Alkanes, Alkenes, Alkynes

• Alkanes


  Saturated organic compounds containing only carbon-carbon and carbon-hydrogen single bonds
• Alkanes


  • Contain strong carbon-carbon and strong carbon-hydrogen single bonds
  • Are non-polar and therefore do not react with polar reagents
• Reactions of alkanes


  • Combustion
  • Cracking
  • Halogenation
• Combustion of alkanes


  Alkane reacts with oxygen to produce carbon dioxide and water
• Combustion of alkanes is an exothermic reaction
• Cracking of alkanes


  Petroleum fractions with 1 to 12 carbon atoms are produced from high molar mass alkanes
• Halogenation of alkanes


  Alkane reacts with halogen in the presence of light to substitute hydrogen atoms
• Free radical


  Atom or group of atoms with a single unpaired electron
• Mechanism of chlorination of methane

  1. Chain initiation

  2. Chain propagation

  3. Chain termination
• Cycloalkanes are alkanes that contain a ring of three or more carbons
• Cycloalkanes


  General molecular formula CnH2n, contain two hydrogen atoms less than the corresponding alkane
• Angle strain


  Distortion of bond angles from the tetrahedral value in cyclic compounds
• Torsion strain


  Strain caused by adjacent pairs of bonds being eclipsed in cyclic compounds
• Cyclopropane


  • Has a bond angle of 60°, making it a planar molecule with high angle and torsion strain
• Cyclobutane


  • Has a bond angle of 90°, adopts a non-planar "puckered" conformation to reduce torsion strain
• Cyclopentane


  • Has a bond angle of 108°, very close to the tetrahedral 109.5° and is very stable
• Cyclohexane


  • Can exist in chair and boat conformations to reduce torsion strain
• Conformational isomers are different arrangements of atoms that are interconverted by rotation about single bonds
• Cis-but-2-ene


  Isomer with higher boiling point
• Trans-but-2-ene


  Isomer with lower boiling point
• Alkenes


  Unsaturated hydrocarbons
• Carbon atom configuration in alkenes


  • Uses a 2s orbital and two 2p orbitals to form sp2 hybridised orbitals, resulting in σ bonds
  • The unhybridised p orbital overlaps with adjacent p orbitals to form a π bond

  
• Delocalised electrons


  Electrons able to move freely around a conjugated system of alternating single and double bonds
• Preparation of alkenes


  1. Dehydration of alcohols

  2. Dehydrohalogenation of halogenoalkanes
• Dehydration of alcohols can be done by passing alcohol vapour over heated aluminium oxide catalyst or using acid catalysts like concentrated phosphoric or sulfuric acid
• Dehydrohalogenation of halogenoalkanes involves refluxing with a strong base like potassium hydroxide in alcohol
• Alcohols dehydrated to form alkenes

  • C2H5OH -> C2H4 + H2O

  2. C6H11OH -> C6H10 + H2O
• Bond energy of C=C double bond is less than twice the bond energy of C-C single bond
• Reactivity of alkenes


  More reactive than alkanes due to the weak π bond, allowing addition reactions
• Electrophilic addition reactions of alkenes


  1. Electrophile attacks the π bond

  2. Carbocation intermediate formed

  3. Nucleophile (e.g. halide ion) attacks carbocation
• Alkenes burn in oxygen to form carbon dioxide, water and heat
• Alkenes burn with a yellow, sooty flame due to incomplete combustion from the double bond
• Ethene


  Molecule with a carbon-carbon double bond
• Electrophilic addition to ethene

  1. Electrons in π bond move towards X

  2. Dative covalent bond formed between Y and carbon

  3. Carbocation produced
• Carbocation


  Positively charged carbon atom
• Hydrogenation of alkenes


  1. Addition of hydrogen gas (H2)

  2. Using nickel catalyst at 150°C and 4 atm

  3. Produces corresponding alkane
• Hydrogenation uses


  • Converting unsaturated plant oils into edible fats
  • Controlling softness of margarine
• Halogenation of alkenes


  1. Addition of halogen (e.g. bromine)

  2. Reaction occurs in organic solvent at room temperature
• Halogenation is used as a test for unsaturation to distinguish alkanes and alkenes
• Reaction of alkenes with bromine water

  Produces 2-bromoethanol, not 1,2-dibromoethane