Chapter 13 - Alkenes

Question 1

Alkenes are which kind of hydrocarbons?

Answer 1

Unsaturated

Question 2

What is the general formula for alkenes?

Answer 2

CnH2n

Question 3

How is a double bond formed?

Answer 3

Three sigma bonds formed.
One electron left in a p-orbital.
A pi bond forms from the sideways overlap of two p-orbitals.

Question 4

What prohibits rotation in alkenes?

Answer 4

The pi bond as it locks the two carbon atoms in position.

The pi bond prevents rotation because of the electron overlap both above and below the plane of the atoms

Question 5

What is the shape around the carbon atoms in the double bond and why?

Answer 5

Trigonal planar
Three regions of electron density
Repel each other equally
In the same plane
120 degree bond angle

Question 6

What is stereoisomerism?

Answer 6

The same structural formula but a different arrangement of the atoms in space.

Question 7

What are the two types of stereoisomerism?

Answer 7

E/Z isomerism

Optical isomerism

Question 8

When is cis-trans isomerism applicable?

Answer 8

Molecules have a C=C double bond

Each carbon in the double bond is attached to two different groups.

Question 9

What is the cis isomer?

Answer 9

Hydrogen atoms on each carbon on the opposite side.

Question 10

What is the trans isomer?

Answer 10

Hydrogen atoms same side.

Question 11

What are the Cahn-Ingold-Prelog rules for deciding E/Z isomerism?

Answer 11

Higher atomic number is higher priority.
Groups of higher priority on the same side of the double bond = Z isomer.
Groups of higher priority diagonally opposite sides of double bond = E isomer

Question 12

Describe the hydrogenation of alkenes

Answer 12

An alkene is mixed with hydrogen and passed over a nickel catalyst at 423K forming an alkane.

Question 13

Describe the halogenation of alkenes

Answer 13

Rapid addition reaction between an alkene and chlorine or bromine at room temperature to form a halogenoalkane.

Question 14

How are alkenes tested for?

Answer 14

When bromine water (orange) is added to the sample of an alkene, the bromine reacts with the alkene and the colour disappears.

Question 15

Describe the hydration of alkenes

Answer 15

An alkene is reacted with steam in the presence of a phosphoric acid catalyst forming an alcohol (two products can be formed).

Question 16

What is the mechanism for addition reactions?

Answer 16

Electrophilic addition

Question 17

Outline an electrophilic addition reaction between an alkene and hydrogen bromide

Answer 17

H-Br is a polar molecule
Electron pair in pi bond attracted to H
Double bond breaks
Bond forms H and C atoms
H-Br bond breaks by heterolytic fission
Bromide ion and carbocation formed
Br- reacts with carbocation

Question 18

How does electrophilic addition take place with molecules that are non polar?

Answer 18

When the molecule approaches the pi bond, the electrons in the bond are repelled and this induces polarity.

Question 19

What is Markownikoff’s rule?

Answer 19

In electrophilic addition reactions, when the carbocation is formed it can be primary, secondary or tertiary.
The tertiary is the most stable so is the most likely product.

Question 20

Why is the tertiary carbocation the most stable?

Answer 20

The connecting alkyl groups donate and push electrons towards the positive charge of the carbocation.
Therefore more alkyl groups means a more stable ion.

Question 21

What is the name of the reaction for a polymers formation from alkenes?

Answer 21

Addition polymerisation

Question 22

How is the monomer unit displayed?

Answer 22

Displayed formula with an n before it to signify many units.

Question 23

How is the repeat unit of the polymer displayed?

Answer 23

Displayed formula
In square brackets
An n placed after to show a large number of repeats

Question 24

What are the conditions for a polymerisation reaction?

Answer 24

High temperature and pressure

Question 25

What are the uses of polypropene?

Answer 25

Childrens toys, packing crates, guttering, uPVC windows, rope fibre.

Question 26

What are the uses of polyphenylethene aka polysterene?

Answer 26

Packaging material, food trays and cups.

Question 27

What are the uses of polytetrafluoroethene aka PTFE or Teflon?

Answer 27

Coating for non stick pans, permeable membrane for clothing and shoes, cable insulation.

Question 28

What is the downside of current disposal of waste polymers?

Answer 28

Very unreactive so are non-biodegradable, with serious environmental effects.

Question 29

What are the benefits of recycling polymers?

Answer 29

Reduces environmental impact by conserving fossil fuels and decreasing waste.

Question 30

What are the weaknesses of recycling polymers?

Answer 30

Polymers have to be sorted by type.

Process is undermined if polymers are mixed as renders product unusable.

Question 31

How are polymers recycled?

Answer 31

Once sorted, they are chopped into flakes, washed, dried, melted and then cut into pellets for use.

Question 32

What are the dangers of PVC disposal?

Answer 32

High chlorine content and range of additives present make it hazardous.
When burnt it releases the corrosive hydrogen chloride gas and toxic dioxins.

Question 33

What is the method of recycling PVC?

Answer 33

Solvents used to dissolve the polymer and the high grade PVC is recovered by precipitation and the solvent can be used again.
Previous recycling, PVC was ground down and then reused.

Question 34

How can polymers be used as fuel?

Answer 34

Waste polymers can be incinerated to produce heat for power plants.

Question 35

What is feedstock recycling?

Answer 35

Chemical and thermal processes that can reclaim monomers, gases and oils from waste polymers.

Question 36

What is a major advantage of feedstock recycling?

Answer 36

Able to handle unsorted and unwashed polymers

Question 37

What are biodegradable polymers?

Answer 37

Produced from renewable sources and can be broken down by microrganisms. Made from starch or cellulose.

Question 38

What are photodegradable polymers?

Answer 38

Contain bonds that are weakened by absorbing light to start degradation or light-absorbing additives are used.