Alkenes and Alcohols

Question 1

Alkenes are ___________ hydrocarbons

why?

Answer 1

unsaturated

they have a c=c double bond which can make more bonds with extra atoms in addition reactions

Question 2

General formula of an alkene?

Answer 2

CnH2n

Question 3

What makes alkenes so reactive?

Answer 3

the C=C double bond has two pairs of electrons which means it has a high electon density

Question 4

___________ addition reactions happen to alkenes

Answer 4

electrophilic

look at 152-153

Question 5

Electrophiles are….

Answer 5

electron pair acceptors

Question 6

How to test for unsaturation?

Answer 6

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

Question 7

Polymers are formed from _______

Answer 7

monomers

Question 8

give examples of synthetic and natural polymers

Answer 8

synthetic - poly(ethene)
natural - silk, cotton

Question 9

What type of polymers are formed from alkenes and why?

Answer 9

Addition polymers

double bonds can open up and join together to form long chains

polyalkenes are unsaturated and the main carbon chain of a polyalkene is non polar.

addition polymers are unreactive

Question 10

The properties of polyalkenes depend on their _______________ forces

Answer 10

intermolecular

Question 11

The longer the polymer chains are and the closer together they can get, the _______ the Van der Waals forces between the chains will be.

This means that polyalkenes made up of long straight chains tend to be _____ and _____ and short branched chains tend to be _____ and _____

Answer 11

stronger

strong rigid

weaker flexible

Question 12

How can the properties of polymers be modified?

Answer 12

Adding a plasticiser makes it more flexible. The plasticiser gets between the polymer chains to push them apart. This reduces the strength of the intermolecular forces so the chains can slide around more making the polymer easy to bend

Question 13

An alcohol is primary, secondary or tertiary depending on which carbon atom the -__ is bonded to

Answer 13

OH

see page 156

Question 14

What happens when you dehydrate an alcohol?

Answer 14

you form an alkene

CnH2n+1OH –> CnH2n + H2O

allows to produce alkenes from renewable resources

elimination on page 156

Question 15

Describe the method to separate chemicals

Answer 15

In the dehydration of alcohols to form alkenes, the mixture at the end contains the product, the reactant, acid and water. To get pure alkene you need to separate it from other substances.

Distillation example - producing cyclohexene from cyclohexanol

Stage 1) distillation
Add concentrated H2SO4 and H 3PO4, to a round-bottom flask containing cyclohexanol. Mix the solution by swirling the flask and add 2-3 carborundum boiling chips (these make the mixture boil more calmly).
reaction mixture
2) The mixture should be gently heated to around 83 °C (the boiling point of cyclohexene) using a water bath or electric heater.
3) Chemicals with boiling points up to 83 °C will evaporate.
The warm gas will rise out of the flask and into the condenser, which has cold water running through the outside, turning it into a liquid.
4) The product can then be collected in a cooled flask.

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

Stage 3 - purification
1) Drain the cyclohexene into a round-bottomed flask.
impu aquec water leg. m
2) Add anhydrous CaCl2 (a drying agent) and stopper the flask.
Let the mixture dry for at least 20 minutes with occasional swirling.
3) The cyclohexene will still have small amounts of impurities so distil the mixture one last time

Question 16

Alcohols are produced by ______ of alkenes

Answer 16

hydration

using steam in the presence of an acid catalyst

mechanism on page 158

Question 17

Describe how ethanol can be produced industrially?

Answer 17

via fermentation

1) exothermic process carried out by yeast in anaerobic conditions
C6H12O(aq) ——-> 2C2H5OH(aq) + 2CO2(g)
30-40 degrees c
yeast
2) yeast produces an enzyme that converts sugars into ethanol and CO2

3) the enzyme works at an optimum temp of 30-40 degrees celsius

4) once formed, ethanol is separated from the rest of the mixture via fractional distillation

5) uses cheap equipment and renewable resources but the fractional distillation that is needed to purify ethanol takes extra time and money

Question 18

Ethanol is a biofuel

define biofuel

Answer 18

a fuel that is made from biological material thats recently died

Question 19

What are the + and - of biofuels ?

Answer 19

+

renewable
sustainable
carbon neutral as they produce CO2 but plants take this in

-

to use biofuels car engines would need to be modified to use fuels with high ethanol concentrations
when you use land to grow crops for fuel, you cannot then use this land to grow food

Question 20

show the chemical equations to support the argument that bioethanol production is carbon neutral

Answer 20

Plants take in CO2

6CO2 + 6H2O –> C6H12O6 +6O2

fermentation process where glucose is converted into ethanol

C6H12O6 –> 2C2H5OH + 2CO2

when ethanol is burned CO2 and water are produced

2C2H5OH + 6O2 –> 4CO2 + 6H2O

Question 21

Argument that bioethanol production by fermentation isn’t carbon neutral?

Answer 21

fossil fuels will need to be burned to power the machinery used to make fertilisers for the crops, harvest the crops and refine and transport the bioethanol. This produces CO2 - therefore not fully carbon neutral

Question 22

Primary alcohols will oxidise to _____ and __________ ______

how?

Answer 22

aldehydes - hydrogen and alkyl group
carboxylic acids - cooh group

1) Gently heating ethanol with potassium dichromate(VI) and sulfuric acid in a test tube produces ethanal (an 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, so 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 vigorously oxidised.
The alcohol is mixed with excess oxidising agent and heated under reflux.

4) Heating under reflux means you can increase the temperature of an organic reaction without losing volatile solvents, reactants or products. Any vaporised compounds cool, condense and drip back into the reaction mixture. So the aldehyde stays in the reaction mixture and is oxidised to carboxylic acid.

Question 23

Secondary alcohols will oxidise to ____

how?

Answer 23

ketones - two alkyl groups

1) refluxing a secondary alcohol with acidified potassium dichromate will produce a ketone

2) ketones cant be oxidised easily so even prolonged refluxing wont produce anything more

Question 24

Tertiary alcohols cannot be oxidised easily
why?

Answer 24

they don’t react with acidified potassium dichromate at all - the only way to oxidise is to burn them

Question 25

How to use oxidising agents to distinguish between aldehydes and ketones?

Answer 25

Fehlings and Benedicts solution are both deep blue Cu2+ complexes which reduce to brick red Cu2O when warmed with an aldehyde but stay blue with a ketone

Tollens reagent is reduced to silver when warmed with an aldehyde but not a ketone