3.3.3 Halogenoalkanes Flashcards
What is a halogenoalkane?
An alkane in which one or more of the hydrogen atoms have been replaced by a halogen atom (fluorine, chlorine, bromine or iodine)
What are the three types of halogenoalkane?
Primary
Secondary
tertiary
How are haloalkanes classified as primary, secondary or tertiary?
According to the position of the halogen atom in the molecule.
What is a primary haloalkane?
When there is one alkyl group attached to the carbon with the halogen atom.
What is a secondary haloalkane?
When there are two alkyl groups attached to the carbon with the halogen atom.
What is a tertiary haloalkane?
When there are three alkyl groups attached to the carbon with the halogen.
What is the important property of the carbon-halogen bond?
It is a polar bond, so has a permanent dipole.
This is because of the large difference in electronegativity, so electron density lies towards the halogen, creating a delta + and delta - region.
What happens to the boiling point of haloalkanes as they increase in size and why?
The boiling points increase as the haloalkanes increase in size (chain gets longer).
This means that there are greater van-der-walls forces between molecules which increases the boiling point.
What happens to the boiling point of haloalkanes compared to equivalent alkanes and why?
Haloalkanes have a higher boiling point than the equivalent alkanes, as the polar halogen-carbon bond means permanent dipole-dipole forces are present as well as van-der-walls forces.
What happens to the boiling point of haloalkanes as you go down group 7?
As you go down group 7, the boiling points of the haloalkanes increases, because the size of the halogen atom increases. This increases the van-der-waals forces between molecules, which increases the boiling point.
How does the c-x bond polarity determine reactivity of a halogenoalkane?
A halogenoalkane that contains a more electronegative halogen would be more reactive than those that contain a less electronegative halogen.
How does c-x bond enthalpy affect the reactivity of a halogenoalkane?
A halogenoalkane with a lower c-x bond enthalpy would be more reactive than those with a higher bond enthalpy.
What is the order of electronegativity of the halogens?
Most electronegative = fluorine
Chlorine
Bromine
Least electronegative = iodine
What is the order of bond enthalpies of the halogens when bonded with carbon?
Highest bond enthalpy = fluorine
Chlorine
Bromine
Lowest bond enthalpy = iodine
What does the reactivity of halogenoalkanes depend on?
The bond enthalpy
What are the conditions for elimination reactions of halogenoalkanes using hydroxide ions?
Warm ethanolic sodium hydroxide (source of OH- ions)
Carried out under reflux
What does elimination in halogenoalkanes using OH- ions form?
Alkene
Water
Halide ion (or metal halide depending on the source of the OH- ions)
What type of halogenoalkanes are the most reactive and why?
Tertiary halogenoalkanes are the most reactive as they form a stable intermediate which allows the nucleophile to react faster.
Describe the elimination of halogenoalkanes using OH- ions?
The OH- ions attacks the hydrogen atom on the carbon adjacent to the carbon with the halogen on.
OH- acts as a base (accepting proton/hydrogen) to form water.
The electrons in the C-H bond move to form a carbon-carbon DOUBLE bond.
The C-X (carbon halogen) bond is broken, as carbon can only form 4 bonds.
The halogen takes both electrons in the C-X bond.
This forms an alkene, a water molecule and a halide ion (or metal halide).
How can we use elimination to test the reactivity of the C-X bond?
We can react the halide ion produced with silver nitrate to form a precipitate.
We can measure how fast the precipitate forms to measure the rate of reaction and therefore reactivity.
What are the results from the elimination reaction test for the C-X bond reactivity?
Silver iodide forms fastest which shows that iodoalkanes are the most reactive halogenoalkanes.
They have the lowest bond enthalpy, which shows that this is the factor that affects halogenoalkanes reactivity.
What must we check for in the alkane produced by elimination of halogenoalkanes by OH- ions?
E-Z isomerism
What is a mechanism?
A pathway that a chemical reaction takes in order to convert the reactants into the products.
Why do halogenoalkanes undergo nucleophilic substitution?
Due to the polar C-X bond
