alkenes Flashcards
Define electrophile
A species that accepts a lone pair of electrons.
What is a π bond? (with diagram
The sideways overlapping of p-orbitals.
Give 2 differences between σ and π bonds
- π bonds have a lower bond enthalpy than σ bonds.
- σ bonds have electron density between bonding atoms
WHEREAS π bond have electron density above and below bonding atoms (so are more exposed/prone to attack).
What are stereoisomers?
Same structural formulae but different spatial arrangement of atoms.
What is required for E-Z stereoisomerism to arise?
- A double C=C bond (which provides limited rotation).
- Two different groups on each carbon atom around the double
bond.
Describe the Cahn-Ingold- Prelog (CIP) priority rule
● Identify the priority groups on either side (the atom with the larger atomic mass).
● If the priorities are on the same (zame) side, it is a Z-isomer. If the priorities are on opposite sides (epposite), it is a E- isomer.
● If the directly attached atoms are the same, move onto the next atom
What is E/Z isomerism?
An example of stereoisomerism where…
● There is restricted rotation about the C=C.
● Two different groups attached to each C of the C=C group.
What is cis-trans isomerism?
A special case of E/Z isomerism (and thus stereoisomerism) where a substituent group on one C of the C=C group is the same as the other C
What is the hydration of alkenes and its conditions?
● Forming alcohols from alkenes. ● Conditions: ○ Steam. ○ Phosphoric acid catalyst. ○ Around 300°C ● Example: ○ C2H4 + H2O → C2H5OH
What is hydrogenation of alkenes and its conditions?
● C2H4 + H2 → C2H6.
● With hydrogen in the presence of a nickel catalyst at around 150°C.
Describe and draw the electrophilic addition between bromine and ethene
Br2 molecule approaches alkene.
2. Electrons in the π bond induce a temporary dipole the Br2
thus causing the Brδ+ to attack the electron dense π bond
whilst the Br-Br bond breaks by heterolytic fission.
3. The positive carbon on the carbocation intermediate attracts the Br- ion
Describe the electrophilic addition of hydrogen bromide and butene
- As the HBr (polar molecule) approaches the double bond, the Hδ+ attacks the electron dense π bond whilst the hydrogen- halogen bond breaks by heterolytic fission.
- This forms a carbocation intermediate which attracts the Br- atom
What is Markovnikov’s rule and what is used to do? (both with and without 1H)
● That the Hδ+ in the hydrogen halide will attach to the carbon with more hydrogens attached.
● It’s used to determine the major and minor products of electrophilic addition between hydrogen halides and asymmetric alkenes.
● For ICl, the Iδ+ will bond with the carbon with more hydrogens attached to it since it is less electronegative (just like Hδ+).
What conditions are required for polymerisation?
● High temperature. ● High pressure.
● A catalyst.
The specifics depend on the product you want to form.
What limits biodegradable plastics
Not having the right conditions to decompose (i.e., moisture and oxygen).