Alkenes Flashcards
What is an aliphatic alkene?
Have one double bond and general formula CnH2n
What two alkenes do not obey the general formula?
Cyclic alkenes and alkenes with more than one double bond.
What is a pi bond.
The sideways overlap of the two p orbital electrons. The electron density is concentrated above and below the line joining the carbon nuclei. One electron from each carbon atom is in a sigma bond, the other is in a pi bond.
What property does a pi bond give an alkene?
|t prevents atoms from rotating around the double bond.
What is the shape of bonds around each carbon atom in the double bond?
There are three regions of electron density around each carbon so it is trigonal planar. 120 degrees.
What two conditions does an alkene have to have to satisfy E/Z isomerism.
- A double bonded carbon atom.
- Different groups attatched to each end of the double bonded carbon atoms.
Which one can form E/Z isomers: but-1-ene or but-2-ene?
Only But-2-ene can form ez isomerism as it has a methyl group and a hydrogen attatched to each end of the double bond. But-1-ene has only hydrogen on one side of its double bond.
Cis-trans isomerism requires the same condiitions as the as E/Z isomerism but one more condition is required, what is this?
One of the groups attatched to each carbon atoms either side of the double bonds must be hydrogen.
What is a cis isomer?
A cis isomer has a hydrogen atom on each carbon atom in the double bond on the same side of the molecule.
What is a trans isomer?
The hydrogen atoms on either carbons in the double bond are diagonally opposite each other.
How can isomers with no hydrogen atoms and no similar groups on either side of the carbon atom be identified as E/Z
Groups are prioritised by their atomic number. If high priority groups are on the same side of the double bond, it is Z. If they are diagonally opposite it is E.
Which has a higher bond enthalpy, sigma bonds or pi bonds?
Sigma bonds have a higher bond enthalpy than pi bonds. This is because the pi orbital exists further away from the positive nucleus so the electron is more exposed.
What is an addition reaction for an alkene?
When the pi bond breaks and allows another small molecule to join. Either: Halogens, Hydrogen halides, hydrogen with nickle catalyst, steam with acid catalyst.
What is hydrogenation of an alkene?
When an alkene is mixed with hydrogen and passed over a nickle catalyst at about 400K. Hydrogen is added and an alkene is formed.
How can you test if a hydrocarbon is saturated or not?
Add the unknown hydrocarbon to bromine water, if the water decolourises (goes from orange to colourless). The bromine molecule is broken and added to the alkene.
