12 Alkanes Flashcards
What is the general formula of alkanes?
CnH2n+2
What are alkanes?
- Alkanes are the main components of natural gas and crude oil.
- They are mainly used as fuels, exploiting their reaction with oxygen.
What is the bonding in alkanes?
- Alkanes are saturated hydrocarbons, containing only carbon and hydrogen atoms joined together by single covalent bonds.
- Each carbon atom in an alkane has four sigma bonds, either C-C or C-H.
What is a sigma bond?
- A sigma bond is the result of the overlap of two orbitals, one from
each bonding atom. - Each overlapping orbital contains one electron, so the sigma bond has two electrons that are shared between the bonding atoms.
What is the shape of alkanes?
- Each carbon atom is surrounded by four electron pairs in four sigma bonds.
- Each bond angle is approximately 109.5^0 (tetrahedral shape)
How do oil refineries seperate crude oil?
- They separate crude oil into fractions by fractional distillation.
- Each fraction contains a range of alkanes.
- Separation is possible because the boiling points of the alkanes are different, increasing as their chain length increases.
What is the effect of chain length on boiling point?
- Between the molecules in alkanes, there are induced dipole-dipole interactions (also called London forces) which hold them all together.
- The longer the carbon chain, the more induced dipole-dipole interactions there are.
- This is because they have a larger molecular surface area so there is more surface contact between the molecules and there are more electrons to interact.
- So as the molecules get longer, it takes more energy to overcome the induced dipole-dipole interactions and separate them, and the boiling point rises.
What is the effect on branching on boiling point?
1) London forces- There are fewer surface points of contact between molecules of the branched alkanes, giving fewer London forces.
2) Shape of molecules- The branches get in the way and prevent the branches molecules getting as close together as straight-chain molecules, decreasing the intermolecular forces further.
Reactivity of alkanes
- Alkanes do not react with most common reagents.
- The low reactivity of alkanes can be explained by:
1) The C-C and C-H sigma bonds all have a large bond enthalpy, making them very strong and therefore difficult to break.
2) The bonds are also non-polar. This means that they won’t attract any positively or negatively charged particles to react with them.
Do alkanes react with oxygen?
- Yes.
- Despite their low reactivity, all alkanes react with a plentiful supply of oxygen to produce carbon dioxide and water.
- This reaction is called combustion.
- All combustion processes give out heat, and alkanes are used as fuels because they are readily available, easy to transport, and burn in a plentiful supply of oxygen without releasing toxic products.
What is incomplete combustion of alkanes?
- If there isn’t much oxygen around, an alkane will still burn, but it will produce carbon monoxide and water.
- This is an incomplete combustion reaction.
Can alkanes react with halogens?
Yes.
- In the presence of sunlight, alkanes react with halogens.
- The high energy ultraviolet radiation present in sunlight provides the initial energy for the reaction to take place.
- During the formation of haloalkanes a hydrogen atom is substituted by a halogen in a radical substitution reaction.
E.g the mechanism for the bromination of methane
CH4 + Br2-> CH3Br + HBr
What three stages does the mechanism take place in?
Initiatio, propagation and termination.
What happens in the initiation stage?
- In the initiation stage, the reaction 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 radiation.
What happens in the propagation stage?
- In the first propagation stage, a bromine radical. Br. reacts with a C-H bond in the methane, forming a methyl radical, .CH3 , and a molecule of hydrogen bromide, HBr.
CH4 + Br. -> .CH3 + HBr - In the second propagation step, each methyl radical reacts with another bromine molecule, forming the organic product bromomethane, CH3Br, together with a new bromine radical (Br.)
.CH3 + Br2 -> CH3Br + Br.
