14.1 Alkenes Flashcards
What are alkenes
Unsaturated hydrocarbons so have one or more carbon carbon double bond
General formula CnH2n
Why are alkenes more reactive than alkanes
What are they used for
The double bond means there is a high concentration of electrons (high electron density) between the two carbon atoms
Can make up polymers , so ethene can make up poly-ethene , polystyrene etc
Also paint and antifreeze
What shape are alkenes
It is a planar molecule, as each carbon is bonded to 3 things (the double bond can count as one thing it is bonded to, and there are no lone pairs) so the shape is trigonal planar. 3 bond pairs no lone pairs.
Bond angles are 120 degrees
Why are alkenes planar
Unlike the C-C bond in alkanes, the molecules can’t rotate around the double bond
Why can’t double bonds rotate
As well as a normal C-C sigma bond, there is a p orbital which contains a single electron on each carbon.
These two orbitals overlap to form an orbital with a cloud of electron density above and below the single bond.
This is a π orbital and its presence means the bond can’t rotate
Remember that the shape of a p orbital is an infinity sign on its side
What types of isomer can alkenes form
Chain isomerism, positional isomers and geometrical isomers
What are position isomers
Isomers with a double bond in different positions, so between a pair of adjacent carbon atoms in different positions on the carbon chain
But-2-ene and But-1-ene
The longer the chain is, the more possibilities there are of isomers
What are geometrical isomers
It is a form of stereoisomerism.
So they have the same structural formula but a different arrangement of atoms in space
It only occurs around the C=C double bond which can’t rotate
What is E-Z isomerism
Type of stereoisomerism, which is geometrical
Where around the double bond, on either side of it there are two molecules eg a hydrogen and another on both
So if the hydrogens go opposite each other on the double bond,
it is E ISOMERISM
If the hydrogens go on the same side of the double bond
IT IT Z ISOMERISM
What is the difference between E-Z and Cis-trans isomerism
Why did this change occur
E and Z is the new version of cis and trans
Cis and trans didn’t allow for more than two different substituents around a double bond
Why might you think that alkenes are less reactive than alkanes,
Why is this wrong
Because the bond enthalpy of a double carbon bond is higher than the enthalpy of a single carbon bond
So it would take more energy to break it
HOWEVER
actually alkenes are more reactive than alkanes
How does the C=C bond make alkenes more reactive
It forms an electron rich area which can easily be attacked by positively charged reagents
These reagents are called electrophiles, so are electron pair acceptors
For example a H+ ion
As alkenes are unsaturated, they can undergo addition reactions, where a molecule is added on
Describe and explain the boiling and melting points of alkenes
The double bond doesn’t affect boiling and melting points of alkenes.
These are dictated by the number of van der waals so intermolecular bonds
So the boiling points are very similar to alkanes. So they increase with the number of carbons
Does E/Z isomerism affect the boiling points of molecules
Yes, because they typically have different physical properties but the same chemical properties
