Alkenes and Alcohols

Question 1

What is an alkene?

Answer 1

A hydrocarbon that contains a double bond.

Question 2

What type of reaction happens to alkenes?

Answer 2

Electrophilic addition.

Question 3

What is the test for an alkene and describe it.

Answer 3

Bromine water

When you shake an alkene with orange bromine water, the solution decolourises.

Question 4

Describe the mechanism of bromine and ethene:

Answer 4

1) The double bond repels the electrons in Br2, polarising Br-Br
2) A pair of electrons in the double bond attracts the Br+ and forms a bond with it. This repels the electrons in the Br-Br bond further, until it breaks.
3) You get a positively charged carboncation immediate. The Br- now moves over.

4) And bonds to the other C atom, forming
1,2-dibromoethane

Question 5

Alkenes undergo what reaction with hydrogen halides?

Answer 5

Electrophilic addition.

Question 6

What is most stable and what is least stable out of the following:
Primary carbocation, Secondary carbocation, Tertiary carbocation

Answer 6

A primary carbocation is it least stable

A tertiary carbocation is most stable

Question 7

When an alkene is added to sulphuric acid what kind of reaction takes place?

Answer 7

Electrophilic addition.

Question 8

What is formed when an alkene reacts with sulphuric acid?

Answer 8

Alkyl hydrogen sulfates

Question 9

If you add water to the alkyl hydrogen sulphate what is formed?

Answer 9

An alcohol

Question 10

What are polymers made of?

Answer 10

Monomers

Question 11

Addition polymers are formed from:

Answer 11

Alkenes

Question 12

The properties of polyalkenes depend on their:

Answer 12

Intermolecular forces

Polyalkene chains are usually non-polar - so the chains are only held together by Van der Waals forces. The longer the polymer chains are and the closer together they are, the stronger Van der Waals forces are. This means that polyalkenes made up of long, straight chains tend to be strong and rigid, polyalkenes that are made up of short, branched chains tend to be weaker and more flexible.

Question 13

How does adding a plasticiser to a polymer alter its properties?

Answer 13

Adding plasticiser to the polymer makes it more flexible.
The plasticiser molecules get between the polymer chains and push them apart. This reduces the strength of the intermolecular forces between the chains-so they can slide around more, making the polymer easy to bend.

Question 14

What happens to alcohols in order for them to form alkenes?

Answer 14

They are dehydrated.

Question 15

How do you dehydrate ethanol to form ethene?

Answer 15

Ethanol is heated with a concentrated sulphuric acid catalyst:
C2H5O —H2SO4–> C2H4 + H2O
The product is usually a mixture of water, acid and reactant so the alkene has to be separated out using distillation.

Question 16

Distillation-

Describe stage 1-distillation of producing cyclohexane from cyclohexanol.

Answer 16

1) Add concentrated H2 SO4 and H3PO4 to a round bottom flask containing cyclohexanol. Mix the solution by swirling the flask and add 2 to 3 carborundum boiling chips
2) The mixture should be gently heated to around 83° C using a water bath or electric heater
3) chemicals with boiling points up to 83° C will evaporate. The cool gas will rise out of the flask into the condenser, turning it liquid.
3) The product can then be collected in a cooled flask.

Question 17

Distillation-

Describe stage 2-separation of producing cyclohexane from cyclohexanol.

Answer 17

1) The product collected after distillation will still contain impurities
2) Transfer the product mixture to a separating funnel and add water to dissolve water soluble impurities and create an aqueous solution.
3) Allow the mixture to settle into layers. Drain the aqueous lower layer, leaving the impure cyclohexene.

Question 18

Distillation-

Describe stage 3-purification of producing cyclohexane from cyclohexanol.

Answer 18

1) Drain the cyclohexane into a round bottom flask
2) Add anhydrous CaCl2 and stopper the flask. Let the mixture dry for at least 20 minutes with occasional swirling.
3) The cyclohexane will still have small amounts of impurities so distil the mixture one last time.

Question 19

Alcohol so produced by what reaction of alkenes?

Answer 19

Hydration

Question 20

Give the conditions of the hydration of alkanes:

Answer 20

Using steam in the presence of an acid catalyst.

Question 21

Describe the industrial production of ethanol by fermentation of glucose:

Answer 21

Fermentation is an exothermic process, carried out by yeast in anaerobic conditions.

This produces an enzyme which converts sugars into ethanol and carbon dioxide is

The enzyme works at 30 to 40° C.

Once formed, ethanol is separated from the rest of the mixture by fractional distillation.

Question 22

What are the benefits of the industrial production of ethanol?

Answer 22

Low-tech, uses cheap equipment and renewable sources.

Question 23

What are the disadvantages of industrial production of ethanol?

Answer 23

The distillation step in the method takes extra time and money.

Question 24

Definition of biofuel:

Answer 24

A biofuel is a fuel that’s made from biological matter that’s recently died.

Question 25

Advantages of using biofuel vs fossil fuels:

Answer 25

Biofuels are renewable energy sources, unlike fossil fuels, biofuels won’t run out, so they’re more sustainable.
Biofuels produce CO2 when burnt, but it’s CO2 that the plants absorbed while growing, so biofuels are classified as carbon neutral.

Question 26

Disadvantages of using biofuel vs fossil fuels:

Answer 26

Petrol, engines would have to be modified to use fuels with high ethanol concentrations
Land used to grow crops for fuel, that land can’t be used to grow food. If countries start using land to grow biofuel crops instead of food, they may be unable to feed their population.

Question 27

Why is bioethanol production not completely carbon neutral?

Answer 27

Because fossil fuels need to be burnt in power station to power the machinery used to make fertilisers for the crops, harvest the crops and refine and transport bioethanol. Burning the fuel to power this machinery produces carbon dioxide. So using bioethanol isn’t completely carbon neutral.

Question 28

Using equations show how bioethanol production is almost carbon neutral:

Answer 28

6CO2 + 6H2O —> C6H12O6 + 6O2

C6H12O6 —> 2C2H5OH + 2CO2

2C2H5OH + 6O2 —> 4CO2 + 6H2O

If you combine all three of these equations you’ll find exactly 6 moles of CO2 are taking in and exactly 6 moles of CO2 are given out.

Question 29

Primary alcohols can be oxidised to give:

Answer 29

Aldehydes then carboxylic acids.

Question 30

Secondary alcohols can be oxidised to give:

Answer 30

Ketones.

Question 31

Tertiary alcohols can be oxidised to give:

Answer 31

Nothing, tertiary alcohols aren’t oxidised.

Question 32

What oxidising agent is used to oxidise primary and secondary alcohols?

Answer 32

The oxidising agent to mildly oxidise is, acidified potassium dichromate (VI), K2Cr2O7.
The orange dichromate (VI) ion, Cr2O7^2-, is reduced to the green chromium (III) ion, Cr3+.

Question 33

Describe an aldehyde is:

Answer 33

They have a hydrogen and one alkyl group attached to the carbonyl carbon atom.
The suffix is -al.

Question 34

Describe a ketone is:

Answer 34

They have a two alkyl group attached to the carbonyl carbon atom.
The suffix is -one. For ketones with five or more carbons, you have to say which carbon the functional group is on.

Question 35

Describe a carboxylic acid:

Answer 35

They have a COOH at the end of the carbon chain.

The suffix is -oic acid

Question 36

Describe the method used when oxidising the primary alcohol to an aldehyde and carboxylic acid:

Answer 36

1) Gently heat ethanol with potasssium dichromate (VI) and sulfuruc acid in a test tube, this produces ethanol (and aldehyde). However, it’s tricky to control the heat and the aldehyde is usually oxidised to form ethanoic acid.
2) To get just the aldehyde, you need to get it out of the oxidising solution as soon as it forms. You do this using distillation apparatus, to the aldehyde (which boils at a lower temperature than the alcohol) is distilled off immediately.
3) To produce the carboxylic acid, the alcohol has to be vigourously oxidised. The alcohol is mixed with XS oxidising agent and heated under reflux.

Question 37

In order to produce a ketone from a secondary alcohol the alcohol has to be heated under what?

Answer 37

Under reflux.