ALKENES

Question 1

Definition: Pi bond

Answer 1

A bond formed by the sideways overlap of two py-orbitals, with the electrons above and below the plane of the bonding atoms

Question 2

Definition: Sigma bond

Answer 2

A bond formed by the overlap of one orbital from each bonding atom, consisting of two electrons and with electron density centred round a line going directly between the nuclei of the two atoms

Question 3

Why is a sigma bond stronger than a pi bond?

Answer 3

Electron density is higher in a sigma bond because it’s directly between the atoms (closer to the nuclei) however in the pi bond, the electrons are spread out so electrostatic attraction is weaker

Question 4

Definition: Stereoisomerism

Answer 4

Molecules that have the same structural formula but a different arrangement of atoms in space

Question 5

How does E/Z isomerism arise?

Answer 5

• Has to have a double bond (restricted rotation)

- Has to have two different groups on each atom of the double bond

Question 6

When is it E isomerism?

Answer 6

When the priority groups are on opposite sides

Question 7

What is cis/trans isomerism?

Answer 7

A special case of E/Z isomerism when one of the groups on each carbon of the double bond is the same. If the identical groups are on the same side it is cis and trans if they are opposite.

Question 8

Products and conditions for hydrogenation

Answer 8

Ni catalyst, 423K

Alkene + H2 -> Alkane

Question 9

Alkene + Halogen

Answer 9

Dihaloalkane

Room temperature

Question 10

Alkene + HX

Answer 10

Haloalkane

Room temperature

Question 11

Alkene + H20 (g)

Answer 11

Alcohol

Phosphoric acid catalyst

Question 12

Definition: Electrophile

Answer 12

An electron pair acceptor

Question 13

Definition: Addition reaction

Answer 13

The double bond (C=C) breaks and something is added

Two reactants become one product

Question 14

Definition: Heterolytic fission

Answer 14

The breaking of a covalent bond forming a cation and an anion (both the electrons go to the same atom)

Question 15

Markownikoff’s rule

Answer 15

When a hydrogen halide is added to an unsymmetrical alkene, the most likely product is the one in which the H atom adds to the carbon that already has the most H atoms attached to it

Question 16

Definition: Atom economy

Answer 16

Sum of molar masses of desired products/Sum of molar masses of all products x100. Measure of how well atoms were utilised

Question 17

Definition: Percentage yield

Answer 17

Actual yield/theoretical yield x100. Shows the percentage of how much product was made

Question 18

Reasons why 100% yield is rarely achieved

Answer 18

1. The reaction may not have gone to completion
2. Side reactions may have taken place
3. Purification of the product may result in some loss of product

Question 19

How to improve atom economy

Answer 19

Find a use for other products or change the synthesis reaction so it produces fewer products

Question 20

How to improve % yield

Answer 20

1. Use one reactant in excess to ensure completion of reaction
2. Use a catalyst that only catalyses the reaction you want
3. Recycle the waste product

Question 21

Problems with vast quantities of plastic used worldwide

Answer 21

Non-biodegradable so hard to dispose of because they don’t break down easily in the environment. Build up in waste disposal sites which means it can be dangerous for wildlife - at sea animals could become tangled or suffocate

Question 22

How are biodegradable and photodegradable polymers made?

Answer 22

Biodegradable polymers are made of starch or cellulose or contain additives that alter the structure so that micro organisms break them down (they contain ester groups)
Photodegradable polymers are oil based and contain bonds that are weakened by absorbing light or light absorbing additives are used

Question 23

Is combustion a good method of disposing polymers?

Answer 23

• It can be used to produce energy for other purposes like generating electricity
• Some produce toxic fumes but this can be rectified by installing scrubbers in chimneys that burn polymers

Question 24

How are addition polymers treated before reusing?

Answer 24

They are sorted, chopped into flakes, washed, dried and melted to then be recasted

Question 25

How can types of polymer be separated for effective processing?

Answer 25

They are given resin identification codes during manufacturing