Alkanes Essentials and Reactions

Overview

This lecture covers the alkane portion of hydrocarbons in organic chemistry. Alkanes are saturated hydrocarbons with single bonds between carbon atoms, following the general formula CnH2n+2. The session emphasizes understanding reaction mechanisms and nomenclature as foundations for organic chemistry.

Classification of Hydrocarbons

• Aliphatic compounds: Open-chain hydrocarbons including alkanes, alkenes, and alkynes; carbon atoms form straight or branched chains without closure
• Alicyclic compounds: Closed-ring structures like cycloalkane, cycloalkene, and cycloalkyne; rings without alternating double bonds
• Aromatic compounds: Closed-ring structures with delocalized pi electrons or alternating double bonds at alternate positions; benzene derivatives; show characteristic smell
• Saturated hydrocarbons: Contain only single bonds between carbon atoms (alkanes)
• Unsaturated hydrocarbons: Contain double or triple bonds between carbon atoms (alkenes and alkynes)

Alkanes - Basic Properties

• Definition: Hydrocarbons with single bonds between carbon atoms; also called paraffins
• General formula: CnH2n+2, where n = number of carbon atoms
• Composition: Made only of carbon and hydrogen atoms
• Bond type: Sigma (σ) bonds only; pi (π) bonds are absent

Degree of Carbon Atoms

• Primary (1°): Carbon bonded to one other carbon atom
• Secondary (2°): Carbon bonded to two other carbon atoms
• Tertiary (3°): Carbon bonded to three other carbon atoms
• Quaternary (4°): Carbon bonded to four other carbon atoms
• The degree depends on how many carbons are directly attached to the carbon in question

Alkyl Groups

• Formed by removing one hydrogen from any alkane
• Methyl (CH3): From methane (CH4)
• Ethyl (C2H5): From ethane (C2H6)
• Propyl (C3H7): From propane (C3H8)
• Pattern continues for longer chains

Nomenclature Systems

• Trivial system: Older naming based on appearance, action, or smell; examples include aromatic compounds named for characteristic aroma
• IUPAC system: International Union of Pure and Applied Chemistry; systematic naming based on rules
• IUPAC order: Write substituent name, then root word (meth, eth, prop, but, pent, hex, hept, oct, non, dec), then bond type (ane for single bonds), then functional group
• Numbering priority: Functional group gets highest priority, followed by bonds, then substituents
• Root words correspond to number of carbons: 1=meth, 2=eth, 3=prop, 4=but, 5=pent, 6=hex, 7=hept, 8=oct, 9=non, 10=dec

Preparation of Alkanes

• Wurtz reaction limitation: Cannot prepare alkanes with odd number of carbons; methane cannot be prepared
• Fluoroalkanes: Avoid using fluorine in reduction reactions due to very strong C-F bond; only two reactions in entire 12th grade chemistry use fluorine
• Kolbe's mechanism: Sodium salt dissociates; anions migrate to anode (positive rod); free radicals form; two radicals combine to form alkane

Physical Properties of Alkanes

• Color: Colorless
• Odor: Odorless
• Polarity: Nonpolar covalent compounds; carbon and hydrogen have minimal electronegativity difference
• Intermolecular forces: Weak van der Waals forces of attraction
• Physical state: C1-C4 are gases; C5-C17 are liquids; >C17 are solids
• Boiling point: Increases with molecular mass; inversely related to branching (more branches = lower boiling point due to decreased surface area and weaker van der Waals forces)
• Melting point: Depends on molecular mass and molecular packing in lattice structure; no smooth increase with mass
• Solubility: "Like dissolves like"; nonpolar alkanes dissolve in nonpolar solvents; insoluble in water (polar solvent); oil does not mix with water
• Density: Increases with increasing molecular mass; becomes constant around 0.8 at higher molecular weights

Chemical Properties of Alkanes

Substitution Reactions (Halogenation)

• Hydrogen atoms replaced by halogen atoms in presence of sunlight or heat
• Chlorination: CH4 + Cl2 → CH3Cl + HCl; reaction continues until all hydrogens replaced (CCl4)
• Bromination: Slower than chlorination but follows same mechanism
• Iodination: Requires strong oxidizing agent (HNO3) because HI is strong reducing agent; reaction becomes reversible otherwise
• Fluorination: Extremely violent and uncontrollable; carbon-fluorine bond is very strong due to small atomic size and low electronegativity difference
• Rate of replacement: 3° > 2° > 1° > methyl; tertiary carbons most reactive due to more stable free radical intermediates (+I effect from alkyl groups)
• Product: CHCl3 (chloroform) formed from methane

Combustion Reactions

• Complete combustion: Alkane + O2 → CO2 + H2O + heat + light; exothermic reaction
• Incomplete combustion: Alkane + insufficient O2 → C (carbon black) + H2O; used in ink manufacture
• Controlled combustion: Three possible products depending on catalyst:
  • With Cu catalyst (523K, 100 atm): Forms alcohols (e.g., 2CH4 + O2 → 2CH3OH)
  • With Mo2O3: Forms aldehydes (e.g., 2CH4 + O2 → 2HCHO + H2O)
  • With Mn carboxylate: Forms carboxylic acids

Isomerization

• Transformation of n-alkane (straight chain) to branched alkane
• Conditions: Anhydrous AlCl3 + HCl gas
• Same molecular formula but different structural arrangement
• Example: n-hexane → isohexane (branched form)

Aromatization

• Conversion of aliphatic or alicyclic hydrocarbons to aromatic hydrocarbons
• Conditions: Cr2O3 or V2O5 (vanadium pentoxide) or MoO2; 773K; 10-20 atm pressure
• Forms benzene ring with delocalized pi electrons or alternating double bonds
• Example: n-hexane → benzene

Reaction with Steam

• CH4 + H2O (steam) → CO + 2H2 (in presence of Ni catalyst at high temperature)
• Industrial method for preparing dihydrogen (H2) gas
• Produces synthesis gas (CO + H2)

Pyrolysis

• Thermal decomposition of alkanes in absence of air at high temperature (773K)
• Compound breaks into smaller fragments
• Example: C6H14 → C6H12 (hexene) + H2, or C3H6 + C2H4 + CH4

Conformations of Alkanes

• Definition: Different spatial arrangements of atoms achieved by rotating around carbon-carbon single bonds
• Eclipsed conformation: Hydrogen atoms on adjacent carbons positioned as close as possible
• Staggered conformation: Hydrogen atoms on adjacent carbons positioned as far apart as possible
• Skew conformations: Intermediate structures between eclipsed and staggered
• Representation methods:
  • Sawhorse projection: Shows two carbons with distance line and attached groups
  • Newman projection: Front carbon shown as dot; rear carbon shown as circle; attached groups drawn from each

Key Terms & Definitions

• Paraffins: Another name for alkanes due to low reactivity
• Hydrogenation: Addition of hydrogen to unsaturated compounds
• Decarboxylation: Removal of carbon dioxide from carboxylic acid salts
• Soda lime: Mixture of NaOH and CaO (calcium oxide/quick lime)
• Free radical: Species with unpaired electron; acts as intermediate in reactions
• Electrolysis: Breakdown of compounds using electrical current; anode (positive rod) and cathode (negative rod)
• Sigma bond: Single covalent bond; stronger than pi bond
• Pi bond: Found in double/triple bonds; weaker than sigma bond; breaks more easily

Action Items / Next Steps

• Practice IUPAC nomenclature with various alkane structures
• Study alkenes and alkynes in the follow-up video
• Memorize preparation methods and their specific conditions
• Understand reaction mechanisms rather than memorizing equations
• Focus on organic chemistry fundamentals established in this alkane chapter for success in grade 12