Alkanes

• What is an alkane?

  A saturated hydrocarbon containing C-H bonds only
• What is the general formula of an alkane?

  CnH$2n+$$2$
• Are alkane bonds polar? Why or why not?

  No, they are nonpolar because carbon and hydrogen have similar electronegativities
• What is the shape and angle of an alkane?

  The shape is tetrahedral with an angle of 109.5°
• Describe the sigma bond in alkanes.

  The sigma bond is a covalent bond with a direct overlap of the electron clouds of the bonding atoms
• What type of intermolecular forces do alkanes have? Why?

  London forces due to induced dipole-dipole interactions, because the bonds are nonpolar
• What happens to the boiling point as alkane chain length increases? Why?

  The boiling point increases due to more surface area and more induced dipole-dipole interactions, requiring more energy to overcome the attraction
• Does a branched molecule have a lower or higher boiling point compared to an equivalent straight chain? Why?

  A branched molecule has a lower boiling point because it has a smaller surface area and fewer induced dipole-dipole interactions
• How reactive are alkanes?

  Alkanes are very unreactive
• Are alkanes soluble in water? Explain your answer.

  No, they are insoluble because hydrogen bonds in water are stronger than the London forces of attraction in alkanes
• What reactions will alkanes undergo?

  Alkanes will undergo combustion and reaction with halogens
• What type of reaction is combustion?

  Combustion is an oxidation reaction
• What is complete combustion?

  Complete combustion occurs with a plentiful supply of air
• What are the products of complete combustion when alkanes are used?

  The products are carbon dioxide and water
• What is the colour of the Bunsen burner flame during complete combustion?

  The flame is blue
• What is incomplete combustion and what products are formed in the case of alkanes?

  Incomplete combustion occurs in a limited supply of oxygen, producing water, carbon dioxide, and carbon monoxide
• Write an equation for the complete combustion of propane.

  C$3H8$ + 5O$2$ → 3CO$2$ + 4H$2O$
• Which type of hydrocarbon is most likely to undergo incomplete combustion?

  Longer chain hydrocarbons are most likely to undergo incomplete combustion
• What is the environmental impact of carbon monoxide?

  Carbon monoxide is toxic and poisonous
• What is the environmental impact of soot (carbon)?

  Soot can cause asthma, cancer, and global dimming
• How are halogenoalkanes formed from alkanes?

  Halogenoalkanes are formed through radical substitution
• In the presence of what do alkanes react with halogens?

  Alkanes react with halogens in the presence of UV light
• What are the three stages of free radical substitution?

  Initiation, propagation, and termination
• What are the three stages of free radical substitution in detail?

  • Initiation: Breaking halogen bond to form free radicals
  • Propagation: Chain part of the reaction where products are formed but free radical remains
  • Termination: Free radicals are removed, stable products are formed
• Write equations for the reaction of CH$4$ with Cl$2$ to form CH$3Cl$.


  Initiation: Cl$2$ → 2Cl• (in presence of UV light); Propagation: Cl• + CH$4$ → HCl + •CH$3$; •CH$3$ + Cl$2$ → CH$3Cl$ + Cl•; Termination: •CH$3$ + Cl• → CH$3Cl$; 2Cl• → Cl$2$; •CH$3$ + •CH$3$ → CH$3CH3$