4.2 Alkanes

• What are alkanes made up of?

  Saturated hydrocarbons of hydrogen and carbon
• What type of bonds join alkanes?

  Sigma (σ) bonds
• What is the general formula for alkanes?

  C_nH_2n+2
• Why are C-H bonds in alkanes considered non-polar?

  Electronegativities of carbon and hydrogen are similar

  
• What does the non-polar nature of alkanes imply about their reactivity?

  They do not react with polar reagents

  
• What areas do alkanes lack that affects their reactivity?

  No electron-deficient or electron-rich areas

  
• How many covalent bonds does each carbon atom in an alkane form?

  Four covalent bonds
• What is the bond angle in alkanes?

  109.5 degrees

  
• What type of intermolecular forces do alkanes exhibit?

  Weak temporary induced dipole-dipole forces
• How does the boiling point of alkanes change with bond length?

  Increases as bond length increases
• What is the shape around each carbon atom in alkanes?

  Tetrahedral shape

  
• What type of electron pairs do alkanes have?

  Only bonding pairs of electrons

  
• What is the order of repulsion among electron pairs?

  Lone pair-lone pair > lone pair-bond pair > bond pair-bond pair
• What factors affect the boiling points of alkanes?

  • Chain length
  • Branching

  
• How does chain length affect the boiling point of alkanes?

  Boiling point increases with molecular size
• How does branching affect the boiling point of alkanes?

  Branched alkanes have lower boiling points
• Why do unbranched alkanes have greater van der Waals' forces?

  Greater surface areas allow more attraction
• What explains the unreactive nature of alkanes?

  High bond enthalpies and low polarity

  
• What is the bond enthalpy of the C-C bond?

  346 kJ/mol
• What is the bond enthalpy of the C-H bond?

  411 kJ/mol
• Why is the C-H bond stronger than the C-C bond?

  C-H bond length is shorter
• What happens during complete combustion of alkanes?

  Carbon and hydrogen oxidized to CO₂ and H₂O

  
• What occurs during incomplete combustion of alkanes?

  Carbon partially oxidized to carbon monoxide

  
• Where does incomplete combustion often occur?

  Inside a car engine
• What are common pollutants in car exhaust fumes?

  CO, NO/NO₂, and VOCs
• What are the effects of carbon monoxide on health?

  Causes dizziness and loss of consciousness
• How does carbon monoxide affect hemoglobin?

  It binds to hemoglobin, preventing oxygen transport

  
• What is required for free radical substitution of alkanes?

  Ultraviolet light (sunlight)
• What are the steps of free radical substitution of alkanes?

  1. Initiation
  2. Propagation
  3. Termination
• What occurs during the initiation step of free radical substitution?

  Halogen bond breaks to form radicals
• What happens during the propagation step?

  Radicals attack alkanes, forming new radicals
• What is the termination step in free radical substitution?

  Two radicals collide to form a stable molecule
• Why is free radical substitution not suitable for specific halogenoalkanes?

  Produces a mixture of substitution products
• What can happen if excess chlorine is present during substitution?

  Further substitution can occur
• What is a limitation of free radical substitution regarding impurities?

  Impurities can form during termination
• How does substitution occur on different carbon atoms in alkanes?

  Substitution can occur on any carbon atom
• What is an important condition for free radical substitution besides UV light?

  High temperature of 400 °C