Chemistry: Class 112 — Chapter 3: Hydrocarbons

Alkanes exhibit structural isomerism, specifically chain isomerism, starting from butane ($C_4H_{10}$):

• Butane ($C_4H_{10}$): Has 2 isomers ($n$-butane and isobutane).
• Pentane ($C_5H_{12}$): Has 3 isomers ($n$-pentane, isopentane, and neopentane).
• Hexane ($C_6H_{14}$): Has 5 isomers.

Carbon atoms in alkanes are classified based on the number of other carbon atoms attached to them:

• Primary ($1^\circ$): Attached to no other or only one other 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.

1. From Unsaturated Hydrocarbons (Hydrogenation):

   Dihydrogen gas adds to alkenes and alkynes in the presence of finely divided catalysts like platinum, palladium, or nickel.

   $$CH_2=CH_2(g) + H_2(g) \xrightarrow{Pt/Pd/Ni} CH_3-CH_3(g)$$
2. From Alkyl Halides:
   • Reduction: Alkyl halides (except fluorides) are reduced with zinc and dilute hydrochloric acid.
     $$CH_3-Cl + H_2 \xrightarrow{Zn, H^+} CH_4 + HCl$$
   • Wurtz Reaction: Alkyl halides in dry ethereal solution react with sodium to give higher alkanes containing an even number of carbon atoms.
     $$2CH_3-Br + 2Na \xrightarrow{\text{dry ether}} CH_3-CH_3 + 2NaBr$$
3. From Carboxylic Acids:
   • Decarboxylation: Sodium salts of carboxylic acids heated with soda lime ($NaOH$ and $CaO$) yield alkanes with one less carbon atom.
     $$CH_3COONa + NaOH \xrightarrow[\Delta]{CaO} CH_4 + Na_2CO_3$$
   • Kolbe's Electrolytic Method: Electrolysis of an aqueous solution of sodium or potassium salt of a carboxylic acid gives alkanes containing an even number of carbon atoms at the anode.
     $$2CH_3COONa + 2H_2O \xrightarrow{\text{Electrolysis}} CH_3-CH_3 + 2CO_2 + H_2 + 2NaOH$$

Physical Properties: Alkanes are non-polar, colourless, and odourless. Boiling points increase regularly with molecular mass due to increasing van der Waals forces. Branching decreases the boiling point by reducing the surface area of contact.

Chemical Properties:

• Substitution Reactions (Halogenation): Occurs via a free radical chain mechanism (Initiation, Propagation, and Termination) in the presence of ultraviolet light or high temperature.
  $$CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl$$
• Combustion: Complete oxidation to carbon dioxide and water with the evolution of large amounts of heat.
  $$C_nH_{2n+2} + \left(\frac{3n+1}{2}\right)O_2 \rightarrow nCO_2 + (n+1)H_2O$$
• Controlled Oxidation: Yields different products depending on catalysts and conditions (e.g., copper at 523 K/100 atm yields methanol; molybdenum oxide yields methanal).
• Isomerisation: $n$-Alkanes isomerise to branched-chain alkanes when heated with anhydrous $AlCl_3$ and $HCl$ gas.
• Aromatization: $n$-Alkanes with six or more carbon atoms cyclise and dehydrogenate to benzene or its derivatives at 773 K and 10-20 atm in the presence of oxides of vanadium, molybdenum, or chromium.
• Pyrolysis (Cracking): Decomposition of higher alkanes into lower alkanes and alkenes upon heating.

Rotation about the carbon-carbon single bond ($\sigma$-bond) in ethane results in different spatial arrangements called conformations or rotamers. The two extreme conformations are:

• Eclipsed Conformation: Hydrogen atoms on adjacent carbons are as close together as possible, resulting in maximum torsional strain and minimum stability.
• Staggered Conformation: Hydrogen atoms are as far apart as possible, resulting in minimum torsional strain and maximum stability.

These conformations are represented using Sawhorse projections and Newman projections. The energy barrier between the two forms is about $12.5\text{ kJ mol}^{-1}$.

The carbon-carbon double bond consists of one strong $\sigma$-bond (formed by head-on overlap of $sp^2$ hybridised orbitals) and one weak $\pi$-bond (formed by lateral overlap of unhybridised $2p$ orbitals).

Alkenes exhibit both structural isomerism (chain and position) and geometrical isomerism (*cis-trans* isomerism):

• *cis*-Isomer: Two identical atoms or groups lie on the same side of the double bond. It is generally more polar and has a higher boiling point.
• *trans*-Isomer: Two identical atoms or groups lie on opposite sides of the double bond. It is non-polar or less polar and has a higher melting point due to symmetry.

1. From Alkynes: Partial reduction with calculated dihydrogen in the presence of palladised charcoal deactivated with quinoline (Lindlar's catalyst) yields *cis*-alkenes. Reduction with sodium in liquid ammonia yields *trans*-alkenes.
2. From Alkyl Halides (Dehydrohalogenation): Heating alkyl halides with alcoholic potash ($KOH$) undergoes $\beta$-elimination.
   $$CH_3-CH_2-X + KOH(\text{alc.}) \xrightarrow{\Delta} CH_2=CH_2 + KX + H_2O$$
3. From Vicinal Dihalides (Dehalogenation): Treatment of vicinal dihalides with zinc dust.
   $$CH_2Br-CH_2Br + Zn \rightarrow CH_2=CH_2 + ZnBr_2$$
4. From Alcohols (Acidic Dehydration): Heating alcohols with concentrated sulphuric acid.
   $$CH_3-CH_2-OH \xrightarrow[\Delta]{\text{Conc. } H_2SO_4} CH_2=CH_2 + H_2O$$

Alkenes primarily undergo electrophilic addition reactions:

• Addition of Dihydrogen: Yields alkanes in the presence of $Ni/Pt/Pd$.
• Addition of Halogens: Addition of bromine water serves as a test for unsaturation (decolourisation of reddish-orange bromine solution).
• Addition of Hydrogen Halides:
  • Markovnikov's Rule: In the addition of unsymmetrical reagents to unsymmetrical alkenes, the negative part of the addendum gets attached to the carbon atom possessing fewer hydrogen atoms.
  • Anti-Markovnikov / Peroxide Effect (Kharash Effect): In the presence of organic peroxides, the addition of $HBr$ (only) to unsymmetrical alkenes occurs contrary to Markovnikov's rule, yielding 1-bromoalkanes via a free radical mechanism.
• Addition of Water: Acid-catalysed hydration yields alcohols in accordance with Markovnikov's rule.
• Oxidation: Cold, dilute, aqueous potassium permanganate (Baeyer's reagent) oxidises alkenes to vicinal glycols. Acidic $KMnO_4$ cleaves alkenes to ketones and/or carboxylic acids.
• Ozonolysis: Addition of ozone followed by cleavage with $Zn$ and $H_2O$ yields aldehydes and/or ketones, which is highly useful in locating the position of double bonds.
• Polymerisation: Alkenes polymerise under high temperature and pressure in the presence of a catalyst (e.g., polythene from ethene).

Each carbon in a triple bond is $sp$ hybridised, forming one $\sigma$-bond along the internuclear axis and two mutually perpendicular $\pi$-bonds by lateral overlap of unhybridised $p$ orbitals. The molecule is linear with a bond angle of $180^\circ$.

1. From Calcium Carbide: Industrial preparation by treating calcium carbide with water.
   $$CaC_2 + 2H_2O \rightarrow Ca(OH)_2 + C_2H_2$$
2. From Vicinal Dihalides: Double dehydrohalogenation using alcoholic $KOH$ followed by sodamide ($NaNH_2$).

• Acidic Character: Hydrogen atoms attached to $sp$ hybridised carbons in terminal alkynes are weakly acidic and can react with strong bases like sodium metal or sodamide to form sodium acetylides.
• Addition Reactions: Alkynes add two molecules of dihydrogen, halogens, or hydrogen halides in a stepwise manner.
• Addition of Water: Hydration in the presence of mercuric sulphate ($Hg^{2+}$) and dilute sulphuric acid at 333 K yields carbonyl compounds via tautomerisation.
  $$HC\equiv CH + H_2O \xrightarrow{Hg^{2+}/H^+} [CH_2=CH-OH] \rightarrow CH_3-CHO$$
• Polymerisation:
  • Linear: Yields polyacetylene (conducts electricity under special conditions).
  • Cyclic: Passing ethyne through a red-hot iron tube at 873 K causes cyclic polymerisation to form benzene.
    $$3C_2H_2 \xrightarrow{\text{Red hot Fe, 873 K}} C_6H_6$$

Benzene ($C_6H_6$) is a planar ring of six $sp^2$ hybridised carbon atoms. The delocalised $\pi$-electron cloud lies above and below the plane of the ring, making all C-C bond lengths equal ($139\text{ pm}$).

Hückel Rule of Aromaticity: For a cyclic, planar, conjugated system to be aromatic, it must contain $(4n+2)\pi$ electrons, where $n$ is an integer ($n = 0, 1, 2, \dots$).

1. Cyclic Polymerisation of Ethyne: (See section 9.4.3).
2. Decarboxylation of Sodium Benzoate: Heating sodium benzoate with soda lime.
   $$C_6H_5COONa + NaOH \xrightarrow[\Delta]{CaO} C_6H_6 + Na_2CO_3$$
3. Reduction of Phenol: Passing phenol vapours over heated zinc dust.
   $$C_6H_5OH + Zn \xrightarrow{\Delta} C_6H_6 + ZnO$$

Benzene undergoes electrophilic aromatic substitution reactions:

• Nitration: Heating with a mixture of concentrated nitric acid and concentrated sulphuric acid (nitrating mixture) to introduce a nitro group ($-NO_2$).
• Halogenation: Reaction with chlorine or bromine in the presence of a Lewis acid catalyst (e.g., anhydrous $FeCl_3$, $AlCl_3$) to yield halobenzenes.
• Sulphonation: Heating with fuming sulphuric acid (oleum) to introduce a sulphonic acid group ($-SO_3H$).
• Friedel-Crafts Alkylation: Treatment with an alkyl halide in the presence of anhydrous $AlCl_3$ to form alkylbenzenes.
• Friedel-Crafts Acylation: Treatment with an acyl halide or acid anhydride in the presence of anhydrous $AlCl_3$ to form acylbenzenes.

Mechanism of Electrophilic Substitution: Proceeds via three steps: (a) Generation of the electrophile ($E^+$), (b) Formation of a carbocation intermediate (sigma complex/arenium ion) stabilised by resonance, and (c) Removal of a proton to restore aromaticity.

When monosubstituted benzene undergoes further substitution, the existing substituent directs the incoming group:

• Ortho and Para Directing Groups: Electron-donating groups (e.g., $-OH$, $-NH_2$, $-OCH_3$, $-CH_3$) increase electron density at the *ortho* and *para* positions via resonance, activating the ring. Halogens are deactivating due to strong $-I$ effect but are *ortho/para* directing due to resonance.
• Meta Directing Groups: Electron-withdrawing groups (e.g., $-NO_2$, $-CN$, $-CHO$, $-COOH$, $-SO_3H$) decrease electron density on the ring, particularly at *ortho* and *para* positions, directing the incoming electrophile to the relatively richer *meta* position.