12 - Alkanes

Question 1

Sigma bonds

Answer 1

Overlap of orbitals directly between the bonding atoms

Question 2

Shape & angle of Alkanes

Answer 2

Electron pair repulsion - tetrahedral shape, 109.5 degrees

Question 3

What does boiling point of Alkanes depend on and why?

Answer 3

1) chain length
Inc chain length = larger SA= more surface points of contact = more and stronger London forces between molecules = more energy to overcome = higher bp

2) branching
no branching = more surface points of contact & can pack together closely = more and stronger London forces = more energy to overcome = higher bp

Question 4

Why are Alkanes not very reactive

Answer 4

High bond enthalpy of sigma bonds

Very low polarity of sigma bonds present (C-C non polar, C-H pretty much non polar)

Question 5

Dangers of CO?

Answer 5

Odourless, colourless, highly toxic

Combines irreversibly with haemoglobin in RBCs to form carboxyhaemoglobin.

Question 6

Limitations of radical substitution in synthesis

Answer 6

• further substitution: radicals can keep reacting with the products, forming mixture of organic products
• reactions at different positions in a carbon chain: for methane, all 4 hydrogens bonded to same C atom, so only 1 monobromo compound, CH3Br. Same with ethane, C2H5Br. But if carbon chain is longer, get mixture of mono substituted isomers by substitution at different positions in the carbon chain. Eg pentane could form 3 monosubstituted isomers