Upload
jewel-hilda-james
View
234
Download
3
Tags:
Embed Size (px)
Citation preview
New Way Chemistry for Hong Kong A-Level Book 3A
1
Alkenes
30.130.1 IntroductionIntroduction
30.230.2 Nomenclature of AlkenesNomenclature of Alkenes
30.330.3 Physical Properties of AlkenesPhysical Properties of Alkenes
30.430.4 Preparation of AlkenesPreparation of Alkenes
30.530.5 Reactions of AlkenesReactions of Alkenes
Chapter 30Chapter 30
New Way Chemistry for Hong Kong A-Level Book 3A
2
30.1 Introduction (SB p.122)
Functional group of alkenes:
The C = C double bond in ethene
New Way Chemistry for Hong Kong A-Level Book 3A
3
Alkenes show geometrical isomerism
30.1 Introduction (SB p.122)
New Way Chemistry for Hong Kong A-Level Book 3A
4
30.2 Nomenclature of Alkenes (SB p.123)
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
Nomenclature of AlkenesNomenclature of Alkenes
New Way Chemistry for Hong Kong A-Level Book 3A
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.
New Way Chemistry for Hong Kong A-Level Book 3A
7
Example 30-1Example 30-1Give the IUPAC names for the following alkenes:
(a)
(b)
Answer
30.2 Nomenclature of Alkenes (SB p.124)
Solution:
(a) trans-3,4-dichlorohept-3-ene
(b) cis-3,4-dimethyloct-3-ene
New Way Chemistry for Hong Kong A-Level Book 3A
8
Check Point 30-1 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 Answer
30.2 Nomenclature of Alkenes (SB p.124)
(a)
(b)
(c)
New Way Chemistry for Hong Kong A-Level Book 3A
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
New Way Chemistry for Hong Kong A-Level Book 3A
10
30.4 Preparation of Alkenes (SB p.125)
• alkenes can be prepared industrially by cracking of high molecular mass alkanes
CrackingCracking
422C600
33 CH2CHCH CHCH2
242223C600
323 HCHCHCHCHCHCH CHCHCH2
New Way Chemistry for Hong Kong A-Level Book 3A
11
30.4 Preparation of Alkenes (SB p.125)
• Dehydrohalogenation is the elimination of a hydrogen halide molecule from a haloalkane
Elimination ReactionsElimination Reactions
Dehydrohalogenation
New Way Chemistry for Hong Kong A-Level Book 3A
13
30.4 Preparation of Alkenes (SB p.126)
The ease of dehydrohalogenation of haloalkanes decreases in the order:
tertiary > secondary > primaryhaloalkane haloalkane haloalkane
New Way Chemistry for Hong Kong A-Level Book 3A
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. KOHCH3CH2CHClCH3 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
New Way Chemistry for Hong Kong A-Level Book 3A
15
30.4 Preparation of Alkenes (SB p.127)
The relative stabilities of alkenes decrease in the order:
New Way Chemistry for Hong Kong A-Level Book 3A
16
30.4 Preparation of Alkenes (SB p.125)
Dehydration is the removal of a water molecule from a reactant molecule
Dehydration of Alcohols
New Way Chemistry for Hong Kong A-Level Book 3A
17
30.4 Preparation of Alkenes (SB p.125)
The experimental conditions of dehydration depend on the structures of alcohols
e.g.
New Way Chemistry for Hong Kong A-Level Book 3A
18
30.4 Preparation of Alkenes (SB p.125)
The relative ease of dehydration of alcohols generally decreases in the order:
tertiary > secondary > primaryalcohol alcohol alcohol
Like dehydrohalogenation, the more highly substituted alkene is formed as the major product
New Way Chemistry for Hong Kong A-Level Book 3A
19
Example 30-2Example 30-2Classify the following alcohols as primary, secondary or tertiary alcohols.
(a) CH3CHOHCH2CH3
(b) CH3CH2CH2OH
(c) (CH3)2COHCH2CH2CH3 AnswerSolution:
(a) Secondary alcohol
(b) Primary alcohol
(c) Tertiary alcohol
30.4 Preparation of Alkenes (SB p.128)
New Way Chemistry for Hong Kong A-Level Book 3A
20
Check Point 30-2 Check Point 30-2
Classify the following haloalkanes as primary, secondary or tertiary haloalkanes.
(a)
(b)
(c)
Answer
30.4 Preparation of Alkenes (SB p.128)
(a) Secondary haloalkane
(b) Primary haloalkane
(c) Tertiary haloalkane
New Way Chemistry for Hong Kong A-Level Book 3A
21
30.4 Preparation of Alkenes (SB p.129)
• hydrogenation of alkynes using Lindlar’s catalyst produces alkenes
• prevent further hydrogenation of the alkenes formed to alkanes
Addition ReactionsAddition Reactions
Hydrogenation
New Way Chemistry for Hong Kong A-Level Book 3A
22
30.5 Reactions of Alkenes (SB p.129)
Alkenes are more reactive than alkanes
Reason: presence of the C = C double bond
alkenes undergo addition reactions and the reactions are exothermic
Energetically favourable!!
New Way Chemistry for Hong Kong A-Level Book 3A
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
New Way Chemistry for Hong Kong A-Level Book 3A
24
Addition of hydrogen bromide to C = C double bond yields a bromoalkane
Electrophilic Addition ReactionsElectrophilic Addition Reactions
Addition of Hydrogen Bromide
30.5 Reactions of Alkenes (SB p.130)
New Way Chemistry for Hong Kong A-Level Book 3A
26
Propene reacts with HBr to give 2-bromopropane (majo
r product) and 1-bromopropane (minor product)
30.5 Reactions of Alkenes (SB p.131)
The formation of two possible products can be explained
by the reaction mechanism.
New Way Chemistry for Hong Kong A-Level Book 3A
27
The mechanism for the addition of HBr to an alkene involves 2 steps
Step 1:
30.5 Reactions of Alkenes (SB p.131)
Reaction Mechanism: Electrophilic Addition Reaction of Hydrogen Bromide to Alkenes
Step 2:
New Way Chemistry for Hong Kong A-Level Book 3A
28
If the alkene is unsymmetrical, two different carbocations
can be formed
30.5 Reactions of Alkenes (SB p.131)
Theoretical Explanation of Markownikoff’s Rule
New Way Chemistry for Hong Kong A-Level Book 3A
29
2-bromopropane is the major product because the more stable
secondary carbocation is formed in the first step
30.5 Reactions of Alkenes (SB p.132)
New Way Chemistry for Hong Kong A-Level Book 3A
30
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.
30.5 Reactions of Alkenes (SB p.132)
Example:
New Way Chemistry for Hong Kong A-Level Book 3A
31
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:
New Way Chemistry for Hong Kong A-Level Book 3A
32
Alkenes react rapidly with Br2 in 1,1,1-trichloroethane
at room temperature and in the absence of light
Addition of Bromine
30.5 Reactions of Alkenes (SB p.133)
e.g.
New Way Chemistry for Hong Kong A-Level Book 3A
33
The behaviour of alkenes towards Br2 in CH3CCl3 is a useful
test for the presence of carbon-carbon multiple bonds
30.5 Reactions of Alkenes (SB p.133)
Add Br2 in CH3CCl3 to excess alkene
The reddish brown colour of Br2 is decolourized
New Way Chemistry for Hong Kong A-Level Book 3A
34
In an aqueous solution of Br2, the following equilibrium exists
Addition of Bromine Water
30.5 Reactions of Alkenes (SB p.133)
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
New Way Chemistry for Hong Kong A-Level Book 3A
35
e.g.
30.5 Reactions of Alkenes (SB p.134)
When bromic(I) acid reacts with alkenes, bromohydrin i
s formed
New Way Chemistry for Hong Kong A-Level Book 3A
36
Alkenes react with cold and concentrated H2SO4 to for
m alkyl hydrogensulphates
Addition of Sulphuric(VI) Acid
30.5 Reactions of Alkenes (SB p.134)
e.g.
New Way Chemistry for Hong Kong A-Level Book 3A
37
The large bulky –OSO3H group makes the alkyl hydrogensulphate
very unstable. Two possible further reactions take place:
1. Regeneration of alkenes
30.5 Reactions of Alkenes (SB p.134)
2. Production of alcohols
New Way Chemistry for Hong Kong A-Level Book 3A
38
In the presence of metal catalysts (e.g. Pt, Pd or Ni), H2 is ad
ded to each atom of C = C double bond to form an alkane
Catalytic HydrogenationCatalytic Hydrogenation
30.5 Reactions of Alkenes (SB p.135)
e.g.
New Way Chemistry for Hong Kong A-Level Book 3A
39
30.5 Reactions of Alkenes (SB p.135)
• 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).
• 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
New Way Chemistry for Hong Kong A-Level Book 3A
40
Check Point 30-3 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
30.5 Reaction of Alkenes (SB p.136)
(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 manganat
e(VII) solution while hexane cannot.
New Way Chemistry for Hong Kong A-Level Book 3A
41
Check Point 30-3 Check Point 30-3
(b) What is the major product of each of the following reactions?
(i)
(ii)
Answer
30.5 Reaction of Alkenes (SB p.136)
(b) (i)
(ii)
New Way Chemistry for Hong Kong A-Level Book 3A
42
Check Point 30-3 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
30.5 Reaction of Alkenes (SB p.136)
(c) (i) CH3CH2CH3
(ii)
(iii)
New Way Chemistry for Hong Kong A-Level Book 3A
43
Check Point 30-3 Check Point 30-3
(d) Arrange the following carbocations in increasing order of stability. Explain your answer briefly.
30.5 Reaction of Alkenes (SB p.136)
Answer
(d) The increasing order of stability of carbocations is :
Tertiary carbocations are the most stable because th
e three alkyl groups release electrons to the positiv
e carbon atom and thereby disperse its charge. Prim
ary carbocations are the least stable as there is only
one alkyl group releasing electrons to the positive c
arbon atom.
New Way Chemistry for Hong Kong A-Level Book 3A
44
Check Point 30-3 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.
30.5 Reaction of Alkenes (SB p.136)
Answer
(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:
New Way Chemistry for Hong Kong A-Level Book 3A
45
Ozonolysis is a widely used method for locating the double bond of an alkene
OzonolysisOzonolysis
30.5 Reactions of Alkenes (SB p.136)
(unstable)
The unstable ozonide is reduced directly by treatment with Zn and H
2O
New Way Chemistry for Hong Kong A-Level Book 3A
46
Overall process of ozonolysis:
30.5 Reactions of Alkenes (SB p.137)
e.g.
New Way Chemistry for Hong Kong A-Level Book 3A
47
Example 30-3Example 30-3Predict the structures of the following hydrocarbons A, B and C using the information given below:
Answer
30.5 Reaction of Alkenes (SB p.137)
HydrocarbonMolecular formula
Products after ozonolysis
A C3H6
B C6H10
C C10H16
New Way Chemistry for Hong Kong A-Level Book 3A
48
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.
30.5 Reaction of Alkenes (SB p.138)
New Way Chemistry for Hong Kong A-Level Book 3A
49
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 .
30.5 Reaction of Alkenes (SB p.138)
New Way Chemistry for Hong Kong A-Level Book 3A
50
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.
30.5 Reaction of Alkenes (SB p.138)
New Way Chemistry for Hong Kong A-Level Book 3A
51
Check Point 30-4 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
30.5 Reaction of Alkenes (SB p.139)
(a)
(b)
New Way Chemistry for Hong Kong A-Level Book 3A
52
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
PolymerizationPolymerization
30.5 Reactions of Alkenes (SB p.139)
New Way Chemistry for Hong Kong A-Level Book 3A
53
Monomer: ethene
Depending on the conditions, two kinds of poly(ethene) are
formed
Poly(ethene)
30.5 Reactions of Alkenes (SB p.139)
New Way Chemistry for Hong Kong A-Level Book 3A
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
New Way Chemistry for Hong Kong A-Level Book 3A
55
30.5 Reactions of Alkenes (SB p.139)
Some products made of poly(ethene)
New Way Chemistry for Hong Kong A-Level Book 3A
56
Reaction Mechanism: Free Radical Addition Polymerization of Ethene
30.5 Reactions of Alkenes (SB p.140)
1. 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
New Way Chemistry for Hong Kong A-Level Book 3A
57
30.5 Reactions of Alkenes (SB p.140)
2. Chain propagation
New Way Chemistry for Hong Kong A-Level Book 3A
58
30.5 Reactions of Alkenes (SB p.140)
3. Chain termination
The radicals react to give a stable molecule and the reaction stops.
New Way Chemistry for Hong Kong A-Level Book 3A
59
Poly(propene)
30.5 Reactions of Alkenes (SB p.141)
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
The helmet is made of poly(propene)
New Way Chemistry for Hong Kong A-Level Book 3A
60
Preparation of monomer (phenylethene):
Poly(phenylethene) (or Polystyrene)
30.5 Reactions of Alkenes (SB p.141)
Formation of poly(phenylethene):
New Way Chemistry for Hong Kong A-Level Book 3A
61
Poly(phenylethene):
Properties:
• Transparent, brittle and chemically inert
Uses:
• Make toys, specimen containers and cassette cases
30.5 Reactions of Alkenes (SB p.142)
New Way Chemistry for Hong Kong A-Level Book 3A
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
Some products made of expanded poly(phenylethene)
New Way Chemistry for Hong Kong A-Level Book 3A
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