Alkenes and Electrophilic Addition

1
Alkenes and Electrophilic Addition
28.1 Introduction 28.2 Nomenclature of
Alkenes 28.3 Physical Properties of
Alkenes 28.4 Preparation of Alkenes 28.5 Reactions
of Alkenes
2
Introduction
3
28.1 Introduction (SB p.167)
Alkenes

• Unsaturated hydrocarbons containing the CC
  double bond
• General formula of alkenes CnH2n
• The carbon atoms involved in the CC double bond
  are sp2-hybridized

4
28.1 Introduction (SB p.167)
Alkenes

• The CC double bond is made up of a ? bond and a
  ? bond
• Trigonal planar geometry
• Bond angle 120o

5
28.1 Introduction (SB p.167)
Alkenes

• ? bond can be broken down easily
• ? alkenes are reactive compounds
• ? undergo mainly addition reactions

6
28.1 Introduction (SB p.167)
Alkenes

• Limited rotation of the CC double bond
• ? alkenes show geometrical isomerism
• e.g.

7
Nomenclature of Alkenes
8
28.2 Nomenclature of Alkenes (SB p.167)
IUPAC Rules of Naming of Alkenes

• Select the longest possible straight chain that
  contains the CC double bond
• ? determine the stem name
• ? use the ending of -ene

9
28.2 Nomenclature of Alkenes (SB p.167)
IUPAC Rules of Naming of Alkenes
2. Number the parent chain so as to include both
carbon atoms of the double bond Begin numbering
with the end of the chain nearer the CC double
bond
10
28.2 Nomenclature of Alkenes (SB p.168)
IUPAC Rules of Naming of Alkenes

• Designate the position of the CC double bond by
  using the number of the first atom of the double
  bond
• Designate the position of the substituents by
  using the numbers obtained

11
28.2 Nomenclature of Alkenes (SB p.168)
IUPAC Rules of Naming of Alkenes
e.g.
12
28.2 Nomenclature of Alkenes (SB p.168)
IUPAC Rules of Naming of Alkenes

• If two identical groups are present on the same
  side of the CC double bond
• ? the compound is designated as cis
• If they are on opposite sides
• ? the compound is designated as trans

13
28.2 Nomenclature of Alkenes (SB p.168)
IUPAC Rules of Naming of Alkenes
e.g.
14
Physical Properties of Alkenes
15
28.3 Physical Properties of Alkenes (SB p.169)
Physical Properties of Alkenes
Some physical properties of several alkenes
16
28.3 Physical Properties of Alkenes (SB p.169)
Physical Properties of Alkenes
Some physical properties of several alkenes
17
28.3 Physical Properties of Alkenes (SB p.169)
Physical Properties of Alkenes

• Alkenes are non-polar
• Dissolve in non-polar solvents or in solvents of
  low polarity
• Only very slightly soluble in water

18
28.3 Physical Properties of Alkenes (SB p.169)
Physical Properties of Alkenes

• M.p. and b.p of alkenes are lower than their
  corresponding alkanes
• Densities of alkenes are less than that of water

19
Preparation of Alkenes
20
28.4 Preparation of Alkenes (SB p.170)
Cracking

• Prepared by the cracking of alkanes of high
  molecular masses
• Give alkenes of low molecular masses

21
28.4 Preparation of Alkenes (SB p.170)
Cracking
e.g.
22
28.4 Preparation of Alkenes (SB p.170)
Elimination Reactions

• Involve removal of atoms or groups of atoms from
  adjacent carbon atoms in the reactant molecule
• Formation of a double bond between carbon atoms

23
28.4 Preparation of Alkenes (SB p.170)
1. Dehyhalogenation

• Elimination of a hydrogen halide molecule from a
  haloalkane
• By heating the haloalkane in an alcoholic
  solution of KOH

24
28.4 Preparation of Alkenes (SB p.170)
1. Dehydrohalogenation
e.g.
25
28.4 Preparation of Alkenes (SB p.171)
1. Dehyhalogenation

• The ease of dehydrohalogenation of haloalkanes
  decreases in the order

26
28.4 Preparation of Alkenes (SB p.171)
1. Dehyhalogenation

• Different classes of haloalkanes or alcohols have
  different reactivities
• May undergo different types of reactions under
  the same reaction conditions

27
28.4 Preparation of Alkenes (SB p.171)
1. Dehyhalogenation

• Dehydrohalogenation of secondary or tertiary
  haloalkanes can take place in more than one way
• A mixture of alkenes is formed

28
28.4 Preparation of Alkenes (SB p.171)
1. Dehyhalogenation
e.g.

• The more highly substituted alkene (i.e.
  but-2-ene) is the major product

29
28.4 Preparation of Alkenes (SB p.172)
1. Dehyhalogenation

• The more highly substituted alkene is the alkene
  with a larger number of alkyl groups bonded to
  the C C group
• The greater the number of alkyl groups that an
  alkene contains
• ? the more stable the molecule

30
28.4 Preparation of Alkenes (SB p.172)
1. Dehyhalogenation

• The relative stabilities of alkenes decrease in
  the order

31
28.4 Preparation of Alkenes (SB p.172)
2. Dehydration of Alcohols

• Removal of a water molecule from a reactant
  molecule
• By heating the alcohols in the presence of
  concentrated sulphuric acid
• Give alkenes and water as the products

32
28.4 Preparation of Alkenes (SB p.172)
2. Dehydration of Alcohols

• Experimental conditions (i.e. temperature and
  concentration of concentrated sulphuric acid)
• ? closely related to the structure of the
  individual alcohol

33
28.4 Preparation of Alkenes (SB p.172)
2. Dehydration of Alcohols

• Primary alcohols generally required concentrated
  sulphuric acid and a relatively high temperature

34
28.4 Preparation of Alkenes (SB p.172)
2. Dehydration of Alcohols

• Secondary alcohols are intermediate in reactivity
• Tertiary alcohols dehydrate under mild conditions
  (moderate temperature and dilute sulphuric acid)

35
28.4 Preparation of Alkenes (SB p.173)
2. Dehydration of Alcohols

• The relative ease of dehydration of alcohols
  generally decreases in the order

36
28.4 Preparation of Alkenes (SB p.173)
2. Dehydration of Alcohols

• Secondary and tertiary alcohols dehydrate to give
  a mixture of alkenes
• The more highly substituted alkene is formed as
  the major product

37
28.4 Preparation of Alkenes (SB p.174)
Addition Reactions
Hydrogenation

• Alkenes can be prepared by hydrogenation of
  alkynes
• ? Depend on the conditions and the catalyst
  employed

38
28.4 Preparation of Alkenes (SB p.174)
Hydrogenation

• Lindlars catalyst is metallic palladium
  deposited on calcium carbonate
• ? further hydrogenation of the alkenes formed
  can be prevented

39
Reactions of Alkenes
40
28.5 Reactions of Alkenes (SB p.174)
Why do Alkenes Undergo Addition Reactions Readily?

• Presence of CC double bond
• CC double bond is made up of a ? bond and a ?
  bond

41
28.5 Reactions of Alkenes (SB p.174)
Why do Alkenes Undergo Addition Reactions Readily?

• In addition reactions,
• ? one ? bond and one ? bond are broken
• ? two ? bonds are formed
• Heat evolved during bond formation gtHeat
  required during bond breaking
• Addition reactions are usually exothermic

42
28.5 Reactions of Alkenes (SB p.174)
Why do Alkenes Undergo Addition Reactions Readily?
43
28.5 Reactions of Alkenes (SB p.174)
Why do Alkenes Undergo Addition Reactions Readily?

• The electrons of the ? bond are
• ? diffuse in shape
• ? less firmly held by the bonding carbon
  nuclei
• Susceptible to the attack by electrophiles

44
28.5 Reactions of Alkenes (SB p.175)
Why do Alkenes Undergo Addition Reactions Readily?

• Electrophiles that attack the CC double bond
• ? protons (H)
• ? neutral species in which the molecule is
  polarized, e.g. bromine

45
28.5 Reactions of Alkenes (SB p.175)
Electrophilic Addition Reactions

• Addition of electrophiles to the CC double bond
  of alkenes

46
28.5 Reactions of Alkenes (SB p.175)
1. Addition of Hydrogen Bromide

• A molecule of HBr adds to the CC double bond of
  an alkene
• Give a bromoalkane

47
28.5 Reactions of Alkenes (SB p.175)
1. Addition of Hydrogen Bromide
e.g. the addition of HBr to ethene produces
bromoethane
48
28.5 Reactions of Alkenes (SB p.175)
1. Addition of Hydrogen Bromide

• When but-2-ene reacts with HBr
• ? 2-bromobutane is formed as the only product

49
28.5 Reactions of Alkenes (SB p.175)
1. Addition of Hydrogen Bromide

• When propene reacts with HBr
• ? the major product is 2-bromopropane
• ? the minor product is 1-bromopropane

50
28.5 Reactions of Alkenes (SB p.176)
Reaction Mechanism Electrophilic Addition
Reactions of Hydrogen Bromide to Alkenes

• Step 1
• The alkene abstracts a proton from hydrogen
  bromide
• ? form a carbocation and a bromide ion

51
28.5 Reactions of Alkenes (SB p.176)
Reaction Mechanism Electrophilic Addition
Reactions of Hydrogen Bromide to Alkenes

• Step 2
• The bromide ion reacts with the carbocation by
  donating an electron pair
• ? a bromoalkane is formed

52
28.5 Reactions of Alkenes (SB p.176)
Regioselectivity of Hydrogen Halide Addition
Markovnikovs Rule
CH3CHCHCH3 is a symmetrical alkene. CH3CHCH2 is
an asymmetrical alkene.
53
28.5 Reactions of Alkenes (SB p.176)
Regioselectivity of Hydrogen Halide Addition
Markovnikovs Rule

• A hydrogen halide can add to an asymmetrical
  alkene in either of the two ways
• The reaction proceeds to give a major product
  preferentially
• ? the reaction is said to exhibit
  regioselectivity

54
28.5 Reactions of Alkenes (SB p.176)
Regioselectivity of Hydrogen Halide Addition
Markovnikovs Rule
55
28.5 Reactions of Alkenes (SB p.177)
Regioselectivity of Hydrogen Halide Addition
Markovnikovs Rule
Markovnikovs rule states that in the addition of
HX to an asymmetrical alkene, the hydrogen atom
adds to the carbon atom of the carbon-carbon
double bond that already has the greater number
of hydrogen atoms
56
28.5 Reactions of Alkenes (SB p.177)
Regioselectivity of Hydrogen Halide Addition
Markovnikovs Rule

• The products formed according to this rule are
  known as Markovnikov products

57
28.5 Reactions of Alkenes (SB p.177)
Stability of Carbocation and Mechanistic
Explanation of the Markovnikovs Rule

• Carbocations are a chemical species that contains
  a positively charged carbon
• Very unstable
• Exist transiently during the reaction
• Classified as primary, secondary or tertiary
• ? according to the number of carbon chains that
  are directly attached to the positively charged
  carbon

58
28.5 Reactions of Alkenes (SB p.178)
Stability of Carbocation and Mechanistic
Explanation of the Markovnikovs Rule

• Carbocations are a reactive intermediate formed
  during the reaction
• ? react to give the product, or
• ? convert back to the reactant
• The more stable the carbocation
• ? the faster its formation

59
28.5 Reactions of Alkenes (SB p.178)
Stability of Carbocation and Mechanistic
Explanation of the Markovnikovs Rule

• The stability of the carbocations increases in
  the order

60
28.5 Reactions of Alkenes (SB p.178)
Stability of Carbocation and Mechanistic
Explanation of the Markovnikovs Rule

• Alkyl groups stabilize the positively charged
  carbocation by positive inductive effect
• A greater number of alkyl groups
• ? release more electrons to the positively
  charged carbon
• ? increase the stability of the carbocation

61
28.5 Reactions of Alkenes (SB p.178)
62
28.5 Reactions of Alkenes (SB p.178)
Stability of Carbocation and Mechanistic
Explanation of the Markovnikovs Rule

• Consider the addition of HBr to propene

63
28.5 Reactions of Alkenes (SB p.178)
Stability of Carbocation and Mechanistic
Explanation of the Markovnikovs Rule

• The hydrobromination of propene involves two
  competing reactions

64
28.5 Reactions of Alkenes (SB p.179)
2. Addition of Halogens

• Halogens normally react with alkenes by
  electrophilic addition

where X2 can be F2, Cl2 or Br2
65
28.5 Reactions of Alkenes (SB p.179)
2. Addition of Halogens

• Alkenes react rapidly with Cl2 (or Br2) in
  1,1,1-trichloroethane at room temp and in the
  absence of light
• Form dichloroalkanes (or dibromoalkanes)

66
28.5 Reactions of Alkenes (SB p.179)
2. Addition of Halogens
67
28.5 Reactions of Alkenes (SB p.179)
2. Addition of Halogens
e.g.
68
28.5 Reactions of Alkenes (SB p.180)
2. Addition of Halogens

• The decolourization of bromine in
  1,1,1-trichloroethane is a useful test for
  unsaturation

69
28.5 Reactions of Alkenes (SB p.180)
3. Addition of Bromine Water (HOBr)

• In an aqueous solution of bromine, the following
  equilibrium is established

70
28.5 Reactions of Alkenes (SB p.180)
3. Addition of Bromine Water (HOBr)

• Bromic(I) acid reacts readily with an alkene at
  room conditions to form a bromohydrin

71
28.5 Reactions of Alkenes (SB p.180)
3. Addition of Bromine Water (HOBr)
e.g.

• The consequent decolourization of the reddish
  brown colour of bromine water is also a test for
  unsaturation

72
28.5 Reactions of Alkenes (SB p.181)
4. Acid-catalyzed Hydration

• Alkenes dissolve in cold and concentrated
  sulphuric acid

73
28.5 Reactions of Alkenes (SB p.181)
4. Acid-catalyzed Hydration
e.g.
74
28.5 Reactions of Alkenes (SB p.181)
4. Acid-catalyzed Hydration

• The presence of the large bulky group (?OSO3H) of
  the alkyl hydrogensulphate makes it very unstable
• Two possible further reactions may take place

75
28.5 Reactions of Alkenes (SB p.181)
1. Regeneration of Alkenes

• On heating, alkyl hydrogensulphates form alkenes
  and sulphuric acid

76
28.5 Reactions of Alkenes (SB p.181)
2. Production of Alcohols

• Alkyl hydrogensulphates can be easily hydrolyzed
  to alcohols by heating with water

77
28.5 Reactions of Alkenes (SB p.182)
Catalytic Hydrogenation

• Alkenes react with hydrogen in the presence of
  metal catalysts (e.g. Ni, Pd, Pt) to give alkanes

78
28.5 Reactions of Alkenes (SB p.182)
Catalytic Hydrogenation
e.g.
79
28.5 Reactions of Alkenes (SB p.182)
Catalytic Hydrogenation

• Useful in analyzing unsaturated hydrocarbons

80
28.5 Reactions of Alkenes (SB p.182)
Catalytic Hydrogenation

• Fats and oils are organic compounds called
  triglycerides
• ? esters formed from glycerol and carboxylic
  acids of long carbon chains
• Fats and oils are either saturated or unsaturated

81
28.5 Reactions of Alkenes (SB p.182)
Catalytic Hydrogenation

• Saturated fats
• ? solids at room temp
• ? usually come from animal sources

82
28.5 Reactions of Alkenes (SB p.182)
Catalytic Hydrogenation

• Unsaturated fats
• ? liquids at room temp
• ? primarily derived from plants

83
28.5 Reactions of Alkenes (SB p.182)
Catalytic Hydrogenation

• Fats and oils are similar in structure
• Only difference is the presence of CC double
  bonds in the acid components of oils
• ? lower their m.p.
• ? make them liquids at room temp

84
28.5 Reactions of Alkenes (SB p.182)
Catalytic Hydrogenation

• Fats are stable towards oxidation by air
• More convenient to handle and store

85
28.5 Reactions of Alkenes (SB p.182)
Catalytic Hydrogenation

• Can be employed to convert the CC double bonds
  present in oils to saturated fats (i.e.
  margarine)
• The conversion is also known as hardening of oils
• Advantage
• ? turning rancid much less readily than
  unsaturated oils

86
28.5 Reactions of Alkenes (SB p.182)
Catalytic Hydrogenation
Hydrogenation of vegetable oils produces margarine
87
28.5 Reactions of Alkenes (SB p.183)
Relative Stability of Alkenes in Terms of
Enthalpy Changes of Hydrogenation

• Hydrogenation of alkenes is exothermic
• From enthalpy changes of hydrogenation
• ? predict the relative stabilities of alkenes

88
28.5 Reactions of Alkenes (SB p.183)
Enthalpy changes of hydrogenation of but-1-ene,
cis-but-2-ene and trans-but-2-ene
89
28.5 Reactions of Alkenes (SB p.184)
Relative Stability of Alkenes in Terms of
Enthalpy Changes of Hydrogenation

• The pattern of the relative stabilities of
  alkenes determined from the enthalpy changes of
  hydrogenation

90
28.5 Reactions of Alkenes (SB p.184)
91
Reactions of Alkenes
Ozonolysis
Ozonolysis is a widely used method for locating
the double bond of an alkene
The unstable ozonide is reduced directly by
treatment with Zn and H2O
92
Reactions of Alkenes
Overall process of ozonolysis
e.g.
93
Reactions of Alkenes
Reaction Mechanism Free Radical Addition
Polymerization of Ethene

• Chain initiation
• The diacyl peroxide molecule undergoes homolytic
  bond fission to generate free radicals

The radical reacts with an ethene molecule to
form a new radical
94
Reactions of Alkenes
2. Chain propagation
95
Reactions of Alkenes
3. Chain termination The radicals react to give
a stable molecule and the reaction stops.
96
Potassium Manganate(VII)
1. Mild Oxidation by Potassium Manganate(VII) Und
er mild oxidation by alkaline KMnO4, alkenes are
oxidized to diols
97
2. Vigorous Oxidation by Potassium Manganate(VII)
Occur in acidic or alkaline medium Heating
is to ensure the vigour of the reaction
98
The END
99
28.2 Nomenclature of Alkenes (SB p.168)
Example 28-2
Give the IUPAC names for the following
alkenes (a)
Answer
(a) trans-3,4-Dichlorohept-3-ene
100
28.2 Nomenclature of Alkenes (SB p.168)
Back
Example 28-2
Give the IUPAC names for the following
alkenes (b)
Answer
(b) cis-3,4-Dimethyloct-3-ene
101
28.2 Nomenclature of Alkenes (SB p.169)
Check Point 28-2
Back
Draw the structural formula for each of the
following alkenes (a) cis-Hex-3-ene (b)
trans-2,3-Dihydroxybut-2-ene (c)
cis-1,2-Dichloroethene
Answer
102
28.4 Preparation of Alkenes (SB p.173)
Example 28-4
Classify the following alcohols as primary,
secondary or tertiary alcohols. (a) CH3CHOHCH2CH3
(b) CH3CH2CH2OH (c) (CH3)2COHCH2CH2CH3
Answer
(a) It is a secondary alcohol. (b) It is a
primary alcohol. (c) It is a tertiary alcohol.
Back
103
28.4 Preparation of Alkenes (SB p.173)
Back
Check Point 28-4
Classify the following haloalkanes as primary,
secondary or tertiary haloalkanes. (a) (c) (
b)
(a) A secondary haloalkane (b) A primary
haloalkane (c) A tertiary haloalkane
Answer
104
28.5 Reactions of Alkenes (SB p.177)
Check Point 28-5A
Of the isomeric C5H11 carbocations, which one is
the most stable?
Answer
Back
105
28.5 Reactions of Alkenes (SB p.179)
Back
Let's Think 1
Both alkanes and alkenes undergo halogenation.
The halogenation of alkanes is a free radical
substitution reaction while the reaction of
alkenes with halogens is an electrophilic
addition reaction. Can you tell two differences
between the products formed by the two different
types of halogenation?
Answer
Alkenes give dihalogenated products while alkanes
usually give polysubstituted products. Another
difference is the position of the attachment of
the halogen atom. For alkenes, the halogen atom
is fixed to the carbon atom of the carboncarbon
double bond. In the substitution reaction of
alkanes, the position of of the halogen atom
varies.
106
28.5 Reactions of Alkenes (SB p.183)
Check Point 28-5B
(a) What chemical tests would you use to
distinguish between two unlabelled bottles
containing hexane and hex-1-ene respectively?
Answer
(a) We can perform either one of the following
tests Hex-1-ene can decolourize bromine water
or chlorine water in the dark while hexane
cannot. Hex-1-ene can decolourize acidified
potassium manganate(VII) solution while hexane
cannot.
107
28.5 Reactions of Alkenes (SB p.183)
Check Point 28-5B

• What is the major product of each of the
  following reactions?
• (i)
• (ii)

Answer
108
28.5 Reactions of Alkenes (SB p.183)
Check Point 28-5B
109
28.5 Reactions of Alkenes (SB p.183)
Check Point 28-5B
Answer
110
28.5 Reactions of Alkenes (SB p.183)
Check Point 28-5B
Back
111
28.5 Reactions of Alkenes (SB p.184)
Check Point 28-5C

• Arrange the following carbocations in increasing
  order of stability. Explain your answer briefly.

Answer
112
28.5 Reactions of Alkenes (SB p.184)
Check Point 28-5C
113
28.5 Reactions of Alkenes (SB p.184)
Check Point 28-5C

• (b) Based on your answer in (a), arrange the
  following molecules in the order of increasing
  rates of reaction with hydrogen chloride.

Answer
114
28.5 Reactions of Alkenes (SB p.184)
Back
Check Point 28-5C