Chapter 12

• What is the main use of alkanes?

  fuels, as they make up natural gas and crude oil
• What is the general formula of alkanes?

  CnH2n+2
• Describe the bonding in alkanes

  they are saturated hydrocarbons with only single bonds between carbon atoms. Each carbon atom forms 4 sigma bonds

  
• How does a sigma bond form?

  the overlap of 2 orbitals, one from each bonding atom. Each overlapping orbital contains one electron, resulting in 2 electrons shared between bonding atoms

  
• What is the shape and bond angle of alkanes?

  tetrahedral shape around each carbon atom, with a bond angle of 109.5
• Explain the trend in boiling points of the alkanes

  it increases with chain length. The larger chain length molecules have more surface contact between molecules, resulting in greater induced dipole-dipole interactions which require more energy to overcome
• How is fractional distillation made possible?

  the variations in boiling points of alkanes
• Why do highly branched alkanes have lower boiling points?

  there are fewer surface points of contact between molecules, meaning fewer induced dipole-dipole interactions which are easily overcome. Molecules can't get as close together, so the intermolecular forces decrease
• Why are alkanes relatively unreactive?

  - the C-C and C-H sigma bonds are strong

  - the C-C bonds are non-polar

  - the electronegativity of C and H are similar, so C-H is considered non-polar
• What are the products of complete combustion in alkanes?

  carbon dioxide and water
• What products are formed from the incomplete combustion of alkanes?

  carbon monoxide, carbon soot and water
• Why is carbon monoxide dangerous?

  It is colourless, odourless, and tasteless and can be fatal. It is a toxic gas
• What conditions are needed for substitution reactions of alkanes with halogens?

  ultraviolet (UV) radiation
• Explain the mechanism of bromination of alkanes

  -it is an example of free radical substitution

  1) Initiation- the covalent bond in a bromine radical is broken by homolytic fission, with the energy provided by UV radiation

  2) Propagation- a continuous chain reaction where radicals react with molecule until all the reactants are used up, or 2 radicals collide

  3) Termination- 2 radicals collide to form a molecule with all electrons paired, stopping the reaction
• What are the limitations of radical substitution in organic synthesis?

  -further substitution= can continue until all hydrogen atoms have been substituted

  -substitution at different positions in the carbon chain= if the carbon chain is 3 carbons or longer, we will get a mixture of monosubstituted isomers by substituting at different positions in the carbon chain