Haloalkanes

Question 1

Write the word equation for the preparation of Chloroethane from an alkane

Answer 1

Ethane + Chlorine→chloroethane

Question 2

What are the conditions under which chloroethane is made from an alkane?

Answer 2

• Room temperature and pressure
• in the presence of UV radiation

Question 3

What is the equation for the formation of chloroethane from an alkane?

Answer 3

CH₃CH₃ + Cl₂→CH₃CH₂Cl + HCl

Question 4

What is the mechanism for the formation of chloroethane from an alkane?

Answer 4

Free radical substitution

Question 5

What are the reagents needed to make chloroethane from an alkene?

Answer 5

Ethene and HCl

Question 6

What are the conditions needed to make chloroethane from an alkene?

Answer 6

Room temperature and pressure

Question 7

What is the mechanism by which chloroethane can be made from an alkene?

Answer 7

Electrophilic addition

Question 8

What is the chemical equation for the reaction of ethene and HCl to produce chloroethane?

Answer 8

H₂C=CH₂ + HCl →CH₃CH₂Cl

Question 9

What is the atom economy for the reaction needed to make chloroethane from an alkene?

Answer 9

100%

Question 10

Why are haloalkanes generally more reactive than alkanes?

Answer 10

• The strength of Carbon-Halogen bonds is lower than Carbon-hydrogen bonds
• So the Carbon-halogen bonds break more easily and allow the haloalkanes to react

Question 11

What is the exception to haloalkanes being more reactive than alkanes? Why?

Answer 11

Fluroalkanes

• Carbon-fluorine bonds have a higher bond enthalpy than Carbon-Hydrogen bonds
• so require more energy to break

Question 12

Are Carbon-Halogen bonds polar?

Answer 12

Yes

Question 13

Why are Carbon-Halogen bonds polar?

Answer 13

Halogens have a higher electronegativity than Carbon

Question 14

Define hydrolysis

Answer 14

A chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule

Question 15

What are the reagents needed to prepare an alcohol from a haloalkane by hydrolysis?

Answer 15

Haloalkane and NaOH(aq) or water

Question 16

What are the conditions to prepare an alcohol from a haloalkane by hydrolysis?

Answer 16

Heat under reflux

Question 17

What is the mechanism to prepare an alcohol from a haloalkane by hydrolysis?

Answer 17

Nucleophilic substitution

Question 18

Give an equation for the reaction of bromoethane and water to produce an alcohol

Answer 18

CH3CH₂Br + H₂O —> CH3CH₂OH + HBr

Question 19

Define nucleophile

Answer 19

A species capable of donating a pair of electrons

Question 20

Give 3 examples of nucleophiles

Answer 20

NH₃(aq), H₂O, OH^-(aq)

Question 21

What do most nucleophiles have in common?

Answer 21

They have lone pairs

Question 22

How do you test for the rate of hydrolysis of haloalkanes?

Answer 22

Place the test tube with haloalkane in a hot water bath, add ethanol and then AgNO₃ (aq)

Question 23

Why is the experiment to test the rate of hydrolysis of haloalkanes conducted in a hot water bath?

Answer 23

To speed up the rate of reaction

Question 24

Why does the experiment to test the rate of hydrolysis of haloalkanes use ethanol?

Answer 24

Ethanol acts as a cosolvent as both water and the haloalkane dissolve in ethanol so the reactants can mix

Question 25

What are the two roles of AgNO₃ (aq) in the experiment to test the rate of hydrolysis of haloalkanes?

Answer 25

- provides H₂O to act as the reactant in hydrolysis -Ag⁺ from AgNO₃ trap halide ions released by the reaction, to form a preciptate - rate of precipitate production allows the rate of reaction to be calculated

Question 26

Why is water used as the nucleophile in the experiment to test the rate of hydrolysis of haloalkanes rather than AgNO₃ (aq)?

Answer 26

- if OH^- ions were present they would react with Ag⁺ to form AgOH - this would form a brown precipitate - Ag⁺ ions would be removed from reacting with halides - any silver halide precipitate formed would be masked

Question 27

How is the rate of hydrolysis of haloalkanes compared?

Answer 27

The time taken for a visible quantity of silver halide to be formed is compared

Question 28

What colour precipitate is formed in the hydrolysis of chloroethane?

Answer 28

White

Question 29

What colour precipitate is formed in the hydrolysis of bromoethane?

Answer 29

Cream

Question 30

What colour precipitate is formed in the hydrolysis of iodoethane?

Answer 30

Yellow

Question 31

What are the relative rates of the hydrolysis of chloroethane, bromoethane, and iodoethane?

Answer 31

- iodoethane is the fastest - then bromoethane - chloroethane is the slowest

Question 32

Why do the rates of hydrolysis of haloalkanes differ?

Answer 32

The C-Cl bond is the strongest, then the C-Br bond, then the C-I is the weakest

Question 33

What were CFCs used for?

Answer 33

- coolants (in fridges) - propellant (in aerosols) - fire extinguishers - blow moulding (making plastics)

Question 34

What properties of CFCs made them good propellants/fire extinguishers/coolants?

Answer 34

They are: - inert - non-toxic - non-flammable - non-irritant

Question 35

Give the equations to show the formation of ozone in the atmosphere

Answer 35

O₂ → O• + O• under UV light O₂ + O• → O₃

Question 36

Why is the ozone layer important?

Answer 36

Protects organisms from high energy UV rays as O₃ readily absorbs UV radiation

Question 37

What are the risks to people without the ozone layer?

Answer 37

Increased risk of genetic damage and skin cancer due to UV radiation exposure

Question 38

Why is there no overall change in concentration of ozone in the atmosphere (without human intervention)?

Answer 38

-ozone is constantly forming and breaking down at the same rate

Question 39

Why do CFCs persist in the lower atmosphere?

Answer 39

C-F and C-Cl bonds are strong and so the molecules do not break down easily

Question 40

Give the equation for the breakdown of ozone naturally within the atmosphere

Answer 40

Under the action of UV radiation O₃→ O₂ + O•

Question 41

Show, using equations, how Cl radicals catalyse the breakdown of ozone

Answer 41

Cl• + O₂→ ClO• + O₂ ClO•+ O→Cl• + O₂

Question 42

Give the overall equation of the breakdown of ozone by Chlorine radicals

Answer 42

O₃ + O→2O₂

Question 43

Give two substances that can catalyse the breakdown of ozone

Answer 43

CF₂Cl₂ NO

Question 44

Where does Nitrogen monoxide come from in the atmosphere?

Answer 44

Lightning strikes and high flying aircrafts

Question 45

What type of catalysts are NO and CFCs?

Answer 45

Homogeneous

Question 46

Show, using equations, how NO radicals can catalyse the breakdown of ozone

Answer 46

NO• + O₃→ NO₂• + O₂ NO₂• + O ---\> NO• + O₂

Question 47

Define homogeneous catalyst

Answer 47

A catalyst which is in the same phase as the reactants of the reaction it catalyses