Bonding and Structure of Hydrocarbons

1
Bonding and Structure of Hydrocarbons
22.1 Saturated Hydrocarbons 22.2 Unsaturated
Hydrocarbons 22.3 Aromatic Hydrocarbons
2
Hydrocarbons

• Compounds containing carbon and hydrogen atoms
  only
• Raw materials for synthesizing other organic
  products

3
Saturated Hydrocarbons
4
22.1 Saturated Hydrocarbons (SB p.29)
Saturated Hydrocarbons

• Contain only C ? C and C ? H single bonds
• Carbon atom is tetravalent (i.e. able to form
  four bonds)
• Structures and shapes can be predicted in terms
  of
• ? hybridization of orbitals
• ? valence shell electron pair repulsion theory

5
22.1 Saturated Hydrocarbons (SB p.29)
sp3 Hybridization

• Four unpaired electrons
• ? Form four bonds with other atoms

6
22.1 Saturated Hydrocarbons (SB p.29)
sp3 Hybridization

• 2s and 2p orbitals with their unpaired electrons
  are mixed to give four hybrid orbitals of the
  same energy level

7
22.1 Saturated Hydrocarbons (SB p.29)
sp3 Hybridization

• The hybrid orbital formed is called sp3 hybrid
  orbital
• ? each hybrid orbital has one part s character
  and 3 parts p character
• The four hybrid orbitals have the same shape and
  same energy level

8
22.1 Saturated Hydrocarbons (SB p.30)
sp3 Hybridization

• There is one electron in each hybrid orbital to
  form a bond
• ? Electrons repel one another
• ? four sp3 hybrid orbitals get as far away as
  possible
• ? tetrahedral arrangement

9
22.1 Saturated Hydrocarbons (SB p.30)
Methane (CH4)
The formation of a methane molecule
10
22.1 Saturated Hydrocarbons (SB p.30)
Methane (CH4)

• Four C?H bonds are arranged tetrehedrally
• All H ? C ? H bond angles are the same, i.e.109.5o

11
22.1 Saturated Hydrocarbons (SB p.31)
Methane (CH4)
An sp3 hybrid orbital

• Has two lobes of unequal sizes

12
22.1 Saturated Hydrocarbons (SB p.31)
Methane (CH4)

• The large lobe overlaps with the 1s orbital of a
  hydrogen atom in a head-on manner to form a C ? H
  bond

Formation of a C ? H ? bond
13
22.1 Saturated Hydrocarbons (SB p.31)
Methane (CH4)

• ? The larger lobe of sp3 hybrid orbitals is
  relatively large in size
• ? The overlap between it and 1s orbital of
  hydrogen atom is large
• ? C ? H bond is strong
• In a ? bond, the bonding electrons are localized
  symmetrically along the internuclear axis of the
  bonding atoms

14
22.1 Saturated Hydrocarbons (SB p.31)
Ethane (C2H6)

• Both carbon atoms are sp3 hybridized

The formation of an ethane molecule from two
sp3-hybridized carbon atoms
15
22.1 Saturated Hydrocarbons (SB p.31)
Ethane (C2H6)
The ball-and-stick model of ethane
16
22.1 Saturated Hydrocarbons (SB p.32)
Valence Shell Electron Pair Repulsion Theory

• Predict the geometry of arrangement of atoms in
  the molecules

17
22.1 Saturated Hydrocarbons (SB p.32)
Valence Shell Electron Pair Repulsion Theory
Step 1. Focus on the central atom of the
molecule 2. Consider all of the valence electron
pairs of the central atom, i.e. bond pairs and
lone pairs
18
22.1 Saturated Hydrocarbons (SB p.32)
Valence Shell Electron Pair Repulsion Theory

• Electron pairs tend to stay as far apart as
  possible
• ? electronic repulsion between lone pairs is
  generally greater than that between bond pairs

19
22.1 Saturated Hydrocarbons (SB p.32)
Valence Shell Electron Pair Repulsion Theory

• The shape of the molecule is referred to the
  positions of the atoms
• ? the shape has minimum repulsion between the
  electron pairs

20
22.1 Saturated Hydrocarbons (SB p.32)
Valence Shell Electron Pair Repulsion Theory
The tetrahedral shape of a methane molecule
resulting in the minimum electronic repulsion
21
22.1 Saturated Hydrocarbons (SB p.33)
22
Unsaturated Hydrocarbons
23
22.2 Unsaturated Hydrocarbons (SB p.34)
Unsaturated Hydrocarbons

• Hydrocarbons containing multiple bonds
• Include alkenes, alkynes and aromatic hydrocarbons

24
22.2 Unsaturated Hydrocarbons (SB p.34)
sp2 Hybridization

• 2s orbital and two 2p orbitals are hybridized to
  form three sp2 hybrid orbitals
• One 2p orbital unhybridized

25
22.2 Unsaturated Hydrocarbons (SB p.34)
sp2 Hybridization

• Take up a trigonal planar arrangement

26
22.2 Unsaturated Hydrocarbons (SB p.35)
sp2 Hybridization

• The unhybridized 2p orbital is at a right angle
  to the plane of the three sp3 hybrid orbitals

An sp2-hybridized carbon atom
27
22.2 Unsaturated Hydrocarbons (SB p.35)
Ethene (C2H4)

• The sp2 hybrid orbitals from each carbon atom
  head-on overlap with each other to form a ? bond
  between the carbon atoms

28
22.2 Unsaturated Hydrocarbons (SB p.35)
Ethene (C2H4)

• The remaining sp2 hybrid orbitals head-on overlap
  with the 1s orbitals of the hydrogen atoms to
  form ? bonds

29
22.2 Unsaturated Hydrocarbons (SB p.35)
Ethene (C2H4)

• Side-way overlap of orbitals is much less
  effective than head-on overlap of orbitals
• ? ? bond is much stronger than ? bond

30
22.2 Unsaturated Hydrocarbons (SB p.35)
Ethene (C2H4)

• ? bond electron cloud consists of two lobes
• ? one above and one below the plane of the
  molecular framework

31
22.2 Unsaturated Hydrocarbons (SB p.35)
Ethene (C2H4)

• The bonding electrons of a ? bond are loosely
  held by the carbon nuclei
• ? they can easily be attacked by electrophiles
  (electron-loving chemical species)
• ? unsaturated hydrocarbons with double bonds
  are reactive compounds
• ? undergo addition reactions readily

32
22.2 Unsaturated Hydrocarbons (SB p.36)
Ethene (C2H4)
33
22.2 Unsaturated Hydrocarbons (SB p.36)
34
22.2 Unsaturated Hydrocarbons (SB p.36)
Cyclohexene (C6H10)

• Four of the carbon atoms are sp3-hybridized
• The remaining two carbon atoms that form the
  carbon-carbon double bond are sp2-hybridized

35
22.2 Unsaturated Hydrocarbons (SB p.36)
Cyclohexene (C6H10)
The structure of cyclohexene
36
22.2 Unsaturated Hydrocarbons (SB p.36)
37
22.2 Unsaturated Hydrocarbons (SB p.38)
sp Hybridization

• 2s orbital and one 2p orbital are hybridized to
  form two sp hybrid orbitals
• Two 2p orbitals unhybridized

38
22.2 Unsaturated Hydrocarbons (SB p.38)
sp Hybridization

• The sp hybrid orbitals are co-linear
• Orientated at an angle of 180o

39
22.2 Unsaturated Hydrocarbons (SB p.38)
sp Hybridization

• The unhybridized 2p orbitals are perpendicular to
  the axis that passes through the centre of two sp
  hybrid orbitals

An sp-hybridized carbon atom
40
22.2 Unsaturated Hydrocarbons (SB p.39)
Ethyne (C2H2)

• The sp hybrid orbitals from each carbon atom
  head-on overlap with each other to form a ? bond
  between the carbon atoms

41
22.2 Unsaturated Hydrocarbons (SB p.39)
Ethyne (C2H2)

• The remaining sp hybrid orbitals head-on overlap
  with the 1s orbitals of the hydrogen atoms to
  form ? bonds

42
22.2 Unsaturated Hydrocarbons (SB p.39)
Ethyne (C2H2)

• The 2py and 2pz orbitals on each carbon atom
  overlap in a side-way manner to form two ? bonds

43
22.2 Unsaturated Hydrocarbons (SB p.39)
Ethyne (C2H2)
44
Aromatic Hydrocarbons
45
22.3 Aromatic Hydrocarbons (SB p.40)
Aromatic Hydrocarbons

• During the latter part of the 19th century,
• Aliphatic compounds
• ? chemical behaviour was fat-like
• Aromatic compounds
• ? low hydrogen to carbon ratio
• ? fragrant

46
22.3 Aromatic Hydrocarbons (SB p.40)
Aromatic Hydrocarbons

• Kekule was the first to recognize the early
  aromatic compounds all contain a six-carbon unit
  (i.e. benzene)

47
22.3 Aromatic Hydrocarbons (SB p.40)
Benzene (C6H6)

• Benzene was highly unsaturated
• ? undergoes addition reactions readily
• Indeed, benzene hardly undergoes addition
  reactions

48
22.3 Aromatic Hydrocarbons (SB p.40)
Benzene (C6H6)

• When benzene undergoes substitution reactions
  with chlorine
• ? Only one form of chlorobenzene is formed
• ? All six hydrogen atoms are identical
• ? More stable than alkenes and alkynes

49
22.3 Aromatic Hydrocarbons (SB p.41)
Benzene (C6H6)

• From X-ray crystallography,
• All C ? C bonds have the same length (0.139 nm)
• C ? C bond in cyclohexane is 0.154 nm in length
• C C bond in cyclohexene is 0.134 nm in length

50
22.3 Aromatic Hydrocarbons (SB p.41)
Benzene (C6H6)

• The stability of benzene and the C ? C bond
  length in benzene can be explained by a resonance
  model

51
22.3 Aromatic Hydrocarbons (SB p.41)
Benzene (C6H6)

• The unhybridized p orbitals sideway overlap to
  form ? bonds
• ? Form a delocalized ? electron cloud

52
22.3 Aromatic Hydrocarbons (SB p.41)
Benzene (C6H6)

• The delocalization of ? electrons imparts extra
  stability to benzene

53
22.3 Aromatic Hydrocarbons (SB p.41)
Benzene (C6H6)

• For simplicity, the structure of benzene can be
  written as

54
22.3 Aromatic Hydrocarbons (SB p.41)
Benzene (C6H6)

• In the real structure, the ? electrons are
  delocalized over the whole system
• The better way to represent the structure of
  benzene

55
22.3 Aromatic Hydrocarbons (SB p.43)
Enthalpy Changes of Hydrogenation of Benzene and
Cyclohexene
Hydrogenation of cyclohexene
56
22.3 Aromatic Hydrocarbons (SB p.43)
Enthalpy Changes of Hydrogenation of Benzene and
Cyclohexene
Hydrogenation of 1,3-cyclohexadiene
57
22.3 Aromatic Hydrocarbons (SB p.44)
Enthalpy Changes of Hydrogenation of Benzene and
Cyclohexene
Hydrogenation of 1,3,5-cyclohexatriene (Kekule
structure)
58
22.3 Aromatic Hydrocarbons (SB p.44)
Relative stabilities of cyclohexene,
1,3-cyclohexadiene, 1,3,5-cyclohexatriene
(hypothetical) and benzene
The real benzene molecule is more stable than the
hypothetical Kekule structure by 150.4 kJ mol-1
59
22.3 Aromatic Hydrocarbons (SB p.45)
Enthalpy Changes of Formation of Benzene and
Cyclohexene
60
22.3 Aromatic Hydrocarbons (SB p.45)
Enthalpy Changes of Formation of Benzene and
Cyclohexene
61
22.3 Aromatic Hydrocarbons (SB p.45)
Enthalpy Changes of Formation of Benzene and
Cyclohexene
62
22.3 Aromatic Hydrocarbons (SB p.46)
Enthalpy Changes of Formation of Benzene and
Cyclohexene
63
22.3 Aromatic Hydrocarbons (SB p.46)
Enthalpy Changes of Formation of Benzene and
Cyclohexene
64
22.3 Aromatic Hydrocarbons (SB p.46)
Enthalpy Changes of Formation of Benzene and
Cyclohexene

• Enthalpy change of formation of benzene is less
  positive than that of cyclohexene
• ? benzene is energetically more stable than
  cyclohexene

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The END
66
22.1 Saturated Hydrocarbons (SB p.33)
Check Point 22-1
(a) Draw a three-dimensional structure for
propane (C3H8).
Answer
67
22.1 Saturated Hydrocarbons (SB p.33)
Check Point 22-1
(b) How many s bonds are there in a molecule
of (i) propane? (ii) butane? (iii)
methylpropane?
Answer

• (i) 10 ? bonds
• (ii) 13 ? bonds
• (iii) 13 ? bonds

Back
68
22.1 Saturated Hydrocarbons (SB p.33)
Let's Think 1
Are ball-and-stick models good representations of
organic molecules? Are there any other models
available?
Answer
69
22.1 Saturated Hydrocarbons (SB p.33)
Back
Let's Think 1
The ball-and-stick model has the following
disadvantages The sizes, bond lengths and
bond angles are restricted to fixed
classifications. It cannot represent important
small variations, especially in angles. When
we pick it up to look at the model, bonds tend to
rotate to unintended positions. For big
molecules, it is practically impossible to pick
it up without distorting it. We can only guess
at steric repulsions, pbonding hindrance to
rotation, etc. There are many other molecular
models, one of them is the computer model which
makes use of visualization softwares (e.g. Chime)
to view the structure of molecules. It overcomes
the disadvantages of the ball-and-stick model and
provides much more information about the
structure of molecules.
70
22.2 Unsaturated Hydrocarbons (SB p.35)
Back
Let's Think 2
Which is a stronger bond, a carbon-carbon single
bond or a carbon-carbon double bond? Why?
Answer
A carbon-carbon double bond is stronger than a
carbon-carbon single bond, as the carbon-carbon
double bond contains one sbond and one pbond
whereas the carbon-carbon single bond contains
only one sbond.
71
22.2 Unsaturated Hydrocarbons (SB p.35)
Let's Think 3
Is the ball-and-stick model of ethene in Fig.
22-10 a good model of ethene?
Answer
It is not a good representation of the ethene
molecule as it fails to represent the s bond
between the two carbon atoms.
Back
72
22.2 Unsaturated Hydrocarbons (SB p.36)
Example 22-2
The bond enthalpy of the carbon-carbon double
bond in ethene is 612 kJ mol1 while the bond
enthalpy of the carbon-carbon single bond in
ethane is 348 kJ mol1. Explain briefly why the
bond enthalpy of the carbon-carbon double bond is
less than twice that of the carbon-carbon single
bond.
Answer
73
22.2 Unsaturated Hydrocarbons (SB p.36)
Back
Example 22-2
A carbon-carbon double bond is composed of a
sbond and a p bond. The sbond is formed by the
head-on overlap of orbitals. The electrons of the
sbond are distributed along the internuclear axis
of the two bonded atoms. sbond is a strong bond
and has a high bond enthalpy. On the other hand,
a pbond is formed by the side-way overlap of
orbitals. As the side-way overlap of orbitals is
less effective than the head-on overlap of
orbitals, a pbond is weaker than a sbond. Hence,
the bond enthalpy of the carbon-carbon double
bond (i.e. a sbond and a pbond) is less than
twice that of the carbon-carbon single bond (i.e.
a sbond).
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22.2 Unsaturated Hydrocarbons (SB p.37)
Check Point 22-2A
(a) State the difference between the s bond and
the p bond in the carbon-carbon double bond.
Answer
75
22.2 Unsaturated Hydrocarbons (SB p.37)
Check Point 22-2A

• The differences between the s bond and the p bond
  in the carbon-carbon double bond are
• 1. The s bond is formed by the head-on overlap
  of the sp2 hybrid orbitals of two carbon atoms,
  while the p bond is formed by the side-way
  overlap of the vacant p orbitals of two carbon
  atoms.
• 2. The bonding electrons in the s bond are
  localized symmetrically along the internuclear
  axis of two bonded carbon atoms. However, the
  electrons in the p bond appear as two lobes, one
  above and one below the internuclear axis of the
  two bonded atoms.
• 3. The s bond is much stronger than the p bond
  as the side-way overlap of orbitals which
  results in the p bond is much less effective
  than the head-on overlap of orbitals which
  results in the s bond.
• 4. The s bond is free to rotate, whereas the p
  bond is not.

76
22.2 Unsaturated Hydrocarbons (SB p.37)
Back
Check Point 22-2A
(b) How many s and p bonds are present in a
molecule of (i) propene? (ii) but-1-ene? (iii)
but-2-ene?
Answer
(b) (i) 8 s bonds and 1 p bond (ii) 11 s bonds
and 1 p bond (iii) 11 s bonds and 1 p bond
77
22.2 Unsaturated Hydrocarbons (SB p.40)
Check Point 22-2B
(a) How many s and p bonds are present in a
molecule of (i) propyne? (ii) but-1-yne? (iii)
but-2-yne?
Answer
(a) (i) 6 s bonds and 2 p bonds (ii) 9 s bonds
and 2 p bonds (iii) 9 s bonds and 2 p bonds
78
22.2 Unsaturated Hydrocarbons (SB p.40)
Check Point 22-2B
(b) What is the hybridization of each carbon atom
in ethanenitrile?
Answer
(b) Carbon 1 is sp3-hybridized, whereas carbon 2
is sp-hybridized.
79
22.2 Unsaturated Hydrocarbons (SB p.40)
Back
Check Point 22-2B
(c) State the bond angles indicated in the
compound below
Answer
(c) x 109.5o y 120o z 180o
80
22.3 Aromatic Hydrocarbons (SB p.41)
Let's Think 4
What leads to the extra stability observed in
benzene?
Answer
In the six-member ring of benzene, six p orbitals
of carbon atoms sideway overlap with each other
to form six p bonds. The p electron clouds are
delocalized in the ring, linking the six carbon
atoms together. This imparts extra stability to
the benzene molecule.
Back
81
22.3 Aromatic Hydrocarbons (SB p.42)
Example 22-3A
Classify the following compounds as saturated,
unsaturated or aromatic hydrocarbons.
A B C
D E
Answer
82
22.3 Aromatic Hydrocarbons (SB p.42)
Example 22-3A
A Unsaturated hydrocarbon B Aromatic
hydrocarbon C Unsaturated hydrocarbon D
Saturated hydrocarbon E Saturated hydrocarbon
Back
83
22.3 Aromatic Hydrocarbons (SB p.42)
Example 22-3B
Give the approximate values of the indicated bond
angles of the following compound
Answer
a 180o b 109.5o c 120o d 120o
Back
84
22.3 Aromatic Hydrocarbons (SB p.46)
Check Point 22-3
(a) Name the following compounds
Answer
(a) A cyclohexane B benzene
85
22.3 Aromatic Hydrocarbons (SB p.46)
Check Point 22-3
(b) State the hybridization of the carbon atoms
in each of the above compounds.
Answer
(b) A sp3 hybridization B sp2 hybridization
86
22.3 Aromatic Hydrocarbons (SB p.46)
Check Point 22-3
(c) State the C ? C ? C bond angle in each of the
above compounds.
Answer
(c) A 109.5o B 120
87
22.3 Aromatic Hydrocarbons (SB p.46)
Check Point 22-3
(d) Which one is an aromatic compound? Explain
your answer.
Answer
(d) B. This is because it possesses the aromatic
ring of benzene.
Back