Unit 3: Section 3 - Alkenes and alcohols MDY * Flashcards
Alkenes Addition polymers Alcohols Ethanol production Oxidation of alcohols
what is the general formula of alkenes?
CnH2n
what do all alkenes have?
at least one C=C double covalent bond. they are unsaturated because they can make more bonds with extra atoms in addition reactions.
what makes alkenes reactive?
there are 2 pairs of electrons in the C=C double bond, so it has a high electron density
what happens in an electrophilic addition reaction?
the double bonds in an alkene open up and atoms are added to the carbon atoms
why do electrophilic reactions happen?
because the double bond has got lots of electrons and are easily attacked by electrophiles
what are electrophiles?
electron-pair acceptors, they usually have fewer electrons than usual so they’re attracted to areas with lots
examples of electrophiles:
positively charged ions e.g. H+
polar molecules, the 𝛿+ end is attracted to the electrons
How can you test for alkenes?
using bromine water
what is the test for alkenes with bromine water?
when an alkene is shaken with bromine water the solution goes from orange to colourless.
this is because electrophilic addition occurs and forms colourless dibromoalkanes
which carbocations are more stable?
the more alkyl groups attached the more stable a carbocation is because the alkyl groups feed electrons towards the positive charge.
primary carbocation = least stable
tertiary carbocation = most stable
which carbocations are more stable?
the more alkyl groups attached the more stable a carbocation is because the alkyl groups feed electrons towards the positive charge.
primary carbocation = least stable
tertiary carbocation = most stable
what happens when alkenes react with cold concentrated sulfuric acid?
an electrophilic addition reaction takes place to form alkyl hydrogen sulphates
what happens when the product of the reaction between cold sulfuric acid and an alkene is warmed with water?
an alkyl hydrogen sulfate is formed and is then warmed with water to form an alcohol and the sulfuric acid acts as a catalyst
what are polymers?
long chain molecules formed when lots of small molecules, called monomers join together. they can be natural and synthetic
what are natural polymers?
e.g. DNA and proteins
natural polymers like silk, cotton and rubber have been used for hundreds of years
what are synthetic polymers?
e.g. poly(ethene)
19th century - properties of natural polymers modified, like hardening rubber
20th century - synthetic polymers like nylon and Kevlar invented
what are addition polymers?
polymers formed from alkenes whose double bonds opened up to join together to make long chains. these addition polymers are called polyalkenes
what are polyalkene chains
saturated molecules, where the main carbon chain is non-polar, this makes addition polymers very unreactive
what are polyalkene chains?
saturated molecules, where the main carbon chain is non-polar, this makes addition polymers very unreactive
what properties do polyalkenes have?
they are usually non-polar so the chains are held together by van der Waals
the longer the polymer chains and the closer together they can get the stronger the van der Waals
so polyalkenes made up of long, straight chains tend to be strong and rigid, short branched chains are weaker and more flexible
what does adding a plasticiser to a polymer do?
the plasticiser molecules get between the polymer chains and push them apart. this reduces the strength of intermolecular forces between the chains so they can slide more and are more flexible
what is PVC?
poly(chloroethene), its formed from chloroethene
what properties does PVC have?
long closely packed polymer chains, making it hard and brittle. rigid PVC is used to make drainpipes and window frames
what properties does plasticised PVC have?
its much more flexible than rigid PVC. its used to make electrical cable insulation, flooring tiles and clothing
How to draw the repeating unit of a polymer from its monomer:
draw the 2 alkene carbons
replace the double bond with a single bond
add a single bond coming out of each carbon
then fill in the rest of the groups as they surrounded the double bond
how to draw the repeating unit from its polymer:
for an addition polymer, the repeating unit should be 2 carbons long.
once you have the repeating unit add the double bond and remove the single bonds from the sides
what is the functional group in an alcohol homologous series?
the hydroxyl group
-OH
what are primary, secondary and tertiary alcohols?
primary - the C the -OH is bonded to, is bonded to 1 other carbon
secondary - the C the -OH is bonded to, is bonded to 2 other carbons
tertiary - the C the -OH is bonded to, is bonded to 3 other carbons
how can alkenes be formed from alcohols?
alcohols can be dehydrated in an elimination reaction to form alkenes
CnH2n+1OH –> CnH2n +H2O
how does the dehydration of alcohol produce alkenes from renewable resources?
alcohol can be obtained by fermentation of glucose.
this means polymers can be produced without using oil
what is one of the main industrial uses for alkenes?
as a starting material for polymers
How do you dehydrate ethanol to form ethene?
ethanol is heated with a concentrated sulphuric or phosphoric acid catalyst.
the product is usually in a mixture with water, acid and reactant in it so the alkene has to separate out
C2H5OH –H2SO4–> C2H4 + H2O
describe the mechanism for the elimination of water from ethanol:
lone pair of electrons from oxygen bond to an H+ from the acid. alcohol is protonated, giving O positive charge
O+ pulls electrons away from carbon, and an H2O is formed, forming a carbocation
the electrons between an adjacent carbon and hydrogen are pulled towards the carbocation, forming a double bond and an H+
describe the mechanism for the electrophilic addition reaction of bromine with an alkene:
the double bond repels the electrons in Br2, polarising it
pair of electrons in double bond attracts Br 𝛿+ and forms bond. this repels electrons in Br-Br bond until it breaks
a carbocation intermediate is formed that the Br- is attracted to
describe the mechanism for the electrophilic addition reaction of bromine with an alkene:
the double bond repels the electrons in Br2, polarising it
pair of electrons in double bond attracts Br 𝛿+ and forms bond. this repels electrons in Br-Br bond until it breaks
a carbocation intermediate is formed that the Br- is attracted to
what can be formed in the dehydration of more complicated alcohols?
it has more than 1 possible product as the double bond can be formed on either side of the carbon with the -OH
how can the products of the dehydration of alcohol be separated to get a pure alkene?
distillation uses the fact that different chemicals have different boiling points to separate them. often further separation and purification is needed.
what’s an example of how an alkene is produced from an alcohol?
producing cyclohexene from cyclohexanol
what are the steps of the separation of alkene from the products of alcohol dehydration?
stage 1 - distillation
stage 2 - separation
stage 3 - purification
what happens in distillation in producing cyclohexene from cyclohexanol?
- add conc. H2SO4 and H3PO4 to a round-bottom flask containing cyclohexanol. swirl with 2-3 carborundum boiling chips
- gently heat to around 83*C using water bath or electric heater
- chemicals with boiling points up to 83*C will evaporate. warm gas rises out of flask into condenser, turning it into a liquid.
- product can be collected in a cooled flask
what happens in separation in producing cyclohexene from cyclohexanol?
- product collected from distillation will contain purities
- transfer product mixture to a separating funnel and add water to dissolve water soluble impurities and create aqueous solution
- allow mixture to settle into layers. drain aqueous lower layer, leaving impure cyclohexene
what happens in purification in producing cyclohexene from cyclohexanol?
- drain cyclohexene into round-bottomed flask
- add anhydrous CaCl2 (drying agent) and stopper the flask. let mixture dry for at least 20 mins with occasional swirling
- cyclohexene will have small amounts of impurities so distil the mixture 1 last time
How can alcohols be produced?
hydration of alkenes
ethanol can be produced through fermentation
what is the standard industrial method for producing alcohols?
to hydrate an alkene using steam in the presence of an acid catalyst
CnH2n + H2O CnH2n+1OH
describe the reaction mechanism for the hydration of alkenes?
a pair of electrons from the double bond bonds to an H+ from the acid
a lone pair of electrons from a water molecule bonds to the carbocation
the water loses an H+ and an alcohol is formed
what conditions are needed for the hydration of ethene?
300*C
70 atm
concentrated solid phosphoric (V) acid catalyst
why is the production of ethanol through fermentation important?
ethene comes from cracking heavy fractions of crude oil, but crude oil will eventually run out.
fermentation uses renewable raw materials like glucose so is a good replacement
what conditions are needed for the fermentation?
anaerobic aqueous yeast 35*C normal atmospheric pressure enzyme in yeast - zymase
what kind of reaction is fermentation?
exothermic process
C6H12O6(aq) —-> 2C2H5OH(aq) +2CO2(g)
what does yeast do in the fermentation?
yeast produces an enzyme which converts sugars such as glucose into ethanol and carbon dioxide
why does fermentation work best at 30-40*C?
its the optimum temperature for yeast, if its too cold the reaction is slow and if its too hot the enzyme is denatured
what is done with the products of fermentation?
ethanol is separated by fractional distillation.
fermentation uses cheap equipment and renewable resources but the fractional distillation takes extra time and money
what is a biofuel?
a fuel thats made from renewable biological material thats recently died
what advantages do biofuels have over fossil fuels?
biofuels are renewable energy sources. so are more sustainable than fossil fuels
biofuels are carbon neutral because all carbon released when burnt was absorbed while growing
what disadvantages do biofuels have over fossil fuels?
when switching from fossil fuels to biofuels in transport petrol car engines would have to be modified to use fuels with high ethanol concentrations.
if land is used to grow crops for fuel, then it can’t be used to grow food
Is bioethanol production carbon neutral?
burning ethanol produces CO2 which contributes to global warming but plants that are grown to produce bioethanol take in carbon dioxide from the atmosphere when they’re growing. they release the same amount that they take in when burnt.
so they are theoretically carbon neutral
what are the equations for bioethanol production/ burning?
6CO2 + 6H2O —-> C6H12O6 + 6O2
C6H12O6 —-> 2C2H5OH + 2CO2
2C2H5OH + 6O2 —-> 4CO2 + 6H2O
so 6 moles of CO2 are taken in and given out
why is bioethanol production not completely carbon neutral?
fossil fuels will need to burn 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 the machinery produce CO2.
what is the simplest way to oxidise alcohols?
to burn them
How should alcohols be oxidised?
using the oxidising agent acidified potassium dichromate (VI) K2Cr2O7 to mildly oxidise alcohols.
the orange dichromate ion Cr2O7 2- is reduced to green chromium (III) ion Cr 3+ when they oxidise an alcohol
what happens when primary, secondary and tertiary alcohols are oxidised?
primary alcohols are oxidised to aldehydes and then to carboxylic acids
secondary alcohols are oxidised to ketones
tertiary alcohols aren’t oxidised
what are carbonyl compounds?
compounds with the functional group C=O
e.g. aldehydes and ketones
what is the general formula for aldehydes and ketones?
CnH2nO
what is the general formula for carboxylic acids?
CnH2n+1COOH
with the functional group COOH
what are aldehydes?
they have a hydrogen and 1 alkyl group attached to the carbonyl carbon atom.
the suffix is -al, and the functional group is always on the 1st carbon
what are ketones?
have 2 alkyl groups attached to the carbonyl carbon atom.
the suffix is -one. for ketones with 5 or more carbons, you must say which carbon the functional group is on
what are carboxylic acids?
they have a COOH group at the end of their carbon chain
their suffix is -oic acid
what is the process to produce aldehydes or ketones?
gently heat alcohol with potassium dichromate and excess sulfuric acid in a test tube.
this must be done using distillation apparatus so the aldehyde (boils at a lower temperature than alcohol) is distilled off immediately, and doesn’t oxidise into carboxylic acid
what is the process to produce carboxylic acid?
vigorously oxidise the alcohol using excess oxidising agent and heated under reflux so the aldehyde stays in the reaction mixture and is oxidised to carboxylic acid
what does heating under reflux mean?
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.
what happens if you reflux a secondary alcohol with acidified dichromate (VI)?
a ketone will be produced. ketones cant be oxidised easily, so even prolonged refluxing won’t produce anything more
how can tertiary alcohols be oxidised?
by burning only
how can you distinguish between aldehydes and ketone?
using oxidising agents such as Fehling’s or benedict’s solution or Tollens reagent.
aldehydes are oxidised easily but ketones aren’t
what does Fehling’s reagent do?
deep blue Cu 2+ complex, which reduces to brick red Cu2O precipitate when warmed with an aldehyde, but remains blue with a ketone
Cu2+ + e- –> Cu+
what does Tollens’ reagent do?
[Ag(NH3)2]+ its reduced to silver when warmed with an aldehyde, but not with a ketone. the silver will coat the inside of the apparatus to form a silver mirror
Ag(NH3)2 + + e- –> Ag + 2NH3
what is the shape of an alkene?
they are planar
the C=C bond is planar and has a bond angle of 120*
how do you name alkenes?
the name is based on the corresponding alkane
ane is changed to ene
a number is given to which bond is a double bond
if bonded to an acid or alcohol, enol or enoic acid is used
the higher priority group is given the smallest position number
alkenes with 2 double bonds are called dienes
what is geometric isomerism?
double bond is resistant to rotation
this means an alkene can be either an E or Z isomer
what is E/Z isomerism?
E = entgagen (highest priority on the opposite sides)
Z = zusammen (highest priority on the same side)
assigned by priority of groups attached to C=C
how is priority assigned to groups attached to carbons in alkenes?
the higher the atomic number of the group attached to the C’s the higher the priority
if they are the same look at the next atoms attached
what is a π bond?
overlap between p orbitals in a carbon-carbon double bond
they result in a high charge density in the double bond
what is a nucleophile?
electron lone pair donator
what is an addition reaction?
a reaction where a molecule joins to an unsaturated molecule to produce a saturated molecule
what is a substitution reaction?
reaction where an atom/ group replaces another atom/group
why is the general formula for an alcohol?
Cn H2n+1 OH
how do you name alcohols?
find largest chain containing OH
number carbons so OH is on lowest number - carbonyl groups have priority
remove -e from parent alkane and add -ol
if there is more than 1 OH use diol, triol, tetrol etc and don’t remove e
e.g. propane - 1,2,3 - triol
what shape are the bonds within alcohols?
the C-O-H bond angle is 104.5*
caused by the 2 lone pairs on the O squeezing the H and carbon chain closer together
what are the physical properties of alcohols?
higher melting and boiling points
-due to hydrogen bonding
water soluble
what is the trend in hydride boiling points?
boiling point is a measure of intermolecular forces present between particles
halogens have a gradual increase in boiling point as van der waals increase
hydrides should show this trend
what is fermentation?
a process where glucose is converted into alcohol as a by-product of anaerobic respiration in yeast
process: batch (stop-start)
reaction rate: slow
expensive
why must the fermentation of glucose be anaerobic?
oxygen can oxidise ethanol to ethanoic acid
C2H5OH + O2 —> CH3COOH +H2O
what raw materials does fermentation of glucose use?
carbohydrate crops (e.g. sugar, maize) renewable raw materials
how pure is the ethanol produced by fermentation?
impure - it must be purified by fractional distillation
what is the hydration of ethene?
ethene is mixed with steam and passed over a catalyst consisting of solid silicon dioxide coated with phosphoric acid (H3PO4)
CH2=CH2 +H2O –>C2H5OH
it an acid catalysed electrophilic addition reaction
process: continuous
reaction rate: fast
what raw materials does the hydration of ethene use?
crude oil
finite raw material (non-renewable)
how pure is the ethanol produced by hydration of ethene?
pure
what is a redox reaction?
a reaction with a simultaneously reduction and oxidation reaction
what is reduction?
Gain of H, loss of O
oxidising reagents [O] are reduced when they oxidise something else
what is oxidation?
Gain of O, loss of H
reducing reagents [H] are oxidised when they reduce something else
what is Fehling’s solution?
blue solution of copper (II) ions dissolved in sodium hydroxide
what is Tollens reagent?
ammoniacal silver nitrate solution
what reducing agents can be used on carboxylic acids and aldehydes/ketones?
strong = LiAlH4 in dry ether milder = NaBH4
what are elimination reactions?
they remove small molecules
this is always water in alcohols, forming alkenes
isomeric products are possible
How to make an alkane from and alkene?
+H2
Nickel catalyst
200*C
