Chapter 13 - Alkenes Flashcards
General formula for alkenes
Cn H2n
How does the pi bond affect the alkene’s rotation
The carbon atoms in the double bond is locked in they can’t rotate around the double bond.
The Rule: 7- on ward
- I >B>CL>F>O>N = priority
- Branch with higher molecular mass
- More branching on C closest next to C=C has priority
What do you need to be a cis/ trans
You need each Carbon atom to be bonded to at least 1 hydrogen atom.
If the hydrogen atoms are on the same size, what are they called?
Cis
If the hydrogen atoms are on opposite sides, what are they called?
Trans
If the higher priority groups are on the same side (not hydrogen), what is the alkene called?
Z
If the higher priority groups are on opposite sides (not hydrogen), what is the alkene called?
E
How should you name an alkene
Name the alkene first.
Then if it’s cis/trans/Z/E.
How do pi-bonds differ from sigma bonds
They differ in terms of:
- formation
- bond angles
- The ability to break
- The ability to rotate
- Their electrodensities
How are sigma bonds formed?
Formed when orbitals directly overlap
How do pi-bond form
2 p-orbitals overlap sideways forming a pi-bond and below the plane between the 2 carbon atoms.
Extra information on the pi-bond
Essentially a delocalised electron can move between 2 p-orbitals as if it can “overlap”, but the p-orbitals don’t actually touch.
Formation of a pi-bond
Draw a normal sigma bond and then a pi-bond above and below the molecule.
Angles with a pi-bond
They create 120 degrees bond angles
because 3 carbon with double bonds are bonded to 3 other atoms
the 3 groups of electrons around these carbons repel away as far away as possible
giving these carbons a trigonal planar shape.
How many sigma bonds surround a double bonded atom?
3 sigma bonds always
Explain a sigma bond
Each C atom has 4 valence electrons
When a Carbon forms 4 single bonds with other atoms
All 4 valence electrons are locked between the nuclei of Carbon and the atoms it bonds to.
Explain a pi-bond (final explanation)
Each Carbon atom has 4 valence electrons
When a Carbon formed 1 double bond and 2 single bonds with other atoms
Only 3 valence electrons are locked between the nuclei of Carbon and the atoms it bonds with.
The last valence electron is allowed to travel back and forth between the double bonded atoms.
What are the 4 additional reactions that alkenes can undergo
- Hydrogenation
- Hydration
- Halogenation
- Hydrohalogenation
What is an addition reaction in an alkene
Adding a small molecules across the double bond.
The pi-bond breaks
a sigma bond forms instead
and the organic compound becomes saturated.
Conditions for hydrogenation
Catalyst: Nickel Temperature: 423K By: Mixing alkene (gas) with hydrogen (gas) passed over the catalyst at 423 K
What is a catalyst
A substance that is used up and then regenerated
Conditions for halogenation
Catalyst: none Temperature: Room (25 degrees C) By: Mix the alkene with halogen at RTP
Conditions for hybridisation and halogenation
Catalyst: none
Temperature: RTP (23 degrees C)
Mix the alkene with hydrogen halide
Conditions for hydration
Temperature: heat Catalyst: H3PO4 (phosphoric acid in aqueous form) By: Mix the alkene with steam In the presence of phosphoric acid.
What is an electrophile
Electron-pair acceptor
What is electrophilic addition
mechanism of how alkenes undergo an addition reaction
Electrophiles can be polar molecules.
Use an alkene and HBr to explain how?
The pi-bond in the alkene has delocalised electrons that can attract partially positive atoms.
The C=C bond undergoes heterolytic fission. This is when the covalent bond breaks and one Carbon atom takes both the electrons from the shared pair.
The polar electrophile undergoes heterolytic fission too. It results in a C-Hydrogen bond, a positive charge on the other carbon of the double bond, and a negative charge on the Bromine. Since opposite charges attract, the bromine in then attracted to the positively charged Carbon, which then forms a bond.
Behind the scenes explanation of how electrophiles can be polar molecules.
One of the carbons in the double bond takes both the electrons (from the shared pair in the pi-bond), and then gives it to hydrogen, which then forms a bond.
The other carbon has 1 less electron, hence a +1 charge. Since bromine has a negative charge, they join together as opposites attract and form a bond.
What happens when 2 atoms get close together?
The electrons repel away from each other, but the positively charged nucleus and the electron cloud attract. This creates a dipole on that atom as the naked nucleus is exposed. This is called an instantaneous dipole induced dipole (aka London forces). This dipole then induces a dipole on neighbouring atoms.
Electrophiles can be non-polar molecules.
Use an alkene and Br2 to explain how?
Non-polar molecules approach the alkene’s carbon=carbon double bond.
The pi-bond electrons polarise the non-polar molecule. This is because the pair of electrons will repel as they get closer, because the pi-bond has high electron density, and the electrons will travel to the other bromine atom, which is further away from the double bond. This causes one Br atom to be positively charged, and the other to be negatively charged. The positively charged Br will be attracted to the carbon atom that is negatively charged (due to the heterolytic fission of the double bond). Then the other carbon atom is positively charged, and the other bromine is negatively charged, so they attract and create a bond.
Electrophiles are induced by double bonds.
Explain how
Double bonds have high electron density.
The double bond’s electron can either:
- directly attract the polar electrophile
OR
- polarise the non-polar molecules
- and then attract the polarised electrophile.
Markownikoff’s Rule
When an HX reacts with an asymmetrical alkene, the X will bond to the carbon bonded to the most carbons.
What is a tertiary carbocation
It is connected to 3 carbons
Explain stability with carbocations
Tertiary carbocations are more stable than secondary or primary because the more carbon atoms there are around the main carbon atom, the more stable it is.
Explain what stability is
The time it takes fr the carbocation to stay long enough for the halogen to bond.
If the question asks for:
“Which product is made in HIGHER AMOUNTS?”
You have to show that you can put the halogen on the correct cation. You have to put it on the carbon that is connected to the most number of carbon atoms.
Give an example for an answer explaining why one isomer is in higher amounts than another
2-Bromo propane is produced in higher amounts because a secondary carbocation is more stable than a primary carbocation.
What is a polymer
They are large molecules made up of many repeating units.
When should you use cis/trans
When both the Carbon atoms in the bond are bonded to at least 1 other hydrogen.
When should you use E/Z isomerism
When you cannot use cis/trans.
AKA, when both the carbon atoms in the double bond aren’t bonded to at least 1 hydrogen atom.
What type of polymerisation do unsaturated alkene molecules go under
addition polymerisation.
What is feedstock recycling
The chemical and thermal processes that can reclaim monomers, gases or oil from waste polymers.
Advantage of feedstock recycling
The materials can be used as raw materials for the production of new polymers. It is able to handle unsorted and unwashed polymers.
Biodegradable polymers
They are broken down by microorganisms into water, CO2 and biological compounds.
They are often made of starch or cellulose.
Photodegradable polymers
Where the use of plant-based polymers can’t happen, these are made instead. They are oil-based and they contain bonds that are weakened by absorbing light to start the degradation.
PVC
1-chloro ethene
Polyvinyl chloride
PS
Polystyrene
HDPE
High density polyethene
LDPE
Low density polyethene
PP
Polypropene
PET
Poly ethylene teraphthalate
