21
21.1
21.2
21.3
21.4
1
1
Introduction to
Organic Chemistry
What is Organic Chemistry?
The Unique Nature of Carbon
Classification of Organic Compounds
Factors Affecting the Physical Properties of
Organic Compounds
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21.1
What is Organic
Chemistry?
2
2
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21.1 What is Organic Chemistry (SB p.2)
Organic Chemistry
3
3
•
Chemistry of the compounds present
in living organisms.
•
They all contain carbon.
•
Organic Chemistry is the Chemistry
of Carbon.
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21.1 What is Organic Chemistry (SB p.4)
Natural Sources of Organic Compounds
Living
things
Carbohydrates /
Proteins / Fats /
Vitamins /
Antibiotics
A variety of
organic products
obtained from
living things
4
4
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21.1 What is Organic Chemistry (SB p.4)
Natural Sources of Organic Compounds
Fractional distillation /
oil destructive distillation
Crude
or coal
Alkanes /
Alkenes /
Alkynes /
Aromatic
hydrocarbons
Check Point 21-1
A variety of
useful products
derived from
crude oil and coal
5
5
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21.1 What is Organic Chemistry (SB p.3)
Development of Organic Chemistry
as a Science
In the past …,
Chemistry
Organic
compounds
obtained from
living organisms
6
6
Inorganic
compounds
obtained from
non-living sources
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21.1 What is Organic Chemistry (SB p.3)
Development of Organic Chemistry
as a Science
In 1828, Wohler (a German chemist)
(Inorganic
compound)
7
7
(Organic
compound)
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21.1 What is Organic Chemistry (SB p.3)
Development of Organic Chemistry
as a Science
Redefining … ...
Organic chemistry is the study of carbon
compounds (except CO, CO2, carbonates,
hydrogencarbonates, carbides and cyanides)
obtained from natural sources or
synthesized in the laboratories.
8
8
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21.2
The Unique
Nature of Carbon
9
9
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21.2 The Unique Nature of Carbon (SB p.5)
Ability to form four strong covalent bonds
• Electronic configuration of carbon
(ground state) : 1s22s22p2
Carbon (ground state)
10
10
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21.2 The Unique Nature of Carbon (SB p.5)
Ability to form four strong covalent bonds
• Each carbon atom has four unpaired
electrons when excited
• Tend to form four strong covalent bonds
Carbon (excited state)
11
11
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21.2 The Unique Nature of Carbon (SB p.5)
Ability to Catenate
• Carbon atoms link together to form
chains of varying length, branched
chains and rings of different sizes
• Catenation:
 Ability of atoms in forming stable
bonds with itself, hence joining up
into chains or rings
12
12
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21.2 The Unique Nature of Carbon (SB p.5)
Ability to Catenate
C – C > Si – Si > Ge – Ge > Sn – Sn
Bond strength  as bond length 
13
13
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21.2 The Unique Nature of Carbon (SB p.5)
Ability to Catenate
C–C>N–N>O–O
Bond strength 
as the number of lone pairs 
14
14
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21.2 The Unique Nature of Carbon (SB p.5)
Ability to Catenate
CnH2n+2
n = 1,2,3,…(no limit for n)
SinH2n+2
n = 1 to 6 only  silanes
GenH2n+2 n = 1 to 3 only  germanes
SnnH2n+2 Only SnH4 (stannane) exists
15
15
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21.2 The Unique Nature of Carbon (SB p.5)
Ability to Form Multiple Bonds
sp3
sp2
sp
4 bonds
1 bond, 3 bonds
2 bonds, 2 bonds
Carbon (excited state)
16
16
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21.2 The Unique Nature of Carbon (SB p.6)
Single bond
Double bond
Triple bond
* X = halogens
17
17
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21.2 The Unique Nature of Carbon (SB p.6)
Example 21-2
18
18
Check Point 21-2
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21.3
Classification of
Organic
Compounds
19
19
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21.3 Classification of Organic Compounds (SB p.7)
Functional Groups
•
20
20
Organic compounds are classified by
the the presence of characteristic
functional groups.
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21.3 Classification of Organic Compounds (SB p.7)
Functional Groups
A functional group is defined as an
atom or a group of atoms that
effectively determines the chemical
properties of an organic compound.
21
21
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21.3 Classification of Organic Compounds (SB p.7)
Functional Groups
22
22
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21.3 Classification of Organic Compounds (SB p.7)
Functional Groups
23
23
•
Propane does not react with sodium
•
Ethanol and propan-1-ol react with
sodium to give hydrogen gas
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21.3 Classification of Organic Compounds (SB p.7)
Functional Groups
and
• have similar chemical properties
 they contain the same functional group –OH
 they are classified into the same
homologous series — alcohols
24
24
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21.3 Classification of Organic Compounds (SB p.12)
Homologous Series
A homologous series is a series of
compounds that have the same functional
group, and each member differs from
the next member by a – CH2 – unit in
their formulae.
CH4
C2H6
CH2
25
25
C3H8
CH2
C4H10
CH2
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21.3 Classification of Organic Compounds (SB p.12)
Number
of carbon
atom(s)
IUPAC
name
Molecular
formula
Condensed
structural
formula
1
Methane
CH4
CH4
2
Ethane
C2H6
CH3CH3
3
Propane
C3H8
CH3CH2CH3
4
Butane
C4H10
CH3CH2CH2CH3
26
26
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Structural
formula
The first four members of straight-chain alkanes
21.3 Classification of Organic Compounds (SB p.12)
Number
of carbon
atom(s)
IUPAC
name
Molecular
formula
Condensed
structural
formula
1
Methanol
CH3OH
CH3OH
2
Ethanol
C2H5OH
CH3CH2OH
3
Propan-1ol
C3H7OH
CH3CH2CH2OH
4
Butan-1ol
C4H9OH
CH3CH2CH2CH2OH
27
27
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Structural
formula
The first four members of straight-chain alcohols
21.3 Classification of Organic Compounds (SB p.13)
Homologous Series
• Members in the same series can be
represented by a general formula.
e.g. alkanes: CnH2n+2
alkenes: CnH2n
alkynes: CnH2n-2
28
28
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21.3 Classification of Organic Compounds (SB p.13)
Homologous Series
• Members in the same series can be
represented by a general formula.
e.g. alkanols: CnH2n+1OH
alkanals: CnH2n+1CHO
alkanoic acids: CnH2n+1COOH
29
29
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21.3 Classification of Organic Compounds (SB p.13)
Homologous Series
Functional group
of an organic
compound
Chemical
properties
Members of a homologous series
have similar chemical properties
30
30
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21.3 Classification of Organic Compounds (SB p.13)
Homologous Series
• The physical properties change gradually
along the homologous series
• e.g. the longer the carbon chain in the
molecule ( or the greater the
molecular mass)
 the greater the attractive force
between molecules
 the higher the melting point,
boiling point and density
31
31
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21.3 Classification of Organic Compounds (SB p.13)
Some physical properties of the first 20 members of
straight-chain alkanes
State (at
Number
room
Density of
of
Molecular
Melting
Boiling
temperature
solid / liquid at
point (°C) point (°C)
carbon formula
20°C (g cm–3)
and
atom(s)
pressure)
1
CH4
Gas
–183
–161
–
2
C2H6
Gas
–172
–89
–
3
C 3H 8
Gas
–188
–42
–
4
C4H10
Gas
–135
0
–
5
C5H12
Liquid
–130
36
0.626
6
C6H14
Liquid
–95
69
0.657
7
C7H16
Liquid
–91
98
0.684
8
C8H18
Liquid
–57
126
0.703
9
C9H20
Liquid
–54
151
0.718
32 10
C10H22
–30
174
0.730
NewLiquid
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21.3 Classification of Organic Compounds (SB p.13)
Some physical properties of the first 20 members of
straight-chain alkanes
State (at
Number
room
Density of
of
Molecular
Melting
Boiling
temperature
solid / liquid at
point (°C) point (°C)
carbon formula
20°C (g cm–3)
and
atom(s)
pressure)
11
C11H24
Liquid
–26
196
0.740
12
C12H26
Liquid
–10
216
0.749
13
C13H28
Liquid
–7
233
0.753
14
C14H30
Liquid
–3
260
0.761
15
C15H32
Liquid
10
271
0.769
16
C16H34
Liquid
18
287
0.773
17
C17H36
Liquid
22
302
0.778
18
C18H38
Solid
28
316
0.777
19
C19H40
Solid
32
330
0.777
33 20
C20H42
37Kong A-Level 344
0.785
NewSolid
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3A
33
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21.3 Classification of Organic Compounds (SB p.13)
34
34
Example 21-3A
Example 21-3B
Example 21-3C
Check Point 21-3
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21.4
Factors Affecting the
Physical Properties of
Organic Compounds
Refer to notes on ‘Bonding and Structure’
pp.77-92 – intermolecular forces
35
35
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.17)
Main Factors Affecting the
Physical Properties of Organic
Compounds
1. Structure of the functional group
1.1 Dipole moment of the molecule
1.2 Formation of hydrogen bonding
2. Length of carbon chains (London
dispersion forces)
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36
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.17)
Structure of Functional Group
• Molecules having a polar functional
group have a higher b.p. than others
with a non-polar functional group of
similar molecular masses
 Stronger intermolecular attraction
among molecules
37
37
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.17)
Structure of Functional Group
Molecule
Molecules
with polar
functional
groups
Molecules
with nonpolar
functional
groups
38
38
Relative
molecular
mass
Boiling point
(oC)
CH3CH2CH2OH
60
97.2
CH3CH2CH2NH2
59
48.6
64.5
12.5
CH3CH2COOH
60
141
CH3CH2CH2CH3
58
-0.5
CH3CH2CH=CH2
56
-6.2
CH3CH2CCH
54
8.1
CH3CH2Cl
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.18)
Dipole Moment of Molecule
• Tetrachloromethane has 4 polar bonds
in the molecule
• M.p. and b.p. are very low
 the molecule is non-polar
 the molecule is tetrahedrally
symmetrical
 the dipole moments of the
C  Cl bond cancel each other
39
39
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.18)
Examples of Polar Molecules
with Net Dipole Moment
40
40
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.18)
Examples of Non-polar Molecules
with No Net Dipole Moment
41
41
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.18)
Solubility of Organic Molecules
• Depends on the polarity of organic
molecules and the solvent
• Non-polar or weakly polar compounds
dissolve readily in non-polar or weakly
polar solvents
• Highly polar compounds dissolve readily
in highly polar solvents
• “Like dissolves like”
42
42
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.18)
Solubility of Organic Molecules
Hexane in
tetrachloromethane
43
43
Hexane in water
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.19)
Why does Hexane Dissolve Readily in
Tetrachloromethane?
Intermolecular forces among
hexane molecules and those 
among tetrachloromethane
molecules
44
44
Intermolecular forces
between hexane and
tetrachloromethane
molecules
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.19)
Why is Hexane Insoluble in Water?
45
45
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.19)
Formation of Hydrogen Bonding
• Molecules having OH or  NH2 groups
are able to form hydrogen bonds
• Hydrogen bonds affect the physical
properties of alcohols and amines with
low molecular masses
46
46
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.20)
Why does Propan-1-ol have a
Higher Boiling Point?
47
47
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.20)
Formation of Hydrogen Bonding
• Also affect the solubility of a molecule
• Molecules with OH groups are able
to form hydrogen bonds with
surrounding water molecules
 Soluble in water
48
48
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.20)
Example 21-4A
Example 21-4B
49
49
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.21)
Length of Carbon Chains
• Molecules with higher molecular masses
have higher m.p., b.p. and density
 Higher molecular masses
 Large molecular sizes
 Stronger London dispersion
forces among molecules
50
50
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.21)
Length of Carbon Chains
• Molecules with branched chains
 b.p. and density lower than its
straight-chain isomer
 Straight-chain isomers have greater
surface area in contact with each other
 Greater attractive force among the
molecules
51
51
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.21)
Length of Carbon Chains
• Molecules with branched chains
 m.p. higher than its straight-chain
isomer
 Branched-chain isomers are more
spherical
 Packed more efficiently in solid state
 Extra energy is needed to break
down the efficient packing
52
52
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21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.21)
Example 21-4C
Check Point 21-4
53
53
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21.3 Classification of Organic Compounds (SB p.8)
Family
General
formula
Functional
group
Alkane
RH
(Nil)
Alkene
RCH = CH2
RCH = CHR
R2C = CHR
R2C = CR2
Alkyne
RC  CH
RC  CR
Aromatic
hydrocarbon
ArH
Carboncarbon
double bond
–CC–
Carboncarbon triple
bond
Example
Formula
IUPAC name
CH3CH3
Ethane
CH2 = CH2
Ethene
HC  CH
Ethyne
Benzene
Phenyl group
54
54
R = CnH2n+1 –New Way Chemistry for Hong Kong A-Level 3A
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21.3 Classification of Organic Compounds (SB p.8)
Example
Formula
IUPAC name
Family
General
formula
Functional
group
Haloalkane
RX
X
halo group
CH3Cl
Chloromethane
Alcohol
ROH
 OH
hydroxyl group
CH3OH
Methanol
Ether
RO  R
O
oxy group
CH3  O 
Methoxymethane
CH3
Aldehyde
Methanal
R = CnH2n+1 –
55
55
carbonyl group
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21.3 Classification of Organic Compounds (SB p.8)
Family
General
formula
Functional
group
Example
Formula
IUPAC name
Ketone
Propanone
carbonyl group
Carboxylic
acid
Ethanoic acid
carboxyl group
Amine
Nitrile
RNH2
R2NH
R 3N
RCN
CH3NH2
Methylamine
CH3CN
Ethanenitrile
amino group
CN
nitrile group
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56
21.3 Classification of Organic Compounds (SB p.8)
Family
General
formula
Functional
group
Example
Formula
IUPAC name
Methyl
ethanoate
Ester
ester group
Ethanoyl
chloride
Acyl halide
acyl halide
group
Amide
Ethanamide
amide group
57
57
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R = CnH2n+1 –
21.3 Classification of Organic Compounds (SB p.9)
Family
Acid
anhydride
58
58
General
formula
Functional
group
Example
Formula
IUPAC name
acid anhydride
group
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Ethanoic
anhydride
R = CnH2n+1 –
The END
59
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21.1 What is Organic Chemistry (SB p.4)
(a) How was organic chemistry defined before 1800s?
(a) The knowledge of organic and inorganic
compounds was raised during the 1780s.
Scientists defined organic chemistry as the
study of compounds that could be obtained
from living organisms. They believed that the
synthesis of organic compounds took place in
living organisms only.
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60
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Answer
21.1 What is Organic Chemistry (SB p.4)
Back
(b) How is organic chemistry defined nowadays?
(b) Nowadays, scientists have discovered that
many organic compounds can be synthesized
from inorganic substances. The updated
definition of organic chemistry is the study of
carbon compounds, except for carbon
monoxide, carbon dioxide, carbonates,
hydrogencarbonates, carbides and cyanides.
These compounds have been traditionally
classified under inorganic chemistry.
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Answer
21.2 The Unique Nature of Carbon (SB p.5)
Why is carbon able to catenate?
The ability to catenate of carbon is chiefly due to
the high strength of the CC single bond (bond
enthalpy of C  C single bond is 356 kJ mol-1).
Back
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62
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Answer
21.2 The Unique Nature of Carbon (SB p.6)
Would you expect silicon, which is just below carbon in the
Periodic Table, to catenate to form diverse molecular
structures? Explain your answer.
Answer
Silicon, unlike carbon, does not catenate to form diverse
molecular structures. Carbon is able to catenate because
carbon atoms have a relatively small atomic size. This
enables a carbon atom to form strong covalent bonds
with other carbon atoms. However, due to the greater
atomic size of silicon, its ability to catenate is much lower
than that of carbon.
Back
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21.2 The Unique Nature of Carbon (SB p.7)
Back
Would you expect sulphur, which has an
electronegativity value very close to carbon, to
catenate? Why?
Answer
The electronic configuration of sulphur is 1s22s22p63s23p4. It
has only two unpaired electrons. Its atomic size is larger than
that of carbon. So it has a much lower tendency to catenate
than carbon.
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21.3 Classification of Organic Compounds (SB p.14)
Identify the functional group(s) in the following compounds:
(a)
Answer
(a) Carbon-carbon double bond (
chloro group (Cl)
65
65
) and
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21.3 Classification of Organic Compounds (SB p.14)
Identify the functional group(s) in the following compounds:
(b)
Answer
(b) Carbonyl group (
66
66
)
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21.3 Classification of Organic Compounds (SB p.14)
Back
Identify the functional group(s) in the following compounds:
(c)
Answer
(c) Amino group (
67
67
) and carboxyl group (
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)
21.3 Classification of Organic Compounds (SB p.15)
To which homologous series does each of the following
compounds belong?
(a)
(a) Ester
Answer
68
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21.3 Classification of Organic Compounds (SB p.15)
To which homologous series does each of the following
compounds belong?
(b)
(b) Amide
Answer
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New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
21.3 Classification of Organic Compounds (SB p.15)
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To which homologous series does each of the following
compounds belong?
(c)
(c) Acid anhydride
Answer
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70
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
21.3 Classification of Organic Compounds (SB p.15)
State whether each of the following pairs of compounds
belongs to the same homologous series. Explain your answer.
(a)
(a) No, the first one is a carboxylic acid and the
second one is an ester.
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New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Answer
21.3 Classification of Organic Compounds (SB p.15)
State whether each of the following pairs of compounds
belongs to the same homologous series. Explain your answer.
(b)
(b) Yes, both of them are alcohols.
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New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Answer
21.3 Classification of Organic Compounds (SB p.15)
Back
State whether each of the following pairs of compounds
belongs to the same homologous series. Explain your answer.
(c)
(c) No, the first one is an amide and
the second one is an amine.
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New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Answer
21.3 Classification of Organic Compounds (SB p.16)
(a) Name the homologous series of organic compounds
that contain oxygen atoms in their functional groups.
(a)
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Alcohol, ether, aldehyde, ketone,
carboxylic acid, ester, acyl halide,
amide and acid anhydride
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Answer
21.3 Classification of Organic Compounds (SB p.16)
(b) Identify and name the functional groups in glucose
which has the following structure.
(b)
OH (hydroxyl group)
and  O  (oxy group)
Answer
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New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
21.3 Classification of Organic Compounds (SB p.16)
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(c) Identify and name the functional groups in the
following compounds:
(c)
Br (bromo),
(aldehyde),
(acyl chloride),
(carbon-carbon
double bond) groups
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New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Answer
21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.18)
Why is oil immiscible with water?
Oil molecules do not have free OH groups,
so they cannot form hydrogen bonds with
water molecules.
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New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
Answer
21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.20)
The relative molecular mass of glucose is 180.0, but it
is soluble in water. Why?
Answer
Glucose molecules have OH groups, so
they are able to form hydrogen bonds with
water molecules. Therefore, glucose is
soluble in water despite it has a high
molecular mass.
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78
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New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.20)
Despite the fact that butan-1-ol and ethoxyethane have the
same relative molecular mass, they have very different
boiling points. The boiling points of butan-1-ol and
ethoxyethane are 117oC and 35oC respectively. Explain the
difference.
Answer
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New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.20)
Back
There is an OH group in butan-1-ol. Thus, butan-1-ol molecules are
able to form hydrogen bonds with one another and the energy required to
separate butan-1-ol molecules would be much greater. Whereas for
ethoxyethane, the attraction among the molecules is weak van der Waals’
forces only. The amount of energy required to break the forces would not
be great. Therefore, the boiling point of ethoxyethane is lower than that of
butan-1-ol.
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80
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.21)
Back
Explain why propan-1-ol is soluble in water but
1-chloropropane is insoluble in water.
Answer
The  OH group of propan-1-ol molecules enables it to form
hydrogen bonds with water molecules. Thus it is soluble in water.
Although 1-chloropropane is a polar molecule, it does not form
hydrogen bonds with water molecules. So it is insoluble in water.
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New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.21)
Which molecule would have a higher boiling point,
1-bromobutane or 2-bromobutane? Why?
Answer
1-bromobutane would have a higher boiling point.
1-bromobutane is a straight-chain molecule while
2-bromobutane is a branched-chain molecule. Straight-chain
molecules have a greater surface area in contact with each other,
so greater intermolecular forces exist among the molecules.
Higher energy is required to break down the intermolecular
forces among the molecules of 1-bromobutane.
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82
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.22)
1-Chlorobutane and 2-chloro-2-methylpropane have the
same molecular mass, yet their melting points differ. The
melting point of 1-chlorobutane is –123oC while that of 2chloro-2-methylpropane is –27.1oC. Explain the difference.
Answer
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New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.22)
Back
Melting point is a measure of how efficient the molecules are packed
together in the solid state instead of just comparing the van der Waals’
forces among molecules. Hence melting point is a function of the
efficient packing of molecules but not the contact surface area.
1-Chlorobutane is a straight-chain molecule while
2-chloro-2-methylpropane is a branched-chain molecule. As
2-chloro-2-methylpropane is more spherical and symmetrical, its
molecules are packed more efficiently in the solid state.
1-Chlorobutane is linear in shape and flattened, its packing in the
solid state is not so efficient. Hence, it has a lower melting point than
2-chloro-2-methylpropane.
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84
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.24)
(a) What are the major factors that affect the physical
properties of organic compounds?
Answer
(a)
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The physical properties of organic compounds
are mainly affected by the structure of the
functional groups, dipole moment of the
molecule, the formation of hydrogen bonding
between molecules, and the length of carbon
chains of the molecule.
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.24)
(b) The melting point and boiling point of pentane are
–130oC and 36.3oC respectively while the melting point
and boiling point of 2,2-dimethylpropane are –15.9oC
and 9.5oC respectively. Account for the difference in
melting point and boiling point between the two
isomers.
Answer
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New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.24)
(b)
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Pentane is a straight-chain molecule, while 2,2-dimethylpropane is
a branched-chain molecule. Straight-chain molecules have a
greater surface area in contact with each other than branchedchain molecules. Straight-chain molecules are held together by
stronger intermolecular forces. Therefore, pentane has a higher
boiling point than 2,2-dimethylpropane. Molecules of 2,2dimethylpropane are more spherical in shape and are packed
more efficiently in the solid state. Molecules of pentane are linear
in shape and flattened, so their packing in the solid state is not
efficient. Since extra energy is required to break down the efficient
packing of 2,2-dimethylpropane, 2,2-dimethylpropane has a higher
melting point than pentane.
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.24)
(c) Which molecule, hexane or cyclohexane, would have a
higher melting point? Explain your answer.
Answer
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New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.24)
(c)
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Cyclohexane has a higher melting point than hexane. Molecules
of cyclohexane are more spherical in shape and are packed more
eff iciently in the solid state. Molecules of hexane are linear in
shape and flattened, so their packing in the solid state is not
efficient. Since extra energy is required to break down the efficient
packing of cyclohexane, cyclohexane has a higher melting point
than hexane.
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.24)
(d) Arrange the following molecules in increasing order of
boiling points. Explain your answer.
Answer
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90
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A
21.4 Factors Affecting the Physical Properties of Organic Compounds
(SB p.24)
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(d)
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The boiling points increase in the order:
butane < propanal < propan-1-ol
Molecules of butane are non-polar. Their molecules are held
together by weak instantaneous dipole-induced dipole interactions.
A relatively small amount of energy is required to separate the
molecules in the process of boiling. Both propanal and propan-1-ol
are polar molecules. Molecules of propanal are held together by
relatively weak dipole-dipole interactions, while molecules of
propan-1-ol are held together by intermolecular hydrogen bonds.
Since the intermolecular forces present in molecules of propan-1ol are stronger than those present in molecules of propanal, a
larger amount of energy is required to separate the propan-1-ol
molecules in the process of boiling.
New Way Chemistry for Hong Kong A-Level 3A
New Way Chemistry for Hong Kong A-Level Book 3A