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Organic Chemistry Year 12 > Alkenes > Flashcards

Alkenes Flashcards

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1
Q

why are alkenes unsaturated hydrocarbons

A

they have at least 1 C=C double bond

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2
Q

what is the formula for alkenes, and under what conditions is it valid

A

CnH2n

  • if aliphatic
  • if branched
  • (not if cyclic or contain more than 1 C=C)
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3
Q

what is each C=C double bond made of

A

3 sigma bonds
1 pie bond

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4
Q

what is a pi bond

A
  • the sideways overlap of 2 p-orbitals, one from each carbon atom of the C=C, and each contributing 1 electron, above and below the plane of C-atoms
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5
Q

where is electron density concentrated in a pi bond

A

concentrated above and below the line joining the nuclei of the bonded atoms

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6
Q

why can pi bonds not rotate

A

the double bond locks the atoms into position, so atoms cannot rotate

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7
Q

what shape forms around the C=C

A

trigonal planar

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8
Q

why is the C=C bond trignal planar

A
  • 3 electron pair regions around C atom
  • electron pair regions repel each other as far as possible
  • 120 degree bond angle
  • no lone pairs, so don’t have to worry
  • all atoms in the same plane
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9
Q

what are stereoisomers, and how are they different to structural isomers

A

have the same molecular and structural formula, but a different arrangement of atoms in space

  • structural = same molecular formula, different structural formula
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10
Q

why can E/Z isomers occur

A

the C=C double bond is fixed due to pi bond

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11
Q

what are the conditions of E/Z isomers

A

1) must have C=C
2) must have 2 different groups on each C atom in the double bond

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12
Q

what is difference between E and Z isomers

A

E= the groups are attached diagonally across double bond
Z= the groups are on the same side of the double bone (Zee Zame Zide)

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13
Q

what are cis/trans isomers

A

special type of E/Z isomer where one of the groups on each carbon atom in double bond must be a H

  • trans = E
  • cis = Z
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14
Q

what is the rule for assigning priority where there are more groups present

A

assign priority based upon highest atomic number, and if one group has more than one atom, read from left to write, comparing atoms
- do separate chains on each C=C separately

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15
Q

how do you determine if an isomer with 4 groups is an E/Z isomer

A

E= the groups with the highest priority are diagonally placed across the C=C
Z= the groups with highest priority are on the same side of the C=C

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16
Q

why are alkenes more reactive than alkanes

A
  • due to presence of pi-bond
  • means the electron density is concentrated above and below the plane of sigma bonds
  • so outside of double bond, means that the pi-electrons are more exposed than sigma electrons
  • pi bond readily breaks to undergo addition reactions
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17
Q

how can you determine the enthalpy of the pi bond alone

A
  • subtract the enthalpy of the C=C to the equivalent C-C bond
18
Q

what is an addition reaction in alkenes

A
  • there is an addition of a small molecule across the double bond
  • breaking the pi-bond and forming new bonds
19
Q

what are the 4 addition reactions of alkenes

A

1) hydrogenation
2) halogenation
3) hydrogen halides
4) hydration

20
Q

explain the hydrogenation of alkenes

A
  • alkenes mix with hydrogen
  • pass over NICKEL CATALYST at 423K
  • breaks ALL C=C
  • forms equivalent alkane
    ( consider balancing the H2 so ensure enough to attach to all C=C)
21
Q

explain the halogenation of alkenes

A
  • rapid addition reaction with
  • CHLORINE/BROMINE
  • at ROOM temperature
22
Q

how can you test the saturation of alkenes

A

BROMINE WATER:
- bromine water is orange
- if bromine is added across double bond
- will turn clear
- if no C=C, will stay orange

23
Q

explain the reaction between alkenes and hydrogen halides ( e.g. chloride and bromide)

A
  • react with GASEOUS hydrogen halides
  • or CONCENTRATED ACID equivalenT (solutions of halides in H2O)
  • at ROOM TEMPERATURE
  • form haloalkanes
  • if liquid alkene, halide is bubble through
  • if gaseous alkene, gases are mixed
24
Q

why can the hydrogen halide reaction with alkenes form more than 1 molecule

A
  • if unsymmetrical alkene reacts with asymmetrical molecule, will form 2 product
25
Q

explain the hydration of alkenes

A
  • react with STEAM (H2O (g))
  • in presence of PHOSPHORIC ACID CATALYST (H3PO4)
  • forms alcohol, as steam is added across double bond (OH to one carbon, H to other)
26
Q

what is the addition reaction mechanism in alkenes

A

electrophilic addition

27
Q

why does electrophilic addition occur in alkenes

A
  • double bond in alkenes represents a region high in electron density
  • due to pi-bond
  • this attracts electrophiles to react
28
Q

what are electrophiles

A

an atom or group of atoms that is attracted to an electron rich centre, and accepts an electron pair (usually a positive ion, or molecule containing atom with slightly positive charge)

29
Q

explain how you would draw out electrophilic addition

A
  • arrow from C=C BOND to atom facing it
  • arrow from molecule BOND to other atom in molecule
  • BREAKS VIA HETEROlYTIC FISSION, where electron PAIR goes to one atom)
  • forms a carbocation, show on arrow
  • line going from LONE PAIR on atom left behind (INCLUDE -ve charge too) to the + charge of unbonded C atom
  • line going to end product
30
Q

what is a carbocation

A

contains a positive carbon atom

31
Q

how does electrophilic addition occur between alkenes and NON-POLAR atoms

A
  • as molecule approaches alkene, the pi-bond electrons interact with the bond
  • interaction causes the polarisation of the molecule, forming an induced dipole
    AS:
  • usually electrons are evenly distributed
  • but as they approach the pi-bond, the electrons in molecule are repelled, so induces polarity, as electrons move towards one atom more

REST IS SAME

32
Q

what is Markinowoff’s rule

A
  • the major product is made from the most stable carbocation ( the positive C is attached to the most alkyl groups)
  • AS more of the positive charge can be spread out between the alkyl groups
33
Q

explain the 3 types of carbocations

A

1) primary = only has 1 alkyl group attached to the positively charged C
2) secondary = has 2 alkyl groups attached to the positively charged C
3) tertiary = has 3 alkyl groups attached to the positively charged C

  • stability increases as you go down, so major product is most likely to be made of the tertiary carbocation
34
Q

what are polymers

A

large molecules made out of lots of small molecules called monomers joined together

35
Q

what are the 2 types of polymers

A
  • addition: no other products formed
  • condensation: small molecule is released
36
Q

how do you draw an equation for polymerisation

A
  • put n in front of monomer unit
  • draw arrow
  • unjoin C=C, add brackets with elongated bonds at side, and n on the outside
  • n= lots of repeating units
37
Q

how do you find the monomer from a polymer

A
  • just remove and replace the C=C in ONLY the chain joining the separate monomers together
38
Q

why are polymers useful, and what are some of the drawbacks

A
  • readily available, cheap to purchase, more convenient
  • challenge to dispose, as has a lack of reactivity
39
Q

how can you usefully recycle and process polymers

A

1) sort and recycle, and remould into new polymers
2) burn to release energy, which can be used to generate electricity
3) cracked into monomers, and used to make reform polymers (used as feedstock)

40
Q

what are types of polymers which are now being usefully made

A

bioplastics, which protect environment and conserve valuable oil reserves

1) BIODEGRADABLE POLYMERS: which are plant based and can be broken down by microorganisms
2) PHOTODEGRADABLE POLYMERS: oil-based polymers, which contain bonds that can be weakened by absorbing light, and start degradation

41
Q

what are impacts of burning polymers

A
  • HCL formed, which causes acid rain
  • can be neutralised with alkali, e.g. NaOH
  • CO formed with incomplete, which is toxic to humans
  • can be burned in excess O2, to prevent ever being made
  • e.g. PVC= HCL produced, PTFE= HfF produced, which is also toxic and acidic

Organic Chemistry Year 12 (6 decks)

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