Alkenes (13)
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What are alkenes?
Alkenes are unsaturated hydrocarbons which means they have at least one double bond.
What is the general formula for alkenes?
CnH2n
Which alkenes don’t obey the general formula?
Cyclic alkenes and alkenes with more than one double bond.
What creates the double bond?
A sigma bond and a pi bond create the double bond
What is a pi bond?
The sideways overlap of 2 p orbitals and its electron density is above and below the bonded atoms
Which is stronger, sigma or pi bond?
Sigma bonds are stronger because they have a stronger electron density and a direct overlap which means the attraction between the nuclei and electron pair is stronger. Pi bonds are weaker because they have a their electron density above and below the sigma bond which makes the attraction between the nucleus and electrons weaker.
Are alkenes reactive?
They are more reactive than alkanes because of their pi bond. They have electrons above and below the sigma bonds which are likely to be attacked by an electrophile
Can alkenes rotate freely?
Alkenes cannot rotate freely as the pi bond locks the carbon atoms in position and prevents them rotating.
What are the shapes of alkenes?
Alkenes have 3 sigma bonds and one pi bond which means they have 3 regions of electron density which repel equally so they form trigonal planars and have bond angles of 120 degrees.
Stereoisomers
These are molecules that have the same structural formula but a different arrangement of atoms in space
Why does stereoisomerism occur?
It occurs because of the double bonds in alkenes which doesn’t allow them to rotate.
What conditions are needed for E/Z isomerism to occur?
A C=C bond Different groups attached to each carbon atom in the double bond e.g a H and a methyl group attached to one carbon
Z isomer
The same groups positioned across
E isomer
The same groups opposite from each other
What is cis-trans isomerism?
E/Z isomerism where at least one of the groups is the same on each carbon e.g. both carbons have an H
Cis
The same principle as Z, the same groups positioned across
Trans
The same principle as E, the same groups are opposite each other
What is the Cahn Ingold prelog?
This is used when there are different groups on each carbon. You have to find which of the groups attached to the carbon has the highest priority
How do you find the priority of groups on C=C bonds?
You find the rfm. The group with the highest rfm on each atom is the high priority the other one is low priority
How can you tell which one is E or Z using the Cahn Ingold Prelog?
If you read across the molecule and you see high-high, it’s a Z isomer. If you read across and see a low-low its an E isomer.
What kind of reactions can alkenes undergo easily and why?
They can undergo addition reactions easily because of the pi bond which exposes the electrons to electrophiles
How can you work out the enthalpy of a pi bond?
If you know the enthalpy of the double bond and single bond just do double - single
What is hydrogenation of alkenes?
This is when H2 is added to an alkene. The hydrogen is added and it breaks the double bond and forms an alkane. All double bonds will be broken, even if there are multiple, you will just have to put a higher number of hydrogens
What are the conditions for a hydrogenation?
Nickel catalyst and 150-degree temp