Alkanes

Question 1

Steps for radical substitution (6)

Answer 1

1. ) initiation;
   - covalent bond of halogen broken - using UV
   - to form 2 radicals
2. ) propagation;
   - radical halogen reacts with alkyl to form radical alkyl and HHalide
   - Radical alkyl reacts with halogen to form haloalkane and radical

3.) termination;
Radicals combine

Question 2

Why do alkanes not react with common reagents (3)

Answer 2

• high bond enthalpy for C-C and C-H due to strong sigma bonds
• do not react with electrophiles and nucleophiles
• The C-H bond has very little polarity.

Question 3

Describe bonding of alkanes

Answer 3

• Single covalent bonds between C-C
• Each carbon is bonded to 4 other atoms in covalent bond
• C-H bond is sigma bond; orbitals overlap between each bonded atom

Question 4

Describe the shape of alkanes

Answer 4

• Shape around Carbon is tetrahedral
• 4 BP and 0LP
• All the electrons repel equally so the angle is 109.5.

Question 5

How does the length of a chain affects its boiling point. (3)

Answer 5

• Greater length = greater boiling point
• Increased surface area of contact means that greater London forces between molecules
• Therefore requires more energy to overcome strong attraction.

Question 6

How does branching affect the boiling point of an alkane?(3)

Answer 6

• Increased branching = lower boiling point
• Less surface area of contact means London forces
• Shape of the molecules also physically prevent molecules from getting closer together, which also decreases intermolecular forces.

Question 7

How are different substances obtained from crude oil? (3)

Answer 7

• Fractional distillation
• Different fraction of crude oil have different boiling points
• The distillation tower is heated to allow the vaporisation of different fractions based on their boiling point so they can be collected.

Question 8

Describe difference in pi and sigma bonds.

Answer 8

Sigma: the overlapping of orbitals
Pi: sideways overlap over p orbitals

Question 9

Alkane to haloalkane (3)

Answer 9

• Radical substitution
• Halogen + alkane
• UV radiation