Alkenes

Question 1

What is the general formula for alkenes

Answer 1

Cn H2n

Question 2

What are the properties of alkenes

Answer 2

Unsaturated hydrocarbons and have at least one C=C double bond
More reactive than alkanes because of the double bond
The C=C double bond is the functional group in an alkene

Question 3

What is the shape of an alkene

Answer 3

Triagonal planar around each carbon of the double bond
Three bonding regions and no lone pairs
So bonding regions repel
C=C

Question 4

What is a double bond made up of

Answer 4

A sigma bond and a pi bond

Question 5

What are the features of sigma bonds

Answer 5

Head on overlap of 2 atomic orbitals directly in a line between the 2 bonded atoms
Occur between C-H and C-C

Question 6

What are the features of the pi bond

Answer 6

Formed through the sideways overlap between 2 adjacent p orbitals

Question 7

Explain how the bonding differs in alkanes and alkenes (2)

Answer 7

Ethane is an alkane so has a single C-C bond madeup of a sigma bond
Ethene is an alkene so has a double C=C bond made up of a sigma bond and a pi bond

Question 8

Why are alkenes more reactive than alkanes

Answer 8

Alkenes will be more reactive than alkanes because the double bond has a high electron density and because the pi bond has a low bond enthalpy so it more easily broken than the C-C sigma bond in ethane

Question 9

How can alkenes form stereoisomers

Answer 9

Single bonds have free rotation but double and triple bonds have restricted rotation

Question 10

What are stereoisomers

Answer 10

Compounds with the same structural formula but a different arrangement of atoms in space

Question 11

When does stereoisomers occur

Answer 11

When 2 double bonded carbon atoms each have 2 different atoms or groups attached to them
The molecule must contain a C=C double bond which restricts rotation

Question 12

What are the 2 types of stereoisomers

Answer 12

E/Z stereoisomerism
Optical

Question 13

What happens if an alkene is an E isomer

Answer 13

Has the groups apart on opposite sides (one above one below)

Question 14

What happens if an alkene is a Z isomer

Answer 14

Has the groups together on the same side

Question 15

How do you use the Cahn Ingold Prelog priority rules to determine E/Z stereoisomers

Answer 15

The atom or group on each side of the double bond with the higher Ar or Mr is given the higher priority

Question 16

When can you use Cis/trans isomers

Answer 16

When the carbon atoms have at least one group in common

Question 17

What is a cis isomer

Answer 17

The same groups are on teh same side of the double bond

Question 18

What is a trans isomer

Answer 18

The same groups are on opposite sides of the double bond

Question 19

Where may the cis/trans isomer s not work

Answer 19

The carbon atoms have different groups attached so theres no way of deciding which isomer is cis and which is trans

Question 20

What are electrophiles

Answer 20

Electron pair acceptors

Question 21

What are the main compounds that react with alkenes

Answer 21

Br2
HBr

Question 22

How does bromine become slightly polar during electrohilic addition

Answer 22

When bromine approaches an alkene a dipole is induced in the bromine molecule because the bond electrons repel the electrons in the Br-Br bond. So its slightly polar

Question 23

How does the double bond break during electrophilic addition

Answer 23

The electron pair in the pi bond is attracted to the slightly positive atom

Question 24

What is heterolytic fission

Answer 24

The bond breaks and both electrons go to one atom

Question 25

How does a bromide ion and carbocation form

Answer 25

The Br-Br bond breaking by heterolytic fission with the electron pair going to the bromine atom

Question 26

What is a carbocation

Answer 26

An organic ion containing a positively charged carbon atom

Question 27

How do you use bromine water to test for carbon double bonds

Answer 27

When you shake an alkene with orange bromine water the solution will quickly decolourise
This is because bromine is added across the double bond to form the colourless dibromoalkane

Question 28

What happens during electrophilic addition with hydrogen halides

Answer 28

Bromine is more electronegative than hydrogen so H-Br is polar
The pi bond breaks so its electron pair is donated to the H atom in HBr
A covalent bond will form between a C from the double bond and the H

Question 29

What happens during electrophilic addition for hydrogen halide (2)

Answer 29

The H-Br bond breaks by heterolytic fission and the carbocation forms

Question 30

What does asymmetrical mean

Answer 30

It is a mixture of 2 products. A major and minor product

Question 31

Why are carbocations with more alkyl groups the most stable

Answer 31

Because the alkyl groups feed electrons towards the positive charge

Question 32

Which carbocations are the most stable

Answer 32

The primary carbocations with one alkyl group are the least stable
Secondary with 2 alkyl is more
Tertiary with 3 alkyl groups is the most stable

Question 33

How do you explain why which is a major/ minor product

Answer 33

_________ is the major product because it is formed from a ________ carbocation intermediate which is more stable than the ____________ carbocation

Question 34

What is a monomer

Answer 34

A small molecule that combines with other molecules to form a polymer

Question 35

What is a polymer

Answer 35

A long moleculer chain built up from monomer units

Question 36

Why can an alkene behave as a monomer

Answer 36

Each alkene can break in double bonds joining alkenes together to make a long carbon chain

Question 37

What is the reaction that occurs between alkene monomers

Answer 37

Addition Polymerisation

Question 38

How must a repeating unit be shown

Answer 38

Brackets
The use of n
Extended bonds through brackets

Question 39

What is the reaction called when a halogen is added to alkene

Answer 39

Halogenation

Question 40

Why are alkenes not soluble in water

Answer 40

They have non polar bonds

Question 41

EXAM QUESTION- Explain the bonding in a C=C double bond Use the orbital overlap model (2)

Answer 41

Sideways overlap of p orbitals (1)
Forming a pi bond (1)
include diagram

Question 42

EXAM QUESTION- Suggest why the average bond enthalpy of a C=C double bond is not twice the bond enthalpy of a C-C bond (1)

Answer 42

The pi bond is weaker than the sigma bond OR VICE VERSA

Question 43

What happens during hydrogenation

Answer 43

Adding hydrogen to an alkene
Need a nickel catalyst and 150 degrees

Question 44

What happens in a hydration reaction

Answer 44

Adding water to an alkene in steam
Need phosphoric acid and makes an alcohol

Question 45

EXAM QUESTION- Explain why one of the organic products forms in much greater quantity than the other organic products (2)

Answer 45

The major product would be formed in much greater quantity as the intermediate in the mechanism would contain a tertiary carbocation which is much more stable than a secondary carbocation