Alkanes and Nomenclature Flashcards
What is the alkanes general formula?
Cn H2n+2.
They’ve only got carbon and hydrogen atoms- they’re hydrocarbons.
How many bonds does each carbon atom in an alkane have?
Four single bonds with other atoms.
It’s impossible for carbon to make more than four bonds, so alkanes are saturated.
Give examples of alkanes.
Methane Ethane Propane Butane Pentane
What are cyclohexanes?
They have a ring of carbon atoms with two hydrogens attached to each carbon.
Cyclohexane’s have two fewer hydrogens than other alkanes so they have a general formula of Cn H2n but they’re still saturated.
1- Stem and Alkane?
Meth
Methane
2-Stem and Alkane?
Eth
Ethane
3- Stem and Alkane?
Prop
Propane
4- Stem and Alkane?
But
Butane
5- Stem and Alkane?
Pent
Pentane
6- Stem and Alkane?
Hex
Hexane
7- Stem and Alkane?
Hept
Heptane
8- Stem and Alkane?
Oct
Octane
9- Stem and Alkane?
Non
Nonane
10- Stem and Alkane?
Dec
Decane
What is the side chain prefix for a side chain with 1 carbon atom?
Methyl
What is the side chain prefix for a side chain with 2 carbon atoms?
Ethyl
What is the side chain prefix for a side chain with 3 carbon atoms?
Propyl
What is the side chain prefix for a side chain with 4 carbon atoms?
Butyl
What is the side chain prefix for a side chain with 5 carbon atoms?
Pentyl
What is the side chain prefix for a side chain with 6 carbon atoms?
Hexyl
What shape do carbon molecules in an alkane form?
A tetrahedral shape around each carbon.
Each carbon atom has four pairs of bonding electrons around it that all repel each other equally.
How is the boiling point of an alkane dependent on its size and shape?
The smallest alkanes, like methane, are gases at room temperature and pressure- they’ve got very low boiling points.
Larger alkanes are liquids- they have higher boiling points.
This is due to differences in there intermolecular forces.
Why is the boiling point of an alkane dependent on its size and shape?
Alkanes have covalent bond inside the molecules.
Between the molecules, there are van der Waals force which hold them all together.
The longer the carbon chain, the more van der Waals forces there are.
This is because they have a larger molecular surface area so there is more surface contact between molecules.
As a result, more electrons interact and it takes more energy to overcome the forces and separate them.
What is the complete combustion reaction?
If you burn (oxidise) alkanes with enough oxygen, you get carbon dioxide and water.
Why do larger alkanes release more energy per mole?
When alkanes burn, some energy is used to break the covalent bonds but more energy is released by forming new product bonds in CO2 and H2O- larger alkanes form more CO2 and H2O.
What is propane used for?
Central heating and cooking fuel.
What is butane used for?
Bottled and sold as camping gas.
What is petrol and diesel used for?
Both made up of a mixture of alkanes and are used as fuels for transport.
What is the incomplete combustion reaction?
If there isn’t much oxygen around an alkane will still burn, but it will produce carbon monoxide and water.
Why is incomplete combustion a problem?
Carbon monoxide is poisonous- the oxygen in your body is carried around by haemoglobin.
Carbon monoxide is better at binding to heamoglobin than oxygen is.
This means less oxygen will reach your cells.
Describe the molecular shape around each carbon atom in an alkane.
Molecules form tetrahedral shapes around each carbon.
Name two factors that affect the boiling point of an alkane.
The length of the carbon-carbon chain and the amount of branching in the molecule.
What are the products of the complete and incomplete combustion of alkanes?
The products of complete combustion are carbon dioxide and water.
The products of incomplete combustion are carbon monoxide and water (and sometimes particulate carbon and carbon dioxide).
The structure of an Alkane, as with all Hydrocarbons, can be represented with what four things?
- A 3D model, with all the atoms and bonds shown, modelled in 3 dimensions.
- Full Structural Formula, with all the atoms and bonds shown.
- Skeletal Formula, with only the bonds between the Carbon atoms shown.
- Shortened Structural Formula, a linear form where all the atoms are shown but no bonds.
What are the ten first alkanes in the homologous series?
Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane Decane
Why are alkanes inert?
They have a lack of polarity due to the fact carbon and hydrogen have similar electronegativities- causing them to be non polar.
What can substitute the hydrogen atoms in an alkane with halogen atoms?
UV light
What is the reaction for chlorine mixing with methane?
CH4 (g) + Cl2 (g) -> CH3Cl(g) + HCl (g)
If a reaction requires ultraviolet light what is it therefore known as?
A photochemical reaction.
What is the absorption of light energy causing a bond to break known as?
Photodissociation.
What happens when UV light breaks a Cl-Cl bond in the initiation step?
Homolytic fission and two chlorine atoms are formed, each having seven electrons in their router shell.
The chlorine atoms each have one unpaired electron and are thus free radicals.
Write the equation for the bond breaking of Cl-Cl in the initiation step.
Cl-Cl (g) -> Cl. (g) + Cl. (g)
What does the reaction of a chlorine free radical with a methane molecule produce?
Hydrogen chloride and a CH3. methyl free radical
What can a methyl free radical and a chlorine molecule react to produce?
Chloromethane and a new chlorine free radical (a chain reaction).
How long will the propagation reaction go on for?
As long as there is a supply of methane molecules and undissociated chlorine.
What is the termination stage?
Take place when two free radicals meet which removes the free radicals and disrupts the propagation steps.
Summarise alkane substitution.
The Cl-Cl or Br-Br bond undergoes homolytic fission in ultraviolet light producing reactive Cl or Br free radicals.
This initation step of free-radical substitution is followed by propagation steps involving a chain reaction which regenerates the halogen free radicals.
Termination of the reaction may occur, for example, when two radicals combine.
What does the boiling point of an alkane depend on?
Its size and shape.
Alkanes have covalent bonds inside the molecules and van der Waals forces between the molecules.
The longer the carbon chain, the stronger the forces because there is a greater molecular surface area and more electrons to interact.
It therefore takes more energy to overcome.
Why does a branched alkane chain have a lower boiling point than straight chained alkanes?
Branched chain alkanes can’t pack closely together and they have smaller molecular surface areas so van der Waals forces are reduced.
