Section 3 Alkenes and Alcohols

Question 1

What are alkenes?

Answer 1

Unsaturated hydrocarbons

Question 2

What is the generla formula for an alkene?

Answer 2

C_nH_2n

Question 3

Why are alkenes reactive?

Answer 3

They have a high electron density around the double bond.

Question 4

What type of reaction happens to alkenes?

Answer 4

Electrophilic addition

Question 5

What are electrophiles?

Answer 5

Electron-pair acceptors

Question 6

What is a test for unsaturation?

Answer 6

Bromine water - goes from orange to colourless in the presence of an alkene.

Question 7

What mechanism is the bromine water test?

Answer 7

Electrophilic addition

Question 8

Why does a polar bond form on the Br₂ molecule?

Answer 8

As the bromine approaches the double bond, the electrons on it are repelled slighty, causing the delta charge.

Question 9

Why are carbocations with more alkyl groups, more stable?

Answer 9

The alkyl groups feed electrons towards the positive charge.

Question 10

Which carbocation is most likely to form in a reaction?

Answer 10

The most stable one.

Question 11

What are the conditions for an electrophilic addition reaction between alkenes and sulfuric acid?

Answer 11

• cold, conc sulfuric acid

Question 12

What are the conditions to hydrolise an alkyl hydrogen sulfate into an alcohol?

Answer 12

• cold water
• warm

Question 13

What are polymers?

Answer 13

Long chain molecules formed when lots of small molecules (monomers) join together.

Question 14

How are addition polymers formed from alkenes?

Answer 14

The double bond of the alkenes are opened up and join to make long chains.

Question 15

How reactive are polymers and why?

Answer 15

Unreactive - the main carbon chain of a polymer is non-polar.

Question 16

What are the IMFs between polymers?

Answer 16

Van der Waals

Question 17

What determines the strength of IMFs between polymers?

Answer 17

The longer the chains, and the closer together they are, the stronger the VdWs.

Question 18

What are the opposing qualities of long, straight chain and short, branched chain polymers?

Answer 18

Long, straight chain - strong, rigid.

Short, branched chain - weak, flexible.

Question 19

How can you make a polymer more flexible?

Answer 19

Add a plasticiser.

Question 20

How do plasticisers work?

Answer 20

They get between the polymer chains and push them apart, reducing the strength of the IMFs.

The polymers can then slide around more and bend easier.

Question 21

What is the IUPAC name for PVC?

Answer 21

polychloroethene

Question 22

What are the properties of PVC?

Answer 22

Long, closely packed chains so hard but brittle at room temperature

Question 23

What is rigid PVC used for?

Answer 23

• drainpipes
• windowframes

Question 24

What is plasticised PVC used for?

Answer 24

• electrical cable insulation
• flooring tiles
• clothing

Question 25

What is the IUPAC name for Teflon?

Answer 25

Polytetrafluoroethene

Question 26

How can you form alkenes from alcohols?

Answer 26

Dehydration

Question 27

What type of reaction is the dehydration of alcohol?

Answer 27

Elimination (of water)

Question 28

How is dehydration of alcohols better for the environment?

Answer 28

Alcohol is a renewable resource - ethanol can be produced by fermentation of glucose, which can be retrieved from plants

Question 29

What are the conditions for the elimination of water from alcohol?

Answer 29

• heat
• conc sulfuric or phosphoric acid

Question 30

What is distillation used for?

Answer 30

To separate chemicals

Question 31

Why must you distil the mixture at the end of an elimination reaction?

Answer 31

It will contain the product, reactant, acid and water.

Question 32

How can alcohols be produced from alkenes?

Answer 32

Hydration

Question 33

What type of reaction is the hydration of alkenes?

Answer 33

Electrophilic addition

Question 34

What are conditions for electrophilic addition of water to alkenes?

Answer 34

• steam
• high pressure and temperature
• acid catalyst (phosphoric acid)

Question 35

How can ethanol be produced industrially?

Answer 35

Fermentation

Question 36

What is the equation and conditions for the fermentation of glucose?

Answer 36

C₆H₁₂O_6(aq) —> 2C₂H₅OH_(aq) + 2CO_2(g)

• yeast
• 30-40^oc

Question 37

Why is yeast used in fermentation?

Answer 37

It produces an enzyme.

Question 38

Why does the temperature have to be 30-40^oc for fermentation?

Answer 38

That is the optimum temperature for the enzyme.

Question 39

Give some positives and negatives of fermentation?

Answer 39

+ low tech

+ cheap equipment

+ renewable resources

• purification takes time and money

Question 40

What is biofuel?

Answer 40

Fuel made from biological material that’s recently died

Question 41

Gives some advantages and disadvantages of biofuel:

Answer 41

+ renewable

+ the carbon dioxide produced is absorbed by plants

• petrol car engines would have to be modified
• crops should be prioritised as food over fuel

Question 42

Give 3 equation to show that bioethanol production is carbon neutral:

Answer 42

Photosynthesis:

6CO₂ + 6H₂O —> C₆H₁₂O₆ + 6O₂

Fermentation:

C₆H₁₂O₆ —> 2C₂H₅OH + 2CO₂

Combustion of Ethanol:

2C₂H₅OH + 6O₂ —> 4CO₂ + 6H₂O

6 moles of carbon dioxide in and out.

Question 43

Why is it debatable whether bioethanol production is actually carbon neutral?

Answer 43

Fossil fuels will need to be burned to power fertilising, harvesting, refining and transporting machinery.

Question 44

What is the oxidising agent used to oxidise alcohols?

Answer 44

Acidifed potassium dichromate

Question 45

What are primary alcohols oxidised to?

Answer 45

Aldehydes then carboxylic acids

Question 46

What are tertiary alcohols oxidised to?

Answer 46

They aren’t

Question 47

What are secondary alcohols oxidised into?

Answer 47

Ketones

Question 48

What are the conditions for the oxidation of primary alcohols to aldehydes?

Answer 48

Distillation

Question 49

What are the conditions for the oxidation of primary alcohols to carboxylic acids?

Answer 49

Reflux

Question 50

What are the conditions for the oxidation of secondary alcohols to ketones?

Answer 50

Reflux

Question 51

What will you observe if you add Fehling’s solution to an aldehyde?

Answer 51

Blue solution to red precipitate

Question 52

What will you observe if you add Fehling’s solution to an ketone?

Answer 52

Nothing - blue solution

Question 53

What happens when Fehling’s solution goes from blue to red?

Answer 53

The Cu²⁺ complex is reduced to brick-red Cu₂O.

Question 54

What will you observe if you add Tollen’s reagent to an aldehyde?

Answer 54

It will go from a colourless solution to a silver mirror.

Question 55

What will you observe if you add Tollen’s reagent to a ketone?

Answer 55

Nothing - colourless solution

Question 56

What happens when Tollen’s reagent goes from colourless to silver?

Answer 56

The [Ag(NH₃)₂]⁺ is reduced

Question 57

What do you observe when potassium dichromate oxidises a substance?

Answer 57

Orange to green

Question 58

What happens to potassium dichromate when it oxidises a substance?

Answer 58

The dichromate ion, Cr₂O₇^2-, is reduced to the chronium ion, Cr³⁺.

Question 59

What is a way to oxidise a tertiary alcohol?

Answer 59

Burn it