Chapter 12 - Alkanes Flashcards
What are alkanes?
Saturated hydrocarbons
General formula of alkanes
Cn H2n+2
What will every carbon atom in an alkane have?
Four single bonds with other carbon atoms
What are saturated hydrocarbons?
All carbon-carbon bonds are single
What is the name of the bond between the carbon atoms in an alkane?
A sigma bond
What is a sigma bond?
The result of an overlap of two orbitals, one from each bonding atom
How many electrons are in a sigma bond?
2
Where is a sigma bond positioned?
On a line directly between two bonding atoms
What are the two bonds where sigma bonds can occur?
C-H or C-C
What is the bond angle in an alkane and why?
109.5, the four pairs of bonding electrons repel eachother equally
What structure does an alkane form?
A tetrahedral structure
Why are alkanes not rigid?
The sigma bonds act as an axes which the shape can rotate around
How is crude oil separated into fractions?
Fractional distillation
As chain length increases, what happens to the boiling point of alkanes?
It increases
Why does fractional distillation work?
Alkanes have different boiling points, dependent on chain length
What type of bonds are responsible for the boiling point of alkanes?
London forces between the molecules
Where are covalent bonds found?
Inside molecules
Where are London forces found?
Between molecules
Why do alkanes with a longer chain length have higher boiling points?
The longer the carbon chain, the larger the surface area available over which London forces can act. There are also more electrons to interact. This means more energy is needed to overcome the London forces
Will branched isomers have higher or lower boiling points?
Lower
Why do branched chain alkanes have lower points?
The chains can’t pack as closely together. There are fewer points of contact and a smaller surface area of the molecules, so the London forces are weaker
Why are alkanes very unreactive? (3)
C-C and C-H sigma bonds are very strong
C-C bonds are non-polar
Carbon and hydrogen have similar electronegativities, so the C-H bond is considered non-polar
What products are produced when an alkane reacts completely with oxygen?
Carbon dioxide and water
What is the name of the reaction between oxygen and alkanes?
Combustion
Why are alkanes commonly used as fuels?
Alkanes release a lot of energy when they combust and don’t release toxic products
Why will global warming mean more methane is released?
Lots of methane is stored in the tundra, which will be released as the tundra thaws
How is methane formed?
Anaerobic respiration
Before liquid alkanes can combust, what must happen?
They have to be vaporised because combustion only occurs between gases
Why will larger alkanes release more energy when burnt?
They have more bonds to react
What will be produced when alkanes incompletely combust?
Carbon monoxide and water
What is it called when alkanes react in limited supplies of oxygen?
Incomplete combustion
Why is carbon monoxide produced when an alkane undergoes incomplete combustion?
The hydrogen atoms are always oxidised to form water but if there isn’t enough oxygen, the carbon will be incompletely oxidised and form CO not CO2
Why is carbon monoxide deadly?
It binds to haemoglobin in cells better than oxygen can, so less oxygen can be carried around the body. This can lead to oxygen deprivation
What is produced when an alkane reacts with a halogen?
A haloalkane
What must be present for a haloalkane to be produced?
UV radiation - potentially from sunlight
Why is the production of a haloalkane an example of a substitution reaction?
A hydrogen atom in the alkane is replaced witha achlroine or bromine atom, e.g.:
CH4 + Cl2 -> CH3Cl + HCl
What is the mechanism for the bromination of methane an example of?
Radical substitution
What are the three stages of this radical substitution?
Initiation
Propagation
Termination
What happens during initiation?
Sunlight provides the energy to break the Br-Br or Cl-Cl bond
Homolytic fission occurs - the bond splits equally and each atom gains one electron from the bond
The atoms are now free radicals because of this unpaired electron
Br - Br -> Br. + .Br
What is photodissociation?
Sunlight providing the energy to break a bond
What happens during the first propagation step?
A bromine (or chlorine) radical reacts with a C-H bond in the methane to form a methyl radical (.CH3) and a molecule of hydrogen(halide)
What are the two reactions that occur during propagation?
1) CH4 + Br. -> .CH3 + HBr
2) .CH3 + Br2 -> Ch3Br + Br.
What happens during the second propagation step?
Each methyl radical reacts with another bromine molecule to form the organic product of bromomethane and another bromine radical. This radical then reacts with a CH4 molecule and the first step repeats
What happens during termination?
Two radicals collide to form a stable molecule. There are lots of ways this could happen, e.g.:
- Br. + .Br -> Br2
- .CH3 + .CH3 -> C2H6
When will termination occur?
When two radicals collide to form a stable molecule
What is radical substitution used for?
Making haloalkanes
What are the limitations of radical substitution?
When CH3Br is formed in the second propagation step, further substitution can occur. Another bromine radical can collide with the molecule and replace a hydrogen atom, forming CH2Br2. This can happen until all of the hydrogen atoms are used up (CBr4)
What is another problem of free radical substitution which is a result of chain length?
It can take place at any point along the carbon chain, so a mixture of isomers can be formed
