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. The C = C double bond in ethene
30.1 Introduction (SB p.122)
Functional group of alkenes:
The C = C double bond in ethene

3. Alkenes show geometrical isomerism
30.1 Introduction (SB p.122)
Alkenes show geometrical isomerism

4. Nomenclature of Alkenes
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. Example 30-1 Give the IUPAC names for the following alkenes: (a) (b)
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-dichloroethene
(a)
(b)
(c)
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
0.595
Pent-1-ene
CH3(CH2)2CH = CH2 30
-165
0.641
Hex-1-ene
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. Elimination Reactions
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. The ease of dehydrohalogenation of haloalkanes decreases in the order:
30.4 Preparation of Alkenes (SB p.126)
The ease of dehydrohalogenation of haloalkanes decreases in the order:
tertiary > secondary > primary haloalkane haloalkane haloalkane

14. Note: the more highly substituted alkene is formed as major product
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. The relative stabilities of alkenes decrease in the order:
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:
tertiary > secondary > primary alcohol alcohol alcohol
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) CH3CHOHCH2CH3
(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)
(a) Secondary haloalkane
(b) Primary haloalkane
(c) Tertiary haloalkane
Answer

21. hydrogenation of alkynes using Lindlar’s catalyst produces alkenes
30.4 Preparation of Alkenes (SB p.129)
Addition Reactions
Hydrogenation
hydrogenation of alkynes using Lindlar’s catalyst produces alkenes
prevent further hydrogenation of the alkenes formed to alkanes

22. Energetically favourable!!
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. Electrophilic Addition Reactions
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. The mechanism for the addition of HBr to an alkene involves 2 steps
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. Theoretical Explanation of Markownikoff’s Rule
30.5 Reactions of Alkenes (SB p.131)
Theoretical Explanation of Markownikoff’s 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)
Markownikoff’s 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. The relative stabilities of carbocations:
30.5 Reactions of Alkenes (SB p.132)
Markownikoff’s 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
Add Br2 in CH3CCl3 to excess alkene
The reddish brown colour of Br2 is decolourized

34. In an aqueous solution of Br2, the following equilibrium exists
30.5 Reactions of Alkenes (SB p.133)
Addition of Bromine Water
In an aqueous solution of Br2, the following equilibrium exists
Br2 + H2O HBr + HOBr Bromic(I) acid
The bromine atom bears a partial positive charge while the oxygen atom bears a partial negative charge ∵ oxygen is more electronegative than bromine

35. When bromic(I) acid reacts with alkenes, bromohydrin is formed
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. 1. Regeneration of alkenes
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. Catalytic Hydrogenation
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. Hydrogenation is useful in analyzing unsaturated hydrocarbons
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
(a) 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.

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)
(b) (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 
(c) (i) CH3CH2CH3
(ii)
(iii)
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.
(d) The increasing order of stability of carbocations is :
Tertiary carbocations are the most stable because the three alkyl groups release electrons to the positive carbon atom and thereby disperse its charge. Primary carbocations are the least stable as there is only one alkyl group releasing electrons to the positive carbon atom.
Answer

44. 30.5 Reaction of Alkenes (SB p.136)
(e) Upon the reaction with hydrogen chloride, it involves the formation of carbocations. Therefore, the order of reaction rates follows the order of the ease of the formation of carbocations, i.e. the stability of carbocations:
Therefore, the rates of reactions of the three compounds with hydrogen chloride increase in the order:
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. The unstable ozonide is reduced directly by treatment with Zn and H2O
30.5 Reactions of Alkenes (SB p.136)
Ozonolysis
Ozonolysis is a widely used method for locating the double bond of an alkene
(unstable)
The unstable ozonide is reduced directly by treatment with Zn and H2O

46. Overall process of ozonolysis:
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
(a)
(b)
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. Depending on the conditions, two kinds of poly(ethene) are formed
30.5 Reactions of Alkenes (SB p.139)
Poly(ethene)
Monomer: ethene
Depending on the conditions, two kinds of poly(ethene) are formed

54. Low density poly(ethene) (LDPE): Molecular mass: 50 000 to 3 000 000
30.5 Reactions of Alkenes (SB p.139)
Low density poly(ethene) (LDPE):
Molecular mass: to
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
Tougher and higher melting temperature
Uses: make more rigid items like milk bottles and water buckets

55. Some products made of poly(ethene)
30.5 Reactions of Alkenes (SB p.139)
Some products made of poly(ethene)

56. Reaction Mechanism: Free Radical Addition Polymerization of Ethene
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. The radicals react to give a stable molecule and the reaction stops.
30.5 Reactions of Alkenes (SB p.140)
3. Chain termination
The radicals react to give a stable molecule and the reaction stops.

59. The helmet is made of poly(propene)
30.5 Reactions of Alkenes (SB p.141)
Poly(propene)
The helmet is made of 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. Poly(phenylethene) (or Polystyrene)
30.5 Reactions of Alkenes (SB p.141)
Poly(phenylethene) (or Polystyrene)
Preparation of monomer (phenylethene):
Formation of poly(phenylethene):

61. Transparent, brittle and chemically inert Uses:
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. Expanded poly(phenylethene): Properties:
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
Some products made of expanded poly(phenylethene)

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
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
Expanded poly(phenylethene): good shock absorbents in packaging, light-weight ceiling tiles in building, disposable foam cups and food boxes

64. The END