Haloalkanes Flashcards
What is the general formula of haloalkanes?
CnH2n+1X
What are haloalkanes?
alkanes that contain a halogen
What are the three ways haloalkanes can be classified?
Primary, secondary, tertiary
What is a primary haloalkane?
A haloalkane where the halogen is attached to a C, which is only attached to one other C
What is a secondary haloalkane?
A haloalkane where the halogen is attached to a C, which is only attached to two other C’s
What is a tertiary haloalkane?
A haloalkane where the halogen is attached to a C, which is attached to three other C’s
How are haloalkanes formed?
When you combine an alkane with a halogen in the presence of bright sunlight or a UV lamp
Why is light essential in the formation of haloalkanes?
it starts the reaction
What is the reaction to form a haloalkane classed as?
a substitution reaction
What is the reaction to form a haloalkane?
it is a chain reaction called a free radical substitution mechanism - it has three stages
What are the three stages of the free radical substitution mechanism?
Initiation, propagation and termination
What is the definition of the initiation stage?
defined as a stage which results in the formation of a free radical
What is a free radical?
a species which has a single unpaired electron on its outer shell
What is a key property of a free radical?
they are extremely reactive as they no longer have a noble gas configuration
What is involved in the initiation stage?
This stage involves the breaking of the covalent bond between the diatomic halogen molecule to form two halogen atoms. UV light provides the energy needed to break this bond. Both halogen atoms have donated one pair of electrons into the covalent bond, and when the bond breaks they each take back their electron. This is called HOMOLYTIC FISSION. As soon as the halide radicals have been formed this triggers the next stage in the mechanism.
What is first stage in a free radical substitution mechanism?
initiation
What is the second stage in a free radical substitution mechanism?
propagation
What is the third stage in a free radical substitution mechanism?
termination
What is the definition of the propagation stage?
defined as a stage which prolongs the life of the free radical
How many steps are involved in the propagation stage?
two
What is involved in the first step of the propagation stage?
The halide radical collides with the alkane and removes a hydrogen atom. This produces HX. The alkane however is now missing one hydrogen atom and so this is now a radical as one carbon will have a single unpaired electron.
What is involved in the second step of the propagation stage?
During the initiation stage not all of the halogen will react and some will remain as the diatomic molecule. In the second propagation step the alkane radical will collide with one of these diatomic molecules and split it apart. This produces another halide radical and haloalkane (the desired product).
How do the propagation steps show how the free radical substitution mechanism is as a chain reaction?
The second propagation reaction forms the desired haloalkane product, but also regenerates another chloride radical. This can then react again and again.
What is the definition of the termination stage?
defined as a stage in which free radicals are removed
When can termination occur?
when any two free radicals collide and form a covalent bond between them - this can happen in one of 3 combinations
What are the three combinations of two free radicals colliding?
- two halide radicals
- two alkane radicals
- one halide radical and one alkane radical
(In an exam if you’re only given enough space to draw out one termination reaction you should always select the reaction which produces the haloalkane product)
