2.5- Hydrocarbons

Question 1

(c)

Mechanism of radical substitution

Answer 1

photochlorination of alkanes

Question 2

(c)

Photochlorination of alkanes

Answer 2

A photochlorination reaction occurs when one of the hydrogen atoms on an alkane is replaced by a chlorine atom. The reaction is initiated by UV light.

Question 3

(c)

Halogenation

Answer 3

Alkanes react with chlorine in uv light to form a halogenoalkane.

Question 4

(c)

Radical halogenation of alkanes

Answer 4

(1) Initiation
Cl2 → 2Cl.
(2a) Propagation
Cl. + CH4 → .CH3 + HCl
(2b) Propagation
CH3 + Cl2 → CH3Cl + Cl.
(3) Termination
2Cl. → Cl2
Cl. + . CH3 → CH3Cl
2 .CH3 → CH3CH3

Question 5

(c)

Radical halogenation of alkanes explaination

Answer 5

(1) Initiation:
UV light provide energy to split the chlorine molecules into free radicals….
(2a) Propagation:
A chlorine radical reacts with a methane molecule. It produces another radical.
(2b) Propagation:
The methyl radical can collide with chloride molecules to produce a chlorine radical.

Question 6

(g)

Electrophilic addition

Answer 6

Halogens react with alkenes to form dihalogenoalkanes.

H2C=CH2 + Br2 → CH2BrCH2Br

Question 7

(h)

Bromine/bromine water and potassium manganate(VII) tests for alkenes

Answer 7

Add bromine water: The orange solution decolourises if the alkene C=C bond is present.
Add acidified potassium dichromate: The purple solution decolourises if the alkene C=C bond is present.

Question 8

(i)

Explain why the electrophilic addition of bromine to propene can produce a major and a minor product

Answer 8

This reaction can produce two products — a major and a minor product. Two types of intermediates can form — a primary and a secondary carbocation. The secondary carbocation is more stable than the primary one. The more stable secondary carbocation leads to the major product, 2-bromopropane. The primary carbocation results in the minor product, 1-bromopropane.

Question 9

(j)

Hydrogenation

Answer 9

Ethene reacts with hydrogen and a nickel catalyst at 150˚C to produce ethane.

H2C=CH2 + H2 → CH3CH3

Question 10

(j)

Importance of hydrogenation

Answer 10

This is commercially important since unsaturated vegetable oils are reacted with hydrogen to form margarine.

Question 11

(k)

Addition polymerisation

Answer 11

The double bonds in alkenes and substituted alkenes (monomers) can open up and join together to form long chains called polymers.

Question 12

(k)

Poly(propene)

Answer 12

Used in textiles, automotive parts, and packaging materials.

Question 13

(k)

Poly(chloroethene) or PVC

Answer 13

Insulation for electrical wires, windows, gutters, pipes

Question 14

(k)

Poly(phenylethene) or
polystyrene

Answer 14

Drinking cups, make disposable plastic cutlery and household appliances