Alkanes and Radical Substitution

1
Alkanes and Radical Substitution
27.1 Introduction 27.2 Nomenclature of
alkanes 27.3 Physical Properties of
Alkanes 27.4 Preparation of Alkanes 27.5 Reactions
of Alkanes
2
Introduction
3
27.1 Introduction (SB p.146)
Classification of Hydrocarbons
4
27.1 Introduction (SB p.146)
Alkanes and Cycloalkanes
Alkanes are hydrocarbons that contain only C?H
and C?C single bonds
Cycloalkanes are alkanes in which all of some of
the carbon atoms are arranged in a ring
5
27.1 Introduction (SB p.146)
Alkanes and Cycloalkanes

• General formula of acyclic alkanes CnH2n2
• General formula of cycloalkanes CnH2n

6
27.1 Introduction (SB p.146)
The First Three Members of Alkanes
7
27.1 Introduction (SB p.147)
Alkanes

• All carbon atoms are sp3 hybridized
• Bonded tetradehrally to hydrogen and other carbon
  atoms
• C?C?C, C?C?H, H?C?H bond angles are 109.5o

8
27.1 Introduction (SB p.147)
Alkanes

• Alkane molecules are actually zigzag in shape

9
27.1 Introduction (SB p.147)
Alkanes

• An alkane molecule with carbon atoms present in
  more than one continuous carbon chain
• ? branched-chain alkane

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Nomenclature of Alkanes
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27.2 Nomenclature of Alkanes (SB p.147)
Straight-Chain Alkanes
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27.2 Nomenclature of Alkanes (SB p.147)
Branched-Chain Alkanes

• Select the longest possible straight chain
• ? give the parent name for the alkane
• Number the parent chain beginning with the end of
  the chain nearer the branched chain
• Use the number obtained to designate the position
  of the branched chain

13
27.2 Nomenclature of Alkanes (SB p.148)
Branched-Chain Alkanes

• When two or more branched chains are present
• ? give each branched chain a number
  corresponding to its position on the parent
  chain
• When two or more branched chains are identical
• ? indicate this by the use of the prefixes
  di-, tri-, tetra-, etc.

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27.2 Nomenclature of Alkanes (SB p.148)
Branched-Chain Alkanes
e.g.
15
27.2 Nomenclature of Alkanes (SB p.148)
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Physical Properties of Alkanes
17
27.3 Physical Properties of Alkanes (SB p.149)
Some physical properties of the first 8 members
of straight-chain alkanes
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27.3 Physical Properties of Alkanes (SB p.149)
Boiling Point
Variation of boiling points with the number of
carbon atoms of straight-chain alkanes
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27.3 Physical Properties of Alkanes (SB p.150)
Boiling Point

• When the number of carbon atoms of alkanes
  increases
• ? the boiling point shows a regular increase

20
27.3 Physical Properties of Alkanes (SB p.150)
Boiling Point

• Explained in terms of the strength of van der
  Waals forces
• Alkanes are non-polar or very weakly polar
• ? held together by weak van der Waals forces

21
27.3 Physical Properties of Alkanes (SB p.150)
Boiling Point

• For straight-chain alkanes
• ? when the molecular mass increases
• ? the size of electron clouds also increases
• ? large electron clouds are more easily
  polarized
• ? instantaneous dipoles are more readily formed

22
27.3 Physical Properties of Alkanes (SB p.150)
Boiling Point

• For straight-chain alkanes
• ? instantaneous dipole-induced dipole
  interactions between molecules are stronger
• ? more energy is required to separate the
  molecules during boiling

23
27.3 Physical Properties of Alkanes (SB p.150)
Boiling Point

• For straight-chain alkanes
• ? as the molecular size increases
• ? the surface area of the molecule also
  increases
• ? van der Waals forces between molecules are
  stronger
• ? higher boiling point

24
27.3 Physical Properties of Alkanes (SB p.150)
Boiling Point

• Branched-chain alkanes have a lower boiling point
  than straight-chain alkanes

25
27.3 Physical Properties of Alkanes (SB p.151)
Boiling Point

• Branched-chain alkanes are more compact
• ? reduce the surface area
• ? van der Waals forces between molecules
  become weaker
• ? lower boiling point

26
27.3 Physical Properties of Alkanes (SB p.151)
Melting Point
Variation of melting points with the number of
carbon atoms of straight-chain alkanes
27
27.3 Physical Properties of Alkanes (SB p.152)
Density

• All alkanes and cycloalkanes are less dense than
  water
• ? petroleum floats on the water surface

28
27.3 Physical Properties of Alkanes (SB p.152)
Solubility

• Alkanes are non-polar compounds
• Insoluble in water and highly polar solvents
• Soluble in non-polar organic solvents
• ? benzene, 1,1,1-trichloroethane, chloroform,
  other liquid hydrocarbons

29
Preparation of Alkanes
30
27.4 Preparation of Alkanes (SB p.152)
Petroleum Refining

• The first step is fractional distillation
• ? separate petroleum into fractions based on
  the volatility of its components
• ? performed in a petroleum refinery
• ? a fractionating tower is used

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27.4 Preparation of Alkanes (SB p.152)
Petroleum Refining
32
27.4 Preparation of Alkanes (SB p.153)
A simplified diagram of a fractionating tower
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27.4 Preparation of Alkanes (SB p.153)
Typical fractions obtained by fractional
distillation of petroleum
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27.4 Preparation of Alkanes (SB p.153)
Typical fractions obtained by fractional
distillation of petroleum
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27.4 Preparation of Alkanes (SB p.154)
Cracking of Petroleum
36
27.4 Preparation of Alkanes (SB p.154)
Cracking of Petroleum

• Convert hydrocarbons from heavier fractions into
  lighter fractions
• Performed in the absence of air
• Providing lighter fractions of petroleum, alkenes
  and sometimes hydrogen
• C11H24 ?? C9H20 CH2CH2
• C14H30 ?? C8H18 2CH2CH2 2C 2H2

37
27.4 Preparation of Alkanes (SB p.154)
Catalytic Cracking

• When a mixture of alkanes from the heavier
  fractions is heated
• ? at very high temperatures
• ? in the presence of catalysts
• ? in the absence of air
• The molecules break down and rearrange into
  smaller, highly branched hydrocarbons

38
27.4 Preparation of Alkanes (SB p.154)
Thermal Cracking

• The process is done in the absence of catalysts
• Tend to form unbranched carbon chains

39
27.4 Preparation of Alkanes (SB p.154)
Cracking of Petroleum
A cracking tower
40
27.4 Preparation of Alkanes (SB p.155)
Reforming

• Straight-chain alkanes are heated under pressure
  in the presence of a platinum catalyst
• The chains break up and reform
• ? give branched-chain molecules

41
27.4 Preparation of Alkanes (SB p.155)
Reforming
e.g.
42
Reactions of Alkanes
43
27.5 Reactions of Alkanes (SB p.156)
Reactions of Alkanes

• Inertness to chemical reactions
• ? strong C?C and C?H bonds
• C and H have nearly the same electronegativity
• ? C?H bonds are only slightly polarized

44
27.5 Reactions of Alkanes (SB p.156)
Reactions of Alkanes

• Unreactive towards polar or ionic reagents
• Low reactivity of alkanes
• ? the reason why alkanes were originally called
  paraffins (little affinity)
• Only undergo a limited number of reactions

45
27.5 Reactions of Alkanes (SB p.156)
Combustion

• When alkanes react with sufficient oxygen
• ? carbon dioxide and water are formed
• ? release a large amount of heat
• ? known as complete combustion

46
27.5 Reactions of Alkanes (SB p.156)
Combustion

• General equation for the complete combustion of
  an alkane

47
27.5 Reactions of Alkanes (SB p.156)
Combustion

• Alkanes are a common fuel
• Methane is the main component of natural gas
• Butane is a component of bottle gas

48
27.5 Reactions of Alkanes (SB p.156)
Combustion

• In limited supply of oxygen,
• ? alkanes burn to give carbon monoxide and
  carbon particles

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27.5 Reactions of Alkanes (SB p.156)
Relationship Between Global Warming and the Use
of Fossil Fuels

• Carbon dioxide, water vapour, methane and
  dinitrogen oxide
• ? allow incoming solar energy to penetrate to
  the Earths surface
• ? reabsorb infrared radiation emitted from the
  Earth
• ? heat energy is trapped
• ? known as greenhouse gases

50
27.5 Reactions of Alkanes (SB p.157)
Apparent connections between increases in fossil
fuel use, atmospheric concentrations of CO2 and
global temperature between 1970 and 2002
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27.5 Reactions of Alkanes (SB p.157)
Relationship Between Global Warming and the Use
of Fossil Fuels

• Increase in global temperature
• ? causes drought
• ? causes a rise in sea level (flooding occurs
• ? threaten the lives of some organisms

52
27.5 Reactions of Alkanes (SB p.157)
Halogenation

• Alkanes react with halogens to give haloalkanes
  and hydrogen halides
• R?H X2 ??R ? X HX
• where X2 denotes F2, Cl2, Br2 or I2

53
27.5 Reactions of Alkanes (SB p.157)
Halogenation

• When methane is mixed with chlorine in the dark,
  no reaction occurs
• If the mixture is placed under diffuse sunlight
  or is heated, it reacts vigorously
• ? forming a mixture of chloromethane,
  dichloromethane, trichloromethane and
  tetrachloromethane

54
27.5 Reactions of Alkanes (SB p.157)
Halogenation

• One or more hydrogen atoms in methane are
  substituted by chlorine atoms
• ? depend on the relative amounts of methane and
  chlorine

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27.5 Reactions of Alkanes (SB p.158)
Halogenation
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27.5 Reactions of Alkanes (SB p.158)
Halogenation

• When methane is in excess
• ? chloromethane predominates in the products
• When chlorine is in excess
• ? tetrachloromethane predominates in the
  products

57
27.5 Reactions of Alkanes (SB p.158)
Halogenation

• The reactivity of halogens decreases in the
  order
• F2 gt Cl2 gt Br2 gt I2

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27.5 Reactions of Alkanes (SB p.158)
Reaction Mechanism Free Radical Substitution
Reaction
1. Chain initiation

• Homolytic fission of a chlorine molecule
• Two chlorine radicals are formed

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27.5 Reactions of Alkanes (SB p.158)
Reaction Mechanism Free Radical Substitution
Reaction
2. Chain propagation

• The highly reactive chlorine radical reacts with
  a methane molecule by abstracting a hydrogen atom

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27.5 Reactions of Alkanes (SB p.158)
Reaction Mechanism Free Radical Substitution
Reaction
2. Chain propagation

• The highly reactive methyl radical attacks a
  chlorine molecule by abstracting a chlorine atom

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27.5 Reactions of Alkanes (SB p.159)
Reaction Mechanism Free Radical Substitution
Reaction

• Further substitution occurs when the chlorine
  radical abstracts a further hydrogen atom from
  chloromethane

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27.5 Reactions of Alkanes (SB p.159)
Reaction Mechanism Free Radical Substitution
Reaction
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27.5 Reactions of Alkanes (SB p.159)
Reaction Mechanism Free Radical Substitution
Reaction
64
27.5 Reactions of Alkanes (SB p.160)
Reaction Mechanism Free Radical Substitution
Reaction
3. Chain termination

• The reactive radicals are converted to molecules
  only
• The reaction stops

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27.5 Reactions of Alkanes (SB p.160)
Reaction Mechanism Free Radical Substitution
Reaction
Possible chain-terminating steps
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27.5 Reactions of Alkanes (SB p.160)
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27.5 Reactions of Alkanes (SB p.161)
Stability of Alkyl Radicals

• According to the number of alkyl groups directly
  attached to the carbon that bears the unpaired
  electron
• ? alkyl radicals are classified as primary,
  secondary or tertiary

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27.5 Reactions of Alkanes (SB p.161)
Stability of Alkyl Radicals
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27.5 Reactions of Alkanes (SB p.162)
Stability of Alkyl Radicals

• Alkyl radical has an unpaired electron
• Electron-deficient
• Stabilized by substituents (e.g. alkyl groups)
• ? release electrons to the carbon atom bearing
  the unpaired electron

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27.5 Reactions of Alkanes (SB p.162)
Stability of Alkyl Radicals

• The stability of the alkyl radicals decreases in
  the order

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27.5 Reactions of Alkanes (SB p.162)
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The END
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27.2 Nomenclature of Alkanes (SB p.148)
Example 27-2
Draw the structures of the following compounds.
Are the names provided correct or incorrect? If
they are incorrect, give the correct
name. (a) 2,4-Dimethylpentane
Answer
74
27.2 Nomenclature of Alkanes (SB p.148)
Example 27-2
Draw the structures of the following compounds.
Are the names provided correct or incorrect? If
they are incorrect, give the correct
name. (b) 1,3-Dimethylpentane
Answer
75
27.2 Nomenclature of Alkanes (SB p.148)
Back
Example 27-2
Draw the structures of the following compounds.
Are the names provided correct or incorrect? If
they are incorrect, give the correct name. (c)
4-Ethyl-1-methylheptane
Answer
76
27.2 Nomenclature of Alkanes (SB p.149)
Back
Check Point 27-2
Draw the structures of all the isomers of C6H14,
and give the IUPAC names for all the structures
drawn.
Answer
77
27.4 Preparation of Alkanes (SB p.155)
Example 27-4
Define each of the following terms and give an
example of each (a) Catalytic cracking
Answer
(a) Catalytic cracking is the process in which a
mixture of alkanes from the heavier fractions is
heated at very high temperatures, in the presence
of catalysts and the absence of air. Smaller and
highly branched hydrocarbons are formed. For
example, C8H18 ?? C5H12 C3H6
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27.4 Preparation of Alkanes (SB p.155)
Example 27-4
Define each of the following terms and give an
example of each (b) Thermal cracking
Answer

• Thermal cracking is the breakdown of large alkane
  molecules in the heavier fractions into lighter
  fractions of smaller molecules in the absence of
  catalysts and air. Straight-chain alkanes are
  usually formed in this process. For example,
• C14H30 ?? C8H18 2CH2 CH2 2C 2H2

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27.4 Preparation of Alkanes (SB p.155)
Back
Example 27-4
Define each of the following terms and give an
example of each (c) Reforming
Answer
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27.5 Reactions of Alkanes (SB p.160)
Example 27-5A
Write down the reaction mechanism involved in the
bromination of ethane in the presence of diffuse
sunlight.
Answer
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27.5 Reactions of Alkanes (SB p.160)
Example 27-5A
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27.5 Reactions of Alkanes (SB p.160)
Example 27-5A
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27.5 Reactions of Alkanes (SB p.160)
Back
Example 27-5A
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27.5 Reactions of Alkanes (SB p.161)
Check Point 27-5A
(a) Explain why cracking has to be carried out in
the absence of air.
Answer
(a) Cracking must be carried out in the absence
of air because combustion occurs instead of
cracking in the presence of air.
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27.5 Reactions of Alkanes (SB p.161)
Check Point 27-5A
(b) An alkane has a relative molecular mass of 72
and produces one product only on
monochlorination. Deduce its structure.
Answer
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27.5 Reactions of Alkanes (SB p.161)
Check Point 27-5A
Back
87
27.5 Reactions of Alkanes (SB p.161)
Let's Think 1
Two isomeric compounds A and B have the
molecular formula C3H7Cl. Chlorination of A gave
a mixture of two dichlorides of formula C3H6Cl2.
Chlorination of B gave three different compounds
of formula C3H6Cl2 (they may not all be different
from the dichlorides from A). What are the
structural formulae of A and B and the
dichlorides obtained from each?
Answer
88
27.5 Reactions of Alkanes (SB p.161)
Let's Think 1
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27.5 Reactions of Alkanes (SB p.161)
Back
Let's Think 1
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27.5 Reactions of Alkanes (SB p.162)
Example 27-5B
Answer
91
27.5 Reactions of Alkanes (SB p.162)
Example 27-5B
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27.5 Reactions of Alkanes (SB p.162)
Example 27-5B
The resulting free radicals react with chlorine
in a succeeding propagation step to give the
corresponding haloalkane. Butyl radical gives
only 1-chlorobutane while secondary butyl radical
gives only 2-chlorobutane. Since the more
stable secondary radical is formed at a higher
rate, 2-chlorobutane is formed as the major
product of the reaction.
Back
93
27.5 Reactions of Alkanes (SB p.162)
Check Point 27-5B
Give the structure of the major product formed by
free radical bromination of each of the
following (a) Methylcyclopentane
Answer
94
27.5 Reactions of Alkanes (SB p.162)
Back
Check Point 27-5B
Give the structure of the major product formed by
free radical bromination of each of the
following (b) 2,2,4-Trimethylpentane
Answer