4.2.2 Haloalkanes

Question 1

Definition of a nuleophile.

Answer 1

Electron pair donor.

Question 2

What is nucleophilic substitution?

Answer 2

A haloalkane reacts with a nucleophile which replaces the halogen in substitution reaction.

Question 3

What’s hydrolysis?

Answer 3

Chemical reaction involving water/aqueous solution of a hydroxide that causes breaking of a bond in a molecule.

Question 4

What is hydrolysis of a haloalkane?

Answer 4

Halogen atom gets replaces by an OH group.

Question 5

Describe the steps in hydrolysis of a haloalkane.

Answer 5

Nucleophile approaches carbon atom attached to halogen on opposite side to halogen (direction minimises repulsion between nucleophile and halogen)
Lone pair from hydroxide attracted and donated to + carbon atom.
Bond formed between oxygen and carbon.
Carbon-halogen bond breaks by heterolytic fission.
Forms alcohol and halide ion.

Question 6

Conditions of converting haloalkanes to alcohols.

Answer 6

Aqueous sodium hydroxide.
Heated under reflux (to obtain good yield)

Question 7

What does rate of hydrolysis depend on?

Answer 7

Strength for carbon-halogen bond.

Question 8

Describe the reactivity of haloalkanes in terms of bond enthalpies.

Answer 8

Carbon-iodine bond is weakest so iodoalkanes react faster than bromo.
Carbon fluorine bonds are strongest so fluoroalkanes are unreactive.
Bromoalkanes react faster than chloroalkanes.

Question 9

Conditions for measuring rate of hydrolysis in primary alcohols.

Answer 9

Aqueous silver nitrate and ethanol.

Question 10

Describe the experiment for measuring rate of hydrolysis in primary alcohols.

Answer 10

Test tube 1= 1cm3 ethanol, two drops 1-chlorobutane
Test tube 2= 1cm3 ethanol, two drops 1-bromobutane
Test tube 3= 1cm3 ethanol, two drops 1-iodobutane

60 degree water bath with test tube of 0.1mol/dm3 silver nitrate, constant temp.

Add 1cm3 silver nitrate to all, start stop-clock and observe.

Chloro= white ppt, slow
Bromo= cream, faster
Iodo= yellow, rapid

Question 11

What happens when CFCs reach the stratosphere?

Answer 11

UV radiation provides energy to break carbon-halogen bond in CFCs by homolytic fission to form radicals (C-Cl bond lowest enthalpy, Cl radicals formed).

Question 12

What is the name for UV radiation breaking bonds in CFCs?

Answer 12

Photodissociation.

Question 13

Equation for photodissociation of CF2Cl2.

Answer 13

CF2Cl2 -> CF2Cl. + Cl.

Question 14

How do chlorine radicals break down ozone?

Answer 14

They’re very reactive and can react with an ozone molecule, breaking it down into oxygen.
Causes a chain reaction.

Question 15

Equations for the breakdown of ozone by Cl radicals.

Answer 15

Step 1: Cl. + O3 -> ClO. + O2
Step 2: ClO. + O -> Cl. + O2

Overall: O3 + O -> 2O2

Question 16

How are nitrogen oxide radicals produced?

Answer 16

Naturally during lightning strikes and as a result of aircraft travel in stratosphere.

Question 17

Equations for the breakdown of ozone by nitrogen oxide radicals.

Answer 17

Step 1: NO. + O3 -> NO2. + O2
Step 2: NO2. + O -> NO. + O2

Overall: O3 + O -> 2O2

Question 18

What do radicals act as in the breakdown of ozone?

Answer 18

Catalysts.