lecture about alkenes and cycloalkenes

1. CHAPTER 3
ALKENES & CYCLOALKENES
CHM258

2. LESSON OUTLINE
1.0 Introduction
✓ General Formula: CnH2n
1.1 Structure and Nomenclature: IUPAC And Common Names
1.2 Methods of Preparation (Alkenes):
✓ Dehydrohalogenation of alkyl halides
✓ Dehalogenation of dihalides
✓ Reduction of alkynes
1.3 Reactions of Alkenes And Cycloalkenes
✓ Addition of symmetric reagents: H2 and X2
✓ Markovnikov and anti-Markovnikov rule.
✓ Addition of unsymmetrical reagents: HX and H2O/H2SO4
✓ Oxidation : Combustion, ozonolysis and diol hydroxylation (KMnO4/H+ cold only)
1.4 Simple Test of Alkenes
1.5 Uses

3. 1.0 INTRODUCTION
ALKENE
Unsaturated
HC
Also known
as olefins
Contain a C=C
bond
Terminal
C=C at the
end
Internal
C=C not at
the end
Cycloalkenes
C=C in a cyclic
structure
sp2
hybridization
Trigonal
planar (120°)
General Formula (CnH2n)
Example
CH2CH2
→ C2H2(2): C2H4
So, CH2CH2 is an alkene which is
ethene

4. 1.1 NOMENCLATURE
Parent chain
→Longest continuous
carbon chain
→ Must include C=C
Numbering
→C=C must at lowest
carbon number
Locate and name
substituents
Arrange
→Alphabetical order

5. BASIC
ALKENES
ALKENOLS
CIS-
TRANS
ALKENES
1.1 NOMENCLATURE

6. ALKENES

7. RULE 1
Find longest continuous carbon chain that must includes a carbon
double bond
Longest Continuous Carbon Chain

8. RULE 1
Longest carbon chain must contain both atoms of the double bond.

9. RULE 2
Double bond must at the lowest
carbon number
Numbering

10. RULE 3
Identify and numbered all the substituent present
Locate and name substituents

11. RULE 4 Alphabetical order
Have 3 methyl group
at C2, C3 & C5
✓ Use prefix
2,3,5-trimethyl-2-hexene or 2,3,5-trimethylhex-2-ene

12. COMMON RULES
Compounds with two double bonds;
✓dienes by changing the “-ane” ending of the parent alkane
to the suffix “–adiene”.
Compounds with three double bonds;
✓atrienes, and so forth.
CH2=CH-CH=CH2 CH2=CH-CH=CH-CH=CH2
1,3-butadiene 1,3,5-hexatriene

13. ALKENOLS
The chain (or ring) is numbered to give the OH group the lower
number.
Double bond and a Hydroxyl group

14. CIS &TRANS ISOMER

15. CYCLOALKENES

16. RULE 1
Find longest continuous carbon chain that must includes a carbon
double bond
Longest Continuous Carbon Chain (must have “=”)
3
5
1
6 4
2
B
r

17. Double bond: C1 and C2.
If substituent at double
bond,
➢ Substituent : C1
RULE 2
1
3
5
4 2
6
Double bond: C1 and C2.
If have > 1, double bond,
➢ Double Bond :
Lowest Carbon
Number
Numbering
3
5
1
6 4
2

18. RULE 3
Identify and numbered all the substituent present
Locate and name substituents
3
5
1
6 4
2 ✓ 1-tert-butyl
✓ 3-isopropyl or 3-sec-propyl
✓ 5-methyl

19. RULE 4
Alphabetical order
3
5
1
6 4
2 ✓ 1-tert-butyl
✓ 3-isopropyl or 3-sec-propyl
✓ 5-methyl
1-tert-butyl-3-isopropyl-5-methylcyclohexa-1,3-dienes
@
1-tert-butyl-5-methyl-3-sec-propylcyclohexa-1,3-dienes

20. Exercise
2,3,4-tribromo-penta-2,3-diene
2-chlorobutan-2-ene
Cis-6-ethyl-2-methylcyclohexa-1,3-diene
2-bromo-3-chloro-5-iodocyclopenta-1,3-diene

21. 1.2 METHOD OF PREPARATION
(ALKENES)
I) Dehydrohalogenation
of alkyl halides
(-HX)
II) Dehalogenation of
dihalides
(-X2)
III) Reduction of
alkynes

22. I) DEHYDROHALOGENATION OF
ALKYL HALIDE
Elimination of hydrogen and halogen from an alkyl halides to form alkene

23. SAYTZEFF RULE
Elimination product that favor formation of an alkene with greatest
number of substituents attached to the C=C group.
CH3CH-CH-CH2
Br
H H
KOH CH3CH=CH-CH3 CH3CH2CH=CH2
alcohol
reflux
2-bromobutane
2-butene
(major product)
1-butene

24. EXAMPLE
BACK

25. II) DEHALOGENATION OF
DIHALIDES
Elimination of two vicinal halide from an alkyl dihalides to form alkene
BACK

26. III) REDUCTION OF ALKYNES
i. Alkynes to cis-alkenes
ii. Alkynes to trans-alkenes

27. EXAMPLE
BACK

28. 1.3 REACTION OF ALKENES
ADDITION
I) Hydrogenation
II) Halogenation
III) Hydration
IV) Hydrohalogenation
OXIDATION
I) Combustion
II) Ozonolysis
III) Hydroxylation

29. ADDITION REACTION
• A reaction in which an unsaturated molecule becomes saturated by
the addition of a molecule across a multiple bond (C=C in alkenes,
-C≡C- in alkynes, C=O in aldehydes and ketones)
• Characteristics:
✓the π bond of the double bond is broken and two single bonds
are formed
✓only one product is obtained at the end of the reaction
✓the product obtained is a saturated organic compound

30. ADDITION REACTION
SYMMETRICAL
REAGENT
I) HYDROGENATION II) HALOGENATION
UNSYMMETRICAL
REAGENT
III) HYDRO-
HALOGENATION
IV) HYDRATION

31. Addition Of
Symmetrical
Reagent

32. I) HYDROGENATION
Hydrogen in the present of catalyst (Pt, Ni or Pd) will be
added to double bond to form an alkanes
Ni, Pt or Pd
Addition of hydrogen to a double bond

33. II) HALOGENATION
Addition of halogen (X2) to a double bond
✓ Alkenes react with halogens at room temperature and in dark.
✓ Halogens is usually dissolved in an inert solvent such as
dichloromethane (CH2Cl2) and tetrachloromethane (CCl4).

34. EXAMPLES

35. Addition of
Unsymmetrical
Reagent

36. STABILITY OF CARBOCATION
The greater the number of alkyl groups on
the carbon atom, the greater its stability

37. MARKOVNIKOV RULES
H atom will be added to the double bond that has
more hydrogen attached to it.
The more electropositive atom of the reagent will be added to the carbon that
has more H atoms.

38. III) HYDRATION
Reagent : H2SO4 or H3PO4 , 300°C
Addition of water (H2O) to a double bond.

39. EXAMPLES

40. IV) HYDROHALOGENATION
Addition of hydrogen halides (HX) to a double bond

41. EXAMPLES

42. ANTI-MARKOVNIKOV RULES
Addition of hydrogen bromide (HBr) to a double bond with
presence of peroxide or H2O2 or ROOR.

43. SUMMARY

44. CYCLOALKENES

45. H2, Pt
X2, CCl4
X2, H2O H-X
H2O,
H2SO4 H Br, ROOR
X
H
H
Br
OH
H
OH
X
X
X
H
H
EXAMPLES

46. OXIDATION REACTION
COMBUSTION OZONOLYSIS
HYDROXYLATION
(reactions that form carbon-oxygen bonds)

47. I) COMBUSTION
Alkenes burn with a more sooty flame than alkanes
1. The double bond means there is less hydrogen to combine
with oxygen.
2. Some carbon will not be oxidized and will produce carbon in the
form of soot.
3. Hence, alkenes are not used a fuels
Reaction with oxygen (O2) to produce carbon dioxide ( CO2) and water (H2O)
C2H4 + 3O2 → 2CO2 + 2H2O

48. II) OZONOLYSIS
C C
R
R
R'
H
O3 C
O O
C
O R'
H
R
R
(CH3)2S
C O
R
R
C
O
R'
H
ozonide ketone aldehyde
or H2O, Zn/H+
Reaction with ozone (O3) to produce ketone and/or aldehyde
Reagent
✓O3, (CH3)2S or H2O, Zinc/ H+ or Zinc dust

49. EXAMPLE

50. III) HYDROXYLATION
Reagent
✓cold, dilute, alkaline potassium permanganate (KMnO4)
✓known as Baeyer’s reagent
Reaction with cold, dilute KMnO4, -OH to produce a diol

51. EXAMPLES
CH CH2
CH3
CH2 CH2 CH2 CH2
OH OH
CH2
OH
CH
CH3
OH
MnO2
MnO2
KMnO4 (aq), OH-
cold, dilute
ethene
1,2-ethanediol
KMnO4 (aq), OH-
cold, dilute
propene
1,2-propanediol

52. 1.4 SIMPLE TEST FOR ALKENES
BAEYER’S TEST
Cold, dilute, alkaline
KMnO4
BROMINE TEST
Bromine in inert solution

53. BAEYER’S TEST
• Test : Baeyer’s test
• Reagent and : Cold, dilute, KMnO4 solution, H+ or OH-
condition
• Observation : i) Alkane : Purple color of KMnO4 remain unchanged
: ii) Alkene : Purple color of KMnO4 decolorized and formation of
brown precipitate of manganese (IV) oxide, MnO2
C C C C
OH OH
MnO2
KMnO4 (aq), OH-
cold, dilute
a diol / glycol brown precipitate

54. BROMINE TEST
• Test : Bromine test
• Reagent and : Bromine in inert solvent (CCl4 or CH2Cl2) condition
• Observation : i) Alkane : Brown/ yellow color of bromine remain unchanged
: ii) Alkene : Brown/ yellow color of bromine decolorized

55. 1.5 USES
• Ethylene and propylene are the largest-volume industrial organic chemicals.
• Used to synthesis a wide variety of useful compounds.

56. POLYETHENE (PE)

57. POLYVINYL CHLORIDE (PVC)
1. Clothing :
✓ Waterproof : coats,
shoes, jackets,
aprons and bags
2. Insulator for electric
wire
3. Pipe
4. Composite for
accessories or housing
portable electronics

58. THE
END