Alkenes

Question 1

Formation of alkane

Answer 1

Hydrogenation (used to manufacture margarine)
[H2(g) added]

Conditions:
- Catalyst: Nickel Catalyst
- Temperature: 100°c
- Pressure: 3 to 5 atm

Question 2

Formation of Halogenoalkane using Br2

Answer 2

Electrophilic addition (e.g Bromination)

Conditions:
-Temperature: Room temperature
- Bromine in a non-polar solvent (or Bromine water, but this forms halogen alcohol as an alternative)

Tests for C=C bond
Colour change: Orange -> Colourless

Mechanism:
1. Br2 induced dipole due to the high electron density of C=C bond. Slightly negative Br end attracted to C=C region.
2. Forms a carbocation with one Br bonded. Lone pair of bromide forms a dative covalent bond to the Carbon centre (+).

Question 3

Formation of Halogenoalkane using HBr to unsymmetrical alkenes

Answer 3

Electrophilic addition
Forms a major and minor product

Conditions:
Temperature: Room temperature
HBr (g) or dissolved in inert solvent

Question 4

Determining Major and Minor Product

Answer 4

Depends on the stability of the carbocation

Stability can be predicted by the carbon in the C=C bond with the most hydrogen atoms bonded to it. This forms a carbocation whereby the Carbon that had the least is the C+.

Order of decreasing stability
Tertiary > Secondary > Primary
- Alkyl groups have an electron-releasing inductive effect.

Question 5

Chemical tests for C=C

Answer 5

1. Bromine water
   Orange to colourless
2. Cold acidified potassium manganate
   Forms diols
   MnO4- (Purple) reduced to Mn^2+ (Colourless/pale pink)

Question 6

Forming a polymer

Answer 6

Addition polymerisation

-A large number of unsaturated alkenes join together to form polyalkenes

Question 7

Forming an alkene from an alcohol

Answer 7

Elimination

• Reflux alcohol with Concentrated Sulphuric acid H2SO4
• Eliminates H and OH off adjacent Carbons -> H2O