3.2 Halogenoalkanes

Question 1

How to seperate individual halogenoalkanes

Answer 1

fractional distillation

Question 2

During CFC breakdown, why are F radicals not formed?

Answer 2

C-Cl bond broken because its weaker / requires less energy to break than C-F AND C-H

Question 3

Ozone formation equation:

Answer 3

O₂ (UV) ➜ 2O.
O₂ + O. (UV) ➜ O₃

Question 4

Equation for CFC breaking up to form Cl radical:

Answer 4

CF₂Cl₂ ➜ .CF₂Cl + Cl.

Question 5

Equation for Cl radicals reacting w/ ozone:
Overall reaction:

Answer 5

O₃ + Cl. ➜ ClO. + O₂
ClO. + O₃ ➜ 2O₂ + Cl.

Overall: 2O₃ ➜ 3O₂

Question 6

CFC usage

Answer 6

• coolants
• solvents

Question 7

State Le Chateliers principle

Answer 7

If any factor is changed which affects an equillibrium, equillibrium will shift as to oppose the change

Question 8

Write overall equation for decomposition of ozone to oxygen

Answer 8

2O₃ ➜ 3O₂

Question 9

When asked for displayed formula containing OH:

Answer 9

O-H

Question 10

what is a nucleophile

Answer 10

an electron pair donor

Question 11

How do Cl. acts as catalysts in decomposition of ozone layer?

Answer 11

• Cl. is regenerated
• role is to lower activation energy

Question 12

cyanide is a

Answer 12

chain extender
eg. 1-bromopropane -> nucelophilic sub by -CN -> butanenitrile

Question 13

Identify the reagent needed for elimination and the conditions required: (2)

Answer 13

• NaOH
• high temperature / ethanolic conditions

Question 14

For nucleophilic substitution, name the:
* Reagent:
* Conditions:
* Mechanism:
* Type of reagent:

Answer 14

Reagent: potassium (or sodium) hydroxide
Conditions: In aqueous solution; Heat under reflux
Mechanism: Nucleophilic Substitution
Type of reagent: Nucleophile, OH-

The aqueous conditions needed is an important point. If the solvent is changed to ethanol an elimination reaction occurs.

Question 15

Nucleophilic substitution with ammonia

Answer 15

Reagent: NH3 dissolved in ethanol
Conditions: Heating under pressure (in a sealed tube)
Mechanism: Nucleophilic substitution
Type of reagent: Nucleophile, :NH3

Question 16

Elimination with alcoholic hydroxide ions

Answer 16

Change in functional group: halogenoalkane -> alkene
Reagents: Potassium (or sodium) hydroxide
Conditions: In ethanol ; heat
Mechanism: Elimination
Type of reagent: Base, OH

Question 17

Note the importance of
the solvent to the type of
reaction here.

Answer 17

Aqueous: substitution
Alcoholic: elimination

Question 18

Nucleophilic substitution is when:

Answer 18

(from aqueous) Nucleophile attacks partially +ve C and replaces the halogen

Question 19

(nucleophilic) Elimination is when:

Answer 19

(from alcoholic) nucleophile attacks adjacent H and creates double bond so halogen is ‘eliminated’