3.3 Halogenoalkanes

Question 1

how many classes of halogenoalkanes are there?

Answer 1

• primary halkanes
• secondary halkanes
• tertiary halkanes

Question 2

explain what happens in nucleophilic substitution reactions

Answer 2

• nucleophile attacks the positive carbon atom
• halogen bond breaks
• halogen replaced with nucleophile

Question 3

what is a nucleophile?

Answer 3

• an electron pair donor
• :OH- :NH3 CN-

Question 4

what does the rate of these substitution reactions depend on?

Answer 4

• strength of the C-X bond
• weaker the bond, easier to break and faster the reaction

Question 5

give the conditions, reagent, mechanism and type of reagent for nucleophilic substitution with aqueous hydroxide ions

Answer 5

• potassium / sodium hydroxide
• aqeueous solution, warm
• nucleophilic substitution
• Nucleophile, OH-

Question 6

give the reagents, conditions, mechanisk and type of reagent for nucleophilic substitution with cyanide ions

Answer 6

• KCN dissolved in ethanol/water
• Heating under reflux
• Nucleophilic substitution
• Nucleophile, :CN-

Question 7

what are the industrial benefits of nucleophilic sub with cyanide ions

Answer 7

• increases length of carbon chain

Question 8

give the reagents, conditions, mechanism and type of reagent for nucleophilic substitution with ammonia

Answer 8

• NH3 dissolved in ammonia
• heating under pressure
• Nucleophilic substitution
• Nucleophile, :NH3

excess ammonia

Question 9

why do we use excess ammonia?

Answer 9

• it helps to minimise further substitution reactions between the halogenoalkanes and the amines formed, leading to a lower yield

Question 10

give the overall equation for bromopropane reacting with KOH

Answer 10

CH3CH2CH2BR + KOH > CH3CH2CH2OH + KBr

Question 11

give the overall equation for reaction of bromopropane with CN

Answer 11

CH3CH2CH2Br + :CN- > CH3CH2CH2CN + Br-

Question 12

give the overall reaction for bromopropane reacting with ammonia

Answer 12

CH3CH2CH2Br + 2NH3 > CH3CH2CH2NH2 + NH4Br

Question 13

what does the term elimination mean

Answer 13

• removal of small molecule from the organic molecule

Question 14

give the reagents, conditions, mechanism and type of reagent for elimination reaction with alcoholic hydroxide ions

Answer 14

• potassium / sodium hydroxide
• in ethanol ; heat
• elimination
• Base, OH-

Question 15

give the reaction for 1-bromopropane with KOH

Answer 15

CH3CH2CHBrH + KOH > CH3CH=CH2 + KBr + H20

Question 16

what are the uses of halogenoalkanes

Answer 16

they can be used as solvents

Question 17

why have these uses been stopped

Answer 17

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

Question 18

what is the naturally occuring ozone layer in the upper atmospheres role?

Answer 18

• filters out much of the suns harmful UV radiation

Question 19

whats the impact of ozone in the lower atmosphere?

Answer 19

• it is a pollutant and contributes towards the formation of smog

Question 20

what are the impacts of man made CFC’s?

Answer 20

• they have caused a hole to form in the ozone layer

Question 21

how does the process of ozone depletion work?

Answer 21

• chlorine radicals are formed in the upper atmosphere when energy from UV radiation causes C-Cl bonds in CFC’s to break
• chlorine free radicals atoms catalyse the decomposition or ozone as they are regenerated.
• these reactions contribute to the formation of a hole in the ozone layer

Question 22

give the equations for depletion of ozone

Answer 22

Cl• + O3 > ClO• + O2
ClO• + O3 > 2O2 + Cl•

2O3 > 3O2

regenerated Cl radical means that one Cl radical could destroy many thousands of ozone molecules

Question 23

why are HCFCs now used for refrigerators?

Answer 23

• they do not contain the C-Cl bond
• the C-F bond is stronger and not affected by UV