1. Alkanes can also form cyclic structures
Cyclopropane
Cyclobutane
Cyclopentane
Cyclohexane
General formula for cycloalkanes: CnH2n
Can be conveniently represented using line segment formulae

2. Cycloalkane nomenclature can be extended to include substitution
Note:
Cycloalkane nomenclature can be extended to include substitution
Methylcyclohexane
1,3-Dimethylcyclohexane

3. Only one cycloalkane has a planar structure: cyclopropane
All others have non-planar structure
Ideal tetrahedral angle is 109.5o
sp3 hybridised carbons with bond angles very different to 109.5o will be less stable (higher in energy)
Bond angle approaching 60o
Cyclopropane
Cyclopropane is said to suffer
from angle-strain
All C-H bonds in cyclopropane are eclipsed

4. Cyclopentane has almost zero angle-strain
To relieve torsional strain due to eclipsed C-H bonds,
cyclopentane relaxes into a non-planar structure
One CH2 group out of the plane of the ring

5. Cyclohexane
A planar structure would have internal bond angles of 120o and eclipsed C-H bonds
Actual structure relaxes into a chair conformation
This reduces the bond angle to 109o
Geometry about each Carbon very close to tetrahedral ideal
Angle strain ~ zero

6. All C-H bonds staggered, i.e. torsional strain ~ zero
Newman projection along any C-C bond
The chair conformation contains two different hydrogen environments
6 Equatorial Hydrogens
6 Axial Hydrogens

7. Exists in trace quantities
At temperatures below 230 K (-43C):
can observe that two different types of hydrogen environment are present on cyclohexane
Above this temperature, observe only one hydrogen environment
Reason: cyclohexane molecules are not static above 230 K
i.e. exist in different conformations
Undergo ring inversion
Boat conformation
Exists in trace quantities
Note: hydrogens axial in one chair conformation equatorial in the other

8. Ball-and-stick model of boat cyclohexane

9. What if one of the cyclohexane hydrogens were replaced by a methyl group?
Methylcyclohexane
The two chair conformations are no longer equivalent
One has the methyl group in an axial position; one in an equatorial position

10. These interconvert by ring inversion (exist in equilibrium)
[Inversion proceeds through boat conformations which exist in trace amounts]
Can simplify diagram by omitting the C-H bonds
Methyl equatorial
Methyl axial

11. Sources of alkanes
Lower Mol. Mt. (~ < 5 Carbons): natural gas
Larger Mol. Wt.: petroluem of crude oil
Crude oil: complex mixture of hydrocarbons
Separated into fractions based on boiling point ranges
Boiling point related to molecular weight, i.e to number of carbons
< 5 Carbons: gases at room temperature
5 Carbons < ~18 Carbons: liquids at room temperature
> 18 Carbons: solids at room temperature

12. Increasing molecular size results in increasing tendency to form condensed phases
Associated with weak intermolecular interactions between alkane molecules
London dispersion forces: weak electrostatic attractions between induced dipoles, i.e. are…
Van der Waals’ forces between electrons of one molecule and nuclei of another
Extent of attraction increases with increasing molecular size
Weak interactions compared to hydrogen bonding or ionic bonding

13. Solubility of alkanes
‘Like dissolves like’: alkanes soluble in other alkanes, e.g petroleum
[Soluble: single liquid phase results upon mixing]
Alkanes insoluble in water, i.e are hydrophobic
Mixtures with water separate into two liquid phases: aqueous and hydrocarbon

15. 2 C4H10 + 13 O2 → 8 CO2 + 10 H2O CO2 → Urea DH = - 2877 kJ mol-1
Reactions of alkanes
Relatively inert; contain only stable C-C and C-H bonds
Some important reactions:
1. Combustion, e.g.
2 C4H O2 → 8 CO H2O
DH = kJ mol-1
i.e. exothermic
2. Steam reforming
CH4 + H2O → 3H2 + CO N2 ↓ ↓
NH3
CO2
→ Urea +

16. 3. Reaction with halogens

17. 4. Catalytic cracking
Fragmentation of alkanes into smaller molecules, e.g:
The products of these reactions are a new type of hydrocarbon
They are said to be ‘unsaturated’ compared to alkanes
i.e., have fewer Hydrogens per Carbon than alkanes, which are said to be ‘saturated’

18. Alkenes: contain Carbon-Carbon double bonds
Unsaturated hydrocarbons contain Carbon-Carbon multiple bonds
Classes of unsaturated hydrocarbons are defined by the types of Carbon-Carbon multiple bonds they contain
Alkenes: contain Carbon-Carbon double bonds
Carbon-Carbon double bond
Alkynes: contain Carbon-Carbon triple bonds
Carbon-Carbon triple bond
Carbon valency of four maintained in alkenes and alkynes

19. Alkenes Older name: Olefins
Characterised by presence of Carbon-Carbon double bonds
General
structural
formula
Where ‘R’ = Hydrogen
or alkyl group
Two Carbons and all four ‘R’ groups are lying on the same plane
Bond angles about each Carbon ~ 120o

20. Three sp2 hybridised orbitals can be arrayed to give trigonal geometry
The remaining 2pz orbital is orthogonal to the three sp2 orbitals
View along z axis
View along xy plane

21. s bond formation results from overlap of two sp2 hybridised orbitals
[A s-antibonding orbital is also formed, but this is not occupied by electrons]
Overlap of the pz orbitals results in formation of a p bond
[A p-antibonding orbital is also formed, but this is not occupied by electrons] p

22. p s p orbital: has a nodal plane on which lies on the bond axis
p electron density lies above and below the plane containing the two Carbons and four ‘R’ groups
View along the
Carbon-Carbon bond
Note:
constitutes one p molecular orbital
i.e. constitutes one p bond when occupied p
Carbon-Carbon double bond:
One s bond; One p bond s
Both occupied by two electrons

23. Rotation about a Carbon-Carbon double bond requires opening up of the p bond
Requires large input of energy (~ 268 kJ mol-1)
Hence, rotation about C=C bonds does not occur at room temperature
Consequently, a new form of isomerism becomes possible for alkenes
Consider an alkene with one Hydrogen and one alkyl group ‘R’ bonded to each Carbon
Two structures are possible or

24. Cis isomer Trans isomer
This form of isomerism is known as Cis-Trans isomerism
[older term: geometrical isomerism]
The cis isomer is that with like groups on the same side of the C=C
The trans isomer is that with like groups on opposite sides of the C=C
Cis isomer
Trans isomer

25. First two members of the alkene series:
Ethene
(Ethylene)
Propene
(Propylene)
Note:
Nomenclature:
Prefix indicates number of carbons
(‘eth…’ = 2C; ‘prop…’ = 3C; etc.)
Suffix ‘…ene’ indicates presence of C=C

26. Could have C=C between C1 and C2
or between C2 and C3
Butene
1-Butene
2-Butene
Note:
1. 1-Butene and 2-butene are structural isomers 2.
3. Number indicates starting point of the C=C, i.e. number through the C=C

27. 4. Cis-Trans isomerism is possible for 2-butene
There are two isomeric 2-butenes
Trans-2-butene
b.p. 3.7oC
m.p. -139oC
Cis-2-butene
b.p. 0.3oC
m.p. -106oC

28. Some other alkenes
4-Methyl-2-pentene
2-Methyl-1-butene
1,3-Pentadiene
Cis-3-heptene
Trans-2-decene

29. Can have cycloalkenes
Cyclohexene
Cyclopentene
3-Methylcyclopentene
Note:
1,4-Cyclohexadiene

30. Lycopene molecular structure

31. p electrons in alkenes are available to become involved in bond formation processes
Essential processes in the synthesis of new molecules:
formation of new covalent bonds
Covalent bonds: pairs of electrons shared between nuclei (atoms)
In the synthesis of organic molecules, a major strategy for forming new covalent bonds is:
donation of an electron pair by one molecular species…
…to form a covalent bond with another, electron deficient molecular species
Electron pair donating species are known as nucleophiles
Electron pair accepting species are known as electrophiles
Reaction of a nucleophile with an electrophile results in the formation of a new covalent bond

32. Alkene hydrogenation
Addition of hydrogen (H2) across a C=C
General reaction
Alkene p bond is lost, and two new C-H s bonds formed
Alkene converted to alkane
No reaction in absence of catalyst
Typical catalysts: Palladium (Pd), Platinum (Pt), Nickel (Ni), Rhodium (Rh) or other metals
Catalysts usually supported on materials such as charcoal
E.g. Pd/C “Palladium on Carbon”

33. Examples 1-Hexene Hexane Hexane 1,3-Hexadiene 2-Methyl-1-butene
2-Methylbutane

34. Reaction occurs at the catalyst surface
H2 molecules adsorbed onto catalyst surface
Both Hydrogens added to same face of C=C
1,2-Dimethylcyclohexene
Cis-1,2-dimethylcyclohexane
Both Hydrogens added to the same face of the cyclohexene C=C
[Cis/Trans naming system can be extended to cyclic systems]

35. C=C p bond lost; new C-H and C-X s bonds formed
Addition of HX to alkenes
General reaction
X = Cl, Br, I
C=C p bond lost; new C-H and C-X s bonds formed
e.g:
2-Chloropropane
(only product)
1-Chloropropane
(not formed)
Propene
To explain this, need to consider the reaction mechanism

36. Reaction mechanism:
detailed sequence of bond breaking and bond formation in going from reactants to products
Addition of HX to alkenes: reaction involves two steps
1st Step: Addition of proton (H+)
2nd Step: Addition of halide (X-)
1st Step
Alkene p electrons attack proton
New C-H s bond results
Remaining Carbon short 1 electron
Carbon positively charged

37. Addition of H+ to the alkene p bond forms a new C-H s bond and a carbocation intermediate
[or carbonium ion]
2nd Step
New C-X s bond results
Halide ion attacks electron deficient carbon

38. Reaction of HCl with CH3-CH=CH2
1st Step: addition of H+ to form a carbocation intermediate
Two possible modes of addition or
I.e. two possible carbocation intermediates

39. Classification of carbocations
Primary (1o)
Carbocation
Secondary (2o)
Carbocation
Tertiary (3o)
Carbocation
2o Carbocation
1o Carbocation

40. Most stable 3o > 2o > 1o Least stable
The relative order of stability for carbocations is:
Most stable 3o > 2o > 1o Least stable
This is because carbocations can draw electron density along s bonds; known as an inductive effect
This effect is significant for alkyl substituents, but weak for Hydrogens
Least stabilised
Most stabilised

41. Addition of HCl to CH3-CH=CH2 proceeds so as to give the more stable of the two possible carbocation intermediates, i.e:
Not formed
Addition of chloride then gives 2-chloropropane exclusively
Cl-
Additions of HX to alkenes which follow this pattern are said to obey Markovnikov’s rule
“Reaction proceeds via the more stable possible carbocation intermediate”

42. Other examples not 2-Methylpropene 2-Bromo-2-methyl- propane
1-Methylcyclohexene
1-Chloro-1-methyl-
cyclohexane
1-Chloro-2-methyl-
cyclohexane
2-Butene
2-Chlorobutane
(Symmetrical alkene)
Same structure

43. Addition of water to alkenes Follows same pattern as addition of HX
Acid catalysis required
Propene
2-Hydroxypropane
(2-Propanol)
Mechanism:
1. Protonation of C=C so as to give the more stable carbocation intermediate

44. 2. Attack on the carbocation by water acting as a nucleophile
3. Loss of proton to give the product and regenerate the catalyst

45. Acid catalysed addition of water often difficult to control
A Mercury (II) mediated version often used - oxymercuration
1-Methylcyclopentene
1-Hydroxy-1-methyl-
cyclopentane
Gives exclusively Markovnikov addition
Hydroboration
1-Methylcyclopentene
1-Hydroxy-2-methyl-
cyclopentane
Gives exclusively anti-Markovnikov addition
Mechanisms of these reactions beyond the scope of this module

46. Alkene p bond lost; two new C-OH s bonds formed
Alkene hydroxylation
Alkene p bond lost; two new C-OH s bonds formed
Alkene epoxidation
Epoxides
Alkene p bond lost; two new C-O s bonds are formed to the same Oxygen

47. Examples Propene Propane-1,2-diol 1,2-Epoxypropane Cyclopentene
1,2-Epoxycyclopentane
Cis-1,2-cyclopentanediol

48. Ozonolysis of alkenes
Ozone (O3): strong oxidising agent
Adds to C=C with loss of both the p and s bonds
Products formed are known as ozonides
Ozonide
Ozonides usually not isolated, but further reacted with reducing agents
Formation of two molecules each containing C=O (Carbonyl) groups

49. Overall process: Examples 1-Butene Aldehydes Ketone
2,3-Dimethyl-2-butene

50. Addition of bromine (Br2) to alkenes
General reaction
Alkene p bond lost; two new C-Br s bonds formed
Stereospecific reaction observed with cycloalkenes
Cyclopentene
Trans-1,2-dibromo-
cyclopentane
(no cis-isomer)

51. Chlorine also adds to alkene C=C bonds
1,2-Dichlorobutane
1-Butene

52. All Carbons and Hydrogens equivalent
Benzene
Molecular formula C6H6
All Carbons and Hydrogens equivalent
Kekulé structure (1865) =
However, does not behave like a typical alkene
Less reactive than typical alkenes
Only reacts with bromine in presence of a catalyst
A substitution rather than an addition reaction occurs
not

53. Styrene

54. Arrangement of 6 p electrons in a closed cyclic p systems is especially stable
Said to possess aromaticity
Aromatic systems very common (e.g. benzene and its derivatives)
Representing the p system in benzene
Represents p system well
Of limited use in describing reactivity
Better to use a combination of Kekulé structures

55. These are NOT independent species existing in equilibrium
The p electrons in benzene are said to be resonance delocalised over the entire ring system
Resonance delocalisation is generally energetically favourable
Resonance delocalisation of 6 p electrons in a closed ring system is especially favourable: aromaticity

56. Alkynes Older name: Acetylenes
Characterised by the presence of Carbon-Carbon triple bonds
General structure of alkynes
Groups R, C, C and R are co-linear
Neither sp3 nor sp2 hybridised Carbon consistent with this geometry

57. Two sp hybridised orbitals can be arrayed to give linear geometry
Two remaining 2p orbitals are mutually orthogonal and orthogonal to the two sp hybridised orbitals
[If the two sp orbitals lies along the z axis, 2px lies along the x axis and 2py along the y axis]

58. The s bond lies along the C-C bond axis
C≡C consists of one s bond and two p bonds
The s bond lies along the C-C bond axis
The bond axis lies along the intersection of orthogonal planes
One p bond lies in each plane, with a node along the bond axis
View along the bond axis

59. First two members of the series of alkynes
Ethyne
(Acetylene)
Propyne
Nomenclature
Prefix indicates number of carbons (‘eth…’, ‘prop…’, etc.)
Suffix ‘…yne’ indicates presence of C≡C
Butyne
Can have C≡C between C1 and C2
or between C2 and C3
1-Butyne
2-Butyne
These are structural isomers

60. 6-Methyl-3-octyne
1-Heptene-6-yne
4-Methyl-7-nonen-1-yne

61. Linear geometry of alkynes difficult to accommodate in a cyclic structure
Hence relatively few cycloalkynes
Smallest stable cycloalkyne is cyclononyne
Cyclononyne

62. Hydrogenation of alkynes
Standard hydrogenation conditions completely remove the p bonds
Both p bonds lost; four new C-H s bonds formed
Heptane
3-Heptyne
[Conversion of alkyne to alkane]

63. Alkyne
Cis-alkene
3-Heptyne
Cis-3-heptene

64. This gives specifically Trans-alkenes
Alkynes can also be converted into alkenes by reaction with sodium or lithium metal in liquid ammonia
[Na, liq. NH3; or Li, liq. NH3]
This gives specifically Trans-alkenes
3-Heptyne
Trans-3-heptene

65. Cis-2-hexene
Trans-2-hexene

66. Addition of bromine (Br2) to alkynes
Can have addition to one or both alkyne p bonds
Alkyne
Trans-1,2-dibromo-
alkene
1,1,2,2-tetra-
bromoalkane
1,1,2,2-Tetrabromoethane
Ethyne
(Acetylene)
Trans-1,2-dibromo-
1-butene
1-Butyne

67. Requires catalysis by mercury (II) salts
Hydration of 1-alkynes
[Addition of water]
Requires catalysis by mercury (II) salts
1-Alkyne
Ketones
4-Methyl-1-hexyne
Ketone