Summary
Summary Alkanes and Alkenes
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- OCR
Covers all points on the OCR Chemistry specification for the alkanes and alkenes topic (4.1.2 and 4.1.3)
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Topic 4.2 Hydrocarbons NotesSaturated hydrocarbons
Alkanes are saturated hydrocarbons because they contain hydrogen and carbon only. Carbon atoms
have only C-C single bonds, meaning the maximum number of hydrogen atoms can bond and each
carbon atom can bond to 4 other atoms.
Shape and bond angle in alkanes
Bond angle: 109.5
Carbon electronic structure: 1s22s22p2
Ground state carbon. One electron ‘promoted’ All four orbitals blended to
Only 2 unpaired from 2s to 2p2 orbital, to create four ‘hybrid’ sp3
electrons, so can only give four single occupied orbitals of equal energy.
make 2 covalent bonds. orbitals. Can now make 4 covalent
bonds.
1) Three 2p orbitals and a single 2s orbital ‘hybridise’ (blend) to make 4 identical sp 3 ‘hybrid’
orbitals
2) 4 unhybridised orbitals (2s, 2px, 2py, 2px) – three higher energy 2p orbitals and one lower
energy 2s orbital
3) Hybridise to make 4 hybridised sp3 orbitals of identical energy which have 25% s character
and 75% p character
4) Oriented at 109.5o relative to each other
Shape: tetrahedral
Methane: a tetrahedral
molecule
H-C-H bond angle: 109.5o
,Topic 4.2 Hydrocarbons Notes
Sigma bonds:
In the next alkane, ethane, C2H6, the C-C bond is formed by overlap of an sp 3 orbital from each C
atom. This type of bond, formed by end-to-end overlap of atomic orbitals, is known as a sigma bond.
Free rotation around
sigma bond.
Ethane:
All HCH and HCC bond angles:
109.5o
Relationship between:
Chain length and boiling point
The longer the chain, the higher the boiling point, this is because there are more points of contact
therefore van der Waals’ forces are stronger.
Amount of branching and boiling point
As amount of branching increasing, boiling point decreases because more branching decreases
surface area, there are as a result, less points of contact, so van der Waals’ forces are weaker.
Low reactivity of alkenes
Alkanes contain only carbon-carbon and carbon-hydrogen single bonds which are strong, these
bonds are difficult to overcome so melting and boiling points are high, making the molecules
unreactive. Alkanes contain no double bonds, so no areas of electron density attract electrophiles.
The C-C bond in alkanes are non-polar and the C-H bonds are of such low polarity that they do no
attract nucleophiles. Alkanes do react with free radicals.
, Topic 4.2 Hydrocarbons Notes
Combustion of alkanes
= burning in air
Combustion may be complete or incomplete, depending upon whether there is a sufficient supply of
oxygen.
Complete combustion:
Occurs in the presence of excess oxygen.
All C ends up as CO2.
All H ends up as water vapour.
Example: butane
C4H10 + 61/2O2 4CO2 + 5H2O
Incomplete combustion:
Occurs when the supply of oxygen is limited.
A range of oxidation products are possible, including carbon monoxide and soot.
Eg. C4H10 + 41/2O2 4CO + 5H2O
Carbon monoxide poisoning:
Sources –
House fires
Car exhausts
Boilers / furnaces
Cigarette smoke
Car left running in attached garage
Portable generators
Corroded or disconnected water heater vent pipe
Portable kerosene or gas heaters
Loose or broken vent pipes
Improperly installed kitchen range or vent
Operating a grill indoors or in a garage
Gas or wood-burning fireplace
CO has 200x the affinity for haemoglobin than O2, eg. an atmosphere of 20% and 0.1% CO
50:50 carboxy/oxy-haemoglobin saturation.
Symptoms –
Headaches
Nausea
Dizziness
Breathlessness
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