13 - Alkenes Flashcards
(48 cards)
Define alkenes - simple way
• Unsaturated hydrocarbons
Define alkenes - complicated
• Aliphatic alkanes that have 1 double carbon bond = CnH2n
Bonding in alkenes
• Each C has 4 electrons in its outer shell which can be used for bonding.
• 3 of the electrons are used in sigma bonds
o One to the other C atom
o Two to other atoms = alkene so (C or H)
• 1 electron remaining = forms a π bond between 2 C atoms
What are pi bonds
• Sideways overlap of two P orbitals – one from each bonding atom
• Each orbital contributes one electron to the electron pair
• The pi bond density is concentrated above and below the line joining the nuclei of the bonding atom
Where is pi bond density concentrated
above and below the line joining the nuclei of the bonding atom
Which one is weaker - pi or sigma
Pi
• Sigma bonds results from head on overlapping whereas pi bonds can only partially overlap
• In sigma bonds there is more overlapping between orbitals compared to pi bonds
Does the pi bond lock the C atoms in place
YES - preventing rotation around the double bond. = therefore diff geometry than alkanes
What is stereoisomerism
• have the same structural formula but a different arrangement of atoms in space.
Types of stereoisomerism
o E/Z isomerism
o Optical isomerism – A2
Where does stereoisomerism occur
• occurs around a double bond
Why does stereoisomerism round a double bond
because rotation about the double bond is restricted
o Therefore the groups attached to each carbon atom are fixed relative to each other.
• The reason for this rigidity is the position of the π bonds electron density above and below the plane of the σ bond.
• For a molecule to show e/z isomerism it must have…
o A c=c double bond
o Different groups attached to each carbon atom of the double bond
What is cis - trans isomerism
• A special type of E/Z isomerism
• Occurs when one of the attached groups of each carbon atom of the double bonds is a hydrogen
Cis
• H atoms on the same side of C=C
• Z isomer
Trans
• H atoms on opposite sides of C=C
• E isomer
Example of cis / trans
How are atoms prioritised for E/Z isomerism
• Atoms on each carbon are prioritized based upon their atomic number
How does this prioritisation effect stereoisomerism
• The higher the atomic number…
The higher priority
Why are alkenes more reactive than alkanes
The pi bond
How does the pi bond make alkenes more reactive
• As the electron density is above and below the bond, the π-electrons are more exposed than the electrons in the σ-bond.
• A π-bond readily breaks and alkenes undergo addition reactions relatively easily.
What does hydrogenation form
(alkene –> alkane)
How does hydrogenation work
What type of reaction is hydrogenation
Addition
