alkenes and alcohols Flashcards
Why are alkenes pretty reactive?
There are two pairs of electrons in the C=C bond, making it an area of really high electron density
What is the general formula of an alkene?
CnH2n
Describe the electrophilic addition mechanism:
- 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).
- 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
- 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
How do you test for unsaturation?
Shake an alkene with bromine water, the solution goes from orange to colourless- bromine added across the double bond to for a colourless dibromoalkane
What happens if you add hydrogen halides to an unsymmetrical alkene?
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
What are polymers?
Long chain molecules formed when lots of small molecules (monomers) join together
How can addition polymers be formed from alkenes?
The double bonds can open up and join together to make long chains (polymers)
What are the properties of polymers?
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
Why is PVC a strong but brittle material?
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.
What do plasticisers do?
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.
What is the general formula of an alcohol?
CnH2n+1OH
What are the differences between primary, secondary and tertiary alcohols?
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
How do you dehydrate an alcohol?
Eliminate water in a dehydration reaction to form an alkene using concentrated sulfuric acid as a catalyst
Outline the mechanism for the elimination of water from ethanol:
- Lone pair of electrons from the oxygen bonds to an H+ from the acid- alcohol is protonated, giving the atom a positive charge
- 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
How would you produce ethanol by hydration of alkenes?
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
Outline the mechanism for the reaction of ethene with steam:
- Pair of electrons from the double bond bond to a H+ from the acid
- A lone pair of electrons from a water molecule bonds to the carbocation
- Water loses a H+ and the alcohol is formed
How do you produce ethanol by fermentation of glucose?
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.
Compare both ethanol production methods:
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
What is a biofuel?
Fuel made from biological material thats recently died
What are the advantages of biofuel?
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
What are the disadvantages of biofuel?
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.
What oxidising agent is used with alcohols?
Acidified potassium dichromate (K2Cr2O7), which goes from orange to green when reacting with 1º and 2º alcohols
What are primary alcohols oxidised to?
Aldehydes and then to carboxylic acids
What are secondary alcohols oxidised to?
Ketones
