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
What’s the mechanism of halogens being reacting with alkenes to form dihaloalkanes?
Draw the alkene eg ethane, and draw the halogen eg bromine molecule near the double bond, the closer bromine atom will have a delta positive charge, as electrons have been repelled away by double bond, making the other Br molecule delta negative
Draw curly arrow from double bond to Delta + Br
Draw curly arrow from bond between Br’s to the delta negative Br
Draw the double bond now split to a single bond, and one C has a Br molecule attached, the other C will have a positive charge, and draw the other Br molecule with a negative charge and a lone pair with a curly arrow to the positively charged carbon
Draw the final dihaloalkane
How do you use Bromine to check for Carbon double bonds?
Shake substance with bromine water, if alkene present orange bromine water decolourises as it’s been added across the double bond to form a haloalkane
How do you make ethanol from Ethene?
Add water as steam at 300 degrees
60-70 atm
Phosphoric acid catalyst
How do you produce a haloalkane from an alkene?
Add a hydrogen halide
It’s an addition reaction
What are the 2 products called when a hydrogen halide reacts with an unsymmetrical alkene?
The major and minor product?
when a hydrogen halide reacts with an unsymmetrical alkene how do you know which is the major and minor product?
Major product is more stable so the halide ion will bind to the Carbon which is bonded directly to the most other carbons ( least amount of hydrogens)
Minor product is less stable, so the halide ion will bind to the Carbon which is bonded directly to less Carbons(more amount of Hydrogens)
When Alkene join up to form polymers how do you draw the polymer, repeat unit, and monomer?
Polymer (add poly in front of alkene name), then draw the whole chain (alkene repeated with double bond broken)
Repeat unit- draw the bit that’s repeated, with bracket going over side bonds, and n= amount of repeats
Monomer= the alkene of the polymer
What’s an advantage/disadvantage of polymers?
Not reactive so good at carrying, however also means very difficult to dispose of
How do we get rid of polymers?
Landfill
Recycled
Burned- heat energy can be used to generate electricity, however produces toxic and greenhouse gases
