Chapter 15 - Haloalkanes

Question 1

What are nucleophiles?

Answer 1

Species that donate a lone pair of electrons.

Question 2

Give three examples of nucleophiles

Answer 2

Hydroxide ions
Water molecules
Ammonia molecules

Question 3

When a haloalkane reacts with a nucleophile, what is the name of the reaction mechanism?

Answer 3

Nucleophilic substitution

Question 4

What is hydrolysis?

Answer 4

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

Question 5

Describe the nucleophilic substitution reaction for the hydrolysis of a haloalkane

Answer 5

OH- approaches carbon bonded to halogen on opposite side to halogen
This minimises repulsion between nucleophile and halogen’s dipole
Lone pair of electrons on OH attracted to and donated to carbon atom
New bond formed betwen oxygen and carbon atom
Carbon-halogen bond breaks
Alcohol and halide ion formed

Question 6

Describe the conditions for the hydrolysis of haloalkanes

Answer 6

Using aqueous sodium hydroxide and is heated under reflux.

Question 7

What determines how fast a specific carbon-halogen bond will break?

Answer 7

Bond enthalpy

Question 8

Which halogenoalkane will react the fastest and why?

Answer 8

Iodoalkanes because C-I bonds are the weakest so require less energy to break.

Question 9

How can hydrolysis of haloalkanes be compared to one another?

Answer 9

Perform the reaction in the presence of aqueous silver nitrate and record the time taken to form a precipitate.

Question 10

What would the results show?

Answer 10

Chloroalkanes react slowest (will form white PPT)
Iodoalkanes react fastest (will form yellow PPT)

Therefore rate of hydrolysis increases as the strength of the carbon-halogen bond decreases.

Question 11

How does primary, secondary or tertiary halogenoalkane status affect rate of hydrolysis?

Answer 11

Tertiary reacts the fastest because the intermediary carbocation that is formed is much more stable than a primary carbocation.

Question 12

How do CFCs deplete the ozone layer?

Answer 12

Once in the stratosphere, the UV radiation provides sufficient energy for the carbon-halogen bond to break by homolytic fission forming radicals. This process is called photodissociation.

The carbon to halogen breaks because of the low bond enthalpy

Question 13

What other radicals are responsible for ozone depletion?

Answer 13

Nitrogen oxide radicals which are formed naturally when lightening strikes or when an aircraft is travelling theogony the stratosphere

Question 14

Describe the test for hydrolysis of haloalkanes:

Answer 14

1) Tube 1: Add 1 cm^3 of ethanol and two drops of 1-chlorobutane (will form white PPT)
Tube 2: Add 1 cm^3 of ethanol and two drops of 1-bromobutane (will form cream PPT)
Tube 3: Add 1 cm^3 of ethanol and two drops of 1-iodobutane (will form yellow PPT)

2) Stand the test tubes in a water bath at 60C
3) Place a test tube containing 0.1 mol dm^-3 silver nitrate in the water bath and allow all tubes to reach a constant temperature.
4) Add 1 cm^3 of the silver nitrate quickly to each of the test tubes. Immediately start a stop-clock.
5) Observe the test tubes for five miniutes and record the time taken for the precipitate to form.

Question 15

Earth’s atmosphere is divided into five layers: what are they in order from the lowest to highest level?

Answer 15

troposphere
tropopause
stratosphere
ozone layer 
ozone layer

Question 16

List three sources of CFCs in the atmosphere, and their reasons why?

Answer 16

1) Refrigerants- Non toxic and non-flammable
2) Dry cleaning- good solvent for organic material
3) Aerosol propellants- because it is volatile

Question 17

why is ozone depletion a major concern to humans

Answer 17

The depletion of ozone layer will allow more harmful UV-b radiation to penetrate the troposphere leading to increased incidence of skin cancer and genetic damage

Question 18

show the reaction of nitrogen oxide(NO) radical reacting with ozone layer

Answer 18

NO• + O3&raquo_space;> NO2• + O2

NO2• + O&raquo_space;> NO• + O2

Question 19

show the reaction of Cl radical reacting with ozone layer

Answer 19

Cl• + O3&raquo_space;> ClO• + O2

ClO• + O&raquo_space;> Cl• + O2

Question 20

what is the Montreal protocol

Answer 20

Ban CFCs
Stop using CCl4
Stop using ‘halon’ fire extinguishers

Question 21

What alternative can we use to CFC’s

Answer 21

HFCs
Supercritical CO2
e.g. CO2(l)