UNIT 3: SECTION 3-ALKENES AND ALCOHOLS

Question 1

A) What is the general formula for alkenes?

B) State the elements alkenes are made from

C) Why are alkene molecules unsaturated

Answer 1

A)CnH2n
B)-made from carbon and hydrogen so are hydrocarbons
C)-all alkene molecules have at least 1 C=C double covalent bond
–>these molecules with double carbon bonds are unsaturated as they can make more bonds with extra atoms in addition reactions

Question 2

D) Outline why alkenes are pretty reactive

Answer 2

D)-this is due to the 2 pairs of electrons in the C=C double bond
–>it has a really high electron density which make sit pretty reactive

Question 3

A) What are “elctrophilic addition” reactions and why do they occur?

Answer 3

A)-the double bond opens up and atoms are added to the carbon atoms
-electrophilic addition reactions happen because the double bond has plenty of electrons and is easily attacked by electrophiles.

Question 4

B) Define “electrophiles”

C) State some examples of electrophiles

Answer 4

B)-are electron-pair acceptors–> they are usually a bit short of electrons
–> so are atrtracted to area where there are lots of them about
C)- (+) charged ions like H+ and NO2 +
-polar molecules–> the delta + atom attracted to places with lots of electrons.

Question 5

D) How is a “cyclic alkene” different from a normal open-chain alkene?

Answer 5

D)-has 2 fewer hydrogens than equivalent open-chain alkene

Question 6

A) Describe and explain the “bromine water test” for unsaturation , including the electrophilic addition reaction that occurs

Answer 6

A)-when shake an alkene with ORANGE bromine water

• ->solution quickly decolourises (goes orange to colourless)
• ->this is as bromine added across C=C double bond to from colourless dibromoalkane.

Question 7

Bromine water test mechanism (use reference to displayed mechanism)

MECHANISM:

H2C=CH2 + Br2 –> CH2BrCH2Br

B) Describe this mechanism

Answer 7

B)1-the double bond repels electrons in Br2
–>polarising Br-Br
2-electron pair in double bond attracts the Br (delta +) and forms bond with it
–>this repels electrons in the Br-Br further until breaks
3-you get (+) charged carbocation intermediate
–> Br- goes over to it
–>and bonds to the other C atom, forming 1,2-dibromoethane.

Question 8

A) What happens when alkenes reacts with hydrogen halides?

B) Outline the reaction between ethene and hydrogen (with reference to displayed mechanism)

Answer 8

A)-alkenes undergo electrophilic addition reactions with hydrogen halides to form halogenoalkanes
B)-C2H4 + HBr –> C2H5Br

Question 9

A) What are the expected products when hydrogen halides react with unsymmetrical alkenes?

B) The amount of each of the 2 possible products that form depends on what?

C) How does the n. of alkyl groups a carbocation contain affect it?

Answer 9

A)-then there are 2 possible products that form
B)-depends on how stable the carbocation formed in middle of reaction is–>known as carbocation intermediate.
C)-carbocations with more alkyl groups are more stable as the alkyl groups feed electrons towards the (+) charge
–>the more stable carbocation much more likely to form.

Question 10

D) Describe a PRIMARY/SECONDARY/TERTIARY carbocation

Answer 10

D)-primary: have 1 R group –> MOST STABLE

• secondary: have 2 R groups
• tertiary: have 3 R groups –> LEAST STABLE

Question 11

Hydrogen Bromide reaction with Propene (with reference to displayed mechanism)

A) What is the mechanism for the MAJOR and MINOR product that is formed?

B) Why is the major product that is formed more stable than the minor product which is formed?

Answer 11

A)-MAJOR: H2C=CHCH3–> CH3CHBrCH3 (2-bromopropane)
-MINOR: H2C=CHCH3–> CH2BrCH2CH3
(1-bromopropane)
B)-secondary carbocation’s more stable as it has 2 alkyl groups–>this carbocation will form most of the time
-primary carbocation’s less stable as it only has 1 alkyl group–>it forms less.

Question 12

A) Briefly describe the reaction of alkenes with sulfuric acid (H2SO4) and how they can then be reacted further

Answer 12

A)-alkenes 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.

Question 13

A) EXAMPLE: Describe the following reactions (with reference to the displayed mechanism)

1- ethene + sulfuric acid
2- Describe the reaction if you then add cold water and warm the product to the reaction in (1)

Answer 13

A)-1: cold sufuric acid reacts with an alkene in an electrophilic addition reaction to form ethyl hydrogen sulfate
–>H2C=CH2 + H2SO4–> CH3CH2OSO2OH
(ethene + sulfuric acid–> ethyl hydrogen sulfate)
2-it hydrolyses to form an alcohol
–>CH3CH2OSO2OH + H2O–> CH3CH2OH + H2SO4
(ethyl hydrogen sulfate + water–> ethanol + sulfuric acid)

Question 14

B) How does the sulfuric acid act in the reaction?

C) If the reaction was being used to produce ethanol what would the overall equation look like?

D) What should you expect if you did this reaction with an unsymmetrical alkene?

Answer 14

B)-it isn’t used up it acts as a catalyst.
C)- H2C=CH2 + H2O–> C2H5OH
H2SO4
D)-you would get a mixture of products
–>the one that’s formed via most stable carbocation intermediate will be the major product.

Question 15

A) Define “polymers”

B) What are “natural” polymers and “synthetic” ones

Answer 15

A)-are long chain molecules formed when lots small molecules called monomers join
B)-natural like DNA + proteins
-synthetic are man-made ones like poly (ethene)

Question 16

C) Describe the historical use of polymers including in the 19th and 20th century up until today

Answer 16

C)-people have been using natural polymers like silk/cotton/rubber for 100’s years

• in 19th century research concentrated on modifying properties of natural polymers like hardening rubber to make more suitable for machine parts/tyres
• 20th century saw invention + production of synthetic polymers like nylon + kevlar
• ->scientists still developing new polymers with new properties all the time.

Question 17

A) Describe “addition polymers”?

EXAMPLE: Poly(phenylethene) formed from phenylthene–>card reference diagram

Answer 17

A)-alkenes can act as monomers and form polymers as their double bonds able to open up and join to together to make long chains

Question 18

B) Why are addition polymers fairly unreactive?

Answer 18

B)-polyalkene chains are saturated molecules (only contain single bonds in carbon chain)

• ->main carbon chain of polyalkene is also non-polar
• ->these factors result in addition polymers being unreactive.

Question 19

A) Usually, which forces hold polyalkene chains together?

Answer 19

A)-polyalkene chains are non-polar so chain only held together via Van der Waals forces

Question 20

B) Explain how the polyalkene chain length affects the strength of the Van der Waal forces

Answer 20

B)-the longer polymer chain are and closer together they get the stronger the van der waals forces between chains will be

• ->mean polyalkenes made up of long, straight chains tend to be strog + rigid
• -> while polyalkenes made up of short, branched chains tend to be weaker + more flexible.

Question 21

A) What is the effect of adding a “plasticiser” to a polymer?

B) Outline how plasticiser molecules work

Answer 21

A)-makes it more flexible
B)-plasticiser molecules get between polymer chains and push them apart
–>this reduces strength of intermolecular forces between chains
–>so they are able to slide around more making polymers easy to bend.

Question 22

A) What is poly (chloroethene)? (refer to diagram)

Answer 22

A)-this is PVC formed from chloroethene

Question 23

B) Describe “normal” PVC and it’s uses

C) Briefly explain the structure of “plasticised PVC” and state it’s uses as a result of its properties

Answer 23

B)-PVC has closely packed polymer chains making it hard BUT brittle at room temp
–>rigid PVC used for drainpipes + window frames
C)-plasticised PVC much more flexible than rigid PVC
–>it’s used to make electrical cable insulation/flooring tiles and clothing.

Question 24

A) Outline the composure of polymers and describe it

Answer 24

A)-are made up of repeating units (bit of molecule that repeats over and over)
-the repeating units of addition polymers look v. similar to the monomer BUT with double bond opened out.

Question 25

B) How would you draw the repeating unit of an addition polymer from it's monomer?

Answer 25

B)-first draw the 2 alkene carbons - ->replace the double bond with single bond - ->add single bond coming out from each of the carbons - ->then just fill in rest of groups in same way as they surrounded the double bond.

Question 26

C) EXAMPLE: Briefly explain how you would draw the repeating unit of the polymer formed from ethene, C2H4

Answer 26

C)1-draw double bond opening up to form the polymer backbone 2-then fill in the groups as they surround the double bond--> poly(ethene) repeating unit.

Question 27

D) Outline how you would draw the repeating unit from it's polymer

Answer 27

D)-just need to look at the chain and work out which part is repeating - ->for addition polymer, the repeating unit should be 2 carbons long (so it looks like alkene monomer) - ->once have repeating unit then easy to draw monomer by adding the double bond.

Question 28

E) EXAMPLE: Draw the repeating unit and monomer of the polymer chain Teflon (poly(tetrafluoroethene)

Answer 28

E) Diagram shows how to do it.

Question 29

A) State what the functional group of an alcohol is B) What are the 3 different types of alcohols and what do they depend on? C) Outline the basic structures of PRIMARY/SECONDARY/TERTIARY alcohols

Answer 29

A)-functional group in homologous series is the hydroxyl (-OH) group B)-an alcohol is primary/secondary or tertiary depending on which carbon atom the -OH group is bonded to C)-PRIMARY: C--> 1 bond to H + 1 bond to H + 1 bond to OH + 1 bond to alkyl group (R group) -SECONDARY: C--> 1 bond to H + 1 bond to OH + 1 bond to alkyl group (R1 group) + 1 bond to alkyl group (R2 group) -TERTIARY: C--> 1 bond to OH + 1 bond to alkyl group (R1 group) + 1 bond to alkyl group (R2 group) + 1 bond to alkyl group (R3 group).

Question 30

D) What is an example of each of the different types of alcohols?

Answer 30

D)-primary: Propan-1-ol - secondary: Propan-2-ol - tertiary: 2-methylpropan-2-ol.

Question 31

A) How may you make alkenes from alcohols? B) Outline the general equation for the production of an alkene from an alcohol

Answer 31

A)-can make alkenes by eliminating water from alcohols in a dehydration reaction (i.e: elimination of water) B)-CnH2n+1OH--> CnH2n + H2O

Question 32

C) Why does this reaction allow you to produce alkenes from renewable resources and what is it's importance? D) What is one of the main industrial uses for alkenes?

Answer 32

C)-this is as you can produce ethanol by fermentation of glucose which you can get from plants -->this is important as it means you can produce polymers (poly(ethene) without the need of oil D)-as a starting material for polymers.

Question 33

A) How would you dehydrate ethanol from ethene?(include the reaction equation) B) Which other catalyst may be used in this reaction and outline its use?

Answer 33

A)-ethanol heated with concentrated sulfuric acid catalyst -the product is usually a mix of water + acid + a reactant in it so the alkene has to be separated out. -EQUATION: C2H5OH--> C2H4 + H2O (H2SO4) B)-phosphoric acid (H3PO4) may also be used as a catalyst -->often the 2 are used together.

Question 34

C) Outline the mechanism and it's description for the elimination of water from ethanol

Answer 34

C)1-lone pair of electrons from the oxygen bonds to an H+ from the acid -->alcohol is protonated to give the oxygen a (+) charge 2-the positively charged oxygen pulls electrons away from the carbon -->an H2O molecule leaves-->to create an unstable carbocation intermediate 3-the carbocation loses an H+ ... 4- ... and the alkene is formed. (mechanism is shows as diagram attached)

Question 35

D) How do the products vary when more complicated alcohols are dehydrated E) EXAMPLE: What are the 2 possible products when butan-2-ol is dehydrated in the presence of H2SO4 catalyst?

Answer 35

D)-there is more than one possible product -->the double bond can form either side of the carbon with the -OH group attached E)-the 2 possible products are but-1-ene OR but-2-ene.

Question 36

A) In what state are the products of an organic reaction and what must be done to them? B) In the dehydration of alcohols to form alkenes what products are made and what is done next?

Answer 36

A)-are often impure so they need to be purified B)-the mixture at end contains the product + the reactant + acid + water -->to get a pure alkene you need a way to separate it from other substances.

Question 37

C) So, what method is used often to purify the mixtures of an organic reaction?

Answer 37

C)-distillation is used as it uses the fact that different chemicals have different boiling points to separate them -->HOWEVER often further separation + purification is needed

Question 38

EXAMPLE: Producing an alkene from an alcohol and separating out the product Producing cycohexene from cyclohexanol A) Describe and explain STAGE 1-DISTILLATION (with reference to diagram)

Answer 38

A)1-add concentrated H2SO4 + H3PO4 to round-bottomed flask containing cyclohexanol -->mix solution by swirling flask AND add 2-3 carborundum boiling chips (make boiling more calm) 2-gently heat mixture to 83 degrees C (cyclohexene boiling point) using water bath OR electric heater 3-chemicals with boiling points up to 83 degrees C will evaporate -->warm gas will rise out of glass in to condenser (which has cold water running through outside turning it into liquid) 4-the product can then be collected in a cooled flask.

Question 39

B) Explain STAGE 2- SEPARATION

Answer 39

B)1-the product collected after distillation will still contain impurities 2-transfer product mixture to separating funnel and add water to dissolve water soluble impurities and create an aqueous layer 3-allow the mixture to settle in to layers -->drain the aqueous layer-->leaving the impure cyclohexene.

Question 40

C) In appropriate detail explain STAGE 3-PURIFICATION

Answer 40

C)1-drain the cyclohexene into round-bottomed flask 2-add anhydrous CaCl2 (drying agent) and stopper flask -->let mixture dry for at least 20 min with occasional swirling 3-the cyclohexene will still have small amounts of impurities so distil mixture one last time.

Question 41

A) What is the standard industrial method to produce alcohols and the general equation for the reaction?

Answer 41

A)-hydrate an alkene using steam in presence of acid catalyst --> CnH2n + H2O---> CnH2n+1OH

Question 42

B) Describe the general mechanism for the reaction (refer to mechanism diagram)

Answer 42

B)1-pair of electrons from the double bond bonds to an H+ from the acid 2-a lone pair of electrons from a water molecule bonds to the carbocation 3-the water loses an H+ ... 4-... and an alcohol formed.

Question 43

C) State the key conditions needed for industrial ethanol production

Answer 43

C)-ethanol can be produced by hydration of ethene by steam at: --> 300 degrees C + 60 atm pressure + solid phosphoric (V) acid catalyst needed.

Question 44

A) What is the problem with the current widely used "steam hydration of ethene" method of industrial ethanol production and so consequently what does that mean?

Answer 44

A)-the ethene comes from cracking heavy fractions of crude oil - ->BUT if future when crude oil supplies start running out, petrochemicals like ethene will be expensive - ->so producing ethanol via fermentation using renewable raw materials like glucose will be much more important.

Question 45

A) Describe "fermentation" and outline the reaction equation for it B) What is the effect of carrying this reaction out in the presence of yeast?

Answer 45

A)-fermentation is exothermic process carried out by yeast in anaerobic conditions (without O2) -C6H12O6 (aq) ---> 2C2H5OH (aq) + 2CO2 (g) (yeast + 30-40 degrees C) B)-yeast produces an enzyme which converts sugars like glucose into ethanol + CO2.

Question 46

C) State the optimum temperature that the enzymes work at how changing the temperature would affect he enzyme (refer to the graph)

Answer 46

C)-enzyme works at optimum (ideal) temp of 30-40 degrees C | -->if it's too cold reaction slow BUT if too hot enzyme denatured (damaged)

Question 47

A) Outline the rate of ethanol use and how it's a "biofuel"

Answer 47

A)-it's increasingly being used as a fuel particularly in countries with few oil reserves - ->E.G: in Brazil, sugars from sugar cane fermented to produce alcohol, which added to petrol - ->this type of ethanol production is called bioethanol/biofuel.

Question 48

B) Define a "biofuel"

Answer 48

B)-fuel that's made from biological material that's recently died.

Question 49

C) Describe the ADVANTAGES of biofuels over fossil fuels

Answer 49

C)1-are renewable energy sources-->unlike fossil fuels biofuels won't run out so are more sustainable 2-biofuels produce CO2 when burnt but it's CO2 that plants absorbed while growing, so biofuels usually still classed as carbon neutral.

Question 50

D) However what are the DISADVANTAGES of biofuel use compared to the use of fossil fuels

Answer 50

D)1-switching to biofuels for transport is that petrol car engines would have to be modified to use fuels with high ethanol concentrations 2-also when land used to grow crops for fuel that land can't be used to grow food -->if countries start using land to grow biofuel crops instead of food-->then may not be able to feed everyone in the country.

Question 51

A) Briefly explain the production of the harmful substance produced from burning ethanol

Answer 51

A)-just like burning hydrocarbons from fossil fuels ethanol combustion produces CO2 -->this is a greenhouse gas which contributes to global warming

Question 52

B) Explain why burning ethanol as a fuel could be argued to be "carbon neutral"

Answer 52

B)-the plants grown to produce bioethanol take CO2 in from atmosphere when they are growing - ->as they grow they take in same amount of CO2 as burning the bioethanol you produce from them gives out - ->so ethanol combustion as fuel could be considered as carbon neutral.

Question 53

Outline the equations for the different parts of ethanol production to support the "carbon neutral" argument A) Glucose production via photosynthesis

Answer 53

C)1-plants take CO2 from atmosphere to produce glucose via photosynthesis - -> 6CO2 + 6H2O --> C6H12O6 + 6O2 - ->equation shows 6 moles of CO2 taken from atmosphere to produce 1 mole of glucose

Question 54

B) In the fermentation process, glucose converted to ethanol C) When ethanol burned, CO2 + H2O produced D) Overall, how do these equations support the "carbon neutral" theory?

Answer 54

B)-C6H12O6--> 2C2H5OH + 2CO2 -->2 moles of CO2 released into atmosphere when 1 mole of glucose converted to 2 moles of ethanol. C)-2CH5OH + 6O2 --> 4CO2 + 6H2O -->4 moles of CO2 released into atmosphere when 2 moles of ethanol burned completely D)-if you combine all 3 of these equations you will find that exactly 6 moles of CO2 are taken in AND exactly 6 moles of CO2 are given out.

Question 55

E) HOWEVER, why do some believe that the "carbon neutral" theory isn't completely true?

Answer 55

E)-fossil fuels still need to be burned to power machinery to make fertilisers for the crops/harvest the crops + refine and transport the bioethanol - ->burning the fuel to power this machinery produces CO2 - ->so using bioethanol made by fermentation isn't completely carbon neutral.

Question 56

A) How may alcohols be oxidised and what is the preferred method to oxidise alcohols?

Answer 56

A)-simple way to oxidise alcohols is to burn them - ->but don't get best products this way so need more sophisticated way to oxidise alcohols - so you can use the oxidising agent acidified potassium dichromate (VI), Kr2Cr2O7 to mildly oxidise alcohols - ->the orange dichromate (VI) ion, Cr2O7 2- is reduced to the green chromium (III) ion, Cr3+.

Question 57

B) What are the products produced from oxidsing a PRIMARY/SECONDARY/TERTIARY ALCOHOL?

Answer 57

B)-PRIMARY: alcohols are oxidised to aldehydes and then to carboxylic acids - SECONDARY: alcohols are oxidised to to ketones only - TERTIARY: alcohols are not oxidised.

Question 58

A) What is the FUNCTIONAL GROUP and GENERAL FORMULA of ALDEHYDES/KETONES/CARBOXYLIC ACIDS?

Answer 58

A)-aldehydes and ketones are carbonyl compounds with functional group C=O - ->their general formula is CnH2nO - carboxylic acids have functional group COOH and general formula CnH2n+1COOH

Question 59

B) Describe the structure of ALDEHYDES and state an example

Answer 59

B)-have hydrogen + 1 alkyl group attached to carbonyl carbon atom - their suffix is -al-->don't have to say which carbon functional group on as is always on carbon-1 - E.G: propanal (CH3CH2CHO)

Question 60

C) In appropriate detail describe KETONES and state an example(s)

Answer 60

C)-have 2 alkyl groups attached to they carbonyl carbon atom - ->their suffix is -one - ->for ketones with 5 + carbons you always have to say which carbon functional group is on - E.G: Propanone (CH3COCH3) /Pentan-2-one (CH3COC3H7)

Question 61

D) Describe CARBOXYLIC ACIDS and outline an example A) The diagram shows the general reaction for oxidising a primary alcohol to an aldehyde and then further to a carboxylic acid

Answer 61

D)-have COOH group at the end of their carbon chain -their suffix is -oic acid A) refer to diagram mechanism

Question 62

Controlling how far an alcohol is oxidised by controlling the reaction conditions (with reference to diagram): B) Overall, what happens in the oxidising of a PRIMARY alcohol (with example)?

Answer 62

B)-gently heating ethanol with potassium dichromate (VI) + sulfuric acid in test tube produces ethanal (aldehyde) -->but it's tricky to control the heat and the aldehyde usually oxidised to form ethanoic acid.

Question 63

C) Explain what must be done to just get the aldehyde from this reaction

Answer 63

C)-need to get it out of the oxidising solution as soon as it forms -->do this using distillation apparatus so that aldehyde (which boils at lower temp than alcohol) distilled off immediately

Question 64

D) What must be done to produce the carboxylic acid from this reaction?

Answer 64

D)-alcohol has to be vigorously oxidised | -->the alcohol is mixed with excess oxidising agent and heated under reflux.

Question 65

E) Define the meaning of "heating under reflux"

Answer 65

E)-means you increase the temp of an organic reaxtion without losing the volatile solvents/reactants or products - ->any vaporised compounds cool + condense + drip back into reaction mixture - ->so the aldehyde stays in reaction mixture and is oxidised to carboxylic acid.

Question 66

A) Outline the basic mechanism for the oxidising of secondary alcohols B) What happens when a secondary alcohol is refluxed with the reaction conditions? C) How easily are ketones oxidised and can any further product be produced from ketones?

Answer 66

A)-diagram shows this B)-refluxing secondary alcohol like propan-2-ol with acidified dichromate (VI) will produce a ketone C)-ketones not easily oxidised so even prolonged refluxing won't produce anything more.

Question 67

A) Describe the oxidising of tertiary alcohols

Answer 67

A)-tertiary alcohols are not oxidised easily - ->this is as they don't react with potassium dichromate (VI)-the solution stays orange - ->the only way to oxidise tertiary alcohols is by burning them.

Question 68

Distinguishing between ALDEHYDES and KETONES: A) How may FEHLING'S and BENEDICTS solution be used? B) Briefly explain how TOLLEN'S REAGENT (Ag(NH3)2)+ may be used?

Answer 68

A)-both of these solutions are deep blue Cu2+ complexes which reduce to brick-red Cu2O when warmed with an aldehyde BUT stay blue with ketone B)-this reagent is reduced to silver when warmed with an aldehyde, BUT not with a KETONE -->the silver coats inside of the apparatus to form a silver mirror.