1
25.1
25.2
25.3
New Way Chemistry for Hong Kong A-Level Book 3A

25.1 Saturated Hydrocarbons (SB p.17) sp 3 Hybridization
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25.1 Saturated Hydrocarbons (SB p.17)
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25.1 Saturated Hydrocarbons (SB p.19)
Methane (CH
4
)
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25.1 Saturated Hydrocarbons (SB p.19) sp 3 hydrid orbital
109.5
°
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25.1 Saturated Hydrocarbons (SB p.20)
Ethane (C
2
H
6
)
6
109.5
°
109.5
°
New Way Chemistry for Hong Kong A-Level Book 3A

25.1 Saturated Hydrocarbons (SB p.20)
Electron Pair Repulsion Theory
• predict the geometry of arrangement of atoms in the molecules
Procedure:
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
3.
Electron pairs tend to stay as far apart as possible (electronic repulsion between lone pairs is generally greater than that between bond pairs)
4. The shape of the molecule is referred to the positions of the atoms
(the shape has minimum repulsion between the electron pairs )
7 New Way Chemistry for Hong Kong A-Level Book 3A

25.1 Saturated Hydrocarbons (SB p.20) e.g. Methane
• tetrahedral orientation
• electron pairs to have the maximum separation of 109.5
°
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25.1 Saturated Hydrocarbons (SB p.21)
(a) Draw a three-dimensional structure for propane (C
3
H
8
).
Answer
(a)
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10
25.1 Saturated Hydrocarbons (SB p.21)
(b) How many
 bonds are there in a molecule of
(i) propane?
(ii) butane?
(iii) methylpropane?
Answer
(b) (i) 10
 bonds
(ii) 13
 bonds
(iii) 13
 bonds
New Way Chemistry for Hong Kong A-Level Book 3A

25.2 Unsaturated Hydrocarbons (SB p.22) sp 2 Hybridization
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25.2 Unsaturated Hydrocarbons (SB p.22)
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25.2 Unsaturated Hydrocarbons (SB p.23)
Ethene (C
2
H
4
)
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25.2 Unsaturated Hydrocarbons (SB p.23)
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25.2 Unsaturated Hydrocarbons (SB p.24)
Cyclohexene (C
6
H
10
)
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25.2 Unsaturated Hydrocarbons (SB p.24)
The bond enthalpy of the carbon-carbon double bond in ethene is +612 kJ mol
–1 whilst the bond enthalpy of the carbon-carbon single bond in ethane is +348 kJ mol
–1
.
16
 bond is a hand, a
 bond 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
 bond is weaker than a
 bond. Hence, the bond enthalpy of the carbon-carbon double bond (i.e. a
 bond and a
 bond) is less than twice that of the carbon-carbon single bond (i.e. a
 bond)
New Way Chemistry for Hong Kong A-Level Book 3A

25.2 Unsaturated Hydrocarbons (SB p.25)
(a) State the difference between the
 bond and the
 bond in the carbon-carbon double bond.
Answer
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25.2 Unsaturated Hydrocarbons (SB p.25)
(a) Differences between the
 bond and the
 bond in the carbon-carbon double bond:
 bond
Head-on overlap of the sp 2 hydridized orbitals of two carbon atoms
The bonding electrons in are localized symmetrically along the internuclear axis of two bonded carbon atoms.
Stronger
 bond
Side-way overlap of the vacant p orbitals of two carbon atoms
The electrons in the
 bond appear as two lobes, one above and one below the internuclear axis of the two bonded atoms
Weaker
Free to rotate Restricted to rotation
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19
25.2 Unsaturated Hydrocarbons (SB p.25)
(b) How many
 and
 bonds are present in a molecule of
(i) propene?
(ii) but-1-ene?
(iii) but-2-ene?
Answer
(b) (i) 8
 bonds and 1
 bond
(ii) 11
 bonds and 1
 bond
(iii) 11
 bonds and 1
 bond
New Way Chemistry for Hong Kong A-Level Book 3A

25.2 Unsaturated Hydrocarbons (SB p.25) sp Hybridization
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25.2 Unsaturated Hydrocarbons (SB p.25)
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25.2 Unsaturated Hydrocarbons (SB p.26)
Ethyne (C
2
H
2
)
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23
25.2 Unsaturated Hydrocarbons (SB p.27)
(a) How many
 and
 bonds are present in a molecule of
(i) propyne?
(ii) but-1-yne?
(iii) but-2-yne?
Answer
(b) (i) 6
 bonds and 2
 bonds
(ii) 9
 bonds and 2
 bonds
(iii) 9
 bonds and 2
 bonds
New Way Chemistry for Hong Kong A-Level Book 3A

25.2 Unsaturated Hydrocarbons (SB p.27)
(b) What is the hybridization of each carbon atom in ethanenitrile?
1 2
CH
3
CN
Answer
(b) Carbon 1 is sp 3 -hybridized, whereas carbon 2 is sp -hybridized.
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25.2 Unsaturated Hydrocarbons (SB p.27)
(c) State the bond angles indicated in the compound below:
25
(c) x: 109.5
° y: 120 ° z: 180 °
New Way Chemistry for Hong Kong A-Level Book 3A
Answer

25.2 Unsaturated Hydrocarbons (SB p.28)
Benzene (C
6
H
6
)
From X-ray crystallography,
• all carbon-carbon bonds are equivalent
26
• between length of C = C and C – C bond
New Way Chemistry for Hong Kong A-Level Book 3A

25.2 Unsaturated Hydrocarbons (SB p.29)
 electrons are not localized as shown above.
They are delocalized and represented in a better way as follows:
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28
25.3 Aromatic Hydrocarbons (SB p.29)
Classify the following compounds as saturated, unsaturated or aromatic hydrocarbons.
(a)
(b)
Solution:
(a) Unsaturated hydrocarbon
(b) Aromatic hydrocarbon
Answer
New Way Chemistry for Hong Kong A-Level Book 3A

29
25.3 Aromatic Hydrocarbons (SB p.29)
Classify the following compounds as saturated, unsaturated or aromatic hydrocarbons.
(c)
Solution:
(c) Unsaturated hydrocarbon
(d)
(d) Saturated hydrocarbon
(e) Saturated hydrocarbon
(e)
Answer
New Way Chemistry for Hong Kong A-Level Book 3A

25.3 Aromatic Hydrocarbons (SB p.30)
Give the approximate values of the indicated bond angles of the following compound:
30
Solution: a = 180 ° ; b = 109.5
° ; c = 120 ° ; d = 120 °
Answer
New Way Chemistry for Hong Kong A-Level Book 3A

25.3 Aromatic Hydrocarbons (SB p.30)
Which is a stronger bond, a carbon-carbon single bond or a carbon-carbon double bond? Explain your answer.
Answer
A carbon-carbon double bond is stronger than a carboncarbon single bond, as the carbon-carbon double bond contains one
 bond and one
 bond whereas the carboncarbon single bond contains only one
 bond.
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