6C: Alkenes

Question 1

What is the general formula for alkenes?

Answer 1

C_n H_2n

Question 2

What sort of hydrocarbons are alkenes and cycloalkenes?

Answer 2

Unsaturated hydrocarbons

Question 3

What is a σ bond?

Answer 3

The overlap of 2 s-orbitals
- Stronger than π bonds

Question 4

What is a π bond?

Answer 4

The overlap of 2 p-orbitals
- Weaker than σ bonds

Question 5

State and explain the relative strength of π bonds and σ bonds.

Answer 5

s-orbitals overlap fully,
p-orbitals overlap partially,
so σ bonds are stronger.

Question 6

What is meant by the term electrophile?

Answer 6

A lone pair acceptor.

Question 7

Reaction of alkenes with hydrogen:
Condition,
Catalyst,
Product,
Application.

Answer 7

T = 150 C,
Nickel,
Alkanes,
Manufacture of margarine from vegetable oils.

Question 8

Describe the reaction
Alkene -> Alkane

Answer 8

React with Hydrogen,
Nickel Catalyst,
T = 150 C.

Question 9

Reaction of alkenes with halogens:
Product,
Mechanism.

Answer 9

Dihalogenoalkanes,
Electrophilic addition

Question 10

Describe the reaction
Alkene -> Dihalogenoalkane

Answer 10

React with halogen,
Electrophilic addition

Question 11

Reaction of alkenes with hydrogen halides:
Product,
Mechanism.

Answer 11

Halogenoalkanes,
Electrophilic addition

Question 12

Describe the reaction
Alkene -> Halogenoalkane

Answer 12

React with hydrogen halide,
Electrophilic addition

Question 13

Reaction of alkenes with water:
Condition,
Catalyst,
Product.

Answer 13

T > 100 C (Steam),
Acid (Dilute phosphoric acid),
Alcohols.

Question 14

Describe the reaction
Alkene -> Alcohol

Answer 14

React with steam,
Phosphoric acid catalyst.

Question 15

Reaction of alkenes with Potassium Manganate(VII):
Condition,
Product,
Observation.

Answer 15

Acidic conditions (H₂SO₄),
Diols,
Purple -> colourless

Question 16

Describe the reaction
Alkene -> Diol

Answer 16

React with potassium manganate,
acidic conditions (H₂SO₄),
Colour change purple -> colourless.

Question 17

What does heterolytic bond fission of a covalent bond result in?

Answer 17

Formation of ions

Question 18

What is the relative stability of primary, secondary and tertiary carbocation intermediates?

Answer 18

Primary = least stable
Tertiary = most stable

Question 19

Describe the qualitative test for a C=C double bond, and state the products.

Answer 19

Br_(l) -> dihalogenoalkane

Br_(aq) -> halogen alcohol (major)
-> dihalogenoalkane (minor)

orange -> colourless

Question 20

What are three ways of disposing of waste polymers?

Answer 20

• Recycling
• Incineration to release energy
• Use as a feedstock for cracking

Question 21

What are two ways chemists limit the problems caused by polymer disposal?

Answer 21

Developing biodegradable polymers,
Using chemical scrubbers to remove toxic waste gases caused by incineration of plastics.

Question 22

Explain how a chemical scrubber can remove toxic waste gases caused by incineration of plastics.

Answer 22

The chemical scrubber is a base which would neutralise the HCl gas.