3.3 alkenes and alcohols

Question 1

why are alkenes fairly reactive?

Answer 1

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

Question 2

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

Answer 2

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

Question 3

what is an electrophile?
what can they be?

Answer 3

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

Question 4

general equation of electrophilic addition
how many products produced?

Answer 4

CH2CH2 + X-Y -> CH2XCH2Y
only one product

Question 5

steps of electrophilic addition

Answer 5

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

Question 6

what are the three electrophiles you need to know?

Answer 6

Br2
HBr
H2SO4 (sulfuric acid) (learn structure)

Question 7

test for alkene

Answer 7

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

Question 8

reaction of alkenes with hydrogen halides

Answer 8

electrophilic addition

Question 9

reaction of unsymmetrical alkenes with hydrogen halides

Answer 9

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

Question 10

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

Answer 10

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

Question 11

what is an addition polymer?

Answer 11

double bond opens up and alkenes can join together

Question 12

what is a substituted alkene?

Answer 12

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)

Question 13

IUPAC naming polymers

Answer 13

poly(alkene)

Question 14

properties of polymers

Answer 14

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

Question 15

what is pvc?

Answer 15

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

Question 16

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

Answer 16

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

Question 17

what is a primary alcohol?

Answer 17

-OH attached to carbon with 1 alkyl group attached

Question 18

what is a secondary alcohol?

Answer 18

-OH attached to carbon with 2 alkyl groups attached

Question 19

what is a tertiary alcohol?

Answer 19

-OH attached to carbon with 3 alkyl groups attached

Question 20

what is dehydrating alcohols?

Answer 20

making alkenes by eliminating water from alcohols

Question 21

2 methods of ethanol production

Answer 21

hydrating alkenes
fermentation

Question 22

hydrating alkenes equation and conditions

Answer 22

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

Question 23

fermentation of glucose equation and conditions

Answer 23

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

Question 24

compare and contrast fermentation and hydration

Answer 24

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

Question 25

what are biofuels?

Answer 25

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

Question 26

advantages of biofuels

Answer 26

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

Question 27

what does carbon neutral mean?

Answer 27

no change in total amount of CO2 in the atmosphere

Question 28

disadvantages of biofuels

Answer 28

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

Question 29

equations to support biofuels being carbon neutral

Answer 29

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

Question 30

why arent biofuels carbon neutral?

Answer 30

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

Question 31

what happens when you oxidise an alcohol?

Answer 31

c=o bond forms

Question 32

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

Answer 32

acidified potassium dichromate(vi) K2Cr2O7
orange
can just burn

Question 33

what happens to orange dichromate ion when oxidised?

Answer 33

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

Question 34

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

Answer 34

aldehyde- 1
ketone- 2

Question 35

equations for oxidising primary alcohols

Answer 35

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

Question 36

how to obtain the aldehyde from oxidising primary alcohols?

Answer 36

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

Question 37

how to produce carboxylic acid from oxidising primary alcohols

Answer 37

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

Question 38

oxidation of secondary alcohols

Answer 38

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

Question 39

oxidation of tertiary alcohols

Answer 39

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

Question 40

why can we distinguish aldehydes and ketones using oxidising agents?

Answer 40

aldehydes easily oxidised ketones arent

Question 41

what is fehlings solution?
results?

Answer 41

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

Question 42

Benedicts solution

Answer 42

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

Question 43

Tollens reagent

Answer 43

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

Question 44

practicals in this topic

Answer 44

purifying product of a reaction