12: Alkanes

Question 1

what is an alkane?

Answer 1

• saturated hydrocarbon
• contains single C-C and C-H bonds
• all bonds are σ-bonds w free rotation

Question 2

what is a σ-bond

Answer 2

• overlap of orbitals directly between bonding atoms

- has free rotation

Question 3

shape and BA in alkanes?

Answer 3

tetrahedral, 109.5˚

bonded pairs of electrons repel to be as far apart as possible

Question 4

how does chain length affect bp of alkanes

Answer 4

• increasing chain length increases surface points of contact between neighbouring molecules
• London Forces between molecules are now stronger
• more energy is required to overcome London Forces
  henceforth higher BP w LONGER CHAINS

Question 5

how does branching affect BP in alkanes

Answer 5

• more branching = fewer surface points of contact between molecules (than straight chains)
• weaker London Forces
• less E is required to overcome LF
  therefore, lower BP for BRANCHED ALKANES

Question 6

why are alkanes relatively unreactive?

Answer 6

• σ-bonds have a relatively high bond enthalpy
  bare E to overcome
• low polarity:
  due to C & H having similar electronegativities
  has no dipole
  can’t attract other molecules

Question 7

what are the dangers arise from incomplete combustion of alkanes?

Answer 7

• production of carbon monoxide CO a toxic gas

- production of C particulates responsible for global dimming

Question 8

why is it hard to obtain mono-substituted molecules from radical substitution?

Answer 8

• it is likely to form poly-substituted molecules due to chain reactions
• forms a mixture of organic products
• wastes reactants
• can’t really control the product you form

Question 9

what fission is involved in radical substituion of alkanes?

Answer 9

homolytic

eg Br2 -> 2 Br•