alkenes and alcohols

Question 1

Why are alkenes pretty reactive?

Answer 1

There are two pairs of electrons in the C=C bond, making it an area of really high electron density

Question 1

What is the general formula of an alkene?

Answer 1

CnH2n

Question 2

Describe the electrophilic addition mechanism:

Answer 2

1. The C=C bond repels the electrons in the electrophile (X-Y) which polarises the X-Y bond (unless its HBr in which case its already polar).
2. Two electrons from the C=C bond attack the ∂+ X atom creating a new bond between one carbon and X. X-Y bond breaks and the electrons from the bond go to Y to make a negative ion. The other carbon left with a positive charge. So a carbocation intermediate is formed
3. The Y- ion acts as a nucleophile, attacking the positive carbocation, donating its lone pair of electrons and forming a new bond with the positive carbon.
   Overall the X-Y molecule has been added to form a saturated compound

Question 3

How do you test for unsaturation?

Answer 3

Shake an alkene with bromine water, the solution goes from orange to colourless- bromine added across the double bond to for a colourless dibromoalkane

Question 4

What happens if you add hydrogen halides to an unsymmetrical alkene?

Answer 4

Can form primary, secondary and tertiary carbocations. All have different stabilities and the most stable option is more likely to form (major product)
3º>2º>1º

Question 5

What are polymers?

Answer 5

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

Question 6

How can addition polymers be formed from alkenes?

Answer 6

The double bonds can open up and join together to make long chains (polymers)

Question 7

What are the properties of polymers?

Answer 7

Once alkene monomers form polymers, they are saturated, and the main carbon chain is usually non polar, so polyalkenes are chemically inert.
Monomers within the chain have strong covalent bonds, however the intermolecular forces are much weaker, which affects the properties of the polymer. Longer chains with fewer branches have stronger intermolecular forces so are stronger and more rigid

Question 8

Why is PVC a strong but brittle material?

Answer 8

The covalent bonds between the chlorine and carbon are polar. The ∂- chlorine and ∂+ carbon means there are permanent dipole dipole forces between the polymer chains.

Question 9

What do plasticisers do?

Answer 9

Make a polymer bendier, as can get between the polymer chains and push them apart. Reduces the strength of the intermolecular forces so chains can slide around more.

Question 10

What is the general formula of an alcohol?

Answer 10

CnH2n+1OH

Question 11

What are the differences between primary, secondary and tertiary alcohols?

Answer 11

Depends on which carbon the hydroxyl group is bonded to. Same rules as with 1º, 2º and 3º carbocations- depends how many alkyl groups attached to carbon attached to hydroxyl group

Question 12

How do you dehydrate an alcohol?

Answer 12

Eliminate water in a dehydration reaction to form an alkene using concentrated sulfuric acid as a catalyst

Question 13

Outline the mechanism for the elimination of water from ethanol:

Answer 13

1. Lone pair of electrons from the oxygen bonds to an H+ from the acid- alcohol is protonated, giving the atom a positive charge
2. The positively charged oxygen pulls electrons away away from the carbon. H2O molecule leaves, creating an unstable carbocation intermediate. The carbocation loses an H+ and the alkene is formed

Question 14

How would you produce ethanol by hydration of alkenes?

Answer 14

Use steam in the presence of an acid catalyst
Ethene is hydrated at temperature of 300ºC and a pressure of 60atm, using a solid phosphoric acid catalyst

Question 15

Outline the mechanism for the reaction of ethene with steam:

Answer 15

1. Pair of electrons from the double bond bond to a H+ from the acid
2. A lone pair of electrons from a water molecule bonds to the carbocation
3. Water loses a H+ and the alcohol is formed

Question 16

How do you produce ethanol by fermentation of glucose?

Answer 16

Exothermic process, carried out by yeast in anaerobic conditions. Yeast produces enzymes which convert glucose into ethanol and carbon dioxide. Enzyme works at an optimum temp. of 30-40ºC. When the solution reaches about 15% ethanol, the yeast dies, so fractional distillation is used to increrase conc. of ethanol.

Question 17

Compare both ethanol production methods:

Answer 17

Hydration. Fermentation
Rate. V. Fast. V. slow
Quality Pure. V impure
Raw. Ethene. Sugars
Process Cont, so. Batch so
Costs. expensive. cheap equip,
but exp labour

Question 18

What is a biofuel?

Answer 18

Fuel made from biological material thats recently died

Question 19

What are the advantages of biofuel?

Answer 19

Renewable energy sources, so more sustainable.
Do produce CO2, but the amount released is the same amount taken in when the plant grows so considered to be carbon neutral

Question 20

What are the disadvantages of biofuel?

Answer 20

Land used to grow crops for fuel can’t be used to grow food- less food for people
Deforestation may occur to create more land to grow crops for biofuels- destroys habitats and the trees that are cut down are often burned, releasing more CO2.
Fertilisers used to increase production often pollute waterways and release nitrous oxide- a greenhouse gas
Current car engines would need to be modified to run on fuels with high ethanol conc.
Although bioethanol is thought of as a carbon neutral fuel, fossil fuels will still need to be burnt to power the machinery used as well asrefining and transporting the biofuel.

Question 21

What oxidising agent is used with alcohols?

Answer 21

Acidified potassium dichromate (K2Cr2O7), which goes from orange to green when reacting with 1º and 2º alcohols

Question 22

What are primary alcohols oxidised to?

Answer 22

Aldehydes and then to carboxylic acids

Question 23

What are secondary alcohols oxidised to?

Answer 23

Ketones

Question 24

What is aldehyde?

Answer 24

A carbonyl compound with the C=O functional group. One alkyl group is attached to the carbonyl carbon atom. Carbonyl group always found on carbon-1

Question 25

What is a ketone?

Answer 25

Carbonyl compound, where the carbonyl carbon is attached to two alkyl group.

Question 26

What is a carboxylic acid?

Answer 26

Have a COOH group at the emd of their carbon chain
Their general formula is CnH2n+1COOH

Question 27

How do you oxidise a 1º alcohol to an aldehyde?

Answer 27

Heat and distill with acidified potassium dichromate and H2SO4

Question 28

How do you oxidise a 1º alcohol to a carboxylic acid?

Answer 28

Reflux with acidified potassium dichromate and H2SO4

Question 29

How do you oxidise a 2º alcohol to a ketone?

Answer 29

Reflux/heat with acidified potassium dichromate and H2SO4

Question 30

How can you test for aldehydes and ketones?

Answer 30

Can use oxidising agents- aldehydes are easily oxidised but ketones aren’t. Fehling’s and Benedict’s solution are both deep blue Cu2+ complexes which reduce to a brick red Cu2O precipitate when warmed with an aldehyde but not with a ketone
Tollens reagent is a colourless [Ag(NH3)2]+ complex thats reduced to silver when warmed with an aldehyde but not a ketone.