Module 4.1.3 - Alkenes Flashcards
What are alkenes?
Unsaturated hydrocarbons (have at least one multiple bond between carbon atoms)
Why are alkenes unsaturated?
In alkanes, there is at least one double bond
What is the general formula for alkenes?
CnH2n
What bond is in a C=C bond?
Made up of 1 sigma bond and 1 pi bond (which restricts its rotation)
How is a sigma bond formed in an alkene?
Directly between two carbon atoms by the direct single overlap of orbitals directly between bonding atoms
How is a pi bond formed in an alkene?
The double, sideways overlap of p-orbitals ABOVE AND BELOW the bonding C atoms
What shape and bond angle is around the carbons in C=C?
Trigonal planar, 120 degrees
3 bonding regions of electrons which repel each other equally to get as far apart as possibly
How do you name alkenes?
- Find the longest C chain that includes the alkene group & name the stem
- Number parent chain from end to lowest number possible for alkene group
- If more than 1 alkene group, use prefix (di-,tri-..) and add an ‘a’ at the end of stem of alkene
- Name and number any alkene groups, with multiple list in alphabetical order by name
- Cyclic alkenes have the prefix ‘-cyclo’
What is stereoisomerism?
Compounds with the same structural formula but with a different arrangement of the atoms in space.
What does E/Z stand for?
Entgegen/Zusammen
What is the criteria for E/Z stereoisomerism? (2)
- Must have a carbon-carbon double bond, so the compound cannot rotate
- EACH carbon atom of the C=C must have two different groups attached to it
How do you draw E/Z stereoisomerism?
Draw the molecule with its trigonal planar shape and attached groups are displayed in structural formula, has to condense only to the C=C
How do you find priority in alkene stereoisomers?
Compare the atoms that are attached on either side of the C=C and look at their atomic number (top and bottom)
Higher atomic number = Higher priority
How do you identify a Z-stereoisomer?
The highest priority groups are on the same side of the double bond (either both highest priority groups are top, top OR bottom, bottom)
How do you identify an E-stereoisomer?
Highest priority groups are on opposite sides of the C=C (one highest priority group on bottom, one on top)
What is the criteria for cis-trans stereoisomerism? (3)
- must have a carbon-carbon double bond because it cannot rotate
- Each carbon on the C=C must have two different groups attached
- Two groups on the C=C must be identical
How do you identify a cis-stereoisomer?
The two same groups from each C atom are adjacent to the C=C
How do you identify a trans-stereoisomer?
Two same groups from each C atom are opposite the C=C bond (across from each other)
Why are alkenes more reactive than alkanes?
Alkenes have C=C bond, which is their functional group
What type of reactions do alkenes undergo?
Addition reactions - A group is added across the C=C double bond
Why does the pi bond break when an alkene reacts?
It has relatively low bond enthalpy
Why is the atom economy 100% for alkenes?
Since addition, only one product is formed so no waste product
What are the different reactions for alkenes? (5)
- Bromination
- Chlorination
- Hydrogenation
- Hydration
- Hydrogen halide
What is the test for identifying alkenes?
Bromine water (bromination) as it goes from orange to colourless
What conditions and reagents are needed for hydrogenation?
Conditions: Nickel catalyst and 150 degrees C
reagents: alkene + H2
Forms an alkane
What conditions and reagents are needed for bromination/chlorination?
Conditions: None
Reagents: Br2/Cl2 + alkene
Forms a haloalkane
What conditions and reagents are needed for halogen halide?
Conditions: None
Reagents: alkene + HX (where X is a halogen)
Forms a haloalkane
What conditions and reagents are needed for hydration?
Conditions: Conc. H3PO4 catalyst, high temperature & pressure, H2O has to be as a gas
Reagents: H2O (g) + alkene
Forms an alcohol
What is electrophilic addition?
The mechanism that alkene reactions undergo, as C=C attracts electrophiles due to its high electron density
What is an electrophile?
Electron pair acceptor
How is the electrophilic addition mechanism shown?
Using curly arrow model (shows movement of electron pair)
What must you show in the electrophilic addition mechanism?
The dipoles on the polar molecules and curly arrows showing where electrons are moving
When is a mixture of two organic products formed from alkenes? (2)
- When either hydrogen halide/steam reacts
- Where alkene is unsymmetrical around the C=C double bond
What are the two products formed from unsymmetrical alkenes catagorised as?
Major / Minor product
What is Markownikoff’s rule?
The major product is the tertiary/secondary carbocation which is more stable than the primary carbocation (minor product), as the tertiary carbocation is the most stable and primary is the least stable
What is the carbocation intermediate stage?
After the pi bond is broken, the carbocation is where one C atom has a positive charge and attracts the negative ion
How many alkyls groups do each primary,secondary and tertiary carbocation have around it?
P - 1
S - 2
T - 3
Which product is more likely to form?
Major product
What are addition polymers?
Long, saturated molecules formed from alkene monomers
What conditions and reagents are needed for addition polymerisation?
Conditions: high temperature and pressure + catalyst
Reagents: One type of alkene monomers
What are the problems with disposing addition polymers? (2)
- Non-biodegradable
- Burning/Incineration of toxic chloro-gases
In what ways are waste addition polymers processed which are more sustainable? (4)
- Combustion for energy production
- Removal of toxic waste products (like HCl)
- Use as an organic feedstock
- Recycles
How can polymers be produced to minimise environmental damage? (2)
- develop biodegradable polymers
- develop photodegradable polymers
