3.3 Halogenoalkanes Flashcards
What is a halogenoalkane?
Compounds which one or more hydrogen atoms in an alkane have been replaced by halogen atoms
What are the halogen prefixes?
Fluoro
Chloro
Bromo
Iodo
What are some nomenclature rules for halogenoalkanes?
The different halogens are listed alphabetically
What are some uses for halogenoalkanes?
Refrigerator Solvents Anaesthetics Aerosol propellants Insecticides
What is a primary, secondary and tertiary structure?
Primary- only has 1 other carbon attached to the carbon with the halogen attached
Secondary- has 2 carbons attached to the carbon with the halogen attached
Tertiary - has 3 carbons attached to the carbon with the halogen attached
What is the ozone layer?
O3 forms naturally in the atmosphere
It protects us from harmful UV radiation
What affects the ozone layer?
CFC’s - Chlorofluorocarbons
They form holes in the layer
What are used instead of CFC’s?
HFC’s - Hydrofluorocarbons
What happens when CFC’s reach the ozone layer?
Chlorine free radicals are formed when CFC’s are exposed to UV radiation
What reaction shows what happens to the ozone layer? Type of reaction?
O3 + Cl• -> ClO• + O2
ClO• + O3 -> 2O2 + Cl•
Chain reaction
Name the types of reactions that can take place with halogenoalkanes?
Free radical substitution
Nucleophilic substitution
Elimination
What is free radical substitution?
A chain reaction
Where one thing replaces another with 3 stages
What are the three stages of free radical substitution?
Initiation
Propagation
Termination
What is the initiation mechanism?
Cl2 ——> 2Cl•
With UV light over the arrow
What takes place in initiation?
Homolytic bond fission
When the bond breaks each atom takes an electron from the bond now called free radicals
What is meant by propagation step?
Where free radicals are used up and then regenerated
What is the mechanism for propagation in free radical substitution? (Use CH4 and Cl)
CH4 + Cl• -> •CH3 + HCl
•CH3 + Cl2 -> CH3Cl + Cl•
What is the mechanism for termination in free radical substitution? (Use CH4 and Cl)
Cl• + Cl• -> Cl2
•CH3 + Cl• -> CH3Cl
•CH3 + •CH3 -> C2H6
In free radical substitution how can you make sure you get more of one particular product?
Use an excess of the reactant that will help produce more of the product
Therefore collisions are more likely between the one in excess and the other free radical
What are the strongest to weakest bonds of carbon-halogens?
Strongest C-F C-Cl C-Br C-I Weakest
What is a nucleophile? What do they do?
Attracts positive charge
Attack a halogenoalkane and form a bond with it
As the carbon will have a delta + charge therefore the nucleophile will be attracted to it
Examples of a nucleophile?
:OH- (hydroxide ion)
:NH3 (ammonia)
:CN- (cyanide ion)
How do you know it’s likely nucleophilic substitution?
A polar molecule - nucleophilic
Saturated - substitution
How does carbon-halogen bonds affect reactions?
The weaker the carbon-halogen bond the more reactive it will be
In nucleophilic substitution how does a nucleophile react with a halogenoalkane? (Where do the arrows go)
From the lone pair of the nucleophile to the carbon
From the middle of the carbon halogen bond to the halogen
In nucleophilic substitution what does the arrow from the middle of the carbon to the halogen show?
Shows the electrons that were in the bond but now broken they go to the carbon
What are the products in nucleophilic substitution?
The nucleophile attached to the alkane
And
A free radical halogen
What are the arrows of nucleophilic substitution if the reaction requires two steps? (Ammonia)
The arrow goes from the middle of the hydrogen/nitrogen bond on to the nitrogen
The arrow goes from the ammonia free radical to the hydrogen
What are the final products in nucleophilic substitution of a halogenoalkane and ammonia?
The alkane with NH2 attached
Ammonium ion
The halogen free radical
What is produced in the nucleophilic substitution of a halogenoalkane and ammonia in the first step?
An unstable intermediate that has to lose something in order to become a stable molecule
An alkane with NH3 attached
What are the conditions for nucleophilic substitution?
In water or aqueous solution
Using ethanol to enable the reactants to mix together
Low temperature
What are nucleophilic reactions useful for making?
Alcohols
Nitriles
Amines
Used for drugs in medicine
What is used to heat any organic chemicals?
Reflux apparatus
What is used in reflux apparatus?
A condenser
To prevent the liquid from evaporating therefore you have product to collect
What is an elimination reaction?
A reaction between a halogenoalkane and an OH- ion
Something is lost and nothing is gained
Where do the arrows go in elimination reactions?
From the :OH- to the hydrogen
From the middle of the c-h bond you drew the other arrow to - to the middle of the c-c bond
From the middle of the carbon-halogen bond to the halogen
What is produced in elimination reactions?
An alkene
Water
A halogen free radical
What conditions do elimination reactions need?
Ethanolic conditions and high temperature
