Transcript Document 7234420
1
Chapter 29 Alkanes
29.1
Introduction
29.2
Nomenclature of Alkanes
29.3
Physical Properties of Alkanes
29.4
Preparation of Alkanes
29.5
Reactions of Alkanes
New Way Chemistry for Hong Kong A-Level Book 3A
29.1
Introduction (SB p.102) 2 New Way Chemistry for Hong Kong A-Level Book 3A
29.1
Introduction (SB p.103)
Ball-and-stick models:
methane ethane propane 3 • • All carbon atoms are
sp
3 -hybridized
All bond angles are
109.5
° New Way Chemistry for Hong Kong A-Level Book 3A
29.1
Introduction (SB p.103) • • The term “straight chain” is somehow misleading The carbon chain is actually
zigzag
in shape 4 New Way Chemistry for Hong Kong A-Level Book 3A
29.1
Introduction (SB p.103)
Branched-chain alkane
is the molecule with carbon atoms present in
more than one continuous carbon chain
e.g.
5 New Way Chemistry for Hong Kong A-Level Book 3A
29.2
Nomenclature of Alkanes (SB p.104)
Straight-Chain Alkanes
6
Name
Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane Decane
Number of carbon atoms
1 2 3 4 5 6 7 8 9 10
Structure
CH 4 CH 3 CH 3 CH 3 CH 2 CH 3 CH 3 (CH 2 ) 2 CH 3 CH 3 (CH 2 ) 3 CH 3 CH 3 (CH 2 ) 4 CH 3 CH 3 (CH 2 ) 5 CH 3 CH 3 (CH 2 ) 6 CH 3 CH 3 (CH 2 ) 7 CH 3 CH 3 (CH 2 ) 8 CH
New Way Chemistry for Hong Kong A-Level Book 3A
29.2
Nomenclature of Alkanes (SB p.104)
Branched-Chain Alkanes
1.
Select the longest possible straight chain; this gives the parent name for the alkane
2.
Number the parent chain beginning with the end of the chain nearer the branched chain
3.
Use the number obtained by application of rule 2 to designate the position of the branched chain
4.
When two or more branched chains are present, give each branched chain a number corresponding to its position on the parent chain 7 New Way Chemistry for Hong Kong A-Level Book 3A
29.2
Nomenclature of Alkanes (SB p.104)
5.
When two or more branched chains are identical, indicate this by the use of the prefixes ‘di-’, ‘tri-’, ‘tetra-’, and so on.
e.g.
8 New Way Chemistry for Hong Kong A-Level Book 3A
29.2
Nomenclature of Alkanes (SB p.105)
Example 29-1
Draw the structure of the following compound. Is the name provided correct or incorrect? If it is incorrect, give the correct
Solution:
name.
(a) (a) 2,4-Dimethylpentane Answer The name is correct.
9 New Way Chemistry for Hong Kong A-Level Book 3A
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29.2
Nomenclature of Alkanes (SB p.105)
Example 29-1
Draw the structure of the following compound. Is the name name.
(b) (b) 1,3-Dimethylpentane Answer The name is incorrect.
The correct name is 3-methylhexane.
New Way Chemistry for Hong Kong A-Level Book 3A
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29.2
Nomenclature of Alkanes (SB p.105)
Example 29-1
Draw the structure of the following compound. Is the name name.
(c)
(c) 4-Ethyl-1-methylheptane Answer The name is incorrect.
The correct name is 4-ethyloctane.
New Way Chemistry for Hong Kong A-Level Book 3A
29.2
Nomenclature of Alkanes (SB p.106)
Check Point 29-1
Draw the structures of all the isomers of C 6 H 14 , and give the IUPAC names for all structures drawn.
Answer 12 New Way Chemistry for Hong Kong A-Level Book 3A
29.3
Physical Properties of Alkanes (SB p.106)
Number of carbon atom(s)
1 2 3 4 5 6 7 8 9 10
Straight chain alkane
Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane Decane
Boiling point (
°
C)
–161 –89 –42 0 36 69 98 126 151 174
Melting point (
°
C)
–183 –172 –188 –135 –130 –95 –91 –57 –54 –30
Density at 20
°
C (g cm –3 )
— — — — 0.626
0.657
0.684
0.703
0.718
0.730
• At R.T., C 1 – C 4 : gases ; C 5 – C 17 : liquids ; > C 18 : waxy solid 13 New Way Chemistry for Hong Kong A-Level Book 3A
29.3
Physical Properties of Alkanes (SB p.106)
Boiling Point
• Higher members have higher boiling points • •
Reason:
Increase in molecular mass Increase in intermolecular force
14 New Way Chemistry for Hong Kong A-Level Book 3A
29.3
Physical Properties of Alkanes (SB p.108) 15
Branched-chain alkanes have lower boiling points than straight-chain alkanes
∵ molecule is more compact van der Waals’ force surface area boiling point New Way Chemistry for Hong Kong A-Level Book 3A
29.3
Physical Properties of Alkanes (SB p.108)
Melting Point
16 • Higher members have higher melting points • •
Reason:
Increase in molecular mass Increase in intermolecular force
New Way Chemistry for Hong Kong A-Level Book 3A
29.3
Physical Properties of Alkanes (SB p.109)
Density
All alkanes and cycloalkanes have densities
less than 1 g cm –3
at 20 ° C.
Petroleum floats on water surface 17 New Way Chemistry for Hong Kong A-Level Book 3A
29.3
Physical Properties of Alkanes (SB p.109)
Solubility
Alkanes:
•
non-polar
compounds • insoluble in water and highly polar solvents •
soluble in non-polar solvents
like benzene, 1,1,1-trichloroethane 18 New Way Chemistry for Hong Kong A-Level Book 3A
29.4
Preparation of Alkanes (SB p.109)
Petroleum Refining
• 1st step:
fractional distillation
• Petroleum is
separated into different fractions in the fractionating tower
• Each fraction is a
simple mixture
and has
specific uses
19 New Way Chemistry for Hong Kong A-Level Book 3A
29.4
Preparation of Alkanes (SB p.110) Simplified diagram of fractionating tower: 20 New Way Chemistry for Hong Kong A-Level Book 3A
29.4
Preparation of Alkanes (SB p.110)
Fraction
Fuel gas Petrol Naphtha Kerosene Gas oil Fuel oil Lubricating oil 21 Bitumen
Boiling point range (
°
C) Number of carbon atoms that the molecules contain Major uses
< 40 40 – 150 150 – 220 220 – 350 1 – 4 5 – 10 10 – 14 14 – 25 As gaseous fuel, raw materials for manufacture of chemicals Fuel for motor vehicles Fuel for manufacture of town gas Fuel for aeroplanes, domestic liquid fuel Fuel for trucks, lorries and locomotives > 350 > 25 Fuel for cargo ships and to generate electricity in power stations > 350 > 25 Lubricating oil for moving parts of machinery > 350 > 25
New Way Chemistry for Hong Kong A-Level Book 3A
29.4
Preparation of Alkanes (SB p.111)
Cracking of Petroleum
22 New Way Chemistry for Hong Kong A-Level Book 3A
29.4
Preparation of Alkanes (SB p.111)
Cracking
is the process of
breaking down of large alkane molecules
in the heavier fractions of petroleum into lighter fractions of smaller molecules in the
absence of air
e.g. C 11 H 24 C 14 H 30 — — C 9 H 20 C 8 H 18 + CH 2 + 2CH 2 = CH 2 = CH 2 + 2C + 2H 2
Catalytic cracking :
in the presence of catalyst, smaller and highly branched hydrocarbons formed
Thermal cracking :
23 in the absence of catalyst, unbranched chains formed Cracking tower New Way Chemistry for Hong Kong A-Level Book 3A
29.4
Preparation of Alkanes (SB p.112)
Reforming
Reforming
is a process in which
straight-chain alkanes
are heated under pressure in the presence of a platinum catalyst. The chains
break up and reform to give branched-chain molecules
.
24 New Way Chemistry for Hong Kong A-Level Book 3A
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29.4
Preparation of Alkanes (SB p.112)
Example 29-2
Define each of the following terms and give an example of each:
Solution:
(a) (a) (b)
Catalytic cracking Catalytic cracking is the process in which a mixture of
(c)
Reforming of air, to form smaller, highly branched hydrocarbons.
For example, C 8 H 18 C 5 H 12 + C 3 H 6 Answer
(b)
Thermal cracking is the breakdown of large alkane molecules in the heavier fractions to lighter fractions of smaller molecules in the absence of catalysts and air. Straight-chain alkanes are usually formed in this process.
For example, C 14 H 30 C 8 H 18 + 2CH 2 = CH 2 + 2C + 2H 2 New Way Chemistry for Hong Kong A-Level Book 3A
29.4
Preparation of Alkanes (SB p.112)
Solution:
(c)
Reforming is a process in which straight-chain alkanes are heated under pressure in the presence of a platinum Answer catalyst. The chains break up and reform to give branched chain molecules. For example, 26 New Way Chemistry for Hong Kong A-Level Book 3A
29.5
Reactions of Alkanes (SB p.113)
Combustion
Complete combustion :
Alkanes react with
sufficient oxygen
to give carbon dioxide and water through a complex series of reaction with the
release of a large amount of energy
.
General formula:
C
n
H 2
n
2 3
n
2 1 O 2
n
CO 2
n
1 H 2 O 27 New Way Chemistry for Hong Kong A-Level Book 3A
29.5
Reactions of Alkanes (SB p.114) Methane: main component of natural gas and domestic gas Butane: component of bottle gas Combustion reaction:
free radical reaction
The energy released during combustion dominates a large part of world’s
transport
,
industry
and
domestic heating
28 In the
limited supply of oxygen
, alkanes burn to give
CO(g)
and
carbon particles
C 4 H 10 (
g
) 7 2 O 2 (
g
) 2CO (
g
) 2 C (
s
) 5 H 2 O(
l
) New Way Chemistry for Hong Kong A-Level Book 3A
29.5
Reactions of Alkanes (SB p.114) 29
Chlorination
• Methane reacts with chlorine
under diffuse sunlight
or
heating
but not in dark • A
mixture of products
(CH 3 Cl, CH 2 Cl 2 , CHCl 3 , CCl 4 ) is formed with the
replacement of hydrogen by one or more chlorine atom
• CH 4 (
g
) Cl 2 (
g
) CH 3 Cl(
g
) HCl(
g
) CH 3 Cl (
g
) Cl 2 (
g
) CH 2 Cl 2 (
g
) HCl(
g
) CH 2 Cl 2 (
g
) Cl 2 (
g
) CHCl 3 (
g
) HCl(
g
) CHCl 3 (
g
) Cl 2 (
g
) CCl 4 (
g
) HCl(
g
) •
If methane is in excess, CH 3 Cl is major product If chlorine is in excess, CCl 4 is major product
New Way Chemistry for Hong Kong A-Level Book 3A
29.5
Reactions of Alkanes (SB p.114)
Reaction Mechanism: Free Radical Substitution Reaction Mechanism of reaction : 1.
Chain initiation
• homolytic fission of chlorine molecules by heat or light into two chlorine radicals
Step 1:
30 New Way Chemistry for Hong Kong A-Level Book 3A
29.5
Reactions of Alkanes (SB p.115)
2.
Chain propagation
Step 2: Step 3:
31 • steps 2 and 3 repeat hundreds or thousands of time due to formation of the
reactive intermediate
chain reaction
in each step New Way Chemistry for Hong Kong A-Level Book 3A
29.5
Reactions of Alkanes (SB p.116)
Further substitution
occurs: 32 New Way Chemistry for Hong Kong A-Level Book 3A
29.5
Reactions of Alkanes (SB p.116) 33 New Way Chemistry for Hong Kong A-Level Book 3A
29.5
Reactions of Alkanes (SB p.116) 34 New Way Chemistry for Hong Kong A-Level Book 3A
29.5
Reactions of Alkanes (SB p.117)
3.
Chain termination
•
two free radicals combine
to form a • the chain reaction is terminated
neutral molecule
35 New Way Chemistry for Hong Kong A-Level Book 3A
29.5
Reactions of Alkanes (SB p.117)
Example 29-3
Solution:
Write down the reaction mechanism involved in the chlorination The reaction mechanism is shown as follows: of ethane in the presence of diffuse sunlight.
1. Chain initiation
Answer
2. Chain propagation
36 New Way Chemistry for Hong Kong A-Level Book 3A
29.5
Reactions of Alkanes (SB p.117)
Solution:
3. Chain termination
37 New Way Chemistry for Hong Kong A-Level Book 3A
29.5
Reactions of Alkanes (SB p.118)
Check Point 29-2
38
produces one product only on monochlorination. Deduce its structure.
n
+ 1.0 (2
n
+ 2) = 72
(R.a.m.: C = 12.0, H = 1.0)
∴ n = 5 The alkane has the molecular formula of C 5 H 12 .
Answer As the alkane produces one product only on monochlorination, all hydrogen atoms of the alkane molecule must be equivalent.
∴ It must be New Way Chemistry for Hong Kong A-Level Book 3A
The END
39 New Way Chemistry for Hong Kong A-Level Book 3A