Alkanes
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- Created by: Kyra Mullarney
- Created on: 19-04-19 11:22
What are alkanes?
- Main components of natural gas and crude oil.
- Amongst the most stable organic compounds, and their lack of reactivity has allowed crude oil deposits to remian in the earth for many millions of years.
- Have the general formula CnH2n+2
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Bonding in alkanes
- Saturated hydrocarbons, containing only carbon and hydrogen atoms joined together by single covalent bonds.
- Each carbon atom is joined to four other atoms by single covalent bonds called sigma bonds.
- A sigma bond is the result of the overlap of two orbitals, one from each atom.
- Each overlapping orbital contains one electron so the sigma bond has two electrons which are shared between bonding atoms.
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Boiling point of alkanes
- Boiling point increases because of the weak intermolecular forces known as London Forces. The greater these forces are, the higher the boiling point.
- As the chain length increases, the molecules have a larger surface area and therefore more surface contact. The London forces will be greater so more energy is required to overcome these forces.
- Branched molecules have fewer points of contact giving fewer London forces so less energy is needed to overcome the forces so boiling point decreases.
- Branches also prevent the molecules getting close together further decreasing the number of intermolecular forces.
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Combustion of alkanes
- All alkanes react with a plentiful supply of oxygen to produce carbon dioxide and water.
- All combustion processes give out heat and alkanes are used as fuels becasue they are readily available, easy to transport, and burn in a plentiful supply of oxygen without releasing toxic waste products.
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Incomplete combustion of alkanes
- In a limited supply of oxygen, there is not enough oxygen for complete combustion.
- When oxygen is limited the hydrogen atoms are always oxidised to water, but combustion of the carbon may be incomplete forming carbon monoxide or even carbon itself as soot.
- Carbon monoxide is a colourless, odourless and highly toxic gas.
- Combines irreversibly with haemoglobin to form carboxyhaemoglobin, which prevents haemoglobin from transporting oxygen around the body.
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Reactions of alkanes with halogens
- In the presence of sunlight, alkanes react with halogens.
- The high energy UV radiation present in the sunlight provides the initial energy for a reaction to take place.
- This is a substitution reaction as a hydrogen atom in tha alkane has been substituted by a halogen atom.
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Mechanism for bromination of alkanes
- The mechanism for the bromination of methane is an example of radical substitution.
Step 1: initiation
- The treaction is started when the covalent bond in a bromine molecule is broken by homolytic fission.
- Each bromine atom takes one electron from the pair, forming two highly reactive bromine radicals. The energy for this bond fission is provided by UV readiation.
Br - Br ---> Br. + Br.
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Mechanism for bromination of alkanes
Step 2: propagation
Propagation step 1: CH4+Br. ---> .CH3+ HBr
Propagation step 2: .CH3+Br2 ---> CH3Br+Br.
- In the first propagation step, a bromine radical reacts with a C-H bond in the methane, froming a methyl radical and a molecule of hydrogen bromide
- In the second propagation step, each methyl radical reacts with another bromine molecule forming bromomethane, and a brand new bromine radical.
- The new bromine radical then reacts with another CH4 molecule as in the first propagation step and the two steps can continue to cycle through in a chain reaction.
- Propagation is terminated when two radicals collide.
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Mechanism for bromination of alkanes
Step 3: termination
Br.+Br. ---> Br2
.CH3+.CH3 ---> C2H6
.CH3+.Br ---> CH3Br
- In the termination stage, two radicals collide, forming a molecule with all eletrons paired.
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