3.3 - Alkenes And Alcohols Flashcards
What are alkenes?
Unsaturated hydrocarbons with formula CnH2n. They are reactive because the double bond has a really high electron density.
What happens in electrophilic addition reactions?
Electrophiles are electron-pair acceptors. E.g positively charged ions and polar molecules. When you shake an alkene with orange bromine water, the solution quickly goes from orange to colourless.
The mechanism is with Br delta positive - Br - . The arrow goes from the double bond to the Br, and another arrow from bond to br-. C now has Br and Arrow from lone pair of br- to other positive C.
What happens when you add hydrogen halides to unsymmetrical alkenes?
Two possible products. Carbocations with more alkyl groups are more stable because the alkyl groups feed electrons towards the positive charge. The more stable carbocation is more likely to form (with the second group). Secondary carbocation more stable than primary carbocation.
Describe the electrophilic addition reactions with H2SO4.
Alkenes will react with cold concentrated sulfuric acid to form alkyl hydrogen sulfates. You can then convert the alkyl hydrogen sulfates formed into alcohols by adding water and warming the reaction mixture. The sulfuric acid isn’t used up and is used a catalyst.
What are polymers? What are addition polymers?
Made from monomers. Natural or synthetic. Alkenes can form polymers because their double bonds can open up. The main carbon chain of a polyalkene is also non-polar, so additionally polymers are very unreactive.
What are the properties of polyalkenes. How can you modify the properties of polymers?
Chains only held together by van der waals forces. The longer the polymer chains are and the closer together they can get, the stronger the van der waals forces between the chains will be. Adding a plasticiser to a 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. PVC is formed from chloroethene. PVC has long, closely packed polymer chains, making it hard but brittle at room temperature. Rigid PVC is used to make drainpipes and window frames. Plasticised PVC is much more flexible than rigid PVC. It’s used to make electrical cable insulation, flooring tiles and clothing.
How do you draw the repeating unit from a monomer or a polymer?
Polymers are made up of repeating units. To draw the polymer, open the double bonds, add brackets and an n on the bottom right.
How can alcohols form alkenes?
Elimination or dehydration. This reaction allows you to produce alkenes from renewable resources, because you can produce ethanol by fermentation of glucose, which you can get from plants. So you can produce polymers without needing oil. One of the main industrial uses for alkenes is a starting material for polymers.
Ethanol is heated with a concentrated sulfuric acid catalyst (or phosphoric acid H3PO4). The product is usually in a mixture of water, acid and reactants in it so the alkene has to be separated out.
Mechanism: a lone pair of electrons from the oxygen bonds to a H+ from the acid. Arrow from C-O bond to O+. Arrow from C-H bond adjacent to C+ to C-C bond. Produces alkene and H2O and H+
More than one possible product as the double bond can form either side of the carbon with OH group.
What happens in distillation?
The products of organic reactions are often impure. E.g producing cyclohexene from cyclohexane
1 - distillation
Add concentrated H2SO4 and H3PO4 to a round-bottom flask containing cyclohexanol. Mix the solution by swirling the flask and add 2-3 carborundum boiling chips (mixture boils more calmly). The mixture should be gently heated to around 83°C (the boiling point of cyclohexene) using a water bath or electric heater. 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. The product can then be collected in a cooled flask.
2- separation
The proud collected will still contain impurities. Transfer the product mixture to a separating funnel and add water 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.
Remove the stopper to run substances out of the separating funnel.
Stage 3 - purification
Drain the cyclohexene into a round-bottomed flask. Add anhydrous CaCl2 (a drying agent) and stopper the flask. Let the mixture dry for at least 20 minutes with occasional swirling. The cyclohexene will still have small amounts of impurities so distill the mixture one last time.
How are alcohols produced by alkenes?
Hydrate an alkene using steam in the presence of an acid catalyst.
Mechanism:
Arrow from bond to H+. Arrow from lone pair of H2O to C+. Arrow from O-H bond to O+. Alcohol formed and H+ removed. Ethanol can be produced by the hydration of ethene by steam at 300°C and 60atm. It needs a solid phosphoric (V) acid catalyst.
How can ethanol be produced?
Steam hydration of ethene. The ethene comes from cracking heavy fractions of crude oil. In the future, petrochemicals like ethene will be expensive - so producing ethanol by fermentation, using a renewable raw material like glucose, will become much more important.
Fermentation is an exothermic process, carried out by yeast in anaerobic conditions. C6H12O6 (aq) -> 2C2H5OH (aq) + 2CO2 (g)
Yeast produces an enzyme which converts sugars, such as glucose, into ethanol and carbon dioxide. The enzyme works at an optimum temperature of 30-40°C. If it’s too cold the reaction is slow - if it’s too hot the enzyme is denatured. Once formed, ethanol is separated from the rest of the mixture by fractional distillation. Fermentation is low-tech and uses cheap equipment and renewable resources. But the fractional distillation needed to purify the ethanol produced using this method takes extra time and moneyz
What are biofuels and advantages and disadvantages?
Ethanol is a biofuel. A biofuel is a fuel that’s made from biological material that’s recently died. Biofuels are renewable energy sources so they’re more sustainable. Biofuels do produce CO2 when they’re burnt, but it’s CO2 that the plants absorbed while growing, so biofuels are usually still classed as carbon neutral. However petrol car engines would have to be modified to use fuels with high ethanol concentrations. Also land is used to grow crops or fuel so land can’t be used to grow food.
What are the equations for bioethanol?
Burning ethanol produces carbon dioxide. Carbon dioxide is a greenhouse gas. But it is carbon neutral.
Photosynthesis:
6CO2 + 6H2O -> C6H12O6 + 6O2
Fermentation:
C6H12O6 -> 2C2H5OH and 2CO2
Burning ethanol:
2C2H5OH + 6O2 -> 4CO2 + 6H2O
6CO2 taken in and 6CO2 taken out.
However 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. Burning the fuel to power this machinery produces carbon dioxide. So using bioethanol made my fermentation isn’t completely carbon neutral.
Which alcohols can and can’t be oxidised?
Use potassium dichromate K2Cr2O7, to mildly oxidise alcohols. The orange dichromate ion, Cr2O7^2- is reduced to the green chromium ion Cr^3+
Primary alcohols are oxidised to aldehydes and then to carboxylic acids, secondary alcohols are oxidised to ketones, tertiary alcohols aren’t oxidised.
R-CH2-OH + [O] -> aldehyde + H20
Add two Os to get to carboxylic acid.
Gently heating ethanol with potassium dichromate and sulfuric acid in a test tube produces ethanal. To get the aldehyde you need to use distillation apparatus so the aldehyde (which boils at a lower temperature than the alcohol) is distilled off immediately. To produce the carboxylic acid, the alcohol is mixed with excess oxidising agent and heated under reflux which 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.
Why can’t tertiary alcohols be oxidised easily?
No H atom bonded to the central carbon/carbon with OH group.
