15 Haloalkanes

Question 1

What is a nucleophile? Give examples of common nucleophiles

Answer 1

Electron pair donor
OH-, H2O and NH3

Question 2

What is the mechanism for a haloalkane reacting with a nucleophile?

Answer 2

Nucleophilic substitution

Question 3

What is hydrolysis?

Answer 3

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

Question 4

What happens in hydrolysis of a haloalkane?

Answer 4

Halogen replaced by the -OH group
haloalkane –> alcohol
Nucleophilic substitution

Question 5

Draw the mechanism for nucleophilic substitution

Answer 5

CH3CH2Cl + OH- –> CH2(OH)CH3 + Cl-

1. Draw on dipoles (negative on Cl and positive on C)
2. Arrow from lone pair on OH- ion to C
3. C-Cl breaks by heterolytic fission, arrow from bond to Cl
4. OH has replaced Cl, Cl is an ion

Question 6

What are the conditions for nucleophilic substitution of haloalkanes?

Answer 6

Heated under reflux with aqueous sodium hydroxide to achieve a good yield (slow at room temp)

Question 7

What determines the reactivity of the halogenoalkane?

Answer 7

The strength of the C-Hal bond

Question 8

Does 1-iodobutane or 1-chlorobutane hydrolyse the fastest and explain why

Answer 8

1-iodobutane as the C-I bond has the lowest bond enthalpy so breaks the fastest

Question 9

Draw the mechanism for water as a nucleophile

Answer 9

1. CH3CH2Cl + H2O
   Dipoles - negative on Cl and O
   Arrow from lone pair on O to C and arrow from C-Cl bond to Cl
2. CH3CH2H2O + Cl-
   Positive O and negative Cl
   Curly arrow from O-H bond to O
3. CH3CH2OH + H+ + Cl-

Question 10

Describe the method to measure the rate of hydrolysis of haloalkanes

Answer 10

1. Add 1cm3 of ethanol and two drops of 1-chlorobutane to a test tube
2. Repeat with 1-bromo and 1-iodobutane in separate test tubes
3. Stand the test tubes in a water bath at 60C along with a test tube containing silver nitrate solution
4. Quickly add 1cm3 of silver nitrate solution to each of the haloalkanes
5. Observe the test tubes for 5 minutes, noting the time taken for a precipitate to form

Question 11

Why is ethanol necessary in the rate of hydrolysis of haloalkanes experiment?

Answer 11

Haloalkanes are insoluble in water; ethanol allows the haloalkane and silver nitrate solutions to mix rather than form 2 layers

Question 12

Do tertiary, secondary or primary haloalkanes hydrolyse the fastest and why?

Answer 12

Tertiary is fastest, primary is slowest
Tertiary is the most relatively stable tertiary carbocation so very strong attraction between OH- lone pair and positive C
Mechanism is SN1

Question 13

What is the ozone layer and what is its purpose?

Answer 13

Edge of stratosphere
Absorb most of damaging UV radiation from the sun’s rays
Depletion of the ozone would mean more UV-B would reach earth leading to more genetic damage and risk of skin cancer

Question 14

How is oxygen broken down in the ozone?

Answer 14

High energy UV
Oxygen molecules into oxygen radicals
O2 –> 2O
Homolytic fission

Question 15

What is set up in the ozone layer involving O2 and O radicals?

Answer 15

A steady state. The rate of formation of ozone is the same as the rate at which it is broken down
O + O2 –><– O3

Question 16

Why is the ozone being depleted?

Answer 16

Chlorofluorocarbons (CFCs) were used as refrigerants, in air-con and in aerosols
Very stable (unreactive) so remain in troposhere
Can be broken down by UV if they reach the stratosphere to Cl radicals which can breakdown the ozone layer

Question 17

How do CFCs deplete the ozone layer?

Answer 17

The UV radiation provides sufficient energy to break a C-Hal bond by homolytic fission

CCl2F2 –> CClF2* + Cl*
C-Cl breaks as has lower bond enthalpy
This is photodissociation

Question 18

What impact does the Cl radicals have on the ozone layer?

Answer 18

The Cl* is a reactive intermediate and can react with ozone
Breaks ozone down into oxygen
Cl* + O3 –> ClO* +O2
ClO* + O –> Cl* +O2
Overall: O3+O –> 2O2 (Removes ozone from the stratosphere)

Second propagation step, Cl* is regenerated (Cl* is a catalyst for breakdown of O3)

Question 19

Write the propagation steps for NO reacting with O3

Answer 19

NO* + O3 –> NO2* + O2
NO2* + O –> NO* + O2
Overall: O3 + O –> 2O2