Halogenoalkanes

Question 1

What types of reactions do halogenoalkanes undergo?

[2]:

Answer 1

• Nucleophilic substitution

- Elimination

Question 2

Nucleophile [definition]:

Answer 2

electron pair donator

e.g. :OH-, :NH3, CN⁻

Question 3

What does the rate of the substitution reaction depend on? [2]:

Answer 3

• the strength of the C-X bond

- The weaker the bond, the easier it is to break and the faster the reaction

Question 4

Halogenoalkane + :OH- =

Answer 4

Nucelophilic substitution

Question 5

Halogenoalkane + :NH3 =

Answer 5

Nucelophilic substitution

Question 6

Halogenoalkane + CN⁻ =

Answer 6

Nucelophilic substitution

Question 7

Nucleophilic substitution with aqueous hydroxide ions

[4]::

Answer 7

• halogenoalkane → alcohol
• Reagent: potassium (or sodium) hydroxide
• Conditions: In aqueous solution; Heat under reflux
• Mechanism: Nucleophilic Substitution

Question 8

Water is a poor nucleophile but…

Answer 8

it can react slowly with halogenoalkanes in a substitution reaction

Question 9

Nucleophilic substitution with cyanide ions [4]:

Answer 9

• halogenoalkane → nitrile
• Reagent: KCN dissolved in ethanol/water mixture
• Conditions: Heating under reflux
• Mechanism: Nucleophilic Substitution

Question 10

Nucleophilic substitution with ammonia [4]:

Answer 10

• halogenoalkane → amine
• Reagent: NH3 dissolved in ethanol
• Conditions: Heating under pressure (in a sealed
  tube)
• Mechanism: Nucleophilic substitution

Question 11

Elimination with alcoholic hydroxide ions [4]:

Answer 11

• halogenoalkane → alkene
• Reagents: Potassium (or sodium) hydroxide
• Conditions: In ethanol ; heat
• Mechanism: Elimination

Question 12

Aqueous:

Answer 12

substitution

Question 13

Alcoholic:

Answer 13

elimination

Question 14

What reaction does a primary halogenoalkane tend to?

Answer 14

substitution

Question 15

What reaction does a secondary halogenoalkane tend to?

Answer 15

elimination

Question 16

Uses of halogenoalkanes [4]:

Answer 16

• Chloroalkanes and chlorofluoroalkanes can be used as solvents
• refrigerants
• pesticides
• aerosol propellants

Question 17

Why have people mostly stopped using halogenoalkanes?

Answer 17

stopped due to the toxicity of halogenoalkanes and also their detrimental effect on the atmosphere

Question 18

What is the ozone layer?

Answer 18

• O3

- layer in the upper atmosphere that filters out much of the sun’s harmful UV radiation

Question 19

What happens when ozone is in the lower layer of the atmosphere? [2]:

Answer 19

• becomes a pollutant

- contributes to the formation of smog

Question 20

What did man made chlorofluorocarbons (CFCs) cause?

Answer 20

A hole to form in the ozone layer

Question 21

What happens to CFCs in the upper atmosphere? [2]:

Answer 21

• UV light causes s C–Cl bonds in CFCs to break

- This forms chlorine radicals in upper atmosphere

Question 22

What do chlorine radicals do to ozone?

Answer 22

catalyse the decomposition of ozone

Question 23

What do we use instead of CFCs and why? [3]:

Answer 23

• CH2FCF3 are now used for refrigerators
  and air-conditioners
• safer as they do not contain the C-Cl bond
• C-F bond is stronger than the C-Cl bond and is not affected by UV