Isomerism

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Isomerism
24.1 Structural Isomerism 24.2 Stereoisomerism
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Isomers
Isomers are compounds that have the same
molecular formula but different linkages or
spatial arrangements of atoms
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Isomerism

• Two main types of isomerism
• Structural isomerism in which the atoms are
  linked together in different ways
• Stereoisomerism in which the atoms are linked
  in the same way but have different spatial
  arrangements

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Categories of Isomerism
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Structural Isomerism
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24.1 Structural Isomerism (SB p.78)
Structural Isomerism
Structural isomers are isomers that differ
because their atoms are connected in a different
order
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24.1 Structural Isomerism (SB p.79)
Structural Isomers with the Same Functional Group
1. Chain isomerism
Chain isomers are isomers that have different
carbon skeletons
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24.1 Structural Isomerism (SB p.79)
1. Chain isomerism

• Different physical properties
• Straight-chain isomers always have a higher
  boiliing point than branched-chain isomers
• ? Straight-chain isomers have a larger surface
  area
• ? Intermolecular forces between molecules
  become stronger

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24.1 Structural Isomerism (SB p.79)
1. Chain isomerism
e.g. Butane and methylpropane (molecular
formula C4H10)
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24.1 Structural Isomerism (SB p.79)
1. Chain isomerism
e.g. Butan-2-ol and methylpropan-2-ol
(molecular formula C4H10O)
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24.1 Structural Isomerism (SB p.79)
Structural Isomers with the Same Functional Group
2. 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
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24.1 Structural Isomerism (SB p.79)
2. Position isomerism
e.g. Butan-1-ol and butan-2-ol (molecular
formula C4H10O)
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24.1 Structural Isomerism (SB p.79)
2. Position isomerism
e.g. 2-Methylpentane and 3-methylpentane
(molecular formula C6H14)
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24.1 Structural Isomerism (SB p.80)
Structural Isomers with the Same Functional Group
3. Metamerism
Metamers are those isomers with the functional
group (such as ethers or ketones) interrupting
the carbon skeleton at different positions
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24.1 Structural Isomerism (SB p.80)
3. Metamerism
e.g. Methoxypropane and ethoxyethane (molecular
formula C4H10O)
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24.1 Structural Isomerism (SB p.80)
3. Metamerism
e.g. Pentan-2-one and pentan-3-one (molecular
formula C5H10O)
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24.1 Structural Isomerism (SB p.80)
Structural Isomers with the Same Functional Group
4. Tautomerism
Tautomers are those isomers with structures
differing in arrangement of atoms. They are in
dynamic equilibrium with each other
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24.1 Structural Isomerism (SB p.80)
4. Tautomerism
e.g. Ethenol and ethanal
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24.1 Structural Isomerism (SB p.81)
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
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24.1 Structural Isomerism (SB p.81)
Functional Group Isomerism
e.g. Butan-1-ol and ethoxyethane (molecular
formula C4H10O)
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24.1 Structural Isomerism (SB p.81)
Functional Group Isomerism
e.g. Propanal and propanone (molecular formula
C3H6O)
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24.1 Structural Isomerism (SB p.81)
Functional Group Isomerism
e.g. Propanoic acid and methyl ethanoate
(molecular formula C3H6O2)
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24.1 Structural Isomerism (SB p.81)
24
Stereoisomerism
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24.2 Stereoisomerism (SB p.82)
Stereoisomerism

• Stereoisomers are not structural isomers
• Stereoisomers differ only in the arrangement of
  the atoms in space
• e.g. cis- and trans-isomers of alkenes

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24.2 Stereoisomerism (SB p.82)
Stereoisomerism
e.g. cis-but-2-ene and trans-but-2-ene
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24.2 Stereoisomerism (SB p.82)
Stereoisomerism

• Two categories of stereoisomerism
• Geometrical isomerism
• Enantiomerism

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24.2 Stereoisomerism (SB p.82)
Stereoisomerism
Geometrical Isomerism
Geometrical isomers are stereoisomers that have
different arrangements of their atoms in space
due to restricted rotation about a covalent bond
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24.2 Stereoisomerism (SB p.82)
Geometrical Isomerism
e.g. cis-but-2-ene and trans-but-2-ene
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24.2 Stereoisomerism (SB p.83)
Geometrical Isomerism

• Cannot be easily interconverted
• ? Large energy barrier to rotation of the CC
  double bond
• The ? bond has to be broken for rotation of CC
  double bond

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24.2 Stereoisomerism (SB p.83)
Geometrical Isomerism
Rotation of a carbon atom of a double bond
through an angle of 90o results in the breaking
of the p bond
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24.2 Stereoisomerism (SB p.83)
More examples of geometrical isomers
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24.2 Stereoisomerism (SB p.83)
More examples of geometrical isomers
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24.2 Stereoisomerism (SB p.83)
Geometrical Isomerism

• Geometrical isomers have different m.p. and b.p.

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24.2 Stereoisomerism (SB p.84)
Why Does trans-but-2-ene Have a Higher Melting
Point than cis-but-2-ene?

• Two CH3? groups are on the same side of CC
  double bond

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24.2 Stereoisomerism (SB p.84)
Why Does trans-but-2-ene Have a Higher Melting
Point than cis-but-2-ene?

• Two CH3? groups are on the opposite side of CC
  double bond

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24.2 Stereoisomerism (SB p.84)
Why Does trans-but-2-ene Have a Higher Melting
Point than cis-but-2-ene?

• trans-but-2-ene has a more regular and
  symmetrical structure than cis-but-2-ene
• Molecules of trans-but-2-ene can pack more
  compactly in the crystal lattice
• Crystal lattice of trans-but-2-ene is more
  difficult to break
• ? trans-but-2-ene has a higher m.p.

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24.2 Stereoisomerism (SB p.84)
Why Does cis-but-2-ene Have a Higher Boiling
Point than trans-but-2-ene?

• CH3? groups are electron-releasing groups
• ? Exert electron-releasing inductive effect

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24.2 Stereoisomerism (SB p.84)
Why Does cis-but-2-ene Have a Higher Boiling
Point than trans-but-2-ene?

• Molecules of cis-but-2-ene are held together by
  dipole-dipole interactions
• Molecules of trans-but-2-ene are held together by
  instantaneous dipole-induced dipole interactions

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24.2 Stereoisomerism (SB p.84)
Why Does cis-but-2-ene Have a Higher Boiling
Point than trans-but-2-ene?

• Instantaneous dipole-induced dipole interactions
  are weaker than dipole-dipole interactions
• ? Less energy is required to separate the
  molecules of trans-but-2-ene during boiling
• ? cis-but-2-ene has a higher b.p.

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24.2 Stereoisomerism (SB p.84)
Another example of geometrical isomers
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24.2 Stereoisomerism (SB p.85)
Why Does trans-Butenedioic Acid Have a Higher
Melting Point than cis-Butenedioic Acid ?

• Intramolecular hydrogen bonds are formed due to
  close proximity of two carboxyl groups
• Intermolecular hydrogen bonds are less extensive

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24.2 Stereoisomerism (SB p.85)
Why Does trans-Butenedioic Acid Have a Higher
Melting Point than cis-Butenedioic Acid ?

• Extensive intermolecular hydrogen bonds are formed

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24.2 Stereoisomerism (SB p.85)
Why Does trans-Butenedioic Acid Have a Higher
Melting Point than cis-Butenedioic Acid ?

• More intermolecular hydrogen bonds have to be
  broken during melting
• ? trans-butenedioic acid has a higher m.p.

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24.2 Stereoisomerism (SB p.85)
Why Does cis-Butenedioic Acid Have a Higher
Solubility than trans-Butenedioic Acid ?

• cis-butenedioic acid has a greater dipole moment
  than trans-butenedioic acid
• ? cis-butenedioic acid has a higher solubility
  in water

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24.2 Stereoisomerism (SB p.85)
Different Chemical Properties of cis-Butenedioic
Acid and trans-Butenedioic Acid

• Upon heating, cis-butenedioic acid dehydrates to
  form butenedioic anhydride at 160oC

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24.2 Stereoisomerism (SB p.85)
Different Chemical Properties of cis-Butenedioic
Acid and trans-Butenedioic Acid

• trans-butenedioic acid remains chemically
  unchanged upon heating and sublimes at 200oC

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24.2 Stereoisomerism (SB p.86)
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24.2 Stereoisomerism (SB p.87)
Stereoisomerism
Enantiomerism
Enantiomerism occurs in those compounds whose
molecules are chiral. A chiral molecule is one
that is superimposable with its mirror image. The
chiral molecule and its mirror image are
enantiomers
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24.2 Stereoisomerism (SB p.87)
Enantiomerism
The mirror image of a left hand is a right hand
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24.2 Stereoisomerism (SB p.87)
Enantiomerism
Left and right hands are not superimposable
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24.2 Stereoisomerism (SB p.87)
Enantiomerism

• For an sp3-hybridized carbon atom
• If two or more of the attached groups are
  identical
• ? there must be a plane of symmetry in the
  molecule

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24.2 Stereoisomerism (SB p.87)
Enantiomerism

• Molecules having a plane of symmetry are
  symmetrical and not chiral

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24.2 Stereoisomerism (SB p.88)
(a) Trichloromethane
(b) Propan-2-ol
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24.2 Stereoisomerism (SB p.88)
Enantiomerism

• When four different groups are bond to an
  sp3-hybridized carbon atom
• ? asymmetric molecule
• ? the carbon centre is chiral
• ? the molecule and its mirror image exist as
  enantiomers

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24.2 Stereoisomerism (SB p.88)
Enantiomerism
The butan-2-ol molecule and its mirror image are
not superimposable
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24.2 Stereoisomerism (SB p.89)
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24.2 Stereoisomerism (SB p.90)
Properties of Enantiomers Optical Activity

• Enantiomers have identical physical properties

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24.2 Stereoisomerism (SB p.90)
Properties of Enantiomers Optical Activity

• When a beam of plane-polarized light passes
  through a solution of an enantiomer
• ? The plane of polarization rotates
• ? Two enantiomers cause the same extent of
  rotation
• ? One in clockwise (), one in anticlockwise (-)

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24.2 Stereoisomerism (SB p.90)
Properties of Enantiomers Optical Activity

• Enantiomers are sometimes called optical isomers
• Said to be optically active

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24.2 Stereoisomerism (SB p.91)
Plane-polarized Light

• Light is an electromagnetic radiation

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24.2 Stereoisomerism (SB p.91)
Plane-polarized Light

• In a beam of ordinary light
• ? electrical oscillations are occurring in all
  possible planes perpendicular to the direction
  of propagation

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24.2 Stereoisomerism (SB p.92)
Plane-polarized Light
Polarizer
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24.2 Stereoisomerism (SB p.92)
Polarimeter

• Device used for measuring the rotation of
  plane-polarized light by optically active
  compounds

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24.2 Stereoisomerism (SB p.92)
Polarimeter
The principal working parts of a polarimeter
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24.2 Stereoisomerism (SB p.92)
Polarimeter

• If the tube of the 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

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24.2 Stereoisomerism (SB p.92)
Polarimeter

• If the tube contains an optically active
  substance
• ? the plane-polarized light will be rotated

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24.2 Stereoisomerism (SB p.92)
Polarimeter

• If the analyzer is rotated in a clockwise
  direction
• ? the rotation is said to be positive ()
• A substance that rotates plane-polarized light in
  a clockwise direction is said to be dextrorotatory

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24.2 Stereoisomerism (SB p.92)
Polarimeter

• If the analyzer is rotated in a anticlockwise
  direction
• ? the rotation is said to be negative (-)
• A substance that rotates plane-polarized light in
  a anticlockwise direction is said to be
  levorotatory

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24.2 Stereoisomerism (SB p.92)
Polarimeter

• The number of degrees that the plane of
  polarization depends on
• ? number of chiral molecules that it encounters
  
• Related to the length of the tube and the
  concentration of enantiomer in the sample analyzed

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24.2 Stereoisomerism (SB p.93)
Specific Rotation

• In order to standardize measured rotations,
  specific rotation is calculated

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24.2 Stereoisomerism (SB p.93)
Specific Rotation

• The specific rotation depends on the temperature
  and the wavelength of the light source

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24.2 Stereoisomerism (SB p.93)
Specific Rotation

• A specific rotation might be given as follows

• the D line of a sodium lamp was used as the light
  source
• a temperature of 25ºC was maintained
• a sample containing 1 g cm3 of the optically
  active substance, in a 1 dm tube, produced a
  rotation of 3.12º in a clockwise direction

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The END
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24.1 Structural Isomerism (SB p.79)
Let's Think 1
Chain isomers have different physical properties.
What about their chemical properties? Would they
be different as well?
Answer
Chain isomers have similar chemical properties
because they have the same functional groups.
Back
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24.1 Structural Isomerism (SB p.80)
Let's Think 2
Which tautomer is generally more stable, the enol
or the keto?
Answer
The keto is more stable. The carbon-carbon double
bond in the enol is more susceptible to attack as
it is rich in electrons, while the electrons in
the keto are delocalized.
Back
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24.1 Structural Isomerism (SB p.81)
Example 24-1
Draw the structural formulae for all the isomers
with the molecular formula C3H6O.
Answer
Back
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24.1 Structural Isomerism (SB p.82)
Back
Check Point 24-1

• State whether the compounds having the following
  molecular formulae exhibit structural isomerism.
• CH4
• C3H6
• C4H10
• C2H6O

• No
• Yes
• Yes
• Yes

Answer
79
24.2 Stereoisomerism (SB p.86)
Example 24-2A
1,2-Dichloroethene has two geometrical isomers
with different melting points and boiling points.
Give explanations for these differences.
Answer
80
24.2 Stereoisomerism (SB p.86)
Example 24-2A
In cis-1,2-dichloroethene, the two Cl? groups are
on the same side of the carbon-carbon double
bond. However, trans-1,2-dichloroethene has the
two Cl? groups on the opposite side of the double
bond. The trans-isomer 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. Melting of
a substance involves the breaking of its crystal
lattice. As the crystal lattice of the
trans-isomer is more difficult to break compared
with that of the cis-isomer, trans-1,2-dichloroeth
ene has a higher melting point than
cis-1,2-dichloroethene.
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24.2 Stereoisomerism (SB p.86)
Example 24-2A
Besides, in cis-1,2-dichloroethene, both
inductive effects from the two Clt groups are
exerted from one side of the molecule to the
other, a net dipole moment is resulted. Since
cis-1,2-dichloroethene is polar in nature, their
molecules are held together by dipole-dipole
interactions. In trans-1,2-dichloroethene, since
the inductive effects are exerted from opposite
sides, they tend to cancel out each other.
Molecules of trans-1,2-dichloroethene are thus
held together by instantaneous dipole-induced
dipole interactions. As instantaneous
dipole-induced dipole interactions are weaker
than dipole-dipole interactions, less energy is
required to separate the molecules in the process
of boiling. Therefore, trans-1,2-dichloroethene
has a lower boiling point than cis-1,2-dichloroeth
ene.
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24.2 Stereoisomerism (SB p.86)
Example 24-2A
Back
83
24.2 Stereoisomerism (SB p.87)
Check Point 24-2A
State whether the following compounds exhibit
geometrical isomerism. (a) 1,2-Dibromoethene (b)
1,1-Dibromopropene (c) Ethene-1,2-diol

• Yes
• No
• Yes

Answer
Back
84
24.2 Stereoisomerism (SB p.89)
Example 24-2B
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
3-dimensional structures for both enantiomers,
showing their relationships. (a)
2,3,3-Trichloropentane
Answer
85
24.2 Stereoisomerism (SB p.89)
Example 24-2B
86
24.2 Stereoisomerism (SB p.89)
Example 24-2B
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
3-dimensional structures for both enantiomers,
showing their relationships. (b) 3-Methylpentane
Answer
87
24.2 Stereoisomerism (SB p.89)
Example 24-2B
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24.2 Stereoisomerism (SB p.89)
Example 24-2B
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
3-dimensional structures for both enantiomers,
showing their relationships. (c) 2-Phenylbutane
Answer
89
24.2 Stereoisomerism (SB p.89)
Example 24-2B
90
24.2 Stereoisomerism (SB p.89)
Example 24-2B
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
3-dimensional structures for both enantiomers,
showing their relationships. (d) trans-But-2-ene
Answer
91
24.2 Stereoisomerism (SB p.89)
Example 24-2B
Back
92
24.2 Stereoisomerism (SB p.90)
Check Point 24-2B
Draw the structural formulae for the following
compounds and decide whether they are chiral or
not. (a) 1-Chloro-2-methylbutane
Answer
93
24.2 Stereoisomerism (SB p.90)
Check Point 24-2B
Draw the structural formulae for the following
compounds and decide whether they are chiral or
not. (b) 2-Chloro-2-methylbutane
Answer
94
24.2 Stereoisomerism (SB p.90)
Check Point 24-2B
Draw the structural formulae for the following
compounds and decide whether they are chiral or
not. (c) 1-Chloro-3-methylbutane
Answer
95
24.2 Stereoisomerism (SB p.90)
Back
Check Point 24-2B
Draw the structural formulae for the following
compounds and decide whether they are chiral or
not. (d) 2-Chloro-3-methylbutane
Answer
96
24.2 Stereoisomerism (SB p.91)
Example 24-2C
3-Methylpent-1-ene is found to be an optically
active compound. However, when the compound
reacts with excess hydrogen, the product of the
reaction is found to be optically inactive.
Explain why.
Answer
97
24.2 Stereoisomerism (SB p.91)
Back
Example 24-2C
98
24.2 Stereoisomerism (SB p.93)
Check Point 24-2C
(a) Bromination of propane (C3H8) yields two
monosubstituted products with the same molecular
formula, C3H7Br. (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.
Answer
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24.2 Stereoisomerism (SB p.93)
Check Point 24-2C
100
24.2 Stereoisomerism (SB p.93)
Check Point 24-2C
(b) Dehydration of butan-2-ol gives but-2-ene as
the major 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?
Explain your answer.
Answer
101
24.2 Stereoisomerism (SB p.93)
Check Point 24-2C
102
24.2 Stereoisomerism (SB p.93)
Check Point 24-2C
(c) Draw the structural formulae for all possible
structural isomers of C4H8O2 containing the ?
COO ? group.
Answer
103
24.2 Stereoisomerism (SB p.93)
Check Point 24-2C
104
24.2 Stereoisomerism (SB p.93)
Back
Check Point 24-2C
(d) Draw the structural formulae for all possible
geometrical isomers of hexa-2,4-diene.
Answer