Alkenes Flashcards
What are Alkenes?
⦾ Unsaturated HCs
⦾ (+)1 C=C - area of high e- density
⦾ Reactive due to C=C
⦾ CnH2n
How can you identify Alkenes?
⦾ Shake Alkene with orange bromine water
⦾ Turns solution orange-brown to colourless (+ve test_
What are electrophiles?
⦾ Electron acceptors
⦾ Electron defficient
⦾ Attracted to areas of high e- density
⦾ +vely charged
Give 3 examples of electrophiles?
⦾ HBr
⦾ Br2
⦾ H2SO4
What generally happens in an electrophillic addition reaction?
e.g. CH2CH2 + X-Y → CH2XCH2Y
1 ⦾ C=C repels e-s in X-Y
2 ⦾ Polaries bond in X-Y
3 ⦾ 2 e- from C=C attack 𝛿+ X creating new bond between C-1 and X
4 ⦾ X-Y bond breaks and e-s from bond taken by Y to form -ve ion with lone e- pair
5 ⦾ C-2 left with +ve charge (bond with C=C that broke took e-s to form bond with X) - carbocation intermediate
6 ⦾ :Y- acts as nucleophile
7 ⦾ It attacks +ve carbocation, donating its lone e- pair + forming bond with C-2
8 ⦾ Overall, X-Y has been added to alkene across C=C to form saturated compound.
What happens if you draw the mechanism involving an unsymmetrical alkene?
⦾ 2 different carbocation form
⦾ More stable carbocation more likely to form (make sure to draw right one)
What is the product for the electrophilic addition reaction for:
Heat: Ethene + Water + conc. sulfuric acid
⦾ Ethanol
⦾ Overall: CH2=CH2 + H2O + conc.H2SO4 → C2H5OH
Explain the reaction of:
CH2=CH2 + H2O + conc.H2SO4 → C2H5OH?
1 ⦾ Conc. H2SO4 reacts with ethene in nucleophillic addition - forms hydrogen sulfate
2 ⦾ Add cold water + warm product - hydrolysed to form ethanol
⦾ H2SO4 not used up - catalyst
Explain (fully) the mechanism for the electrophillic addition reaction below:
CH2=CH2 + H2O + conc.H2SO4 → C2H5OH?
1 ⦾ C=C attacks 𝛿+H on H2SO4
2 ⦾ Bond formed between Cs and H
3 ⦾ e-s from O-H bond taken by O to form lone pair
4 ⦾ 2nd C left with +ve charge as e- lost from double bond
5 ⦾ -ve ion created acts as nucleophile + attacks carbocation creating new intermediate
6 ⦾ Water then added to produce sulfuric acid + ethanol
Explain how alkenes are able to decolourise bromine water?
What does it form?
Via which mechanism?
⦾ Bromine added across double bond
⦾ Form colourless dibromoalkane
⦾ Electrophillic Addition
What is the equation for the reaction of bromine water decolourising ethene?
H2C=CH2 + Br2 → CH2BrCH2Br
Explain (fully) the mechanism for the reaction between bromine water and ethene?
1 ⦾ C=C repels e- in Br2, polarising 𝛿+Br-𝛿+Br - induced dipole
2 ⦾ e- pair in C=C attracts 𝛿+Br and forms bond with it
3 ⦾ Repels e- in Br-Br further until bond breaks + 𝛿+Br bonds with it
4 ⦾ You get +vely charged carbocation intermediate
5 ⦾ Br- attracted
6 ⦾ Bonds to other C
⦾ Forms: 1,2-dibromoethane
What is the mechanism for the reaction between alkenes and hydrogen halides?
⦾ Electrophilic Addition
What does the reaction between alkenes and hydrogen halides form?
⦾ Hydrogen halides
Draw the mechanism for the reaction between ethene and hydrogen bromide?
⦾ pg. 218
⦾ C2H4 + HBr → C2H5Br
What happens if a hydrogen halide is added to an unsymmetrical alkene?
⦾ 2 possible products
Draw the mechanism for the reaction between propene and hydrogen bromide and the 2 possible products?
⦾ pg. 218
⦾ Br could be added to 1st or 2nd C
⦾ 1-bromopropane or 2-bromopropane
What are the 3 possible carbocation intermediates?
⦾ Primary carbocation (1 X R)
⦾ Secondary carbocation (2 X R)
⦾ Tertiary carbocation (3 X R)
Which of the 3 carbocations is the most stable?
⦾ Tertiary
Why is the tertiary carbocation the most stable?
⦾ Because the alkyl groups feed e-s towards the +ve charge
How can you show an alkyl group is donating electrons to the +ve charge?
⦾ Draw arrow on bond that points to where e-s are being donated
⦾ pg. 219
What can the product that there’s most of also be called?
⦾ Major product
⦾ (minor product)
Draw out the mechanism for the reaction between Hydrogen bromide and propene?
⦾ H2C=CHCH3 + HBr → CH3CHBrCH3
⦾ H2C=CHCH3 + HBr → CH2BrCH2CH3
⦾ Secondary carbocation more stable - 2 alkyl groups
Draw out the mechanism for the reaction between Hydrogen bromide and 2-methylbut-2-ene?
⦾ pg. 220
What can also affect how much of the major product forms in the end?
⦾ bigger difference in stability of carbocations
Define polymer?
⦾ Natural e.g. DNA
⦾ Synthetic e.g. polyethene
How are addition polymers formed?
⦾ Alkenes where the double bond is broken to form repeating unit.
What should you remember when drawing a monomer in polymer form?
⦾ -(-x-x-)-n
What does n stand for?
⦾ N = no. of repeating units in a polymer
What should you do if you’re asked for the formula of a polymer?
⦾ Include brackets and n
What should you do if you’re asked for the repeating unit?
⦾ Just bit inside bracket
How do you find the monomer used to form an addition polymer?
⦾ Take repeating unit
⦾ Remove side/trailing bonds
⦾ Add double bond
How do you name addition polymers?
⦾ Find monomer
⦾ poly(X)
How would you name the addition polymer for
but-2-ene?
⦾ poly(but-2-ene)
What will altering the reaction conditions for the production of plastic polymer chains do?
⦾ Give different properties
What conditions produce branched chain polymers?
⦾ Weak IMFs between branched chain polymers
⦾ High pressures
⦾ High temps
What conditions produce straight polymers?
⦾ Strong IMFs
⦾ Lower pressures
⦾ Lower temps
What are the features of polymers?
⦾ Unreactive HC chains
⦾ Lots of strong
⦾ Non-polar
⦾ Covalent bonds
Give an everyday use of polymers? Which poly(X)?
⦾ Shopping bags
⦾ poly(ethene)
What is a disadvantage of addition polymers (for plastic in particular)?
⦾ Unreactive nature of bonds
⦾ Therefore not biodegradable
⦾ Cannot be broken down
What is PVC?
⦾ Poly(chloroethene)
⦾ Addition polymer
⦾ Waterproof properties
How does PVC gain its waterproof property?
⦾ Addition of plasticisers during reaction
How does plasticiser make a polymer bendier?
⦾ Gets between polymer chains + pushes apart
⦾ Reduces strength on IMFs
What are the uses of PVC?
⦾ Electrical cable insulation
⦾ Floor tiles
⦾ Clothing
