alkenes

Question 1

Give 2 differences between σ and π bonds

Answer 1

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 agack).

Question 2

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

Answer 2

● identify the priority groups on either side (the atom with the larger atomic mass).
● If the priori:es are on the same (zame) side, it is a Z-isomer. If the priori:es are on opposite sides (epposite), it is a E-isomer.
● If the directly agached atoms are the same, move onto the next atom.

Question 3

What is required for E-Z stereoisomerism to arise?

Answer 3

1. A double C=C bond (which provides limited rotation).

2. Two different groups on each carbon atom around the double bond.

Question 4

What is the hydration of alkenes and its conditions?

Answer 4

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

Question 5

What is hydrogenation of alkenes and its conditions?

Answer 5

● C2H4 + H2 → C2H6.

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

Question 6

Describe the electrophilic adittion between bromine and ethene

Answer 6

1. Br2 molecule approaches alkene.
2. Electrons in the π bond induce a temporary dipole the Br2 thus causing the Brδ+ to agack the electron dense π bond whilst the Br-Br bond breaks by heterolytic fission.
3. The positive carbon on the carboca:on intermediate agracts the Br- ion

Question 7

Describe the electrophilic addition of hydrogen bromide and butene

Answer 7

1. As the HBr (polar molecule) approaches the double bond, the Hδ+ agacks the electron dense π bond whilst the hydrogen- halogen bond breaks by heterolytic fission.
2. This forms a carbocation intermediate which agracts the Br- atom.

Question 8

What conditions are required for polymerisation?

Answer 8

● High temperature.
● High pressure.
● A catalyst.

The specifics depend on the product you want to form.

Question 9

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

Answer 9

● That the Hδ+ in the hydrogen halide will attach to the carbon with more hydrogens agached.
● 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 agached to it since it is less electronega:ve (just like Hδ+).

Question 10

What limits biodegradable plastics?

Answer 10

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

Question 11

Give 3 ways of dealing with waste polymers

Answer 11

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

Question 12

What are the problems of combusting polymers?

Answer 12

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

Question 13

Give examples of homogeneous catalysis

Answer 13

• Acid catalyst in esterification.

* Depletion of atmospheric ozone by chlorine radicals.