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A Levels > 3-2: Alkanes And Halogenoalkanes > Flashcards

3-2: Alkanes And Halogenoalkanes Flashcards

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1
Q

What does a curly arrow show?

A
  • The formation or break of a covalent bond through the movement of electrons
  • they go from lone pairs or bonds to an atom or bond
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2
Q

What is petroleum and how is it separated?

A

a mixture of mainly alkane hydrocarbons that can be separated by fractional distillation

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3
Q

What is the order of fractionated gases from crude oil, smallest to largest?
Green penis needles kill gassy man-bitches
What are their uses?

A
  • gases - camping gas
  • petrol
  • naphtha - petrochemicals
  • kerosene - jet fuel/central heating
  • gas oil - diesel
  • mineral oil - lube/candles
  • bitumen - roofing/road surfaces
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4
Q

Describe thermal cracking

A
  • high temperatures (1000C)
  • high pressures (70 atm)
  • produces high percentage of alkenes
  • alkenes make many valuable products like polymers
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5
Q

Describe catalytic cracking

A
  • zeolite catalyst (lower cost)
  • slight pressure
  • high temperature (500C)
  • aromatic hydrocarbons
  • motor fuels
  • branched alkanes
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6
Q

What are the products of complete combustion?

A
  • CO2 + H2O
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7
Q

What are the products of incomplete combustion and why may it occur?

A
  • CO + H2O
  • limited oxygen
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8
Q

Why is CO poisonous?

A
  • binds to haemoglobin in place of oxygen
  • suffocate
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9
Q

What do catalytic converters remove from car exhausts and why?

A
  • CO: poisonous
  • NOx: acid rain and smog
  • unburnt hydrocarbons: smog and health problems
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10
Q

How do unburnt hydrocarbons cause smog and health issues?

A
  • react with NOx in sunlight to form ground level ozone (smog)
  • causes respiratory problems
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11
Q

What is the equation for the removal of CO from car exhausts by catalytic converters?
What kind of reaction is this?

A

2CO + O2 —> 2CO2
Oxidation

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12
Q

What is the equation for the removal of NOx from car exhausts by catalytic converters?
What kind of reaction is it removed by?

A

2NO + 2CO —> N2 + 2CO2
Reduction

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13
Q

What is the equation for the removal of unburnt hydrocarbons from car exhausts by catalytic converters?
What type of reaction is it removed by?

A

Hydrocarbon + XO2 —> CO2 + H2O
Oxidation

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14
Q

How is sulfur dioxide produced from the burning of fuel and why is it damaging?

A
  • contained in hydrocarbon fuel
  • power station flue gas
  • produced from combustion in car engines
  • reacts with oxygen to form SO2
  • acid rain
  • respiratory problems
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15
Q

What is the equation for the removal of SO2 from power station flue gases and how is it done?

A

CaO (s) + SO2 (g) —> CaSO3
- calcium oxide or calcium carbonate (limestone) is mixed with water to make a slurry
- the acidic SO2 reacts with calcium compounds to form a harmless salt (calcium sulfite)

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16
Q

Describe is a photochemical reaction for the formation of a halogenoalkane

A
  • The formation of halogenoalkanes
  • started by UV
  • H atom substituted by Br or Cl
  • free radical substitution reaction
  • covalent splits equally = homolytic fission
    -dot next to Cl/Br in mechanism
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17
Q

What is the synthesis of chloromethane?

A
  • a mixture of chlorine and methane exposed to UV light reacts to form chloromethane
  • there is a reaction mechanism for each step
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18
Q

Explain the initiation step of the synthesis of chloromethane

A
  • homolytic fission: sunlight (UV) provides enough energy to break some Cl-Cl bonds (photodissociation)
  • each Cl has one electron
  • highly reactive due to unpaired electron
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19
Q

Explain the propagation step of the synthesis of chloromethane

A
  • Cl• attacks methane molecule and takes a hydrogen, CH3 now has an unpaired electron- H’s bonding e-
  • Cl• + CH4 —> •CH3 + HCl
  • methyl free radical attacks another Cl2 molecule, causing homolytic fission and taking one Cl
  • •CH3 + Cl2 —> CH3Cl + Cl•
  • repeats until all CH4 is used up
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20
Q

Explain free-radical substitution from methane to tetrachloromethane

A
  • if the chlorine is in excess, the H atoms on methane will be used up, replaced by Cl atoms
  • CH3Cl —> CH2Cl2 —> CHCl3 —> CCl4
  • e.g: CH2Cl2 + Cl• —> CHCl3
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21
Q

Explain the termination step of the synthesis of chloromethane

A
  • two free radicals join together, covalent bond
  • 3 possible termination stages
  • •CH3 + Cl• —> CH3Cl
  • •CH3 + CH3• —> C2H6
  • Cl• + Cl• —> Cl2
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22
Q

Which molecules can undergo free radical substitution?

Study These Flashcards
A
  • chlorine
  • bromine
23
Q

What is homolytic fission?

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A
  • a covalent bond splits into two free radicals
24
Q

What are CFCs?

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A
  • halogenoalkanes in which all of the H has been replaced with chlorine and fluorine atoms
25
Why is ozone beneficial and how is it naturally formed?
- absorbs UV - O2 is broken into 2 free radicals by UV - O2 + UV —> O• + O• - free radicals attack O2, forming ozone - O2 +O• —> O3
26
How are chlorine free radicals formed in the atmosphere from CFCs?
CCl3F(g) + UV —> CCl2F(•) (g) + Cl•(g)
27
How do Cl free radicals react with ozone?
- act as catalysts - react with ozone to form an intermediate, then oxygen - Cl• + O3 —> O2 + ClO• - ClO• + O3 —> 2O2 + Cl• - Cl• then repeats
28
What is the overall reaction for the destruction of ozone, and what is the catalyst?
- 2O3 —> 3O2 - Cl•
29
What were the uses of CFCs and what has been done to reduce their use?
- used in aerosols and coolants in fridges - damage discovered/ proved in 1970s - banned - safer alternatives developed (HCFCs)
30
What makes carbon-halogen bonds polar?
- halogens are more electronegative than carbon - halogens have a higher electron density (d-)
31
What is a nucleophile?
‘positive loving’ electron-pair donor
32
What do nucleophiles attack?
Positive charges, e.g d+
33
What are three examples of nucleophiles that will react with halogenoalkanes?
- -:CN (cyanide ion, - on C) - :NH3 (ammonia) - -:OH (hydroxide ion, - on O)
34
Describe nucleophillic substitution of halogenoalkanes with :OH-
- OH- nucleophile attacks d+ on C of halogenoalkane - halogen bond breaks, taking the bonding pairs it’s attracting - halogen becomes -1 ion with lone pair - alcohol produced
35
Describe nucleophillic substitution of a bromoethane with potassium cyanide. What are the names of the reactants and products?
- heat reaction under reflux - potassium cyanide forms K and -:CN (dissociated in solution) - -:CN attacks C d+, Br d- takes bonding electrons, forms KBr (Br-/K+ dissociated in solution) - forms propanenitrile and potassium bromide
36
What are flue gases give examples
Waste gases - CO2 - CO - H2O (g) - N2 (from air) - NOx - sO2
37
How do catalytic converters work?
- platinum coated honeycomb structure - provide reaction surface - large SA - reduces nitrous oxides - oxidises CO and hydrocarbons (Redox reactions to reduce pollution)
38
What is the structure of an amine nucleophile?
:NH3
39
What are the conditions required for making amines from halogenoalkanes?
- heat - sealed tube - excess ethanolic ammonia - ethanol
40
What is ethanolic ammonia?
Ammonia dissolved in ethanol
41
What is the first reaction when making amines from halogenoalkanes?
- normal nucleophillic substitution - :NH3 nucleophile - attacks C d+ - C loses one electron to bromine ion which takes both bonding electrons - NH3 bonds, but one electron is missing so N gets +1 charge - :Br- in solution
42
What is the second reaction when making amines from halogenoalkanes?
- another ammonia molecule (hence excess) removes a hydrogen from NH3+ - both bonding electrons stay with N, so it loses its charge (neutral) - H+ then bonds to ammonia’s lone pair in solution, forming NH4+ - ethylene is formed (NH2 on hydrocarbon)
43
What is the third (unrelated) step in making amines from halogenoalkanes?
- NH4+ ammonium ion can react with :Br- ion in solution to form an ionic bond - NH4Br
44
What is the overall equation for making amines from halogenoalkanes?
C2H5Br + 2:NH3 —> C2H5:NH2 + NH4Br
45
Why do flouroalkanes undergo nucleophillic substitution more slowly than bromoalkanes for instance?
- the C—F bond has higher enthalpy, so is harder to break than C—Br
46
What are the conditions necessary for an elimination reaction?
- ethanol solvent with OH- ions dissolved in - warmed - halogenoalkane present - reflux
47
What happens when a bromoalkane is reacted with KOH dissolved in ethanol in an elimination reaction?
- :OH- ion takes H+ from the halogenoalkane, leaving C with a +1 charge and forming H2O - it’s C now has a pair of electrons, so forms a double bond with the carbon next to it - that carbon breaks its bond with the halogen, producing a halide ion in solution, water and an alkene
48
What happens (basically) in an elimination reaction?
- a neighbouring H atom and halogen atom are eliminated from a halogenoalkane - an alkene c=c forms in their place, and water and halogen ions are produced.
49
Which conditions will result in nucleophillic substitution, and which will result in elimination? What if you use both conditions?
- nucleophillic substitution: water, reflux (still a bit of elimination) - elimination: ethanol, reflux - mixture of the two products, alchohol and alkene
50
Why did it take time for CFCs to be banned?
- lack of evidence that ozone was being depleted - lack of CFC alternatives - commercial interests to use CFCs - hard to obtain international agreement
51
Why do CFCs absorb IR wavelengths?
their bonds are polar due to asymmetric vibrations
52
What is a base?
Something that removes H+ ions (by accepting them), so is therefore always alkaline. * neutralise acids by accepting their H+ ions, these are bases*
53
What kind of reaction is alchohol to alkene?
- elimination - double bond formed - water molecule removed

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