Chapter 12 - Alkanes Flashcards
What are alkanes?
Saturated hydrocarbons
General formula of alkanes
CnH2n+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?
There are fewer surface points of contact between molecules of the branched out, giving fewer London forces. The shape of the molecule prevents the branched molecules getting as close together as straight-chain molecules hence decreasing the intermolecular force further
Why are alkanes very unreactive?
- C-H and C-C Sigma bond are strong
- C-C Bonds are non-polar
- The electronegativity of carbon and hydrogen is so similar that the C-H bond is non-polar
What are alkanes uses?
•fuel as readily available/easy to transport/burn in plentiful supplies of oxygen
What is produced with the combustion of alkanes and oxygen?
Carbon dioxide and water
What is produced in an incomplete combustion of alkanes and oxygen?
Carbon monoxide/soot and water
What is carbon monoxide?
Colourless, odourless and highly toxic gas which combines with haemoglobin preventing the transport oxygen around the body
In radical substitution what are the three stages take place?
- Initiation
- propagation
- termination
What is initiation phase from radical substitution?
The first stage in a radical reaction in which a radical starts when a covalent bond is broken by homolytic fission of a covalent bond
What is propagation phase from radical substitution?
The 2nd phase that continue a free radical reaction, in which a radical reacts with a reactive molecules to form a new molecule and another radical , causing a chain reaction
What is termination phase from radical substitution?
The step at the end of the radical substitution which two radicals combine to form a molecule
What are the limitations of a 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 the mechanism for the bromination of methane an example of?
Radical substitution
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
What is radical substitution used for?
Making haloalkanes
When will termination occur?
When two radicals collide to form a stable molecule
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 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
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 are the two reactions that occur during propagation?
1) CH4 + Br. -> .CH3 + HBr
2) .CH3 + Br2 -> Ch3Br + 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)
