27.1
27.2
1 New Way Chemistry for Hong Kong A-Level Book 3A
27.1 Structural Isomerism (SB p.60)
Isomers are different compounds that have the same molecular formula . They have different linkages or spatial arrangements of atoms .
2 New Way Chemistry for Hong Kong A-Level Book 3A
27.1 Structural Isomerism (SB p.61)
Structural Isomers with the Same Functional Group
Chain Isomerism
Chain isomers are isomers that have different carbon skeletons .
e.g.
3 New Way Chemistry for Hong Kong A-Level Book 3A
27.1 Structural Isomerism (SB p.62)
Position Isomerism
Position isomers are isomers that have the same carbon skeleton and functional group . They differ only in the position of the functional group .
e.g.
4 New Way Chemistry for Hong Kong A-Level Book 3A
27.1 Structural Isomerism (SB p.62)
Metamerism
Metamers are those isomers with the functional group interrupting the carbon skeleton at different positions .
e.g.
5 New Way Chemistry for Hong Kong A-Level Book 3A
27.1 Structural Isomerism (SB p.63)
Tautomerism
Tautomers are those isomers with structures differing in arrangement of atoms . They are in dynamic equilibrium with each other.
e.g.
6 New Way Chemistry for Hong Kong A-Level Book 3A
27.1 Structural Isomerism (SB p.63)
Structural Isomers with Different Functional Groups
Functional Group Isomerism
Functional group isomers are isomers that have the same molecular formula but contain different functional groups .
e.g.
7 New Way Chemistry for Hong Kong A-Level Book 3A
27.1 Structural Isomerism (SB p.63)
More examples of functional group isomers:
8 New Way Chemistry for Hong Kong A-Level Book 3A
27.1 Structural Isomerism (SB p.64)
Solution:
Draw the structural formulae for all the isomers with the molecular formula C
3
H
6.
O
Answer
9 New Way Chemistry for Hong Kong A-Level Book 3A
10
27.1 Structural Isomerism (SB p.64)
Check Point 27-1
State whether the compounds having the following molecular formulae exhibit structural isomerism.
(a) CH
4
(b) C
3
H
6
(c) C
4
H
10
(d) C
2
H
6
O
(a) No
Answer
(b) Yes
(c) Yes
(d) Yes
New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.65)
Geometrical Isomerism
Geometrical isomers are stereoisomers that have different arrangements of their atoms in space due to restricted rotation about a covalent bond .
e.g.
11 New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.66)
Rotation of a carbon atom of a C = C bond through an angle of 90 ° causes the breaking of the
bond .
12 New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.66)
More examples of geometrical isomers:
13 New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.66)
Geometrical isomers have different physical and chemical properties .
Physical properties
Melting point ( ° C)
Boiling point ( ° C)
cis-But-2-ene
–139
4
trans-But-2-ene
–106
1
trans-But-2-ene has higher melting point
more regular and symmetrical structure
molecules pack more compactly in crystal lattice
difficult to break the lattice
higher melting point
14 New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.67)
cis-But-2-ene has higher boiling point
∵ net dipole moment
15
molecules are held together by dipole-dipole interactions
trans-isomer has no net dipole moment , their molecules are held by instantaneous dipole-induced dipole interactions
dipole-dipole interactions are stronger
cis -but-2-ene has higher boiling point
New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.67)
Another example:
cis-butenedioic acid and trans-butenedioic acid
16
Physical properties
Melting point ( ° C)
Solubility in water
(gram of solution per 100 g of water at 25
C)
cis-Butenedioic acid
130
78.8
New Way Chemistry for Hong Kong A-Level Book 3A
trans-Butenedioic acid
286
0.7
27.2 Stereoisomerism (SB p.68)
trans-butenedioic acid has higher melting point
∵ form more extensive intermolecular hydrogen bonding
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While the cis -isomer forms intramolecular hydrogen bonding
less extensive intermolecular hydrogen bonding
molecules are less strongly held
lower melting point
New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.68)
Although trans -butenedioic acid can form more extensive hydrogen bonds with water molecules, cis-butenedioic acid is more soluble in water than the trans -isomer because of the greater dipole moment .
18 New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.68) cis - and trans -butenedioic acids have different chemical properties
19 New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.69)
1,2-Dichloroethene has two geometrical isomers with different melting points and boiling points. Give explanations for these differences.
20 New Way Chemistry for Hong Kong A-Level Book 3A
Answer
27.2 Stereoisomerism (SB p.69)
Solution:
• trans -1,2-dichloroethene has a more regular and symmetrical structure than the cis-isomer
• Molecules of trans -1,2-dichloroethene can pack more compactly in the crystal lattice
• The crystal lattice of the trans -isomer is more difficult to break compared with that of the cis-isomer
• trans -1,2-dichloroethene has a higher melting point than cis -1,2-dichloroethene
21 New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.70)
Solution:
• cis -1,2-dichloroethene has a net dipole moment
• Molecules are held together by dipole-dipole interactions
• Molecules of trans -1,2-dichloroethene are held together by instantaneous dipole-induced dipole interactions
• Instantaneous dipole-induced dipole interactions are weaker than dipole-dipole interactions
• Less energy is required to separate the trans -1,2-dichloroethene molecules in the process of boiling.
• trans -1,2-dichloroethene has a lower boiling point than cis -1,2-dichloroethene .
22 New Way Chemistry for Hong Kong A-Level Book 3A
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27.2 Stereoisomerism (SB p.70)
Check Point 27-2
State whether the following compounds exhibit geometrical isomerism.
(a) 1,2-Dibromoethene
(b) 1,1-Dibromopropene
(c) Ethene-1,2-diol
Answer
(a) Yes
(b) No
(c) Yes
New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.70)
Enantiomerism
• Enantiomerism occurs in those compounds whose molecules are chiral .
• A chiral molecule is one that is not superimposable with its mirror image .
• The chiral molecule and its mirror image are enantiomers .
24 New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.71)
Mirror
25
Mirror image of a left hand is a right hand
Not superimposable
New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.72) sp 3 -hybridized carbon atom with two or more identical groups attached is achiral and contains a plane of symmetry
26
Trichloromethane Propan-2-ol
New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.72) sp 3 -hybridized carbon atom with four different groups attached is chiral and do not contain a plane of symmetry e.g. butan-2-ol
Chiral centres
27 New Way Chemistry for Hong Kong A-Level Book 3A
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27.2 Stereoisomerism (SB p.73)
Draw the structural formulae for the following compounds.
Determine whether the compounds are chiral or not. If they are, denote the chiral carbon atom by an asterisk (*). Draw the 3dimensional structure for both enantiomers showing their relationship.
(a) 2,3,3-Trichloropentane
(b) 3-Methylpentane
(c) 2-Phenylbutane
(d) trans -But-2-ene
Answer
New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.73)
Solution:
(a) 2,3,3-Trichloropentane is a chiral compound. The chiral carbon atom is shown by an asterisk.
Their enantiomers are:
29 New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.74)
Solution:
(b) There is no chiral carbon atom in the 3-methylpentane molecule. The compound has no enantiomers.
30
Note that carbon-3 is not a chiral centre because it has two identical groups ( –CH
2
CH
3
) bonded to it .
New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.74)
Solution:
(c) 2-Phenylbutane is a chiral compound. The chiral carbon atom is shown by an asterisk.
Their enantiomers are:
31 New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.74)
Solution:
(d) There is no chiral carbon atom in the trans -but-2-ene molecule. The compound has no enantiomers.
32 New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.74)
Properties of Enantiomers: Optical Activity
Enantiomers have identical physical properties
Physical properties (+) Butan-2-ol (–) Butan-2-ol
Boiling point ( ° C)
Density (g cm
–3 at 20 ° C)
99.5
0.808
99.5
0.808
Refractive index (at
20 ° C)
1.397
1.397
One easily observable difference of a pair of enantiomers is their properties towards plane-polarized light .
33 New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.74)
When a beam of plane-polarized light passes through a solution of an enantionmer , the plane of polarization rotates .
Two enantiomers that are mirror images of one another cause exactly the same extent of rotation , but in opposite directions , clockwise in one and anticlockwise in the other.
Enantiomers are also called optical isomers and said to be optically active .
34 New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.75)
Solution:
There is a chiral carbon atom which is denoted by an asterisk
3-Methylpent-1-ene is found to be an optically active compound.
(*) in 3-methylpent-1-ene. Thus, the compound is optically
However, when the compound reacts with excess hydrogen, the product of the reaction is found to be optically inactive. Explain why.
is 3-methylpentane. The original chiral carbon atom becomes
Answer an achiral one as it links up with two identical groups of
( –CH
2
CH
3
). As the compound is symmetrical , it is optically inactive .
35 New Way Chemistry for Hong Kong A-Level Book 3A
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27.2 Stereoisomerism (SB p.76)
Check Point 27-3
Draw the structural formula for the following compound and decide whether it will show optical activity.
(a) 1-Chloro-2-methylbutane
Answer
(a)
The carbon atom marked with an asterisk (*) is a chiral centre as it is attached to four different groups.
Therefore, the compound is optically active.
New Way Chemistry for Hong Kong A-Level Book 3A
37
27.2 Stereoisomerism (SB p.76)
Check Point 27-3
Draw the structural formula for the following compound and decide whether it will show optical activity.
(b) 2-Chloro-2-methylbutane
Answer
(b)
This compound is optically inactive because there is no chiral centre.
New Way Chemistry for Hong Kong A-Level Book 3A
38
27.2 Stereoisomerism (SB p.76)
Check Point 27-3
Draw the structural formula for the following compound and decide whether it will show optical activity.
(c) 1-Chloro-3-methylbutane
Answer
(c)
This compound is optically inactive because there is no chiral centre.
New Way Chemistry for Hong Kong A-Level Book 3A
39
27.2 Stereoisomerism (SB p.76)
Check Point 27-3
Draw the structural formula for the following compound and decide whether it will show optical activity.
(d) 2-Chloro-3-methylbutane
(d)
Answer
The carbon atom marked with an asterisk (*) is a chiral centre as it is attached to four different groups. Therefore, the compound is optically active.
New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.76)
Plane-polarized Light
Light is an electromagnetic radiation .
40 New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.76)
Polarizer
41
Some possible planes of electrical oscillations in a beam of ordinary light.
Plane-polarized light
New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.77)
Polarimeter
Polarimeter is a device used for measuring the effect of optically active compounds on planepolarized light .
42 New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.78)
If the tube of polarimeter is empty, or if an optically inactive substance is present, the axes of the plane-polarized light and the analyzer will be exactly parallel .
If the tube contains an optically active substance, the planepolarized light will rotate as it passes through the tube.
If the analyzer is rotated in a clockwise direction, the rotation is said to be positive (+).
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If the rotation is anticlockwise , the rotation is said to be negative (–).
New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.78)
The substance that rotates the plane-polarized light in clockwise direction is said to be dextrorotatory .
The substance that rotates the plane-polarized light in anticlockwise direction is said to be levorotatory .
44
Specific rotation is used to measure the rotations on a standard basis.
[
]
where
c
[
]
specific rotation
observed rotation c
concentrat ion of the solution in g cm
3
length of the tube in dm (1 dm
10 cm)
New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.78)
The specific rotation depends on the temperature and the wavelength of light that is employed.
A specific rotation might be given as follows:
[
]
25
D
3 .
12
D = D line of a sodium lamp that was used as light source
25 = temperature of 25 ° C was maintained
[
] = 1 g cm
–3 of the optically active substance was contained in a 1 dm tube
+3.12
° = rotation of 3.12
° in a clockwise direction
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27.2 Stereoisomerism (SB p.78)
Check Point 27-4
(a) (i)
(a) Bromination of propane (C
3
H
8
) yields two monosubstituted products with the same molecular formula, C
3
H
7
Br.
(i) Name the products and draw their structural formulae.
(ii) State the relationship between the two products.
(iii) Do you think the products are optically active?
Explain your answer.
(ii) They are structural isomers.
Answer
(iii) No, because they do not have chiral centres.
New Way Chemistry for Hong Kong A-Level Book 3A
47
27.2 Stereoisomerism (SB p.78)
Check Point 27-4 product. A careful study shows that there are two forms of but-2-ene formed in the reaction.
(i) Name the products and draw their structural formulae.
(ii) State the relationship between the two products.
(iii) Do you think the products are optically active?
(iii) No, because they do not have chiral centres.
Answer
New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.78)
Check Point 27-4
(c)
(c) Draw the structural formulae for all possible structural isomers of C
4
H
8
O
2 containing the –COO group.
Answer
48 New Way Chemistry for Hong Kong A-Level Book 3A
27.2 Stereoisomerism (SB p.78)
Check Point 27-4
(d) Draw the structural formulae for all possible geometrical isomers of hexa-2,4-diene.
(d)
Answer
49 New Way Chemistry for Hong Kong A-Level Book 3A
50 New Way Chemistry for Hong Kong A-Level Book 3A