Alkenes

Question 1

Are alkenes saturated or unsaturated?

Answer 1

Unsaturated

Question 2

What is the generic formula of alkenes?

Answer 2

CnH2n

Question 3

Do cyclic alkenes and alkenes with more than one double bond have the generic formula of CnH2n?

Answer 3

No

Question 4

Describe the formation of a pi-bond.

Answer 4

One electron from each carbon atom is in a p-orbital. When two p-orbitals overlap sideways, one from each carbon atom of the double bond, a pi-bond is formed.

Question 5

Describe the pi-bond electron density.

Answer 5

It is concentrated above and below the line joining the nuclei of the bonding atoms.

Question 6

How does the pi-bond affect the geometry of alkenes?

Answer 6

The pi-bond locks the two carbon atoms in position and prevents them from rotating around the double bond.

Question 7

Explain why the shape around each of the carbon atoms in the double bond is trigonal planar.

Answer 7

There are areas of electron density around each of the carbon atoms. The three regions repel each other as far apart as possible so the bond angle around each carbon atom is 120. All of the atoms are in the same plane.

Question 8

What is stereoisomerism?

Answer 8

When molecules have the same structural formula but a different arrangement of the atoms in space.

Question 9

When can E/Z isomerism occur?

Answer 9

When there is a C=C double bond.

Question 10

Why does stereoisomerism occur?

Answer 10

It arises because rotation around the double bond is restricted and the groups attached to each carbon atom are therefore fixed relative to each other.

Question 11

What two things must a molecule have for it to be able to display E/Z isomerism?

Answer 11

A C=C double bond and different groups attached to each carbon atom of the double bond.

Question 12

What is cis-trans isomerism?

Answer 12

E/Z isomerism for H molecules. There must be a hydrogen group on each carbon.

Question 13

What is the relationship between cis-trans isomerism and E/Z isomerism?

Answer 13

The cis isomer is the Z isomer and the trans isomer is the E isomer.

Question 14

How do you know which group is the priority group when naming molecules?

Answer 14

The higher the atomic number, the higher priority.

Question 15

What happens if the two atoms attached to a carbon atom are the same?

Answer 15

You have to look for the first point of difference.

Question 16

Give four addition reactions of alkenes.

Answer 16

Hydrogen (in the presence of a nickel catalyst, halogens, hydrogen halides, steam (in the presence of a nickel catalyst)

Question 17

Describe hydrogenation reactions of alkenes.

Answer 17

When an alkene such as propene is mixed with hydrogen and passed over a nickel catalyst at 423K, an addition reaction takes place to form an alkane.

Question 18

Describe halogenation reactions of alkenes.

Answer 18

Alkenes undergo a rapid reaction with the halogens chlorine or bromine at room temperature.

Question 19

Describe what bromine water is used for and the process involved.

Answer 19

Can identify if there is a C=C bond present (unsaturation). The bromine water will go from orange to colourless.

Question 20

Describe the reactions of alkenes with hydrogen halides.

Answer 20

Alkenes react with gaseous hydrogen halides at room temperature to form haloalkanes. If the alkene is a gas they can just be mixed if not it must be bubbled through.

Question 21

Describe hydration reactions of alkenes.

Answer 21

Alcohols are formed when alkenes react with steam in the presence of a phosphoric acid catalyst.

Question 22

What is the reaction mechanism for addition reactions with alkenes?

Answer 22

Electrophilic addition

Question 23

What is an electrophile?

Answer 23

An atom or group of atoms that is attracted to an electron rich centre and accepts an electron pair.

Question 24

What is Markownikoff’s rule?

Answer 24

When a hydrogen halide reacts with an unsymmetrical alkene the hydrogen hailed attaches itself to the carbon atom of the alkene with the greater number of hydrogen atoms and the smaller number of carbon atoms.

Question 25

Which carbocation is the most stable? (primary, secondary or tertiary?)

Answer 25

Tertiary

Question 26

What conditions do polymerisation reactions require?

Answer 26

High temperatures and high pressures using catalysts.

Question 27

How do you represent a repeating unit of a polymer?

Answer 27

With the monomer drawn with the double bond broken and the atoms at right angles encased in square brackets.

Question 28

In the general equation for any addition polymerisation reaction, how many n’s should be written and where?

Answer 28

Two; one before the monomer and one after the polymer

Question 29

Types of chains in HDPE?

Answer 29

Linear

Question 30

Types of chains in LDPE?

Answer 30

Branched

Question 31

Which monomer makes poly(chloroethene)?

Answer 31

Chloroethene

Question 32

What are the common uses of poly(chloroethene), PVC?

Answer 32

Pipes, filmes and sheeting, insulation and sable sheathing, bottles, flooring.

Question 33

What are the common uses of poly(propene)?

Answer 33

Children toys, packing crates, guttering

Question 34

What are the common uses of poly(phenylethene) or poly(styrene)?

Answer 34

Packing material and as cups or food trays.

Question 35

What are the common uses of poly(tetrafluoroethene)?

Answer 35

Used as a coating for non-stick pans, permeable membrane for clothing and shoes and for cable insulation.