12.1 Alkanes Flashcards
What are alkanes
How are they used
Saturated hydrocarbons, so only contain carbon - carbon and carbon - hydrogen single bonds.
NO DOUBLE BONDS
. Used in industry as fuels and lubricants and as starting materials for other compounds
What is the general formula of alkanes
CnH2n+2
And they can be in the shape of branched chains, unbranched chains and rings
Describe the unbranched chain structure of alkanes
What is the C - C bond angle
They are straight chains
C-C bond angle is 109.5
So chains aren’t technically straight
How do you write branched chains of alkanes molecular structure
What is the structure of ringed alkanes
(what functional group do they have the same structure as)
When there is a branch, it is in brackets
. General molecular formula CnH2nwhich is the same as alkenes.
Because the end hydrogens are not required
How do you name straight chain hydrocarbons
. They are named from the root, which tells us the number of carbon atoms
The suffix is ane, which tells us it is an alkane
How are branched chain hydrocarbons named
When naming a hydrocarbon with a branched chain, find the longest carbon chain
This gives us the root name
Then, to give us the prefix, name to branches or side chains, eg methyl, ethyl , propyl etc
Then finally add the numbers to say which carbon atoms they are attached to
What does the number of possible isomers depend on
. The more carbons there are on the alkane chain, the more possible isomers there are
eg methane, ethane and propane have no isomers
But butane has 4 carbons and 2 isomers
Pentane has 5 carbons and 3 isomers
Describe polarity of the alkanes
Alkanes are almost non - polar
Because the electronegativities of carbon and hydrogen are very similar
So the intermolecular forces between their molecules are weak van der walls
So the larger the alkane, the stronger the van der walls
Describe boiling points of the alkanes
unbranched chains
The bigger that alkanes get/ the longer the chain length, the more van der walls in them, so the higher the boiling points are
Eg the shorter chains are gases at room temp, whilst pentane has 5 carbons and is a liquid at room temp.
At 18 carbon chain length, they are solid at room temp
They all have low boiling points
Why are the boiling points of branched hydrocarbon chains in alkanes lower than unbranched ones
. Alkanes with branched chains have lower melting points
Because they can’t pack as closely together as unbranched chains
And there are fewer points of contact
So there are weaker van der walls
Describe solubility of alkanes
They are insoluble in water
Because the water molecules are held together by hydrogen bonds which are much stronger than the van der walls forces between alkane molecules.
So the water molecules won’t break apart
However alkanes do dissolve in other non polar liquids
Describe reactivity of alkanes
Do they react with acids, bases, oxidising agents and reducing agents?
Relatively unreactive
Due to strong hydrogen - carbon bonds
They don’t react with acids, bases, oxidising agents and reducing agents
However they burn with halogens under suitable conditions in lots of oxygen to form carbon dioxide and water.
Or incomplete combustion to make carbon monoxide
