New Way Chemistry for Hong Kong A- Level Book 3A 1 1 Introduction to Introduction to Organic Organic Chemistry Chemistry 21.1 21.1 What is Organic Chemistry? What is Organic Chemistry? 21.2 21.2 The Unique Nature of Carbon The Unique Nature of Carbon 21.3 21.3 Classification of Organic Classification of Organic Compounds Compounds 21.4 21.4 Factors Affecting the Physical Factors Affecting the Physical Properties of Properties of Organic Compounds Organic Compounds 21 21
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New Way Chemistry for Hong Kong A-Level Book 3A1 1 Introduction to Organic Chemistry 21.1What is Organic Chemistry? 21.2The Unique Nature of Carbon 21.3Classification.
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New Way Chemistry for Hong Kong A-Level Book 3A11
Introduction to Introduction to Organic ChemistryOrganic Chemistry
21.121.1 What is Organic Chemistry?What is Organic Chemistry?
21.221.2 The Unique Nature of CarbonThe Unique Nature of Carbon
21.321.3 Classification of Organic CompoundsClassification of Organic Compounds
21.421.4 Factors Affecting the Physical Properties of Factors Affecting the Physical Properties of
Organic CompoundsOrganic Compounds
2211
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21.21.11 What is What is
Organic Organic Chemistry?Chemistry?
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21.1 What is Organic Chemistry (SB p.2)
Organic ChemistryOrganic Chemistry
• Chemistry of the compounds present in living organisms.
• They all contain carbon.
• Organic Chemistry is the Chemistry of Carbon.
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Natural Sources of Organic Natural Sources of Organic CompoundsCompounds
A variety of useful products
derived from crude oil and
coal
Check Point 21-1Check Point 21-1
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In the past …,
ChemistryChemistry
Organic compound
s
Organic compound
sobtained from
living organisms
Inorganic compound
s
Inorganic compound
sobtained from
non-living sources
21.1 What is Organic Chemistry (SB p.3)
Development of Organic Development of Organic Chemistry as a ScienceChemistry as a Science
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(Inorganic compound)
(Organic compound)
In 1828, Wohler (a German chemist)
21.1 What is Organic Chemistry (SB p.3)
Development of Organic Development of Organic Chemistry as a ScienceChemistry as a Science
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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.
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.
Development of Organic Development of Organic Chemistry as a ScienceChemistry as a Science
21.1 What is Organic Chemistry (SB p.3)
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21.21.22 The Unique The Unique
Nature of Nature of CarbonCarbon
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21.2 The Unique Nature of Carbon (SB p.5)
Ability to form four Ability to form four strongstrong covalent bonds covalent bonds
Carbon (ground state)
• Electronic configuration of carbon (ground state) : 1s22s22p2
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21.2 The Unique Nature of Carbon (SB p.5)
• Each carbon atom has four unpaired electrons when excited
• Tend to form four strong covalent bonds
Carbon (excited state)
Ability to form four Ability to form four strongstrong covalent bonds covalent bonds
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21.2 The Unique Nature of Carbon (SB p.5)
• 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
Ability to Ability to CatenateCatenate
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21.2 The Unique Nature of Carbon (SB p.5)
Ability to Ability to CatenateCatenate
C – C > Si – Si > Ge – Ge > Sn – SnBond strength as bond length
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21.2 The Unique Nature of Carbon (SB p.5)
Ability to Ability to CatenateCatenate
C – C > N – N > O – O
Bond strength
as the number of lone pairs
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21.2 The Unique Nature of Carbon (SB p.5)
Ability to Ability to CatenateCatenate
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
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Carbon (excited state)
21.2 The Unique Nature of Carbon (SB p.5)
Ability to Form Multiple Ability to Form Multiple BondsBonds
sp2 bonds, 2 bonds
sp2
1 bond, 3 bonds
sp3
4 bonds
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Single bond Double bond Triple bond
21.2 The Unique Nature of Carbon (SB p.6)
* X = halogens
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21.2 The Unique Nature of Carbon (SB p.6)
Example 21-2Example 21-2 Check Point 21-2Check Point 21-2
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21.21.33Classification Classification
of Organic of Organic CompoundsCompounds
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• Organic compounds are classified by the the presence of characteristic functional groups.
21.3 Classification of Organic Compounds (SB p.7)
Functional Functional GroupsGroups
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A functional group is defined as an atom or a group of atoms that effectively determines the chemical properties of an organic compound.
A functional group is defined as an atom or a group of atoms that effectively determines the chemical properties of an organic compound.
21.3 Classification of Organic Compounds (SB p.7)
Functional Functional GroupsGroups
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21.3 Classification of Organic Compounds (SB p.7)
Functional Functional GroupsGroups
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21.3 Classification of Organic Compounds (SB p.7)
Functional Functional GroupsGroups
• Propane does not react with sodium
• Ethanol and propan-1-ol react with sodium to give hydrogen gas
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• have similar chemical properties
they contain the same functional group –OH
they are classified into the same homologous series — alcohols
and
21.3 Classification of Organic Compounds (SB p.7)
Functional Functional GroupsGroups
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Homologous SeriesHomologous Series
21.3 Classification of Organic Compounds (SB p.12)
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.
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 C3H8 C4H10
CH2 CH2 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
Structural formula
1 Methane CH4 CH4
2 Ethane C2H6 CH3CH3
3 Propane C3H8 CH3CH2CH3
4 Butane C4H10 CH3CH2CH2CH3
The first four members of straight-chain alkanes
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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
Structural formula
1 Methanol CH3OH CH3OH
2 Ethanol C2H5OH CH3CH2OH
3 Propan-1-ol
C3H7OH CH3CH2CH2OH
4 Butan-1-ol
C4H9OH CH3CH2CH2CH2OH
The first four members of straight-chain alcohols
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• Members in the same series can be represented by a general formula.
e.g. alkanes: CnH2n+2
alkenes: CnH2n
alkynes: CnH2n-2
21.3 Classification of Organic Compounds (SB p.13)
Homologous SeriesHomologous Series
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• Members in the same series can be represented by a general formula.
21.3 Classification of Organic Compounds (SB p.13)
Homologous SeriesHomologous Series
e.g. alkanols: CnH2n+1OH
alkanals: CnH2n+1CHO
alkanoic acids: CnH2n+1COOH
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Functional group of an
organic compound
Chemical properties
Members of a homologous series have similar chemical
properties
21.3 Classification of Organic Compounds (SB p.13)
Homologous SeriesHomologous Series
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• 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
21.3 Classification of Organic Compounds (SB p.13)
Homologous SeriesHomologous Series
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Number of
carbon atom(s)
Molecular formula
State (at room
temperature and
pressure)
Melting point (°C)
Boiling point (°C)
Density of solid / liquid at 20°C (g cm–3)
1
2
3
4
5
6
7
8
9
10
CH4
C2H6
C3H8
C4H10
C5H12
C6H14
C7H16
C8H18
C9H20
C10H22
Gas
Gas
Gas
Gas
Liquid
Liquid
Liquid
Liquid
Liquid
Liquid
–183
–172
–188
–135
–130
–95
–91
–57
–54
–30
–161
–89
–42
0
36
69
98
126
151
174
–
–
–
–
0.626
0.657
0.684
0.703
0.718
0.730
Some physical properties of the first 20 members of straight-chain alkanes
21.3 Classification of Organic Compounds (SB p.13)
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Number of
carbon atom(s)
Molecular formula
State (at room
temperature and
pressure)
Melting point (°C)
Boiling point (°C)
Density of solid / liquid at 20°C (g cm–3)
11
12
13
14
15
16
17
18
19
20
C11H24
C12H26
C13H28
C14H30
C15H32
C16H34
C17H36
C18H38
C19H40
C20H42
Liquid
Liquid
Liquid
Liquid
Liquid
Liquid
Liquid
Solid
Solid
Solid
–26
–10
–7
–3
10
18
22
28
32
37
196
216
233
260
271
287
302
316
330
344
0.740
0.749
0.753
0.761
0.769
0.773
0.778
0.777
0.777
0.785
Some physical properties of the first 20 members of straight-chain alkanes
21.3 Classification of Organic Compounds (SB p.13)
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21.3 Classification of Organic Compounds (SB p.13)
Example 21-3AExample 21-3A Example 21-3BExample 21-3B
Example 21-3CExample 21-3C Check Point 21-3Check Point 21-3
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21.21.44Factors Affecting Factors Affecting the Physical the Physical Properties of Properties of
Organic Organic CompoundsCompounds
Refer to notes on ‘Bonding and Structure’ pp.77-92 – intermolecular forces
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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)
21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.17)
Main Factors Affecting the Main Factors Affecting the Physical Properties of Physical Properties of Organic CompoundsOrganic Compounds
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• 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
21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.17)
Structure of Functional Structure of Functional GroupGroup
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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.17)
Structure of Functional Structure of Functional GroupGroup
Molecule Relative molecular
mass
Boiling point (oC)
Molecules with polar functional
groups
CH3CH2CH2OH 60 97.2
CH3CH2CH2NH2 59 48.6
CH3CH2Cl 64.5 12.5
CH3CH2COOH 60 141
Molecules with non-
polar functional
groups
CH3CH2CH2CH3 58 -0.5
CH3CH2CH=CH2 56 -6.2
CH3CH2CCH 54 8.1
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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.18)
Dipole Moment of MoleculeDipole 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
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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.18)
Examples of Polar Molecules Examples of Polar Molecules with Net Dipole Momentwith Net Dipole Moment
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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.18)
Examples of Non-polar Examples of Non-polar Molecules with No Net Dipole Molecules with No Net Dipole MomentMoment
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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.18)
Solubility of Organic Solubility of Organic MoleculesMolecules
• 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”
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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.18)
Solubility of Organic Solubility of Organic MoleculesMolecules
Hexane in tetrachloromethane
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 Why does Hexane Dissolve Readily in Tetrachloromethane?in Tetrachloromethane?
Intermolecular forces among hexane molecules and those among tetrachloromethane
molecules
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 Why is Hexane Insoluble in Water?Water?
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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.19)
Formation of Hydrogen Formation of Hydrogen BondingBonding• 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
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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.20)
Why does Propan-1-ol have Why does Propan-1-ol have a Higher Boiling Point?a Higher Boiling Point?
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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.20)
Formation of Hydrogen Formation of Hydrogen BondingBonding• Also affect the solubility of a
molecule
• Molecules with OH groups are able to form hydrogen bonds with surrounding water molecules
Soluble in water
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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.20)
Example 21-4AExample 21-4A
Example 21-4BExample 21-4B
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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.21)
Length of Carbon Length of Carbon ChainsChains
• 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
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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.21)
Length of Carbon Length of Carbon ChainsChains• 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
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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.21)
Length of Carbon Length of Carbon ChainsChains• 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
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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.21)
Example 21-4CExample 21-4C
Check Point 21-4Check Point 21-4
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FamilyGeneral formula
Functional group
Example
Formula IUPAC name
Alkane RH (Nil) CH3CH3 Ethane
Alkene
RCH = CH2
RCH = CHR
R2C = CHR
R2C = CR2
Carbon-carbon
double bond
CH2 = CH2 Ethene
AlkyneRC CH
RC CR
– C C –
Carbon-carbon triple
bond
HC CH Ethyne
Aromatic hydrocarbon
ArH
Phenyl group
Benzene
21.3 Classification of Organic Compounds (SB p.8)
R = CnH2n+1 –
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FamilyGeneral formula
Functional group
Example
Formula IUPAC name
Haloalkane RX X
halo groupCH3Cl Chloromethane
Alcohol ROH OH
hydroxyl groupCH3OH Methanol
Ether RO R O
oxy groupCH3 O CH3
Methoxymethane
Aldehyde
carbonyl group
Methanal
R = CnH2n+1 –
21.3 Classification of Organic Compounds (SB p.8)
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FamilyGeneral formula
Functional group
Example
Formula IUPAC name
Ketone
carbonyl group
Propanone
Carboxylic acid
carboxyl group
Ethanoic acid
Amine
RNH2
R2NHR3N amino group
CH3NH2 Methylamine
Nitrile RCN C N
nitrile groupCH3CN Ethanenitrile
R = CnH2n+1 –
21.3 Classification of Organic Compounds (SB p.8)
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FamilyGeneral formula
Functional group
Example
Formula IUPAC name
Ester
ester group
Methyl ethanoate
Acyl halide
acyl halide group
Ethanoyl chloride
Amide
amide group
Ethanamide
R = CnH2n+1 –
21.3 Classification of Organic Compounds (SB p.8)
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FamilyGeneral formula
Functional group
Example
Formula IUPAC name
Acid anhydride
acid anhydride group
Ethanoic anhydride
R = CnH2n+1 –
21.3 Classification of Organic Compounds (SB p.9)
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The END
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21.1 What is Organic Chemistry (SB p.4)
(a)How was organic chemistry defined before 1800s? Answer
(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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21.1 What is Organic Chemistry (SB p.4)
(b) How is organic chemistry defined nowadays?
Back
Answer(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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21.2 The Unique Nature of Carbon (SB p.5)
Why is carbon able to catenate?
Back
AnswerThe ability to catenate of carbon is chiefly due to
the high strength of the CC single bond (bond
enthalpy of C C single bond is 356 kJ mol-1).
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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)
Would you expect sulphur, which has an electronegativity value very close to carbon, to catenate? Why?
Back
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 ( ) and
chloro group (Cl)
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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 ( )
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21.3 Classification of Organic Compounds (SB p.14)
Identify the functional group(s) in the following compounds:
(c)
Answer(c) Amino group ( ) and carboxyl group
( )
Back
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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)
Answer
(a) Ester
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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)
Answer
(b) Amide
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21.3 Classification of Organic Compounds (SB p.15)
To which homologous series does each of the following compounds belong?
(c)
Answer
(c) Acid anhydride
Back
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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)
Answer(a) No, the first one is a carboxylic acid and the
second one is an ester.
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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)
Answer(b) Yes, both of them are alcohols.
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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.
(c)
Answer(c) No, the first one is an amide and
the second one is an amine.
Back
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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. Answer(a) Alcohol, ether, aldehyde, ketone,
carboxylic acid, ester, acyl halide,
amide and acid anhydride
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21.3 Classification of Organic Compounds (SB p.16)
(b) Identify and name the functional groups in glucose which has the following structure.
Answer
(b) OH (hydroxyl group)
and O (oxy group)
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21.3 Classification of Organic Compounds (SB p.16)
(c) Identify and name the functional groups in the following compounds:
Answer
Back
(c) Br (bromo),
(aldehyde),
(acyl chloride),
(carbon-carbon
double bond) groups
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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.18)
Why is oil immiscible with water?
AnswerOil molecules do not have free OH groups,
so they cannot form hydrogen bonds with
water molecules.
Back
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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?
AnswerGlucose 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. Back
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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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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.20)
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.
Back
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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.21)
Explain why propan-1-ol is soluble in water but
1-chloropropane is insoluble in water.
AnswerThe 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.
Back
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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?
Answer1-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.
Back
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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 2-chloro-2-methylpropane is –27.1oC. Explain the difference.
Answer
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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.22)
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.
Back
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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) 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.
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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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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.24)
(b) 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 branched-
chain molecules. Straight-chain molecules are held together by
stronger intermolecular forces. Therefore, pentane has a higher
boiling point than 2,2-dimethylpropane. Molecules of 2,2-
dimethylpropane 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.
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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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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.24)
(c) 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.
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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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21.4 Factors Affecting the Physical Properties of Organic Compounds (SB p.24)
(d) 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-1-ol 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.