alkenes

Question 1

Define electrophile

Answer 1

A species that accepts a lone pair of electrons.

Question 2

What is a π bond? (with diagram

Answer 2

The sideways overlapping of p-orbitals.

Question 3

Give 2 differences between σ and π bonds

Answer 3

1. π bonds have a lower bond enthalpy than σ bonds.
2. 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).

Question 4

What are stereoisomers?

Answer 4

Same structural formulae but different spatial arrangement of atoms.

Question 5

What is required for E-Z stereoisomerism to arise?

Answer 5

1. A double C=C bond (which provides limited rotation).
2. Two different groups on each carbon atom around the double
   bond.

Question 6

Describe the Cahn-Ingold- Prelog (CIP) priority rule

Answer 6

● 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

Question 7

What is E/Z isomerism?

Answer 7

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.

Question 8

What is cis-trans isomerism?

Answer 8

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

Question 9

What is the hydration of alkenes and its conditions?

Answer 9

● Forming alcohols from alkenes. ● Conditions:
○ Steam.
○ Phosphoric acid catalyst.
○ Around 300°C
● Example:
○ C2H4 + H2O → C2H5OH

Question 10

What is hydrogenation of alkenes and its conditions?

Answer 10

● C2H4 + H2 → C2H6.

● With hydrogen in the presence of a nickel catalyst at around 150°C.

Question 11

Describe and draw the electrophilic addition between bromine and ethene

Answer 11

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

Question 12

Describe the electrophilic addition of hydrogen bromide and butene

Answer 12

1. 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.
2. This forms a carbocation intermediate which attracts the Br- atom

Question 13

What is Markovnikov’s rule and what is used to do? (both with and without 1H)

Answer 13

● 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+).

Question 14

What conditions are required for polymerisation?

Answer 14

● High temperature. ● High pressure.
● A catalyst.
The specifics depend on the product you want to form.

Question 15

What limits biodegradable plastics

Answer 15

Not having the right conditions to decompose (i.e., moisture and oxygen).

Question 16

Give 3 ways of dealing with waste polymers

Answer 16

1. Separating and recycling.
2. Used as feedstock for cracking.
3. Combusting for energy production.

Question 17

What are the problems of combusting polymers?

Answer 17

Can lead to toxic fumes (e.g., HCl).