Alkenes and Alcohols Flashcards
Alkenes are ___________ hydrocarbons
why?
unsaturated
they have a c=c double bond which can make more bonds with extra atoms in addition reactions
General formula of an alkene?
CnH2n
What makes alkenes so reactive?
the C=C double bond has two pairs of electrons which means it has a high electon density
___________ addition reactions happen to alkenes
electrophilic
look at 152-153
Electrophiles are….
electron pair acceptors
How to test for unsaturation?
When you shake an alkene with orange bromine water the solution decolourises.
Polymers are formed from _______
monomers
give examples of synthetic and natural polymers
synthetic - poly(ethene)
natural - silk, cotton
What type of polymers are formed from alkenes and why?
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
The properties of polyalkenes depend on their _______________ forces
intermolecular
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 _____
stronger
strong rigid
weaker flexible
How can the properties of polymers be modified?
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
An alcohol is primary, secondary or tertiary depending on which carbon atom the -__ is bonded to
OH
see page 156
What happens when you dehydrate an alcohol?
you form an alkene
CnH2n+1OH –> CnH2n + H2O
allows to produce alkenes from renewable resources
elimination on page 156
Describe the method to separate chemicals
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
Alcohols are produced by ______ of alkenes
hydration
using steam in the presence of an acid catalyst
mechanism on page 158
Describe how ethanol can be produced industrially?
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
Ethanol is a biofuel
define biofuel
a fuel that is made from biological material thats recently died
What are the + and - of biofuels ?
+
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
show the chemical equations to support the argument that bioethanol production is carbon neutral
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
Argument that bioethanol production by fermentation isn’t carbon neutral?
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
Primary alcohols will oxidise to _____ and __________ ______
how?
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.
Secondary alcohols will oxidise to ____
how?
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
Tertiary alcohols cannot be oxidised easily
why?
they don’t react with acidified potassium dichromate at all - the only way to oxidise is to burn them
How to use oxidising agents to distinguish between aldehydes and ketones?
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
