Alkenes 4.13 Flashcards
Pi bond
Pi bond is the sideways overlap of p orbitals. It locks the two carbon atoms in position and prevents them from rotating around the double bond.
What is the shape of alkenes?
Trigonal planar. 120
The three bonding regions repel each other as far possible
All atoms are in the same plane.
Stereoisomers
Stereoisomers have the same structural formula but a different arrangment of atoms in space.
How does stereoisomerism across double bonds arise?
Stereoisomerism across double bonds arises because rotation around the double is restricted and the group attatched to each carbon atom are therefore fixed relative to each other.
Entegen
Opposite (epposite) - trans
Zusaman
Same side (zame zide) - cis
What conditions must be satisfied for it to have E/Z isomerism?
- A C=C double bond
- Different groups attatched to each carbon atom of the double
What conditions must be satisfied for a molecule to have cis-trans?
- C=C double bond
- Each of the carbon of the double bond must be attachment to two different groups
- One of the attached groups on each carbon of the double bond must be the same
- Each of atom of the double bond must have a H on it (for alkenes)
What determines the priority?
- The higher the atomic number (protons), the higher the priority
- Look for the atom attatched to the carbon then assign priority.
Electrophile
A species that must be positively charged. An electron pair acceptor.
What does the double bond in an alkene represent?
The double bond in an alkene represents a region of high electron density because of the presence of the the Pi bond
Why are alkenes more reactive than alkane?
A high electron density around the double bond and the fact that the pi bond is easier to break because it is weaker.
What conditions are required for the hydrogenation of alkenes?
alkene + H2 —> alkane
Conditions: H2g and Ni 423K
Halogenation of alkene
Alkene + X2 (halogen) —> Haloalkane
How do you test for unsaturation?
Add bromine water into the solution, if the orange colour disappears (colourless) in the presence of a C=C bond in an organic compound.
Hydration of alkenes, how is alcohol formed from alkenes
Alkenes react with steam H2O(g) in the presence of a phosphoric acid catalyst H3PO4
What do curly arrows show?
Show movement of a pair of electrons.
Polymer
Extremely large molecules made up of repeating units know as monomers.
What happens when PVC is burnt
It released hydrogen chloride
How does stability of a carbocation increase? How does this affect the major and minor product?
The more alkyl groups attatched to the positively charged carbon, the more charges is spread, making the ion more stable. The one that is more stable is the major product as it will be made more.
What condition is needed for an addition reaction with a hydrogen halide? (HX)
Alkenes react with HX at room temperature
How does HX react with a liquid alkene
HX(g) is bubbled through the liquid alkene.
How are electrophiles attracted to Pi bonds?
The high electron density of the pi electrons attracts the electrophile.
How do you reduce the negative impact of polymers?
- make them photodegradable or biodegradable
- recycling them - chemically reclaimed as feedstock
