4.3 - Alkenes Flashcards
Describe the structure of alkanes
They are unsaturated hydrocarbons
Can be branched contain more than one double bond and be cyclic
What is the general formula of aliphatic alkenes
CnH 2n
Describe the nature of the double bond in a carbon atom
Each C atom has 4 electrons in its outer shell
3 of these are used in sigma bonds one of which is used in the double bond
The final electron is used in a pi bond
The double bond fixes rotation
How is a pi bond formed
The sideways overlap of 2 p orbitals one from each C of the double bond
What is the electron density in a pi bond
It is concentrated above and below the line joining the nuclei of the bonding atoms
What is the shape around a double bond + why
Trifocals planar as:
There are 3 regions of electron density around each of the carbon atoms
The 3 regions repel each other as far apart as possible so the bond angle is 120
All of the atoms are in the same plane
Define stereoisomers
They have the same structural formula but a different arrangement in space
What are E/Z isomers
Stereoisomers arise around a double bond as rotation is restricted
Also both C must have a diff group attached to each
What is cis-trans isomerism
Same as ez but one of the attached groups on both carbons is hydrogen
Cis- both Hydrogens on the same side
Trans- hydrogens on opposite sides
What is the khan- ingold- prelog law
All atoms are given priority based of atomic number
The bigger the higher priority
When is a isomer e when is it z
If the 2 higher priority groups are on the same side it’s z
If they are on opposite sides it’s e
Why are alkenes more reactive than alkanes
They have a pi bond which is on the outside of the double bond the pi bond electrons are more exposed than a sigma bond
Pi bonds are weaker than sigma bonds and readily break to react
What are the 4 reactions alkenes can undergo
Hydrogenation
Halogenation
Reactions with halogen halides
Hydration
