Halogenoalkanes Flashcards
How are halogenoalkanes classified
They are either primary, secondary or tertiary depending on the number of Carbon atoms that the carbon bonded to the halogen is bonded to
Primary 1
Secondary 2
Tertiary 3
What are the products of a free radical substitution mechanism
A halogenoalkane and a hydrogen halide
What is step 1 of free radical substitution
Initiation
UV light provides energy to break the Cl2 bond the halogen bond
What is step 2 of free radical substitution
Propagation (part 1)
The Cl free radical reacts with CH4 to form a CH3 free radical (methyl radical) and HCl
What is step 3 of free radical substitution
The CH3 free radical react with chlorine molecules o for, chloromethane and more chlorine free radicals
What is step 4 of free radical substitution
Termination
Free radicals react with each other to form stable molecules
How do you write the overall equation for free radical substitution
By combining the 2 equations from the propagation stage
What does Ozone do? (3)
It reduces the amount of UV radiation that reaches the earth which is useful because too much causes skin cancer, eye damage and possible harm to crops
What destroys the Ozone
Chemicals called CFC’s (chlorofluorocarbons)
What are 3 properties of chlorofluorocarbons
Less reactive than alkanes since C-F bond is so strong
They diffuse into the upper atmosphere unchanged
Under the influence of intense ultraviolet radiation in the stratosphere they decompose to from radicals
Describe the chemical reaction between Ozone and CFC’s
(5)
UV causes chlorine atom to break away from CFC molecule
The free chlorine atom hits an Ozone molecule
The chlorine atom pulls one Oxygen atom away
A free oxygen atoms hits the chlorine monoxide molecule
Another free radical chlorine atom is formed
What does chlorine acts as in the breakdown of Ozone
A catalyst as its not used up in the reaction and is regenerated
It also is not shown in the overall equation
What are safer alternatives to CFC’s and why are they safer alternatives
Hydrochlorofluorocarbons (HCFC)
Hydrofluorocarbons (HFC)
They are safer because the C-F bond us stronger than the C-H bond so they can’t produce free radicals as ,Amy break down before even reaching the stratosphere
How have aerosols been made safer
They use pump spray systems or use nitrogen as the propellant
How have industrial fridges been made safer
They use ammonia or hydrocarbons as the coolant gas
How are foamed polymers made safely
They use carbon dioxide or pentane to make them
What are nucleophiles
They are lone pair donors to electron deficient carbons / species
When hydroxide ions are the nucleophile, what is they reagent and the condition
Potassium hydroxide
Warm aqueous solvent
When cyanide ions are the nucleophile in nucleophilic substitution, what is the reagent and the conditions
Potassium cyanide CN-
Ethanol solvent
When ammonia is the nucleophile in nucleophilic substitution, what is the reagent and the conditions
Ammonia is reagent
Excss ammonia in a warm ethanol solvent in a sealed tube
Why is excess ammonia used in nucleophilic substitution when used as a nucleophile
Because it forms prpan -1- amine which also has a lone pair and can therefore act as a nucleophile and react with unreacted halogenoalkanes the excess ammonia minimises further substitutions from occurring
What 2 factors affect how easily the carbon-halogen bond breaks/ the reactivity halogenoalaknes in terms of nucleophilic substitution and which one is the more important factor
The Polarity of the carbon-halogen bond (more polar means easily attacked by nucleophiles)
The strength of tye carbon halogen bond (weaker bond means breaks easier and is more reactive)
More important is the bond enthalpy as experiments show reactivity increases down the halogen group
What are the products when halogenoalkanes undergo elimination reactions (3)
Alkenes
Water
A salt
What reaction conditions favour elimination (3)
Ethanolic solvent
Higher temperatures
Increased branching
What reaction conditions favourite substitution
Aqueous solvent
Lower temperatures
Little / No branching
What does the hydroxide ion act as in elimination reactions and why
What must you remember when outlining elimination mechanisms
A base because it’s dissolved in ethanol
The hydrogen us removed from the carbon next to a carbon bonded to the halogen
What happens if the carbon chain length is 4 or more in terms of elimination
A mixture of positional isomers may be formed when it undergoes elimination reactions
