Alkanes,Alkenes and hydrocarbons

• Alkanes

  Saturated hydrocarbons
• Shape around carbon atom in saturated hydrocarbons


  • Tetrahedral
  • Bond angle 109.5°
• The shape is tetrahedral as a result of the four bond pairs of electrons equally repelling
• Sigma (σ) bond

  Single C-C bond formed by overlap of sp3 orbitals
• Rotation can occur around a sigma bond
• Reactivity of alkanes

  • Low reactivity due to high bond enthalpies of C-C and C-H bonds and low polarity of σ-bonds
• Cracking

  1. Conversion of large hydrocarbons to smaller molecules by breakage of C-C bonds

  2. Requires high temperatures
• Cracking makes use of excess larger hydrocarbons and supplies demand for shorter ones
• The products of cracking are more valuable than the starting materials
• Complete combustion of alkanes

  C8H18(g) + 12.5 O2(g) → 8CO2(g) + 9 H2O(l)
• Incomplete combustion of alkanes

  1. CH4(g) + 3/2 O2(g) → CO(g) + 2 H2O(l)

  2. CH4(g) + O2(g) → C(s) + 2 H2O(l)
• Carbon monoxide is highly toxic as it forms a stronger bond with haemoglobin than oxygen
• Catalytic cracking

  1. Turns straight chain alkanes into branched/cyclic alkanes and aromatic hydrocarbons

  2. Conditions: Slight pressure, High Temperature (450°C), Zeolite Catalyst
• Branched and cyclic hydrocarbons burn more cleanly and have higher octane numbers
• Substitution reactions of alkanes with chlorine/bromine

  Alkane + Cl2/Br2 (UV light) → Haloalkane + HCl/HBr
• Mechanism of alkane substitution with chlorine

  1. Initiation: Cl2 → 2Cl·

  2. Propagation: CH4 + Cl· → CH3· + HCl, CH3· + Cl2 → CH3Cl + Cl·

  3. Termination: CH3· + Cl· → CH3Cl, CH3· + CH3· → CH3CH3
• The same mechanism applies to substitution with bromine
• Further substitution of haloalkanes

  1. CH3Cl + Cl2 → CH2Cl2 + HCl

  2. CH2Cl2 + Cl2 → CHCl3 + HCl

  3. CHCl3 + Cl2 → CCl4 + HCl
• HCl is always the side product, never H2
• Overall reaction equations


  • CH4 + 4 Cl2 → CCl4 + 4 HCl
  • CH3F + 3 Cl2 → CFCl3 + 3 HCl
• Alkenes

  Unsaturated hydrocarbons
• Alkenes

  • Contain a carbon-carbon double bond somewhere in their structure
• Alkenes


  • Ethene
  • Propene

  
• Numbers need to be added to the name when positional isomers can occur
• C=C double covalent bond

  Consists of one sigma (σ) bond and one pi (π) bond
• π bonds

  • Are exposed and have high electron density
  • They are therefore vulnerable to attack by species which 'like' electrons: these species are called electrophiles
• Formation of π bond in alkenes

  Sideways overlap of two p orbitals on each carbon atom forming a π-bond above and below the plane of molecule
• π bond

  • Is weaker than the σ bond
• Arrangement of bonds around >C=C<

  • Planar and has the bond angle 120o
• Formation of σ bond in alkenes

  Two sp2 orbitals (one from each carbon) overlap to form a single C-C bond called a sigma σ bond
• Stereoisomers

  Have the same structural formulae but have a different spatial arrangement of atoms
• 1. Z stereoisomerism

  Alkenes can exhibit this type of isomerism due to restricted rotation about the C=C bond

  
• But-1-ene is a structural isomer of But-2-ene but does not show E-Z isomerism
• Naming E-Z stereoisomers


  1. Determine the priority groups on both sides of the double bond

  2. If the priority atom is on the same side of the double bond it is labelled Z

  3. If the priority atom is on the opposite side of the double bond it is labelled E
• Cis-trans isomerism


  A special case of E-Z isomerism in which two of the substituent groups are the same
• 1. 1,2-dichloroethene


  Polar molecule with polar C-Cl bonds on the same side, higher boiling point due to dipole-dipole attractions

  
• 1. 1,2-dichloroethene


  Non-polar molecule with polar C-Cl bonds on opposite sides, lower boiling point due to only London forces
• Alkenes


  • Relatively reactive because of the relatively low bond enthalpy of the π-bond
• Addition reaction of alkenes with hydrogen

  Alkene + H2 → Alkane
• Electrophilic addition reaction of alkenes with bromine/chlorine

  Alkene + Br2 → Dihaloalkane