$):

\nCH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl\n \nCH_3Cl + Cl_2 \xrightarrow{h\nu} CH_2Cl_2 + HCl\n \nCH_2Cl_2 + Cl_2 \xrightarrow{h\nu} CHCl_3 + HCl\n \nCHCl_3 + Cl_2 \xrightarrow{h\nu} CCl_4 + HCl\n \n\text{Overall: } CH_4 + 4Cl_2 \xrightarrow{h\nu} CCl_4 + 4HCl\n

Reactivity Patterns and Halogen Selectivity

• Order of Halogen Reactivity: $F_2 > Cl_2 > Br_2 > I_2$

  • Fluorination ($F_2$): Highly exothermic and violently explosive; leads to carbon-carbon bond cleavage unless extensively diluted.

  • Iodination ($I_2$): Extremely slow and reversible because the co-product $HI$ is a powerful reducing agent that reverses the substitution. To drive the reaction forward, oxidizing agents such as nitric acid ($HNO_3$) or iodic acid ($HIO_3$) are added to destroy $HI$:     \n    5HI + HIO_3 \rightarrow 3I_2 + 3H_2O\n    

• Order of Ease of Replacement of Hydrogens: $3^\circ > 2^\circ > 1^\circ$   Reflects the relative thermodynamic stability of the intermediate free radicals: tertiary radicals ($3^\circ$) > secondary ($2^\circ$) > primary ($1^\circ$).

Chlorination of Propane

Propane chlorination produces a mixture of two isomeric monochloroalkanes:

\nCH_3-CH_2-CH_3 + Cl_2 \xrightarrow{h\nu} CH_3-CH(Cl)-CH_3\text{ (Major)} + CH_3-CH_2-CH_2-Cl\text{ (Minor)} + HCl\n

• Total number of monochloro derivatives = 2 (2-chloropropane as the major product and 1-chloropropane as the minor product).

Detailed Three-Step Free Radical Mechanism

1. Chain Initiation:    Photochemical homolytic cleavage of the halogen molecule with high bond dissociation energy:    \n   Cl-Cl \xrightarrow{h\nu} 2\dot{C}l\n   

2. Chain Propagation:    \n   \dot{C}l + CH_4 \rightarrow \dot{C}H_3 + HCl\n       \n   \dot{C}H_3 + Cl_2 \rightarrow CH_3Cl + \dot{C}l\n   

3. Chain Termination:    Exothermic combination of active chain carriers:    \n   \dot{C}l + \dot{C}l \rightarrow Cl_2\n       \n   \dot{C}H_3 + \dot{C}l \rightarrow CH_3Cl\n       \n   \dot{C}H_3 + \dot{C}H_3 \rightarrow H_3C-CH_3\n   

Combustion

In the presence of excess oxygen, complete combustion converts alkanes into carbon dioxide and water with the evolution of substantial thermal energy:

• General Alkane Combustion Formula:   \n  C_n H_{2n+2} + \left(\frac{3n+1}{2}\right)O_2 \rightarrow n\,CO_2 + (n+1)H_2O\n  

• General Hydrocarbon Combustion Formula:   \n  C_x H_y + \left(x + \frac{y}{4}\right)O_2 \rightarrow x\,CO_2 + \frac{y}{2}\,H_2O\n  

• Combustion of Methane ($x=1, y=4$):   \n  CH_4 + \left(1 + \frac{4}{4}\right)O_2 \rightarrow 1\,CO_2 + \frac{4}{2}\,H_2O \implies CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O\n  

Controlled (Incomplete / Catalytic) Oxidation

Depending on the specific transition-metal catalyst, temperature, and operating pressure, partial oxidation yields oxygenated derivatives:

1. Oxidation to Alcohol (Copper Catalyst):    \n   2CH_4 + O_2 \xrightarrow{Cu,\ 523\,K,\ 100\,\text{atm}} 2CH_3OH\text{ (Methyl alcohol)}\n   

2. Oxidation to Aldehyde (Molybdenum Oxide Catalyst):    \n   CH_4 + O_2 \xrightarrow{Mo_2O_3, \Delta} HCHO\text{ (Formaldehyde)} + H_2O\n   

3. Oxidation to Carboxylic Acid (Manganese Catalyst):    \n   2CH_3-CH_3 + 3O_2 \xrightarrow{(CH_3COO)_2Mn, \Delta} 2CH_3COOH\text{ (Acetic acid)} + 2H_2O\n   

4. Oxidation of Tertiary ($3^\circ$) Hydrogens by Potassium Permanganate:    While alkanes are generally resistant to aqueous $KMnO_4$, alkanes containing a tertiary hydrogen atom undergo selective oxidation to tertiary alcohols:    \n   2(CH_3)_3CH + O_2 \xrightarrow{KMnO_4} 2(CH_3)_3C-OH\text{ (tert-Butyl alcohol / } 3^\circ\text{ alcohol)}\n   

Isomerisation

Straight-chain alkanes on heating with anhydrous aluminium chloride ($AlCl_3$) and dry hydrogen chloride ($HCl$) gas undergo skeletal rearrangement to branched-chain constitutional isomers:

\n\text{n-Hexane} \xrightarrow{\text{Anhydrous } AlCl_3 / HCl\text{ (gas)}, \Delta} \text{2-methylpentane} + \text{3-methylpentane}\n

Aromatisation

Acyclic alkanes having 6 to 8 carbon atoms on heating to $773\,K$ ($500^\circ\text{C}$) at $10\text{--}20\,\text{atm}$ pressure over supported transition metal oxide catalysts ($Cr_2O_3$, $MoO_3$, $V_2O_5$, or $Fe_2O_3 / Al_2O_3$) undergo simultaneous cyclization and dehydrogenation to yield aromatic hydrocarbons:

\n\text{n-Hexane } (C_6H_{14}) \xrightarrow{Cr_2O_3 / MoO_3 / V_2O_5,\ 773\,K,\ 10-20\,\text{atm}} \text{Benzene } (C_6H_6) + 4H_2\n \n\text{n-Heptane } (C_7H_{16}) \xrightarrow{Cr_2O_3 / MoO_3 / V_2O_5,\ 773\,K,\ 10-20\,\text{atm}} \text{Toluene } (C_6H_5-CH_3) + 4H_2\n

Alkenes: Structure, Bonding, and Hybridisation

Alkenes are unsaturated aliphatic hydrocarbons containing at least one carbon-carbon double bond ($C=C$), consisting of one strong $\sigma$ bond and one weaker $\pi$ bond formed by lateral overlap of unhybridized $2p$ orbitals.

• Functional Group (F.G.): $>C=C<$

• Carbon-Carbon Double Bond Length ($C=C$): $1.34\,\text{\AA}$

• General Formula (G.F.): $C_n H_{2n}$

  • When $n = 2$: $C_2H_4$ (Ethene / Ethylene)

  • When $n = 3$: $C_3H_6$ (Propene / Propylene)

• Hybridisation: Each doubly bonded carbon atom is $sp^2$ hybridised with trigonal planar geometry and bond angles of approximately $120^\circ$.

Preparation Methods of Alkenes

Controlled Reduction of Alkynes

Alkynes can be stereoselectively reduced to either cis- or trans-alkenes depending on the reagent chosen:

1. Syn-Hydrogenation via Lindlar's Catalyst (cis-Alkene):    Hydrogenation of an internal alkyne with molecular hydrogen over Lindlar's catalyst results in stereospecific syn-addition, generating exclusively the cis-alkene isomer:    \n   R-C \equiv C-R + H_2 \xrightarrow{\text{Lindlar's catalyst}} \text{cis-alkene}\n   

   • Lindlar's Catalyst Definition: Palladium metal deposited on solid calcium carbonate ($Pd-CaCO_3$) or barium sulphate ($Pd-BaSO_4$), or carbon ($Pd-C$), partially deactivated (poisoned) with lead salts, quinoline, or sulphur compounds to prevent further reduction of the alkene to an alkane.

   • Example with 2-Butyne:      \n     H_3C-C \equiv C-CH_3 + H_2 \xrightarrow{Pd-C / Pd-BaSO_4} \text{cis-2-butene}\n     

2. Anti-Hydrogenation via Birch Reduction (trans-Alkene):    Chemical reduction using alkali metals (sodium $Na$ or lithium $Li$) dissolved in liquid ammonia ($\text{liq. } NH_3$) proceeds via radical anion intermediates to provide anti-addition, yielding the thermodynamically stable trans-alkene isomer:    \n   R-C \equiv C-R \xrightarrow{Na / \text{liq. } NH_3 \text{ or } Li / \text{liq. } NH_3} \text{trans-alkene}\n   

   • Example with 2-Butyne:      \n     H_3C-C \equiv C-CH_3 \xrightarrow{Na / \text{liq. } NH_3} \text{trans-2-butene}\n     

3. Uncontrolled Catalytic Hydrogenation:    Treatment of alkynes with hydrogen in the presence of unpoisoned transition metal catalysts ($H_2 / Pd$) leads to non-stop reduction straight to the saturated alkane:    \n   R-C \equiv C-R \xrightarrow{H_2 / Pd} R-CH_2-CH_2-R\n   

Elimination Reactions: Mechanisms ($E_1$ and $E_2$)

An elimination reaction involves the cleavage of two single $\sigma$ bonds to generate a new multiple bond ($\pi$ bond) through the loss of atoms or molecular fragments.

Positional Classification of Elimination Reactions

• $\alpha$-Elimination ($1,1$-Elimination): Both departing atoms/groups leave from the same carbon atom (e.g., generation of dichlorocarbene from chloroform).

• $\beta$-Elimination ($1,2$-Elimination): Departing groups leave from two adjacent carbon atoms, categorized as the $\alpha$-carbon (bearing the leaving group) and the $\beta$-carbon (bearing the departing proton). This is the standard pathway for alkene formation.

• $\gamma$-Elimination ($1,3$-Elimination): Departing groups are separated by three carbons, leading to three-membered ring systems (cyclopropanes).

• $\delta$-Elimination ($1,4$-Elimination): Departing groups leave carbons 1 and 4, producing cyclobutanes or conjugated dienes.

Vicinal Dihalide Dehalogenation

Treating vicinal dihalides with zinc dust in alcohol promotes $\beta$-elimination of $ZnX_2$:

\n-C(X)-C(X)- + Zn \rightarrow >C=C< + ZnX_2\n

Unimolecular Elimination ($E_1$ Mechanism)

$E_1$ is a two-step unimolecular elimination taking place under neutral or mildly acidic solvolytic conditions without requiring an added base.

• Solvent Environment: Proceeds efficiently in polar protic solvents having high dielectric constants and strong hydrogen-bonding ability: water ($H_2O / HOH$), ammonia ($NH_3$), thiols ($R-SH / HSH$), alcohols ($R-OH$), and carboxylic acids ($R-COOH$).

• Thermal Requirement: Favored at high temperatures ($T > 25^\circ\text{C}$), as the entropy change ($\Delta S > 0$) dominates the free energy profile ($\Delta G = \Delta H - T\Delta S$).

• Base Dependency: An external base is not required; weak solvent molecules can abstract the proton in the final step.

Kinetic Steps of $E_1$

1. Step 1 (Slow, Rate-Determining Step): Heterolytic dissociation of the leaving group ($LG$) generates an open carbocation intermediate:    \n   CH_3-CH(Br)-CH_2-CH_3 \xrightarrow{\text{slow}} CH_3-\overset{+}{C}H-CH_2-CH_3 + Br^-\n   

2. Step 2 (Fast, Product-Determining Step): Proton transfer from an adjacent $\beta$-carbon to solvent forms isomeric alkenes:    \n   CH_3-\overset{+}{C}H-CH_2-CH_3 \xrightarrow{-\beta\text{-H}^+} CH_3-CH=CH-CH_3\text{ (Major, Saytzeff, more stable)} + CH_2=CH-CH_2-CH_3\text{ (Minor)}\n   

Key Features of the $E_1$ Pathway

• Substrate Reactivity Order: $3^\circ\ (R-X) > 2^\circ\ (R-X) > 1^\circ\ (R-X)$   Matches carbocation stability. Tertiary substrates react fastest; primary halides virtually never undergo $E_1$.

• Leaving Group Effect: The rate of reaction increases with better leaving group ability ($I^- > Br^- > Cl^- > F^-$).

• Rearrangements: Because carbocation intermediates have a finite lifetime, they can undergo skeletal rearrangement via 1,2-hydride shifts, 1,2-methyl shifts, or ring expansions to reach more thermodynamically stable cationic states prior to elimination.

Bimolecular Elimination ($E_2$ Mechanism)

$E_2$ is a concerted, single-step bimolecular elimination requiring anti-periplanar alignment between the $\beta$-hydrogen and the leaving group ($\alpha$-halogen).

• Reaction Condition: Occurs in basic solutions requiring strong bases:

  • Alcoholic sodium hydroxide (alc. $NaOH$)

  • Alcoholic potassium hydroxide (alc. $KOH$)

  • Sodium amide ($NaNH_2$)

  • Sodium methoxide ($CH_3ONa$)

  • Potassium tert-butoxide ($t\text{-BuOK}$: $(CH_3)_3C-O^-K^+$)

  • Triethylamine ($(Et)_3N$)

• Concerted Mechanism: Base abstracts the $\beta$-proton simultaneously with double-bond formation and departure of the leaving group from the $\alpha$-carbon.

Regioselectivity: Saytzeff vs. Hofmann Elimination

1. Saytzeff (Zaitsev) Rule (Small Bases):    When dehydrohalogenation is carried out with small, unhindered bases (such as alc. $KOH$), the major product is the more highly substituted, thermodynamically more stable alkene:    \n   CH_3-CH(Br)-CH_2-CH_3 \xrightarrow{\text{alc. } KOH} CH_3-CH=CH-CH_3\text{ (Major, Saytzeff product)} + CH_2=CH-CH_2-CH_3\text{ (Minor)}\n   

2. Hofmann Rule (Sterically Bulky Bases):    When a bulky, sterically hindered base is employed (such as potassium tert-butoxide, $(CH_3)_3CO^-$), steric hindrance prevents the base from attacking the more substituted, internal $\beta$-carbon. The base abstracts the most accessible, least sterically hindered $\beta$-proton from the terminal methyl group, yielding the less substituted alkene as the major product:    \n   CH_3-CH(Br)-CH_2-CH_3 \xrightarrow{(CH_3)_3CO^-} CH_2=CH-CH_2-CH_3\text{ (Major, Hofmann product)} + CH_3-CH=CH-CH_3\text{ (Minor, Saytzeff product)}\n   

Electrophilic Addition and Oxidation Reactions of Alkenes

Because the loosely held $\pi$-electron cloud of an alkene lies exposed above and below the molecular plane, alkenes function as nucleophiles and predominantly undergo electrophilic addition reactions ($Ad_E$):

\n>C=C< + E-Nu \rightarrow -C(E)-C(Nu)-\n

Addition of Halogens

Alkenes react smoothly with halogens ($Cl_2$, $Br_2$) dissolved in inert non-polar solvents ($CCl_4$ or $CS_2$) to produce vicinal dihalides:

\nCH_2=CH_2 + Cl_2 \xrightarrow{CCl_4} CH_2(Cl)-CH_2(Cl)\text{ (1,2-dichloroethane)}\n \nCH_3-CH=CH_2 + Br_2 \xrightarrow{CS_2 / CCl_4} CH_3-CH(Br)-CH_2Br\text{ (1,2-dibromopropane)}\n

Addition of Hydrogen Halides ($HX$) and Markovnikov's Rule

• Addition to Symmetrical Alkenes:   \n  CH_2=CH_2 + HBr \rightarrow CH_3-CH_2-Br\n  

• Markovnikov's Rule:   When an unsymmetrical reagent ($HX$ or $E^+-Nu^-$) adds to an unsymmetrical alkene, the positive electrophilic part ($H^+$ or $E^+$) attaches to the double-bonded carbon atom that carries the greater number of hydrogen atoms. Concurrently, the negative nucleophilic part ($X^-$ or $Nu^-$) bonds to the double-bonded carbon atom carrying the smaller number of hydrogen atoms (proceeding via the more stable carbocation intermediate).

\nCH_3-CH=CH_2 + HBr \xrightarrow{CS_2} CH_3-CH(Br)-CH_3\text{ (Major, } 2^\circ\text{)} + CH_3-CH_2-CH_2-Br\text{ (Minor, } 1^\circ\text{)}\n \nPh-CH=CH_2 + HCl \rightarrow Ph-CH(Cl)-CH_3\n

Anti-Markovnikov Addition (Peroxide Effect / Kharasch Effect)

When addition of hydrogen bromide ($HBr$) to an unsymmetrical alkene occurs in the presence of organic peroxides ($R-O-O-R$, $H_2O_2$) and light ($h\nu$), the regiochemical outcome reverses: the hydrogen atom adds to the carbon having fewer hydrogen atoms, and the bromine atom adds to the carbon having more hydrogen atoms.

\nCH_3-CH=CH_2 + HBr \xrightarrow{H_2O_2,\ h\nu} CH_3-CH_2-CH_2-Br\text{ (1-bromopropane, Major)}\n

Free Radical Mechanism

1. Chain Initiation (CI):    Homolysis of the weak peroxy single bond triggers hydrogen abstraction from $HBr$:    \n   HO-OH \xrightarrow{h\nu} 2\dot{O}H\n       \n   \dot{O}H + H-Br \rightarrow H_2O + \dot{B}r\n   

2. Chain Propagation (CP):    The electrophilic bromine radical attacks the alkene to generate the more substituted, resonance- or hyperconjugation-stabilized radical intermediate:    \n   \dot{B}r + CH_3-CH=CH_2 \rightarrow CH_3-\dot{C}H-CH_2Br\text{ ( } 2^\circ\text{ free radical, Major)} \gg CH_3-CH(Br)-\dot{C}H_2\text{ ( } 1^\circ\text{ radical, Minor)}\n       \n   CH_3-\dot{C}H-CH_2Br + H-Br \rightarrow CH_3-CH_2-CH_2Br + \dot{B}r\n   

3. Chain Termination (CT):    Coupling of radical species terminates the cycle:    \n   \dot{B}r + \dot{B}r \rightarrow Br_2\n       \n   \dot{H} + \dot{B}r \rightarrow HBr\n   

Representative Substrates

• Isobutylene (2-Methylpropene):   \n  (CH_3)_2C=CH_2 + HBr \xrightarrow{h\nu} (CH_3)_2CH-CH_2Br\text{ (1-bromo-2-methylpropane)}\n  

• 1-Methylcyclohexene:   Addition of $HBr$ with $h\nu$ yields 1-bromo-2-methylcyclohexane.

Critical Thermodynamic Limitation

Anti-Markovnikov free-radical addition is strictly applicable to $HBr$ only and is never observed with $HCl$, $HF$, or $HI$:

• For $HF$ and $HCl$, the $H-X$ bond strength is too high ($H-F = 562\,\text{kJ\,mol}^{-1}$, $H-Cl = 431\,\text{kJ\,mol}^{-1}$); the second initiation step ($\dot{O}H + HX \rightarrow H_2O + \dot{X}$) is strongly endothermic.

• For $HI$, the $H-I$ bond is weak and easily broken, but the resulting iodine radical ($\dot{I}$) is unreactive toward alkene addition (endothermic step). Instead, iodine radicals preferentially recombine with one another to regenerate molecular iodine ($2\dot{I} \rightarrow I_2$).

Hydroboration-Oxidation

Hydroboration-oxidation effects the net hydration of an alkene with overall Anti-Markovnikov regiochemistry without carbocation rearrangements, occurring through concerted syn-stereospecific addition of boron and hydrogen:

1. Reagents: Borane-tetrahydrofuran complex ($BH_3 / \text{THF}$ or $B_2H_6 / \text{THF}$) followed by basic hydrogen peroxide ($H_2O_2 / NaOH$ or $H_2O_2 / OH^-$).

2. Reactions:    \n   R-CH=CH_2 \xrightarrow{(i)\ BH_3 / \text{THF},\ (ii)\ H_2O_2 / OH^-} R-CH_2-CH_2-OH\n       \n   CH_3-CH=CH_2 \xrightarrow{(i)\ BH_3 / \text{THF},\ (ii)\ H_2O_2 / NaOH} CH_3-CH_2-CH_2-OH\text{ (Propan-1-ol)}\n   

3. Stereospecificity: Syn-addition to 1-methylcyclopentene yields trans-2-methylcyclopentan-1-ol.

Oxymercuration-Demercuration (OMDM)

Oxymercuration-demercuration carries out the net addition of water across an alkene with strict Markovnikov regiochemistry while avoiding carbocation rearrangements:

• Reagents: Mercuric acetate ($Hg(OAc)_2$, where $OAc = -O-CO-CH_3$) in aqueous solution ($H_2O$), followed by reductive demercuration using sodium borohydride ($NaBH_4$) in basic medium ($OH^-$).

• General Reaction:   \n  R-CH=CH_2 \xrightarrow{(i)\ Hg(OAc)_2,\ H_2O,\ (ii)\ NaBH_4 / OH^-} R-CH(OH)-CH_3\n     \n  CH_3-CH=CH_2 \xrightarrow{(i)\ Hg(OAc)_2,\ H_2O,\ (ii)\ NaBH_4 / OH^-} CH_3-CH(OH)-CH_3\text{ (Propan-2-ol)}\n  

• Absence of Skeletal Rearrangement:   Because addition proceeds through a cyclic mercurinium ion rather than a free carbocation:

  • 2-Methylpropene gives 2-methylpropan-2-ol:     \n    (CH_3)_2C=CH_2 \xrightarrow{\text{OMDM}} (CH_3)_2C(OH)-CH_3\n    

  • 3-Methylbut-1-ene gives 3-methylbutan-2-ol (normal acid hydration would rearrange to 2-methylbutan-2-ol):     \n    (CH_3)_2CH-CH=CH_2 \xrightarrow{\text{OMDM}} (CH_3)_2CH-CH(OH)-CH_3\n    

  • 1-Methylcyclohexene gives 1-methylcyclohexan-1-ol:     \n    \text{1-methylcyclohexene} \xrightarrow{\text{OMDM}} \text{1-methylcyclohexan-1-ol}\n    

Oxidation with Cold Dilute Permanganate (Baeyer's Reagent)

Treatment of an alkene with cold, dilute, neutral or alkaline potassium permanganate ($KMnO_4$, known as Baeyer's Reagent) leads to syn-dihydroxylation, producing syn-diols (vicinal cis-diols / glycols):

• The reaction proceeds through a cyclic five-membered permanganate ester intermediate, enforcing syn-addition of both hydroxyl groups to the same face of the alkene.

• The purple color of $KMnO_4$ discharges, producing a brown precipitate of manganese dioxide ($MnO_2$); this serves as a diagnostic qualitative test for carbon-carbon unsaturation.

Stereochemical Outcomes

• Cyclohexene: Cold, dilute $KMnO_4$ delivers cis-cyclohexane-1,2-diol (both $-OH$ groups wedged or both dashed, generating a meso stereoisomer):   \n  \text{Cyclohexene} \xrightarrow{\text{cold \& dil } KMnO_4} \text{cis-cyclohexane-1,2-diol}\n  

• 1-Methylcyclohexene: Produces a racemic pair of enantiomers of cis-1-methylcyclohexane-1,2-diol (where $CH_3$ and adjacent $OH$ are trans to each other, while both $OH$ groups are mutual syn):   \n  \text{1-methylcyclohexene} \xrightarrow{\text{cold \& dil } KMnO_4} \text{1-methylcyclohexane-1,2-diol (pair of enantiomers)}\n  

Ozonolysis: Reductive vs. Oxidative Cleavage

Electrophilic addition of ozone ($O_3$) cleaves the carbon-carbon double bond via a molozonide to form a stable cyclic ozonide. The resulting products depend on the workup conditions:

1. Reductive Ozonolysis ($O_3$ followed by $Zn / H_2O$ or $Me_2S$):    Zinc reduces hydrogen peroxide formed during cleavage, preserving aldehydes ($R-CHO$) from oxidation to carboxylic acids:    \n   R-CH=CH-R \xrightarrow{(i)\ O_3,\ (ii)\ Zn / H_2O} 2R-CHO\n       \n   R_2C=CH-R \xrightarrow{(i)\ O_3,\ (ii)\ Zn / H_2O} R_2C=O + R-CHO\n   

   • Example:      \n     CH_3-C(CH_3)=CH-CH_3 \xrightarrow{O_3 / Zn,\ H_2O} (CH_3)_2C=O\text{ (Acetone)} + CH_3CHO\text{ (Acetaldehyde)}\n     

2. Oxidative Ozonolysis ($O_3$ followed by $H_2O_2$ or $H_2O$ without Zinc):    Aldehydes formed during cleavage are oxidized to carboxylic acids, while ketones remain unchanged:    \n   R-CH=CH-R \xrightarrow{(i)\ O_3,\ (ii)\ H_2O_2} 2R-COOH\n       \n   R_2C=CH-R \xrightarrow{(i)\ O_3,\ (ii)\ H_2O_2} R_2C=O + R-COOH\n   

   • Example:      \n     CH_3-C(CH_3)=CH-CH_3 \xrightarrow{O_3 / H_2O_2} (CH_3)_2C=O\text{ (Acetone)} + CH_3COOH\text{ (Acetic acid)}\n     

Vigorous Oxidation with Hot Acidic or Alkaline Permanganate

When alkenes react with hot, concentrated acidic or alkaline $KMnO_4$, complete oxidative cleavage of the double bond takes place, mirroring the outcome of oxidative ozonolysis:

• Terminal $=CH_2$ groups oxidize to carbon dioxide and water ($CO_2 + H_2O$).

• Internal monosubstituted carbons ($=CH-R$) oxidize to carboxylic acids ($R-COOH$).

• Disubstituted carbons ($=CR_2$) oxidize to ketones ($R_2C=O$).

\n2R_2C=CR_2 \xrightarrow{\text{hot conc. acidic / alkaline } KMnO_4} 2R_2C=O\n

Cyclic Cleavage

1,2-Dimethylcyclohexene undergoes ring cleavage by hot acidic $KMnO_4$ to produce a diketone (octane-2,7-dione):

\n\text{1,2-dimethylcyclohexene} \xrightarrow{\text{hot acidic } KMnO_4} CH_3-CO-(CH_2)_4-CO-CH_3\text{ (octane-2,7-dione)}\n

Alkynes: Structure, Bonding, and General Characteristics

Alkynes are unsaturated acyclic hydrocarbons containing one or more carbon-carbon triple bonds ($-C \equiv C-$), consisting of one axial $\sigma$ bond and two mutually perpendicular $\pi$ bonds.

• General Formula (G.F.): $C_n H_{2n-2}$

  • When $n = 2$: $C_2H_2$ ($HC \equiv CH$, Ethyne / Acetylene)

  • When $n = 3$: $C_3H_4$ ($CH_3-C \equiv CH$, Propyne)

• Functional Group (F.G.): $-C \equiv C-$

• Carbon-Carbon Triple Bond Length ($C \equiv C$): $1.20\,\text{\AA}$

• Hybridisation: Each triply bonded carbon atom is $sp$ hybridised with linear molecular geometry and bond angles of $180^\circ$.

Preparation Methods of Alkynes

From Calcium Carbide ($CaC_2$)

Acetylene is produced industrially from limestone ($CaCO_3$) through a three-step high-temperature process:

1. Thermal Decomposition of Limestone:    \n   CaCO_3 \xrightarrow{\Delta} CaO + CO_2\n   

2. Carbide Synthesis in an Electric Furnace:    \n   CaO + 3C \rightarrow CaC_2 + CO\n   

3. Hydrolysis of Calcium Carbide:    \n   CaC_2 + 2H_2O \rightarrow Ca(OH)_2 + C_2H_2\text{ (Acetylene)}\n   

Double Dehydrohalogenation of Vicinal Dihalides

Vicinal dihalides undergo sequential elimination of two equivalents of hydrogen halide ($HX$):

1. Step 1: Mild elimination with alcoholic potassium hydroxide removes the first mole of $HX$ to generate a haloalkene (vinyl halide):    \n   CH_2(Br)-CH_2(Br) \xrightarrow{\text{alc. } KOH, -HBr} CH_2=CH-Br\text{ (Vinyl bromide)}\n   

2. Step 2: Because the remaining halogen atom is conjugated with the double bond (shortened $\text{C-X}$ bond length with partial double-bond character), the second elimination requires a considerably stronger base, such as sodium amide ($NaNH_2$), to form the triple bond:    \n   CH_2=CH-Br \xrightarrow{NaNH_2, -HBr} CH \equiv CH\text{ (Ethyne)}\n   

Synthesis of Propyne

\nCH_3-CH(Br)-CH_2Br \xrightarrow{\text{alc. } KOH, -HBr} CH_3-C(Br)=CH_2 \xrightarrow{NaNH_2, -HBr} CH_3-C \equiv CH\n

Chemical Reactions and Acidity of Alkynes

Electrophilic Addition Reactions of Alkynes

Because alkynes contain two orthogonal $\pi$ bonds, they undergo stepwise addition reactions, incorporating up to two equivalents of reagent.

Catalytic Hydrogenation

\nHC \equiv CH \xrightarrow{H_2 / Ni, Pt, Pd} H_2C=CH_2 \xrightarrow{H_2 / Ni, Pt, Pd} H_3C-CH_3\n \nCH_3-C \equiv CH + 2H_2 \xrightarrow{Ni} CH_3-CH_2-CH_3\n

Halogen Addition

\nHC \equiv CH + Br_2 \rightarrow CH(Br)=CH(Br)\text{ (1,2-dibromoethene)} \xrightarrow{Br_2} CH(Br)_2-CH(Br)_2\text{ (1,1,2,2-tetrabromoethane)}\n \nCH_3-C \equiv CH + Br_2 \rightarrow CH_3-C(Br)=CH(Br) \xrightarrow{Br_2} CH_3-C(Br)_2-CH(Br)_2\text{ (1,1,2,2-tetrabromopropane)}\n

Hydrogen Halide Addition

Addition of $HX$ follows Markovnikov's rule across both addition steps, leading specifically to geminal dihalides (gem-dihalides):

\nHC \equiv CH + HBr \rightarrow CH_2=CH-Br \xrightarrow{HBr} CH_3-CHBr_2\text{ (1,1-dibromoethane)}\n \nCH_3-C \equiv CH + HBr \rightarrow CH_3-C(Br)=CH_2 \xrightarrow{HBr} CH_3-C(Br)_2-CH_3\text{ (2,2-dibromopropane)}\n

Catalytic Hydration (Kucherov Reaction)

Alkynes add water in the presence of dilute sulphuric acid ($40\%\ H_2SO_4$) and mercuric sulphate ($1\%\ HgSO_4$) at $333\,K$ (or $Hg^{2+} / H^+$) to yield an unstable enol intermediate, which rapidly undergoes keto-enol tautomerisation:

• Hydration of Acetylene (Yields Aldehyde):   \n  HC \equiv CH + H_2O \xrightarrow{HgSO_4 / H_2SO_4} [H_2C=CH-OH]\text{ (Ethenol / enol)} \xrightarrow{\text{tautomerisation}} CH_3-CHO\text{ (Acetaldehyde / Ethanal)}\n  

• Hydration of Propyne and Higher Alkynes (Yields Ketones):   Hydration follows Markovnikov's rule, delivering methyl ketones:   \n  H_3C-C \equiv CH + H_2O \xrightarrow{Hg^{2+} / H^+} [CH_3-C(OH)=CH_2] \xrightarrow{\text{tautomerisation}} CH_3-CO-CH_3\text{ (Acetone / Propan-2-one)}\n  

Cyclic Polymerisation (Trimerisation)

Passing alkynes through a red-hot iron ($Fe$) tube at $873\,K$ results in cyclic trimerisation:

• Trimerisation of Ethyne:   \n  3HC \equiv CH \xrightarrow{\text{Red hot } Fe\text{ tube, } 873\,K} C_6H_6\text{ (Benzene)}\n  

• Trimerisation of Propyne:   Three moles of propyne trimerise symmetrically to give mesitylene ($1,3,5$-trimethylbenzene):   \n  3CH_3-C \equiv CH \xrightarrow{\text{Red hot } Fe\text{ tube, } 873\,K} \text{Mesitylene } (1,3,5\text{-trimethylbenzene})\n  

Acidity of Terminal Alkynes

Terminal alkynes have weakly acidic acetylenic hydrogens ($pKa \approx 25$):

• Hybridisation and Electronegativity Relationship:

  • In $sp$ hybridized carbon, the s-character is $50\%$.

  • In $sp^2$ hybridized carbon, the s-character is $33.3\%$.

  • In $sp^3$ hybridized carbon, the s-character is $25\%$.

  • Greater s-character brings the bonding electron pair closer to the carbon nucleus, increasing its effective electronegativity:     \n    \text{Electronegativity: } sp > sp^2 > sp^3\n         Consequently, the $C-H$ bond of a terminal alkyne is highly polarized, facilitating proton removal by strong bases.

• Reactions Demonstrating Acidity:

  • With metallic sodium:     \n    H-C \equiv C-H + 2Na \rightarrow Na^+-C \equiv C^-Na^+ + H_2\n         \n    H-C \equiv C-H + Na \rightarrow H-C \equiv C^-Na^+ + \frac{1}{2}H_2\n    

  • With sodamide ($NaNH_2$):     \n    CH_3-C \equiv C-H + NaNH_2 \rightarrow CH_3-C \equiv C^-Na^+ + NH_3\n    

• Acidity Hierarchy:

  • Across Hybridization States:     \n    HC \equiv CH > H_2C=CH_2 > H_3C-CH_3\n    

  • Among Alkynes:     \n    HC \equiv CH > CH_3-C \equiv CH > CH_3-C \equiv C-CH_3\n         Alkyl substituents exert an electron-donating inductive effect ($+I$), increasing electron density on the acetylenic carbon and destabilizing the conjugate carbanion. Non-terminal alkynes lack an acetylenic hydrogen and do not exhibit acidic behavior.

Arenes and Aromaticity: Criteria and Benzene Structure

Aromatic compounds are formally known as arenes.

Conditions for Aromatic Character (Hückel's Rule)

For a molecule to display aromatic stabilization, it must fulfill four structural criteria:

1. Cyclic: The structure must be a ring system.

2. Conjugated: Every atom in the ring must have an unhybridized p-orbital, creating an uninterrupted, cyclic conjugated $\pi$-electron loop.

3. Planar: The ring system must be planar ($sp^2$ or $sp$ hybridised) to ensure parallel alignment and maximum lateral overlap of p-orbitals.

4. Hückel's Electron Count: The conjugated $\pi$-system must contain $(4n + 2)\pi$ electrons, where $n$ is a non-negative integer ($n = 0, 1, 2, 3\ldots$).

Structural Profile of Benzene

• Molecular Formula: $C_6H_6$

• Geometry and Hybridisation: All six ring carbons are $sp^2$ hybridised; every carbon-carbon bond length is identical ($1.39\,\text{\AA}$, intermediate between a single bond of $1.54\,\text{\AA}$ and a double bond of $1.34\,\text{\AA}$).

• Electron Count: Contains $6\pi$ electrons, fulfilling Hückel's rule for $n = 1$:   \n  4n + 2 = 4(1) + 2 = 6\pi\text{ electrons}\n  

Preparation Methods of Benzene

1. Reduction of Phenol with Zinc Dust:    Heating phenol with metallic zinc dust yields benzene through oxygen abstraction:    \n   C_6H_5OH + Zn \xrightarrow{\Delta} C_6H_6 + ZnO\n   

2. Decarboxylation of Sodium Benzoate:    Heating sodium benzoate with soda lime ($NaOH + CaO$) eliminates the carboxylate group:    \n   C_6H_5-COONa + NaOH \xrightarrow{CaO, \Delta} C_6H_6 + Na_2CO_3\n   

3. Cyclic Polymerisation of Ethyne:    \n   3HC \equiv CH \xrightarrow{\text{Red hot } Fe\text{ tube, } 873\,K} C_6H_6\n   

4. Protonolysis of Grignard Reagents:    Reaction of phenylmagnesium halide ($C_6H_5-MgX$) with any active hydrogen donor ($H_2O$, $CH_3COOH$, $HCl$, $HNO_3$, or $H_2SO_4$) protonates the aryl carbanion to yield benzene:    \n   C_6H_5-MgX + H_2O \rightarrow C_6H_6 + Mg(OH)X\n   

5. Aromatisation of n-Hexane:    \n   \text{n-Hexane } (C_6H_{14}) \xrightarrow{Cr_2O_3 / MoO_3 / V_2O_5,\ 500^\circ\text{C}} C_6H_6 + 4H_2\n   

Electrophilic Aromatic Substitution ($S_E Ar$) Mechanisms and Reactions

Although benzene possesses an electron-rich $\pi$-system, it resists electrophilic addition reactions. Addition converts planar, aromatic $sp^2$ carbons into non-planar $sp^3$ centers, disrupting cyclic conjugation and destroying the resonance stabilization energy of the ring ($150.6\,\text{kJ\,mol}^{-1}$). Consequently, benzene predominantly undergoes electrophilic aromatic substitution ($S_E Ar$), preserving the aromatic core.

General Three-Step Mechanism of $S_E Ar$

1. Step 1: Generation of the Electrophile ($E^+$):    A strong electrophile is generated, typically via coordination with a Lewis or Brønsted acid.

2. Step 2: Attack of the Nucleophilic Ring (Formation of the Carbocation Intermediate):    The $\pi$-cloud attacks the electrophile, generating a resonance-stabilized arenium ion (Wheland intermediate / $\sigma$-complex). The positive charge is delocalized over the ortho and para positions relative to the point of electrophilic attack.

3. Step 3: Removal of Proton ($H^+$):    A weak conjugate base abstracts the proton from the $\sigma$-complex's $sp^3$ carbon, restoring aromaticity and regenerating the acid catalyst.

Halogenation

Benzene reacts with halogens ($X_2 = Cl_2, Br_2, I_2$) in the presence of anhydrous Lewis acids ($AlCl_3$, $FeCl_3$, $AlBr_3$):

\nC_6H_6 + Cl_2 \xrightarrow{\text{Anhydrous } AlCl_3} C_6H_5Cl + HCl\text{ (Chlorobenzene)}\n

Mechanism of Chlorination

1. Generation of Chloronium Ion:    \n   Cl-Cl + AlCl_3 \rightarrow Cl^+ + [AlCl_4]^-\n   

2. Arenium Ion Formation:    The electrophile ($Cl^+$) attacks benzene, breaking aromaticity to form the resonance-stabilized arenium ion:    \n   C_6H_6 + Cl^+ \rightarrow [C_6H_6Cl]^+\n   

3. Deprotonation and Aromatic Regeneration:    \n   [C_6H_6Cl]^+ + [AlCl_4]^- \rightarrow C_6H_5Cl + HCl + AlCl_3\n   

Nitration

Heating benzene with a mixture of concentrated nitric acid ($\text{conc. } HNO_3$) and concentrated sulphuric acid ($\text{conc. } H_2SO_4$), known as the nitrating mixture, yields nitrobenzene:

\nC_6H_6 + \text{conc. } HNO_3 \xrightarrow{\text{conc. } H_2SO_4, \Delta} C_6H_5NO_2\text{ (Nitrobenzene)} + H_2O\n

Mechanism and Acid-Base Roles

1. Generation of the Nitronium Ion ($NO_2^+$):    Sulphuric acid is a stronger acid than nitric acid; it donates a proton to $HNO_3$. Thus, in the nitrating mixture, $H_2SO_4$ acts as a Brønsted acid and $HNO_3$ acts as a Brønsted base:    \n   H_2SO_4 + H-O-NO_2 \rightleftharpoons HSO_4^- + H_2\overset{+}{O}-NO_2\text{ (Protonated nitric acid)}\n       \n   H_2\overset{+}{O}-NO_2 \rightarrow NO_2^+\text{ (Nitronium ion, electrophile)} + H_2O\n   

2. Electrophilic Attack:    \n   C_6H_6 + NO_2^+ \rightarrow [C_6H_6NO_2]^+\text{ (}\sigma\text{-complex intermediate)}\n   

3. Proton Abstraction:    \n   [C_6H_6NO_2]^+ + HSO_4^- \rightarrow C_6H_5NO_2 + H_2SO_4\n   

Sulphonation

Heating benzene with oleum (fuming sulphuric acid, $H_2S_2O_7$, which represents $H_2SO_4 + SO_3$) yields benzenesulphonic acid:

\nC_6H_6 + H_2S_2O_7 \xrightarrow{\Delta} C_6H_5-SO_3H\text{ (Benzenesulphonic acid)}\n

• Active Electrophile: Neutral sulphur trioxide ($SO_3$), in which the central sulphur atom carries a strong partial positive charge ($\delta^+$) owing to the high electronegativity of the three attached oxygen atoms.

Friedel-Crafts Alkylation

Treating benzene with an alkyl halide ($R-X$) in the presence of an anhydrous Lewis acid catalyst ($AlCl_3$ or $FeBr_3$) forms alkylbenzene:

\nC_6H_6 + R-X \xrightarrow{\text{anhyd. } AlX_3} C_6H_5-R + HX\n \nC_6H_6 + CH_3Cl \xrightarrow{\text{anhyd. } AlCl_3} C_6H_5-CH_3\text{ (Toluene)} + HCl\n \nC_6H_6 + CH_3-CH_2-Cl \xrightarrow{AlCl_3} C_6H_5-CH_2-CH_3\text{ (Ethylbenzene)} + HCl\n

Carbocation Rearrangement During Alkylation

Whenever the alkyl group contains three or more carbon atoms, the primary carbocation initially generated can undergo skeletal rearrangement via 1,2-hydride or 1,2-methide shifts to form a more stable secondary or tertiary carbocation, leading to rearranged alkylbenzenes as the major products.

• Example with 1-Bromopropane:

  1. Coordination with the catalyst forms the primary carbocation:      \n     CH_3-CH_2-CH_2-Br + FeBr_3 \rightarrow CH_3-CH_2-\overset{+}{C}H_2\ (1^\circ) + [FeBr_4]^-\n     

  2. A rapid $1,2$-hydride shift produces the more stable secondary carbocation:      \n     CH_3-CH_2-\overset{+}{C}H_2 \xrightarrow{1,2-\text{H}^-\text{ shift}} CH_3-\overset{+}{C}H-CH_3\ (2^\circ)\n     

  3. Attack by the secondary carbocation gives isopropylbenzene (cumene) as the major product:      \n     C_6H_6 + CH_3-\overset{+}{C}H-CH_3 \rightarrow C_6H_5-CH(CH_3)_2\text{ (Isopropylbenzene / Cumene)}\n     

Friedel-Crafts Acylation

Heating benzene with an acyl chloride (e.g., acetyl chloride, $CH_3COCl$) or an acid anhydride (e.g., acetic anhydride, $(CH_3CO)_2O$) in the presence of anhydrous $AlCl_3$ produces an acylbenzene (ketone):

\nC_6H_6 + CH_3COCl \xrightarrow{\text{anhyd. } AlCl_3} C_6H_5-CO-CH_3\text{ (Acetophenone / Acetylbenzene)} + HCl\n \nC_6H_6 + (CH_3CO)_2O \xrightarrow{\text{anhyd. } AlCl_3} C_6H_5-CO-CH_3 + CH_3COOH\n

Acylation Mechanism and the Acylium Ion

1. Generation of the Acylium Ion:    \n   CH_3-CO-Cl + AlCl_3 \rightarrow CH_3-\overset{+}{C}=O + [AlCl_4]^-\n       The acylium ion is resonance-stabilized by lone-pair back-donation from oxygen, making it resistant to carbocation rearrangements:    \n   CH_3-\overset{+}{C}=O \longleftrightarrow CH_3-C \equiv \overset{+}{O}\n   

2. Attack on Benzene:    \n   C_6H_6 + CH_3-\overset{+}{C}=O \rightarrow [C_6H_6-CO-CH_3]^+\n   

3. Deprotonation:    \n   [C_6H_6-CO-CH_3]^+ + [AlCl_4]^- \rightarrow C_6H_5-CO-CH_3 + HCl + AlCl_3\n   

Exhaustive Chlorination (Hexachlorobenzene Formation)

In the presence of excess chlorine and anhydrous aluminium chloride in the dark, benzene undergoes exhaustive substitution of all six ring hydrogens to yield hexachlorobenzene:

\nC_6H_6 + 6Cl_2 \xrightarrow{\text{anhyd. } AlCl_3, \text{ dark}} C_6Cl_6\text{ (Hexachlorobenzene)} + 6HCl\n

Addition Reactions of Benzene

Under energetic or photochemical forcing conditions, benzene can be driven to undergo addition reactions, sacrificing its aromatic resonance stabilization:

Catalytic Hydrogenation

At elevated temperatures under high hydrogen pressure, benzene is fully hydrogenated to cyclohexane:

\nC_6H_6 + 3H_2 \xrightarrow{Ni, \Delta} C_6H_{12}\text{ (Cyclohexane)}\n

Photochemical Halogen Addition

Under intense ultraviolet ($h\nu$) radiation without Lewis acid catalysts, benzene adds three equivalents of chlorine via a free radical pathway to form benzene hexachloride (BHC / Lindane / Gammexane / 1,2,3,4,5,6-hexachlorocyclohexane):

\nC_6H_6 + 3Cl_2 \xrightarrow{h\nu} C_6H_6Cl_6\text{ (Benzene hexachloride / BHC)}\n