1. Bonding in methane, ethane and ethene
AS Chemistry
Bonding in methane, ethane and ethene
 and  bonds

2. Learning Objectives Candidates should be able to:
describe covalent bonding in terms of orbital overlap, giving  and  bonds.
explain the shape of, and bond angles in, ethane and ethene molecules in terms of  and  bonds.

3. Starter activity

4. Alkenes pent-2-ene CH3CH=CHCH2CH3 hex-3-ene CH3CH2CH=CHCH3
2,3-dimethylpent-2-ene
cyclopenta-1,3-diene
3-ethylhept-1-ene CH2=CHCH2CH(CH2CH3)CH2CH2CH3

5. Hybridisation of orbitals
The electronic configuration of a carbon atom is 1s22s22p2 1 1s 2 2s 2p

6. HYBRIDISATION OF ORBITALS
If you provide a bit of energy you can promote (lift) one of the s electrons into a p orbital. The configuration is now 1s22s12p3 1 1s 2 2s 2p
The extra energy released when the bonds form more than compensates for the initial input.

7. Hybridisation of orbitals in alkanes
The four orbitals (an s and three p’s) combine or HYBRIDISE to give four new orbitals. All four orbitals are equivalent.
Because one s and three p orbitals are used, it is called sp3 hybridisation.
2s22p2
2s12p3
4 x sp3

8. Hybridisation of orbitals in alkanes
In ALKANES, the four sp3 orbitals repel each other into a tetrahedral arrangement.
sp3 orbitals

9. Bonding in methane

10. Bonding in ethane

11. Bonding in ethene
Alternatively, only three orbitals (an s and two p’s) combine or HYBRIDISE to give three new orbitals. All three orbitals are equivalent. The remaining 2p orbital is unchanged.
2s22p2
2s12p3
3 x sp2 2p

12. What about ethene?
sp2 hybrids

13.  - bonds

14. AS Chemistry
Geometric Isomerism

15. Learning Objectives Candidates should be able to:
describe cis-trans isomerism in alkenes, and explain its origin in terms of restricted rotation due to the presence of π bonds.
deduce the possible isomers for an organic molecule of known molecular formula.
identify cis-trans isomerism in a molecule of given structural formula.

16. Starter activity

17. What is stereoisomerism?
In stereoisomerism, the atoms making up the isomers are joined up in the same order, but still manage to have a different arrangement in space
ISOMERISM
STRUCTURAL ISOMERISM
STEREOISOMERISM
GEOMETRIC ISOMERISM
OPTICAL ISOMERISM

18. Geometric Isomerism?

19. GEOMETRIC ISOMERISM RESTRICTED ROTATION OF C=C BONDS
Single covalent bonds can easily rotate. What appears to be a different structure in an alkane is not. Due to the way structures are written out, they are the same.
ALL THESE STRUCTURES ARE THE SAME BECAUSE C-C BONDS HAVE ‘FREE’ ROTATION
Animation doesn’t work in old versions of Powerpoint

20. Geometric Isomerism?

21. Geometric isomers of but-2-ene

22. Geometric Isomerism? X 

23. GEOMETRIC ISOMERISM How to tell if it exists    
Two different atoms/groups attached
Two different atoms/groups attached 
GEOMETRICAL ISOMERISM 
Two similar atoms/groups attached
Two similar atoms/groups attached
Once you get two similar atoms/groups attached to one end of a C=C, you cannot have geometrical isomerism
Two similar atoms/groups attached
Two different atoms/groups attached 
Two different atoms/groups attached
Two different atoms/groups attached 
GEOMETRICAL ISOMERISM

24. GEOMETRIC ISOMERISM Isomerism in butene
There are 3 structural isomers of C4H8 that are alkenes*. Of these ONLY ONE exhibits geometrical isomerism.
BUT-1-ENE
cis BUT-2-ENE
trans BUT-2-ENE
2-METHYLPROPENE
* YOU CAN GET ALKANES WITH FORMULA C4H8 IF THE CARBON ATOMS ARE IN A RING

25. Summary To get geometric isomers you must have:
restricted rotation (involving a carbon-carbon double bond for A-level purposes);
two different groups on the left-hand end of the bond and two different groups on the right-hand end. It doesn't matter whether the left-hand groups are the same as the right-hand ones or not.

26. The effect of geometric isomerism on physical properties
melting point (°C)
boiling point (°C)
cis
-80 60
trans
-50 48
You will notice that:
the trans isomer has the higher melting point;
the cis isomer has the higher boiling point.

27. Why is the boiling point of the cis isomers higher?
The difference between the two is that the cis isomer is a polar molecule whereas the trans isomer is non-polar.

28. Why is the melting point of the cis isomers lower?
In order for the intermolecular forces to work well, the molecules must be able to pack together efficiently in the solid.
Trans isomers pack better than cis isomers. The "U" shape of the cis isomer doesn't pack as well as the straighter shape of the trans isomer.

29. AS Chemistry
Optical Isomerism

30. Learning Objectives Candidates should be able to:
explain what is meant by a chiral centre and that such a centre gives rise to optical isomerism.
deduce the possible isomers for an organic molecule of known molecular formula.
identify chiral centres in a molecule of given structural formula.

31. Starter activity

32. Optical isomerism Chiral centre Chiral molecule
When four different atoms or groups are attached to a carbon atom, the molecules can exist in two isomeric forms known as optical isomers. These are non-superimposable mirror images.

33. Optical Isomerism What is a non-superimposable mirror image?
Animation doesn’t work in old versions of Powerpoint

34. Optical isomerism
Amino acids (the building blocks of proteins) are optically active. They affect plane polarised light differently.

35. Butan-2-ol

36. Optical Isomerism The polarimeter A B C D E F
A Light source produces light vibrating in all directions
B Polarising filter only allows through light vibrating in one direction
C Plane polarised light passes through sample
D If substance is optically active it rotates the plane polarised light
E Analysing filter is turned so that light reaches a maximum
F Direction of rotation is measured coming towards the observer
If the light appears to have turned to the right turned to the left
DEXTROROTATORY LAEVOROTATORY

37. Enantiomers – how do they differ?
Usually have the same chemical and physical properties – but behave differently in presence of other chiral compounds.

38. Enantiomers – how do they differ?

39. TYPES OF ISOMERISM STRUCTURAL ISOMERISM
CHAIN ISOMERISM
STRUCTURAL ISOMERISM
POSITION ISOMERISM
Same molecular formula but different structural formulae
FUNCTIONAL GROUP ISOMERISM
GEOMETRICAL ISOMERISM
Occurs due to the restricted rotation of C=C double bonds... two forms - CIS and TRANS
STEREOISOMERISM
Same molecular formula but atoms occupy different positions in space.
OPTICAL ISOMERISM
Occurs when molecules have a chiral centre. Get two non- superimposable mirror images.

40. Electrophilic Addition to Alkenes
AS Chemistry
Electrophilic Addition to Alkenes

41. Learning Objectives Candidates should be able to:
describe the mechanism of electrophilic addition in alkenes, using bromine/ethene as an example.
describe the chemistry of alkenes as exemplified, where relevant, by the following reactions of ethene: addition of hydrogen, steam, hydrogen halides and halogens.

42. Starter activity

43. Electrophilic addition
CH2=CH Br  CH2BrCH2Br

44. Electrophilic addition
bromine with ethene
CH2=CH2
+ Br2
CH2BrCH2Br
1,2-dibromoethane
hydrogen bromide with ethene
CH2=CH2
+ HBr
CH3CH2Br
bromoethane

45. Electrophilic addition mechanism
bromine with ethene H C H C Br +
carbocation + - Br Br - H C Br Br
1,2-dibromoethane

46. hydrogen bromide with ethene
Electrophilic addition mechanism
hydrogen bromide with ethene H C H C +
carbocation - + Br H H C Br Br -
bromoethane

47. Electron flow during electrophilic addition

48. Addition reactions of alkenes
EQUATION
TEMPERATURE
(OC)
PRESSURE
CATALYST
PHASE
NOTES
hydrogen
CH2=CH2 + H2 → CH3CH3
~150
Finely divided nickel on support material
Gas
Never carried out industrially. Analogous reaction used to produce some margarines from oils (see later).
steam
CH2=CH2 + H2O→ CH3CH2OH
330
6MPa
Phosphoric (V) acid (H3PO4) adsorbed onto the surface of silica.
Major industrial process for the manufacture of ethanol.
hydrogen halides
(e.g. HBr)
CH2=CH2 + HBr → CH3CH2Br
Room temperature
Aqueous solution
Reactivity increases from HF to HI.
halogens
CH2=CH2 +Br2→ CH2BrCH2Br
Liquid bromine or solution (both aqueous and non-polar solvent.
Chlorine and iodine produce similar addition products. Fluorine is too powerful an oxidizing agent.
Addition reactions of alkenes

49. Addition to unsymmetrical alkenes
Electrophilic addition to propene
2-bromopropane
1-bromopropane

50. Addition to unsymmetrical alkenes
In the electrophilic addition to alkenes the major product is formed via the more stable carbocation (carbonium ion)
least stable most stable
methyl < primary (1°) < secondary (2°) < tertiary (3°)

51. Addition to unsymmetrical alkenes
SECONDARY
CARBOCATION
PATH A
MAJOR PRODUCT
PRIMARY
CARBOCATION
PATH B
MINOR PRODUCT

52. AS Chemistry
Polymerisation

53. Learning Objectives Candidates should be able to:
describe the chemistry of alkenes including polymerisation.
describe the characteristics of addition polymerisation as exemplified by poly(ethene) and PVC.
Recognize the difficulty of the disposal of poly(alkene)s, i.e. non-biodegradability and harmful combustion products.

54. Starter activity

55. Poly(ethene) Temperature: about 200°C Pressure: about 2000 atmospheres
Conditions
Temperature:
about 200°C
Pressure:
about 2000 atmospheres
Initiator:
often a small amount of oxygen as an impurity

56. Free radical addition
Initiation
Propagation
Propagation
Termination

57. LDPE or HDPE

58. LDPE or HDPE
Sandwich bags, cling wrap, car covers, squeeze bottles, liners for tanks and ponds, moisture barriers in construction
Freezer bags, water pipes, wire and cable insulation, extrusion coating

59. Polymerisation of alkenes
ETHENE
POLY(ETHENE)
CHLOROETHENE
POLY(CHLOROETHENE)
POLYVINYLCHLORIDE PVC
PROPENE
POLY(PROPENE)
TETRAFLUOROETHENE
POLY(TETRAFLUOROETHENE)
PTFE “Teflon”

60. Disposal of polymers Method Comments Landfill
Emissions to the atmosphere and water; vermin; unsightly. Can make use of old quarries.
Incineration
Saves on landfill sites and produces energy. May also release toxic and greenhouse gases.
Recycling
high cost of collection and re- processing.
Feedstock recycling
Use the waste for the production of useful organic compounds. New technology can convert waste into hydrocarbons which can then be turned back into polymers.

61. AS Chemistry
Oxidation of alkenes

62. Learning Objectives
Candidates should be able to describe the oxidation of alkenes by:
cold, dilute, acidified manganate(VII) ions to form the diol, and
hot, concentrated, acidified manganate(VII) ions leading to the rupture of the carbon-to-carbon double bond in order to determine the position of alkene linkages in larger molecules.

63. Starter activity

64. Oxidation of alkenes
In the presence of dilute (acidified or alkaline) potassium manganate (VII).
Alkenes react readily at room temperature (i.e. in the cold).
The purple colour disappears and a diol is formed.
CH2=CH H2O [O]  HOCH2CH2OH
ethane – 1,2-diol

65. Oxidation of alkenes
In the presence of a hot, concentrated solution of acidified potassium manganate (VII), any diol formed is split into two fragments which are oxidized further to carbon dioxide, a ketone or a carboxylic acid.
Fragment
Product
=CH2
CO2
R-CH=
Aldehyde → carboxylic acid
R2C=
Ketone

66. Oxidation of alkenes CH2=CH2 CH3CH=CH2 (CH3)2C=CH2
2 products – both contain ketone
1 product only
2 products – one contains 2 ketone groups and one contains 2 acid groups.

67. AS Chemistry
Halogenoalkanes

68. Learning Objectives
Candidates should be able to recall the chemistry of halogenoalkanes as exemplified by the following nucleophilic substitution reactions of bromoethane:
hydrolysis;
formation of nitriles;
formation of primary amines by reaction with ammonia.

69. Starter activity

70. Naming Halogenoalkanes
CHCl3 trichloromethane
CH3CHClCH3 2-chloropropane
CF3CCl3 1,1,1-trichloro-2,2,2-trifluoroethane F Cl

71. Physical Properties
1-chloropropane is polar and has permanent dipole-dipole intermolecular forces that are stronger than the temporary dipole-induced dipole forces in non-polar butane.
1-chloropropane is polar and has permanent dipole-dipole intermolecular forces that are stronger than the temporary dipole-induced dipole forces in non-polar butane.

72. Nucleophilic substitution
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eat given
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electronegative
polar
attracted
negative
deficient
nucleophiles
halide
substitution

73. Nucleophilic substitution
This is known as an SN2 reaction.
S stands for substitution,
N for nucleophilic, and
2 because the initial stage of the reaction involves two species.

74. ANIMATION SHOWING THE SN2 MECHANISM
Nucleophilic substitution - mechanism
Attack by nucleophile is to the back of the molecule – away from the negatively charged halogen atom.
ANIMATION SHOWING THE SN2 MECHANISM

75. Rate of reaction Halogen F Cl Br I Electronegativity 4.0 3.0 2.8 2.5
Bond strength (C-X) kJ mol-1
484
338
276
238
You may expect the fluoroalkane to react more quickly as the C-F bond is the most polar and therefore more susceptible to attack by nucleophiles. However, the C-F bond is the strongest. A nucleophile may be more attracted more strongly to the carbon atom but, unless it forms a stronger bond to carbon, it will not displace the halogen.
Actually the reaction with the iodoalkane is the most rapid. This suggests that the strength of the C-X bond is more important than its polarity. Note that the C-I bond is not polar. However, it is easily polarisable.

76. Measuring the rate of reaction
Experiment
Water is a poor nucleophile but it can slowly displace halide ions
C2H5Br(l) H2O(l)  C2H5OH(l) H+(aq) + Br¯(aq)
If aqueous silver nitrate is shaken with a halogenoalkane (they are immiscible) the displaced halide combines with a silver ion to form a precipitate of a silver halide. The weaker the C-X bond the quicker the precipitate appears.

77. Nucleophilic substitution
hydroxide ion with bromoethane
warm
CH3CH2Br
+ OH-
(aqueous)
CH3CH2OH + Br-
ethanol
Water with bromoethane
warm
CH3CH2Br
+ H2O
(aqueous)
CH3CH2OH + HBr
ethanol
This is a slower reaction – water is not such a good nucleophile.

78. hydroxide ion with bromoethane
Nucleophilic substitution mechanism
hydroxide ion with bromoethane
CH3 H Br C
CH3 H OH C + - Br - - OH
ethanol

79. water with bromoethane
Nucleophilic substitution mechanism
water with bromoethane
CH3 H Br C H
CH3 OH C + Br - + -
H2O
CH3 H OH C
HBr
ethanol

80. Nucleophilic substitution
propanenitrile
CH3CH2Br
+ CN-(ethanol)
CH3CH2CN + Br-
cyanide ion with bromoethane
reflux
ammonia with bromoethane
Heat /
pressure
aminoethane
CH3CH2Br
+ NH3(ethanol)
CH3CH2NH2
+ HBr
Heat /
pressure
CH3CH2Br
+ NH3(ethanol)
CH3CH2NH2 2
+ NH4+Br-

81. cyanide ion with bromoethane
Nucleophilic substitution mechanism
cyanide ion with bromoethane
CH3 H Br C
CH3 H CN C + - Br - CN -
propanenitrile

82. ammonia with bromoethane
Nucleophilic substitution mechanism
ammonia with bromoethane
CH3 H Br C H
CH3
NH2 C + Br - + -
NH3
NH3
CH3 H
NH2 C
H NH3+Br -
aminoethane

83. Past paper question Cl2 U.V. /sunlight Ethanolic KCN reflux Br2

84. Substitution vs. Elimination
AS Chemistry
Substitution vs. Elimination

85. Learning Objectives Candidates should be able to:
recall the chemistry of halogenoalkanes as exemplified by the elimination of hydrogen bromide from 2-bromopropane.
describe the mechanism of nucleophilic substitution (by both SN1 and SN2 mechanisms) in halogenoalkanes.

86. Starter activity

87. Type of halogenoalkane Position of halogeno- group
Example
primary
at end of chain:
bromoethane
secondary
in middle of chain:
2-bromopropane
tertiary
attached to a carbon atom which carries no H atoms:
2-bromo-2-methylpropane

88. SN1 – tertiary halogenoalkanes
Nucleophilic attack at the back of the molecule is hindered by bulky CH3 groups. Tertiary carbocation is stabilised by electron donating effect of CH3 groups.

89. SN1 or SN2 ? Halogenoalkane Mechanism Primary SN2 Secondary
SN1 and SN2
Tertiary
SN1

90. CH3CH2Br + NaOH  CH2=CH2 + NaBr + H2O
Elimination
You need to be aware that the hydroxide ion can act as a strong base as well as a nucleophile.
An alternative reaction can take place in which HBr is removed and an alkene is formed. This is known as elimination.
CH3CH2Br + NaOH  CH2=CH NaBr + H2O

91. Elimination of HX from haloalkanes
Elimination of HBr from 2-bromopropane
CH3CHBrCH3
+ OH-
CH3CH=CH2 + H2O + Br-
(in ethanol)
CH3 H C Br
CH3 H C
propene Br -
H OH OH -
acting as a base

92. Substitution or Elimination?
alcohol
nucleophilic substitution
RCH3CH2OH + Br-
+ OH-
(aqueous)
hydroxide acts as a nucleophile
RCH2CH2X
+ OH-
(ethanol)
hydroxide acts as a base
elimination
RCH=CH2 + H2O + X-
alkene

93. AS Chemistry
Pros and Cons

94. Learning Objectives Candidates should be able to:
interpret the different reactivities of halogenoalkanes e.g. CFCs; anaesthetics; flame retardants; plastics with particular reference to hydrolysis and to the relative strengths of the C-Hal bonds;
explain the uses of fluoroalkanes and hydrofluorooalkanes in terms of their relative chemical inertness;
recognise the concern about the effect of chlorofluoroalkanes on the ozone layer.

95. Starter activity

96. Chlorofluorocarbons - CFCs.
Properties:
Non-flammable
Low toxicity
Unreactive
Liquefy easily when compressed

97. Uses Refrigerants Propellants for aerosols
Solvents (including dry-cleaning)
Degreasers

98. The ozone layer

99. Natural ozone layer

101. Replacements
Hydrochlorofluorocarbons, HCFCs: shorter life in the atmosphere.
Hydrofluorocarbons, HFCs: don’t contain chlorine so zero affect on ozone layer.
Hydrocarbons: zero effect on ozone layer but flammable and lead to photochemical smog.

102. C. Why is BCF good at extinguishing fires?
The presence of a bromine confers flame – retarding qualities on the product.
The high temperature in fires break this compound down, producing free radicals such as Br∙. These react with other free radicals produced during combustion, quenching the flames.