Alkenes Flashcards
What do alkenes contain?
One or more carbon-to-carbon double bond
What is the general formula of aliphatic hydrocarbons?
C(n)H(2n)
Where is a pi bond formed from?
The sideways overlap of two p-orbitals one from each carbon atom of the double bond
Where is pi bond electron density concentrated?
Above and below the plane of nuclei of bonded atoms
What do pi bonds prevent? How?
Rotation around the double bond, due to the lock of the two carbon atoms leading to concentrated electron density above and below the plane of atom.
What is the shape around the carbon atoms in double bond of an alkene?
Trigonal Planar- three areas of electron density around each of the carbon atoms
Define stereoisomers
Same structural formula but a different arrangement of the atoms in space
Why does stereoisomerism occur?
Due to the restricted rotation around the double bond, and therefore the groups attached to each carbon are fixed relative to each other.
What two conditions have to be fulfilled for E/Z isomerism?
A C=C double bond
Different groups attached to each carbon atom of the double bond
What condition has to be different for cis-trans isomerism than E/Z isomerism?
In cis-trans isomerism one of the attached groups on the each carbon atom on the double bond must be the same.
What does a cis-isomer have?
The hydrogen atoms and methyl groups on each carbon in the double bond are on the same side.
What doe a trans-isomer have?
The hydrogen atoms and methyl groups on each carbon in the double bond are opposite each other.
What occurs in a Z isomer?
The groups with higher priority are on the same side
What occurs in a E isomer?
The groups with higher priority are diagonally placed across the double bond
How is priority assigned?
Based on atomic number. The higher the atomic number, the higher the priority.
What happens if the atom attached to carbon atom is the same when assigning priority?
Keep going along the molecule until reaches a difference
Why are alkenes more reactive than alkanes?
Pi bonds are the overlap of p orbitals above and below the plane of the sigma bond. Being above and below means more exposed electrons. Reacts more easily.
What conditions are needed in the hydrogenation of an alkene?
423K
Nickel Catalyst
What is hydrogenation?
The addition of H2 across a double bond
How is unsaturation tested for?
Addition of bromine water. Colour change from orange to colourless if a double bond is present.
What conditions are required for hydration of alkenes?
Steam
Concentration phosphoric acid catalyst
Define ‘electrophilic addition’
An addition reaction in which the first step is attack by an electrophile on a region of high electron density
Where is high electron density found?
Double bond of the alkene
Define ‘electrophile’
An atom or group of atoms that is attracted to an electron rich centre and accepts an electron pair
