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Alkenes

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Chapter 30 Alkenes 30.1 Introduction 30.2 Nomenclature of Alkenes 30.3 Physical Properties of Alkenes 30.4 Preparation of Alkenes 30.5 Reactions of Alkenes – PowerPoint PPT presentation

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Title: Alkenes


1
Chapter 30
Alkenes
30.1 Introduction 30.2 Nomenclature of
Alkenes 30.3 Physical Properties of
Alkenes 30.4 Preparation of Alkenes 30.5 Reactions
of Alkenes
2
30.1 Introduction (SB p.122)
Functional group of alkenes
3
30.1 Introduction (SB p.122)
Alkenes show geometrical isomerism
4
30.2 Nomenclature of Alkenes (SB p.123)
Nomenclature of Alkenes
1. Determine the stem name by selecting the
longest possible straight chain containing the C
C double bond and use the ending -ene
2. Number the parent chain so as to include both
carbon atoms of the double bond, and begin
numbering with the end of the chain nearer the C
C double bond 3. Designate the position of the
C C double bond by using the number of the
first atom of the double bond 4. Designate the
positions of the substituents by using the
numbers obtained by application of rule 2
5
30.2 Nomenclature of Alkenes (SB p.123)
Examples
6
30.2 Nomenclature of Alkenes (SB p.123)
5. If two identical groups are present on the
same side of the C C double bond, the compound
is designated as cis if they are on opposite
sides, the compound is designated as trans. e.g.
7
30.2 Nomenclature of Alkenes (SB p.124)
Example 30-1 Give the IUPAC names for the
following alkenes (a) (b)
Answer
Solution (a) trans-3,4-dichlorohept-3-ene (b) cis
-3,4-dimethyloct-3-ene
8
30.2 Nomenclature of Alkenes (SB p.124)
Check Point 30-1 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-dichlo
roethene
Answer
9
30.3 Physical Properties of Alkenes (SB p.124)
Name Formula Boiling point (C) Melting point (C) Density at 20 C (g cm-3)
Ethene CH2 CH2 -104 -169
Propene CH3CH CH2 -47.7 -185 0.514
But-1-ene CH3CH2CH CH2 -6.3 -185 0.595
Pent-1-ene CH3(CH2)2CH CH2 30 -165 0.641
Hex-1-ene CH3(CH2)2CH CH2 62.9 -140 0.673
cis-But-2-ene CH3CH CHCH3 (cis) 4 -139 0.621
trans-But-2-ene CH3CH CHCH3 (trans) 1 -106 0.604
2-Methylbut-1-ene CH3CH3C(CH3) CH2 31 -138 0.650
10
30.4 Preparation of Alkenes (SB p.125)
Cracking
  • alkenes can be prepared industrially by cracking
    of high molecular mass alkanes

11
30.4 Preparation of Alkenes (SB p.125)
Elimination Reactions
Dehydrohalogenation
  • Dehydrohalogenation is the elimination of a
    hydrogen halide molecule from a haloalkane

12
30.4 Preparation of Alkenes (SB p.125)
Examples
13
30.4 Preparation of Alkenes (SB p.126)
The ease of dehydrohalogenation of haloalkanes
decreases in the order
14
30.4 Preparation of Alkenes (SB p.126)
Dehydrohalogenation of 2 and 3 haloalkanes can
take place in more than one way and a mixture of
alkenes is formed
alc. KOH CH3CH2CHClCH3 ???? CH3CH CHCH3
CH3CH2CH CH2 heat 2-chlorobutane But-2-ene
But-1-ene (80) (20) Note the more highly
substituted alkene is formed as major product
15
30.4 Preparation of Alkenes (SB p.127)
The relative stabilities of alkenes decrease in
the order
16
30.4 Preparation of Alkenes (SB p.125)
Dehydration of Alcohols
Dehydration is the removal of a water molecule
from a reactant molecule
17
30.4 Preparation of Alkenes (SB p.125)
The experimental conditions of dehydration depend
on the structures of alcohols e.g.
18
30.4 Preparation of Alkenes (SB p.125)
The relative ease of dehydration of alcohols
generally decreases in the order
Like dehydrohalogenation, the more highly
substituted alkene is formed as the major product
19
30.4 Preparation of Alkenes (SB p.128)
Example 30-2 Classify the following alcohols as
primary, secondary or tertiary alcohols. (a) CH3CH
OHCH2CH3 (b) CH3CH2CH2OH (c) (CH3)2COHCH2CH2CH3
Answer
Solution (a) Secondary alcohol (b) Primary
alcohol (c) Tertiary alcohol
20
30.4 Preparation of Alkenes (SB p.128)
Check Point 30-2 Classify the following
haloalkanes as primary, secondary or tertiary
haloalkanes. (a) (b) (c)
Answer
21
30.4 Preparation of Alkenes (SB p.129)
Addition Reactions
Hydrogenation
  • hydrogenation of alkynes using Lindlars catalyst
    produces alkenes
  • prevent further hydrogenation of the alkenes
    formed to alkanes

22
30.5 Reactions of Alkenes (SB p.129)
Alkenes are more reactive than alkanes Reason
presence of the C C double bond
Energetically favourable!!
? alkenes undergo addition reactions and the
reactions are exothermic
23
30.5 Reactions of Alkenes (SB p.129)
Electrons of ? bond are more diffuse and less
firmly held ? susceptible to attack by
electrophiles Electrophiles such as H, neutral
reagents such as bromine (can be polarized) react
with C C double bond
24
30.5 Reactions of Alkenes (SB p.130)
Electrophilic Addition Reactions
Addition of Hydrogen Bromide
Addition of hydrogen bromide to C C double bond
yields a bromoalkane
25
30.5 Reactions of Alkenes (SB p.130)
Examples
26
30.5 Reactions of Alkenes (SB p.131)
Propene reacts with HBr to give 2-bromopropane
(major product) and 1-bromopropane (minor product)
The formation of two possible products can be
explained by the reaction mechanism.
27
30.5 Reactions of Alkenes (SB p.131)
Reaction Mechanism Electrophilic Addition
Reaction of Hydrogen Bromide to Alkenes
The mechanism for the addition of HBr to an
alkene involves 2 steps Step 1
Step 2
28
30.5 Reactions of Alkenes (SB p.131)
Theoretical Explanation of Markownikoffs Rule
If the alkene is unsymmetrical, two different
carbocations can be formed
29
30.5 Reactions of Alkenes (SB p.132)
2-bromopropane is the major product because the
more stable secondary carbocation is formed in
the first step
30
30.5 Reactions of Alkenes (SB p.132)
Markownikoffs rule states that in the addition
of HX to an unsymmetrical alkene, the hydrogen
atom adds to the carbon atom of the carbon-carbon
double bond that already has the greater number
of hydrogen atoms.
Example
31
30.5 Reactions of Alkenes (SB p.132)
Markownikoffs rule is related to the stability
of the carbocation intermediate formed in the
electrophilic addition reaction. The relative
stabilities of carbocations
32
30.5 Reactions of Alkenes (SB p.133)
Addition of Bromine
Alkenes react rapidly with Br2 in
1,1,1-trichloroethane at room temperature and in
the absence of light
e.g.
33
30.5 Reactions of Alkenes (SB p.133)
The behaviour of alkenes towards Br2 in CH3CCl3
is a useful test for the presence of
carbon-carbon multiple bonds
34
30.5 Reactions of Alkenes (SB p.133)
Addition of Bromine Water
In an aqueous solution of Br2, the following
equilibrium exists
The bromine atom bears a partial positive charge
while the oxygen atom bears a partial negative
charge ? oxygen is more electronegative than
bromine
35
30.5 Reactions of Alkenes (SB p.134)
When bromic(I) acid reacts with alkenes,
bromohydrin is formed
e.g.
36
30.5 Reactions of Alkenes (SB p.134)
Addition of Sulphuric(VI) Acid
Alkenes react with cold and concentrated H2SO4 to
form alkyl hydrogensulphates
e.g.
37
30.5 Reactions of Alkenes (SB p.134)
The large bulky OSO3H group makes the alkyl
hydrogensulphate very unstable. Two possible
further reactions take place 1. Regeneration of
alkenes
2. Production of alcohols
38
30.5 Reactions of Alkenes (SB p.135)
Catalytic Hydrogenation
In the presence of metal catalysts (e.g. Pt, Pd
or Ni), H2 is added to each atom of C C double
bond to form an alkane
e.g.
39
30.5 Reactions of Alkenes (SB p.135)
  • Hydrogenation is useful in analyzing unsaturated
    hydrocarbons
  • The number of double or triple bonds present in
    the unsaturated hydrocarbon molecule can be
    deduced by the number of moles of hydrogen reacted
  • Catalytic hydrogenation is used to convert liquid
    vegetable oil to semi-solid fats in making
    margarine and solid cooking fats (known as
    hardening of oils).

40
30.5 Reaction of Alkenes (SB p.136)
Check Point 30-3 (a) What chemical tests would
you use to distinguish between two unlabelled
bottles containing hexane and hex-1-ene
respectively?
Answer
41
30.5 Reaction of Alkenes (SB p.136)
Check Point 30-3 (b) What is the major product
of each of the following reactions? (i) (ii)
Answer
42
30.5 Reaction of Alkenes (SB p.136)
Check Point 30-3 (c) Give the reaction products
for the following reactions Ni (i) CH3CH
CH2 H2 ?? conc. H2SO4 (ii) CH3CH CHCH3
?????? (iii) CH3CH CHCH3 Br2 ??
Answer
43
30.5 Reaction of Alkenes (SB p.136)
Check Point 30-3 (d) Arrange the following
carbocations in increasing order of stability.
Explain your answer briefly.
Answer
44
30.5 Reaction of Alkenes (SB p.136)
Check Point 30-3 (e) Based on your answer in
(d), arrange the following molecules in the
order of increasing rate of reaction with
hydrogen chloride.
Answer
45
30.5 Reactions of Alkenes (SB p.136)
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
46
30.5 Reactions of Alkenes (SB p.137)
Overall process of ozonolysis
e.g.
47
30.5 Reaction of Alkenes (SB p.137)
Example 30-3 Predict the structures of the
following hydrocarbons A, B and C using the
information given below
Hydrocarbon Molecular formula Products after ozonolysis
A C3H6
B C6H10
C C10H16
Answer
48
30.5 Reaction of Alkenes (SB p.138)
Solution A As C3H6 can be expressed as CnH2n,
the hydrocarbon is a molecule with one C C
double bond. When A undergoes ozonolysis,
and are formed. ?The possible
structure of A is CH3CH CH2.
49
30.5 Reaction of Alkenes (SB p.138)
Solution B As C6H10 can be expressed as CnH2n-2
and only one dicarbonyl compound is formed on
ozonolysis, the hydrocarbon is an alicyclic
molecule with one C C double bond. ? The
possible structure of B is .
50
30.5 Reaction of Alkenes (SB p.138)
Solution C As C10H16 can be expressed as
CnH2n-4. Two products with totally five carbon
atoms are formed. So the original compound is an
acyclic molecule with three C C double
bonds. ? The possible structure of C is
CH3CH CHCH2CH CHCH2CH CHCH3.
51
30.5 Reaction of Alkenes (SB p.139)
Check Point 30-4 Draw the structures of the
alkene molecules that give the following products
on ozonolysis. (a) CH3CH2CH2CHO and
CH3CHO (b) CH3CH2CHO and CH3COCH3
Answer
52
30.5 Reactions of Alkenes (SB p.139)
Polymerization
Polymers Compounds that consist of very large
molecules made up of many repeating
units Monomer Each repeating unit Polymerization
The reaction by which monomers are joined
together Addition polymerization alkene
monomers are joined together without the
elimination of small molecules Addition
polymer The polymer produced by addition
polymerization
53
30.5 Reactions of Alkenes (SB p.139)
Poly(ethene)
Monomer ethene Depending on the conditions, two
kinds of poly(ethene) are formed
54
30.5 Reactions of Alkenes (SB p.139)
  • Low density poly(ethene) (LDPE)
  • Molecular mass 50 000 to 3 000 000
  • Light, flexible and low melting temperature
  • Uses make soft items like wash bottles, plastic
    bags and food wraps
  • High density poly(ethene) (HDPE)
  • Molecular mass up to 3 000 000
  • Tougher and higher melting temperature
  • Uses make more rigid items like milk bottles and
    water buckets

55
30.5 Reactions of Alkenes (SB p.139)
Some products made of poly(ethene)
56
30.5 Reactions of Alkenes (SB p.140)
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
57
30.5 Reactions of Alkenes (SB p.140)
2. Chain propagation
58
30.5 Reactions of Alkenes (SB p.140)
3. Chain termination The radicals react to give
a stable molecule and the reaction stops.
59
30.5 Reactions of Alkenes (SB p.141)
Poly(propene)
Properties more rigid than HDPE, high
mechnical strength, strong resistance to
abrasion Uses make moulded furniture make
crates, kitchenware, food containers make
ropes and hard-wearing carpets
60
30.5 Reactions of Alkenes (SB p.141)
Poly(phenylethene) (or Polystyrene)
Preparation of monomer (phenylethene)
Formation of poly(phenylethene)
61
30.5 Reactions of Alkenes (SB p.142)
  • Poly(phenylethene)
  • Properties
  • Transparent, brittle and chemically inert
  • Uses
  • Make toys, specimen containers and cassette
    cases

62
30.5 Reactions of Alkenes (SB p.142)
  • Expanded poly(phenylethene)
  • Properties
  • Extremely light, while solid foam
  • Uses
  • Make light-weight ceiling tiles in buildings,
    food boxes and shock absorbers for packaging

63
30.5 Reactions of Alkenes (SB p.142)
Name and structural formula of monomer Name and structural formula of polymer Uses
Ethene CH2 CH2 Poly(ethene) LDPE plastic bags, wash bottles, food wraps, pipes and tubing
Ethene CH2 CH2 Poly(ethene) HDPE tough bottles and jugs, buckets, washing-up trays, toys, pipes and tubing
Propene CH2 CHCH3 Poly(propene) Moulded furniture, crates, kitchenware, food containers, fibres for making ropes and hard-wearing carpets and athletic wear
Phenylethene CH2 CHC6H5 Poly(phenylethene) Poly(phenylethene) moulded objects (combs, toys, cups, brush and pot handles), refrigerator parts and insulating materials
Phenylethene CH2 CHC6H5 Poly(phenylethene) Expanded poly(phenylethene) good shock absorbents in packaging, light-weight ceiling tiles in building, disposable foam cups and food boxes
64
The END
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