2.8- Haloalkanes

Question 0

What is a reaction’s mechanism?

Answer 0

How it proceeds step by step

Question 1

Why do alkanes not react with many reagents?

Answer 1

The C-C and the C-H bonds are relatively strong

Question 2

How are chloro/bromo alkanes synthesised?

Answer 2

Mechanism is called a free radical substitution

Step one: Initiation
Step two: Propagation
Step three: Termination

Eg
CH4 + Cl2 –> CH3Cl + HCl

Question 3

What is the initiation step?

Answer 3

Essential condition: UV light

Cl2 –> 2Cl.

UV light supplies the energy to break the Cl-Cl bond. It is broken in preference to others as it’s the weakest.

Bond is broken in a process called homolytic fission

Question 4

What is homolytic fission?

Answer 4

When a bond breaks and each atom gets one electron from the covalent bond forming free radicals

Question 5

Define free radical?

Answer 5

A free radical is a reactive species which possess an unpaired electron

Free radicals do not have a charge and are represented by a .

Question 6

What is the propagation step?

Answer 6

The chlorine free radicals are very reactive and remove a H from the methane leaving a methyl free radical

The methyl free radical reacts with a Cl2 molecule to produce the main product and another Cl free radical

As the Cl free radical is regenerated, it can react with several more alkane molecules in a chain reaction

Question 7

What is the termination step?

Answer 7

Collision of two free radicals does not generate further free radicals and the chain is terminated

Write this step using structural formulae and don’t use molecular formulae

Question 8

What are some of the uses of Haloalkanes?

Answer 8

Chloroalkanes and CFC’s can be used as solvents

CH3CCl3 was used as the solvent in dry cleaning

Refrigerants, pesticides and aerosol propellants

Question 9

Why have many of the uses of Haloalkanes now been stopped?

Answer 9

Due to the toxicity of them and their detrimental effect on the atmosphere

Question 10

Why is ozone beneficial?

Answer 10

Naturally occurring ozone (O3) in the upper atmosphere is beneficial as it filters out much of the sun’s harmful UV radiation

Ozone in the lower atmosphere however is a pollutant and contributed towards the formation of smog

Question 11

Why are CFC’s bad for the environment?

Answer 11

Cause holes to form in the ozone layer

Question 12

How do chlorine atoms catalyse the decomposition of ozone?

Answer 12

Chlorine atoms are formed in the upper atmosphere when energy from the ultraviolet radiation causes C-Cl bonds in CFC’s to break

Cl. + O3 –> ClO. + O2
ClO. + O3 –> 2O2 + Cl.

Overall: 2O3 –> 3O2

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

Question 13

How can Haloalkanes be classified?

Answer 13

Classified as primary, secondary or tertiary depending on the number of carbon atoms attached to the C-X functional group

Question 14

What is a nucleophile?

Answer 14

Electron pair donor

Always have a lone pair

Eg
:OH-, :NH3, CN-

Question 15

What is nucleophillic substitution?

Answer 15

Swapping a halogen atom for another atom or group of atoms (nucleophile)

Carbon has a small positive charge because of the electronegativity difference between the carbon and the halogen

The nucleophiles attack the positive carbon atom

Question 16

What is a curly arrow and why do we use it?

Answer 16

Used to show the movement of two electrons

It will always start from a lone pair of electrons or the centre of a bond

Question 17

What does the rate of reaction of Haloalkanes depend on?

Answer 17

Strength of the C-X bond

The weaker the bond, the easier it is to break and the faster the reaction

Bond strength decreases going down group 7

Question 18

What is hydrolysis?

Answer 18

The splitting of a molecule (eg Haloalkane) by a reaction with water

Question 19

How can the rate of hydrolysis reactions be compared?

Answer 19

CH3CH2I + H2O –> CH3CH2OH + I- + H+

Aqueous silver nitrate is added to a Haloalkane and the halide leaving group combines with a silver ion to form a silver halide precipitate

The quicker the precipitate is formed, the faster the substitution reaction

Question 20

Explain nucleophillic substitution with hydroxide ions?

Answer 20

Change in functional group: Haloalkane –> alcohol

Reagent: potassium (or Na) hydroxide

Conditions: in aqueous solution, heat under reflux

Mechanism: nucleophillic substitution

Type of reagent: Nucleophile, :OH-

Question 21

Explain nucleophillic substitution with cyanide ions?

Answer 21

Change in functional group: Haloalkane –> nitrile

Reagent: KCN dissolved in ethanol/water mixture

Conditions: heating under reflux

Mechanism: nucleophillic substitution

Type of reagent: Nucleophile, :CN-

Question 22

How do you name nitriles?

Answer 22

Nitrile groups have to be at the end of a chain. Start numbering the chain from the C in the CN

eg
3-methylbutanenitrile

Question 23

Explain nucleophillic substitution with ammonia?

Answer 23

Change in functional group: Haloalkane –> amine

Reagent: NH3 dissolved in ethanol

Conditions: Heating under pressure (in a sealed tube)

Mechanism: nucleophillic substitution

Type of reagent: Nucleophile, :NH3

Question 24

How do you name amines?

Answer 24

3 C’s in carbon chain:

Propylamine or propan-1-amine

Question 25

Define elimination?

Answer 25

Removal of small molecule (often water) from the organic molecule

Question 26

Explain elimination with alcoholic hydroxide ions?

Answer 26

Change in functional group: Haloalkane –> alkene

Reagents: Potassium (or sodium) hydroxide

Conditions: in ethanol, heat

Mechanism: elimination

Type of reagent: base, OH-

Question 27

How do you determine whether a reaction is going to be a substitution or elimination?

Answer 27

Type of reagent:
Aqueous- substitution
Alcoholic- elimination

Structure of Haloalkane:
Primary tends towards substitution
Tertiary tends towards elimination

Often a mixture of products from both elimination and substitution occurs