Alkanes

Question 1

What is the general formula for an alkane?

Answer 1

CnH2n+2

Question 2

Define sigma bond.

Answer 2

Lengthways overlap of orbitals between two bonding atoms.

Question 3

Properties of an alkane?

Answer 3

• All C-C and C-H bonds in alkanes are sigma bonds + can freely rotate
• Sigma are the strongest covalent bonds
• Alkanes unreactive due to strong bonds.
• Tetrahedral structure (109.5.)

Question 4

How does carbon chain length affect b.p of alkanes?

Answer 4

(All alkanes symmetrical so dipoles cancel out so non polar. LFs BM. Compare using no. of e- OR surface area contact points.)
- “Butane has more e- so more LFs BM so more energy required to break forces. Higher mp + bp.”

Question 5

How does branching affect boiling point of alkanes?

Answer 5

More branching so less surface area contact points - fewer LFs BM, easier to break. Bp decreases.

Question 6

How do you know whether to talk about chain length or branching when asked about alkane boiling points?

Answer 6

Branching when molecular formula is the same.

Question 7

Define homolytic fission.

Answer 7

A covalent bond is broken, and each of the bonding atoms received an electron, to form 2 radicals.

Question 8

Define free radical.

Answer 8

Reactive species due to the presence of an unpaired electron.

Question 9

Limitations of free radical substitution?

Answer 9

Yield is often low as:
1. further substitution/various terminations can occur forming a mixture of products,
2. substitution can occur in different positions forming position isomers.

Question 10

What conditions are needed for free radical substitution?

Answer 10

UV light.