Alkanes Flashcards
What is a hydrocarbon?
A molecule that only contains carbon and hydrogen
And is thus organic
Alkane
A homologous series of hydrocarbon that does not have a functional group
Represented as being saturated
General formula for alkanes
CnH2n+2
Properties of alkanes
Saturated
Non polar
Increasing boiling and melting points up the homologous series with increasing carbon chain
Increasing viscosity with increasing carbon chain
Decreasing volatility with increasing carbon chain
types: cycloalkanes, branched alkanes and linear alkanes
Saturated meaning
Alkanes contain no double bonds and thus the maximum possible number of hydrogen atoms
How does the boiling/melting point in an alkane change?
Increases up the homologous series so more energy required to melt/boil it with a longer carbon chain
Because larger molecules means larger surface area for more London forces
Thus requires more energy to overcome
How does branching affect the melting and boiling points?
Decreases it because branching causes less points of contact between molecules, so less London forces due to less induced dipoles
Therefore lowers bp/mp
What do alkanes reacting with a halogen form?
A halogenoalkane
How do alkanes and halogens react?
In photochemical reactions ie under the presence of UV light
Why does the reaction between a halogen and an alkane require UV light?
Because UV light provides energy to break the bond between the halogens in the dihalogen molecule to form 2 radicals
What is the name for halogen + alkane reaction?
Radical substitution
Because a hydrogen atom on the alkane is substituted for the radical halogen (a halogen atom with an unpaired electron)
3 stages in formation of halogenoalkanes
Initiation
Propagation
Termination
Initiation: summary
The formation of halogen radicals where the bond between halogen molecule is broken in presence of UV (photochemical reaction)
Initiation: the bond breaks between halogen molecules details?
Named homolytic fission
Because the bond (shared pair of 2 electrons) is broken by one electron going to one halogen atom and the other going to other halogen atom = 2 radicals
Initiation: how is the homolytic fission shown in a mechanism?
There are 2 curly arrows from the bond to each chlorine atom. With only half an arrow head to show 1 electron moved
Then arrow to show how in presence of UV light, 2 radicals are formed with an unpaired electron
Propagation: summary
Radicals are used up only to produce more radicals
Propagation: alkane + halogen radical
Forms a hydrogen halide because the hydrogen from the alkane is removed to form hydrogen halide because it reacts with halogen radical
And thus the remaining alkane becomes a radical because it lost the hydrogen
Propagation: alkane radical + halogen
One of the halogen atoms react with alkane radical to form the halogenoalkane and the other halogen atom remains a radical
Propagation: too much halogen issue
If there is too much of the halogen:
The desired halogenoalkane can react further with the halogen radical (because too much of it) to form a dihalogenoalkane by substituting it from another hydrogen
Can repeat to form trihalogenoalkanes etc
Propagation: when does this stop?
When all reactant molecule have been wiped so only radicals remain
Termination: summary
The removal of radicals by reacting radicals together to form stable molecules
Termination: What are the 3 possible reactions?
Halogen radical + halogen radical = halogen diatomic molecule
Halogen radical + alkane radical = halogenoalkane (desired product)
Alkane radical + alkane radical = long chain alkane molecule
Main problems with radical substitution
You form a mixture of products which is not the desired product
You can form a mixture of structural isomers
How can a mixture of products be formed?
Having excess halogens
So the halogen radicals react with desired product (halogenoalkane) by displacing a hydrogen from it or form a dihalogenoalkane
Then repeated = trihalogenoalkane, tetrahalogenoalkane which is repeated till no more hydrogen left to be displaced
How to reduce the problem of having a mixture of products
Reacting with excess alkane
So a halogen radical has greater chance of colliding with excess alkane molecule than the desired product
How can a mixture of structural isomers be formed?
If you start with a long chain alkane eg propane then the halogen radical can substitute any of the hydrogen along the chain to form a mixture of structural isomers eg 1-chloropropane or 2-chloropropane
