2.8- Haloalkanes Flashcards
What is a reaction’s mechanism?
How it proceeds step by step
Why do alkanes not react with many reagents?
The C-C and the C-H bonds are relatively strong
How are chloro/bromo alkanes synthesised?
Mechanism is called a free radical substitution
Step one: Initiation
Step two: Propagation
Step three: Termination
Eg
CH4 + Cl2 –> CH3Cl + HCl
What is the initiation step?
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
What is homolytic fission?
When a bond breaks and each atom gets one electron from the covalent bond forming free radicals
Define free radical?
A free radical is a reactive species which possess an unpaired electron
Free radicals do not have a charge and are represented by a .
What is the propagation step?
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
What is the termination step?
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
What are some of the uses of Haloalkanes?
Chloroalkanes and CFC’s can be used as solvents
CH3CCl3 was used as the solvent in dry cleaning
Refrigerants, pesticides and aerosol propellants
Why have many of the uses of Haloalkanes now been stopped?
Due to the toxicity of them and their detrimental effect on the atmosphere
Why is ozone beneficial?
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
Why are CFC’s bad for the environment?
Cause holes to form in the ozone layer
How do chlorine atoms catalyse the decomposition of ozone?
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
How can Haloalkanes be classified?
Classified as primary, secondary or tertiary depending on the number of carbon atoms attached to the C-X functional group
What is a nucleophile?
Electron pair donor
Always have a lone pair
Eg
:OH-, :NH3, CN-
What is nucleophillic substitution?
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
What is a curly arrow and why do we use it?
Used to show the movement of two electrons
It will always start from a lone pair of electrons or the centre of a bond
What does the rate of reaction of Haloalkanes depend on?
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
What is hydrolysis?
The splitting of a molecule (eg Haloalkane) by a reaction with water
How can the rate of hydrolysis reactions be compared?
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
Explain nucleophillic substitution with hydroxide ions?
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-
Explain nucleophillic substitution with cyanide ions?
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-
How do you name nitriles?
Nitrile groups have to be at the end of a chain. Start numbering the chain from the C in the CN
eg
3-methylbutanenitrile
Explain nucleophillic substitution with ammonia?
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
How do you name amines?
3 C’s in carbon chain:
Propylamine or propan-1-amine
Define elimination?
Removal of small molecule (often water) from the organic molecule
Explain elimination with alcoholic hydroxide ions?
Change in functional group: Haloalkane –> alkene
Reagents: Potassium (or sodium) hydroxide
Conditions: in ethanol, heat
Mechanism: elimination
Type of reagent: base, OH-
How do you determine whether a reaction is going to be a substitution or elimination?
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
