3.3 alkenes and alcohols Flashcards by Evie Ravenscroft

why are alkenes fairly reactive?

2 pairs of electrons in the c=c bond so really high electron density

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what happens in electrophilic addition? why is double bond easily attacked by electrophiles?

double bond in an alkene opens up and atoms are added to the carbon atom
double bond has lots of electrons so easily attacked by electrophiles

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what is an electrophile?
what can they be?

an electron pair acceptor
can be positively charged ions or polar molecules

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general equation of electrophilic addition
how many products produced?

CH2CH2 + X-Y -> CH2XCH2Y
only one product

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steps of electrophilic addition

C=C bond repels electrons in X-Y which polarises X-Y or it might be permanently polar anyway
2 electrons from C=C attack +X so X-Y breaks and electrons from bond taken by Y to form - with 1 LP
C2 left + so now have carbocation intermediate
Y- acts as nucleophile attacking + carbocation and donating LP to form new bond with C2

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what are the three electrophiles you need to know?

Br2
HBr
H-O-SO2OH

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test for alkene

shake with bromine water it goes orange to colourless as colourless dibromoalkane formed by electrophilic addition

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reaction of alkenes with hydrogen halides

electrophilic addition

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reaction of unsymmetrical alkenes with hydrogen halides

2 possible products eg Br could go to first or second carbon in propene
amount of each product formed depends on how stable carbocation intermediate formed is
3 possible are primary, secondary, tertiary (least -> most stable)
more alkyl groups= more stable as electrons fed to + charge

eg CH3CHCH3 is more stable than CH2CH2CH3 as more alkyl groups

ie you want + charge to be in the middle not on the end

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what is a polymer?
what 2 things can it be?

long chain formed of lots of monomers joined together
can be natural (DNA) or synthetic (polyethene)

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what is an addition polymer?

double bond opens up and alkenes can join together

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what is a substituted alkene?

alkene where one H has been replaced for another atom/ group of atoms
they can be used in addition polymerisation eg to form poly(chloroethene)

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IUPAC naming polymers

poly(alkene)

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properties of polymers

saturated

main chain usually non-polar
so addition polymers very unreactive- chemically inert

monomers within have strong covalent bonds but intermolecular forces between chains are weaker

long chain w fewer branches have stronger imf so stronger and more rigid

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what is pvc?
what im forces are present?
uses?

poly(chloroethene)
addition polymer
polar bonds between c and cl
permanent dipole dipole forces between chains so hard but brittle
used as drain pipes and window frames

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what is a plasticiser?
how do they work?
uses of the products?

a chemical added to polymers to alter properties making them bendier

molecules get between chains pushing them apart so reduces strength of forces meaning they can slide

plasticised pvc used as electrical cable insulation, flooring tiles, clothes

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what is a primary alcohol?

-OH attached to carbon with 1 alkyl group attached

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what is a secondary alcohol?

-OH attached to carbon with 2 alkyl groups attached

what is a tertiary alcohol?

-OH attached to carbon with 3 alkyl groups attached

what is dehydrating alcohols?

making alkenes by eliminating water from alcohols

2 methods of ethanol production

hydrating alkenes
fermentation

hydrating alkenes equation and conditions

alkene + steam <-> alcohol
300 degrees c
60atm
phosphoric acid catalyst
reversable and low yield

fermentation of glucose equation and conditions

C6H12O6 -> 2CH3CH2OH +2CO2
exothermic carried out by yeast in anaerobic conditions
1)yeast
2)warm (30-40 for optimum enzymes)
3)anaerobic
4)aqueous

compare and contrast fermentation and hydration

HYDRATION
fast ror
pure product
ethene is raw material- finite
continuous process so £ equipment but low labour costs

FERMENTATION
very slow ror
very impure product- further processing needed
sugars are raw material- renewable
batch process so cheap equipment but high labour costs

what are biofuels?

fuels produced from renewable, biological resources eg ethanol produced from fermenting sugars (bioethanol)

advantages of biofuels

renewable energy sources so more sustainable than crude oil CO2 produced when burnt is equal to CO2 taken in from plant when growing so considered carbon neutral

what does carbon neutral mean?

no change in total amount of CO2 in the atmosphere

disadvantages of biofuels

food vs fuel debate deforestation for land to grow crops for biofuels- destroys habitats, trees cant take in CO2 and CO2 produced when burning trees fertilisers added to soil- pollute waterways and may release nitrous oxide most current engines cant run on fuels with high ethanol conc wo being modified

equations to support biofuels being carbon neutral

photosynthesis: 6CO2 + 6H2O -> C6H12O6 + 6O2 fermentation: C6H12O6 -> 2C2H5OH + 2CO2 burning ethanol: 2C2H5OH + 6O2 -> 4CO2 + 6H2O if combined, 6 mol of CO2 in and 6 mol of CO2 out

why arent biofuels carbon neutral?

need to burn fossil fuels to power machinery for making fertilisers and harvesting crops refining and transporting bioethanol uses energy so CO2 produced

what happens when you oxidise an alcohol?

c=o bond forms

what is the oxidising agent and what colour is it? what else can you do to oxidise?

acidified potassium dichromate(vi) K2Cr2O7 orange can just burn

what happens to orange dichromate ion when oxidised?

Cr2O7^2- is reduced to green chromium (iii) ion Cr^3+

how many alkyl groups do aldehydes and ketones have attached to their carbonyl carbon atom

aldehyde- 1 ketone- 2

equations for oxidising primary alcohols

primary alcohol + [O] -> aldehyde + H20 aldehyde + [o] -> carboxylic acid (reflux)

how to obtain the aldehyde from oxidising primary alcohols?

immediately distill it off by gently heating excess alcohol with controlled amount of oxidising agent in distillation apparatus aldehyde has lower bp than alcohol

how to produce carboxylic acid from oxidising primary alcohols

vigorously oxidise alcohol mixed w oxidising agent and heated under reflux vapourised compounds are condensed and drip back in so aldehyde stays in reaction mixture and oxidises into carboxylic acid

oxidation of secondary alcohols

secondary alcohol + [O] -> ketone + H2O (reflux, acidic conditions)

oxidation of tertiary alcohols

dont react with acidified potassium dichromate so solution stays orange can only oxidise them by burning

why can we distinguish aldehydes and ketones using oxidising agents?

aldehydes easily oxidised ketones arent

what is fehlings solution? results?

deep blue Cu2+ complex which reduces to brick red Cu2O precipitate when warmed with aldehyde but blue with ketone

Benedicts solution

deep blue Cu2+ complex which reduces to brick red Cu2O precipitate when warmed with aldehyde but blue with ketone

Tollens reagent

colourless [Ag(NH3)2]+ complex which forms silver mirror when warmed with aldehyde but not ketone

practicals in this topic

purifying product of a reaction