Alkenes Module 4 Flashcards
What are Alkenes?
Have general formula CnH2n
Must have at least one C=C bond
How do you name alkenes?
In between longest C chain and “ene” ending add where a C=C bond occurs and have “,” between them and all numbers separated by 2 “-“
What’s a sigma bond?
Forms when 2 s orbitals overlap in a straight line, this gives the highest possible electron density between the 2 nuclei, this is a single covalent bond
The high electron density between the nuclei means there is a strong electrostatic attraction between the nuclei and shared pair of electrons
Strongest type of covalent bond
Draw these by having 2 spheres with a dot in the middle for a nucleus, and draw them overlapping in a diablo shape
What’s a pi bond?
Formed from the sideways overlap of 2 adjacent p orbitals
It’s bellow and above the molecular axis, because the pi orbitals that overlap are dumbbell shaped
Pi bonds are weaker than sigma bonds because electron density spread above and bellow the nuclei, so weaker electro static attraction between nuclei and shared pair of electrons, so break easier than sigma bonds
Draw this by having 2 dumbell shapes with curved lines connecting each top of the dumbell
Which forms 2 dots with a curved pi bond above and bellow
Why are Alkenes more reactive than Alkanes?
Alkanes only have C-C and C-H sigma bonds, whislt alkenes also have C=C pi bonds
C=C has a higher electron density as there are 4 electrons, pi bonds stick out, so they are more likely to attract electrophiles or nucleophiles, also pi bonds break easier
Why can’t C=C bonds rotate and what does this cause?
Can’t rotate due to the way the p orbitals overlap to form a pi bond
Causes stereoisomers
When does stereoisomerism occur?
When the two double bonded carbon atoms have each have 2 different atoms or groups bonded to them
What’s a Z isomer?
When the same atom/group with the priority are both above or both bellow on each side
What’s an E isomer?
When the same atom/group with the priority are on diagonal sides
In steroisomerism how do you determine which is the priority group?
On each side it’s the group with the highest Mr, if they are the same will have to look at next Carbon in the chain and find which one has the highest Mr now
What’s a cis isomer?
If the same groups are on the same side on each side of the C=C bond (don’t have to be priority)
What’s a trans isomer?
If the same groups are on different sides of the C=C bond (don’t have to be priority)
What type of reaction happens in alkenes?
Electrophillic addition, because double bond has plenty of electrons, so electrophiles (electron pair acceptors) are easily attracted
Examples of electrophiles?
H+, NO2+ and polar molecules
How do you produce an Alkane from an Alkene?
Add hydrogen gas
Ni catalyst and temperature around 150 degrees
