Isomerism
Isomers – Organic molecules with same molecular formula but different arrangement of atoms
Two types of Isomerism
Structural Isomerism • Same molecular formula • Different structural formula • Different arrangement of atoms
Hydrocarbon Chain Isomer • Different hydrocarbon chain skeleton
Stereoisomerism • Same molecular formula • Same structural formula • Different spatial arrangement of atoms
Positional Chain Isomer • Different functional group position
Functional Group Isomer • Different functional group
Geometric Isomers
Optical Isomers
Cis Isomers Atoms are located on
the same side
Trans Isomers Atoms are located on
the different side
CH3 -CH –CH2 –CH3 │
OH
CH3 -CH –CH –CH3 │
OH
CH3 –CH2–CH2 – O – CH3
Isomerism
Isomers – Organic molecules with same molecular formula but different arrangement of atoms
Structural Isomerism • Same molecular formula • Different structural formula • Different arrangement of atoms
Hydrocarbon Chain Isomer • Different hydrocarbon chain skeleton
Positional Chain Isomer • Different functional group position
Functional Group Isomer • Different functional group
CH3 –CH2 –CH2 –CH3 │
OH
CH3 –CH2 –CH –CH3 │
OH
CH3 –CH2–CH2 – O – CH3
Hydrocarbon Chain Isomer • Different hydrocarbon chain skeleton
Positional Chain Isomer
• Different functional group position
Functional Group Isomer • Different functional group
CH3 –CH2 –CH2 –CH3 │
OH
CH3 - CH – CH2 – OH │
CH3
CH3 │ CH3 - C – OH │
CH3
CH3 –CH2 –CH2 –CH3 │
OH
CH3 –CH2 –CH –CH3 │
OH
CH3 –CH2 –CH2 –CH3 │
OH
CH3 –CH2–CH2 – O – CH3
CH3 –CH2 – O – CH2 -CH3
Structural Isomerism for C4H9
Isomerism
Isomers – Organic molecules with same molecular formula but different arrangement of atoms
Stereoisomerism • Same molecular formula
• Same structural formula • Different spatial arrangement of atoms
Geometric Isomers Optical Isomers
Cis Isomers Atom are located on
the same side
Trans Isomers Atom are located on
the different side
Geometric Isomers • Same chemical properties – Same functional group • Different physical properties – Different spatial arrangement (Different density, polarity, solubility, melting point/boiling point)
Optical Isomers • Same chemical properties – Same functional group • Same physical properties – (Same density, polarity, solubility, melting point/boiling point) • Can only be differentiated by rotation of plane polarised light by optically active isomers
Vs
CI H │ │ C = C │ │
H CI
CI CI │ │ C = C │ │
H H
Cis 1, 2 dichloroethene Trans 1, 2 dichloroethene
Enantiomer Mirror image of each other
Enantiomer Mirror image of each other
Geometrical Isomerism
Geometric Isomers • Same chemical properties – Same functional group • Different physical properties – Different spatial arrangement (Different density, polarity, solubility, melting point /boiling point)
Geometric Stereoisomerism • Same molecular formula • Same structural formula • Different spatial arrangement of atoms
Requirement for geometrical isomers • Presence of C=C double bond to prevent bond rotation • Presence of ring structure to prevent bond rotation • Carbon atom in double bond must be bonded to different atoms
Cis Isomers Atoms are located on the same side
Trans Isomers Atoms are located on the different side
Presence of Double Bond C=C CI –CH=CH –CI
CI CI │ │ C = C │ │
H H
CI H │ │ C = C │ │
H CI
Presence of ring structure
Cis
1, 2 dichloroethene
Trans
1, 2 dichloroethene Cis
1, 2 dichlorocyclopropane
Trans
1, 2 dichlorocyclopropane
Carbon atom bonded to different atoms
H CI │ │ C = C │ │
H H
H CI │ │ C = C │ │
H H
CI CI │ │ C = C │ │
H H
CI H │ │ C = C │ │
H CI
NO Cis/Trans Cis
1, 2 dichloroethene
Trans
1, 2 dichloroethene
X X
Optical Isomerism
Isomers having same Molecular Formula and Structural Formula but different spatial arrangement • At least 1 asymmetric / chiral carbon which is bonded to 4 different groups
• NH2CH(R)COOH show optical isomerism
• Two optical isomers/mirror images are called enantiomers (cannot superimpose on each other)
• Have similar physical and chemical properties except for the effect on rotation of plane of polarised light
• Optically active – enantiomers able to rotate plane polarised light to one direction (clockwise/anticlockwise)
• Optically inactive – enantiomers present in equal amount (equimolar) – racemic mixture and rotation cancel out each other
http://chemwiki.ucdavis.edu/Organic_Chemistry/Chirality/Optical_Activity
Rotate to right Rotate to left
or
rotation cancel out each other
• Optical activity is the ability of solution to rotate the plane of polarised light Substance are said to be optically active – able to rotate plane of polarised light Polarimeter • Two polarising filters that can be rotated to each other, separated by a compartment in which the plane polarise light passes through a liquid • Plane of polarised light – oscillation of wave is restricted to one plane – vertically by passing through a polarised filter • Optically active isomers – able to rotate plane of polarised light to one direction – presence of asymmetrical/chiral carbon bonded to 4 different groups • An equimolar mixture of enantiomers is called racemic mixture – Racemic mixture is optically inactive
Products/Drugs from natural sources/catalysed by enzyme
• gives only one pure optically active enantiomer
• chiral and found in single enantiomers – optically active
Optical Isomerism
Products synthesised chemically • gives two enantiomers in equal amount or racemic mixture
• optically inactive
Optically active Optically inactive
=
• Butan -2-ol (CH3CH(OH)C2H5) has asymmetrical chiral carbon • Enantiomers are mirror image and non superimposable on each other •Two enantiomers both rotate plane of polarised light in different directions
1. Light pass through 1st polariser – plane polarised light produced
2. Sample introduced into sample tube. If sample is optically active – will rotated plane of polarised light to one direction
3. Turn the analyzer either clockwise/anticlock wise to give light of maximum intensity again
4. If one enantiomer rotates light 120 clockwise – Analyzer will need to be rotated anticlock wise 120 – sample is said to be optically active
5. If one enantiomer rotates light 120 clockwise – Another enantiomer rotates light anticlock wise 120 – sample is said to be optically inactive
How polarimeter works
Optical Isomerism
Optically active enantiomer
Optically inactive enantiomer cancel each other rotation
6. Racemic Mixture = enantiomers present in equal amount (equimolar) and cancel each other rotation
Write the structural formula for all isomers for C4H9OH and state which isomer shows optical isomerism
CH3 –CH2 –CH2 –CH2 │
OH
CH3 –CH –CH2 –CH3 │
OH
CH3 - CH – CH2 – OH │
CH3
CH3 │ CH3 - C – OH │
CH3 Butan -1-ol Butan-2-ol
2-methylpropan-2-ol
2-methylpropan-1-ol
Questions on Isomerism
All structural isomers
Stereoisomers
Optical isomers
Write the structural formula of cyclic isomers for C3H4CI2, and state the type of isomerism
Both are Structural formula
Geometric Isomers Cis/Tans isomerism
All structural isomers
Optical Isomers Enantiomers, mirror image
Chiral center – optical isomers
Cyclic ring – geometric isomers
Write the structural formula for all isomers for C4H9OH and state which isomer shows optical isomerism
CH3 –CH2 –CH2 –CH2 │
OH
CH3 –CH –CH2 –CH3 │
OH
CH3 - CH – CH2 – OH │
CH3
CH3 │ CH3 - C – OH │
CH3 Butan -1-ol Butan -2-ol
2-methylpropan-2-ol
2-methylpropan-1-ol
Which of the following pairs are stereoisomers?
CH3 CH3
│ │ C = C │ │
CI CI
CH3 CI
│ │ C = C │ │
CI CH3
CH3 Br
│ │ C = C │ │
H CI
CH3 H
│ │ C = C │ │
Br CI
H │ CH3 - C – COOH │
NH2
H │ HOOC - C – CH3
│
NH2
Questions on Isomerism
All structural isomers
Stereoisomers
Optical isomers
A B C
Geometrical isomers
(Stereoisomers)
Structural isomers Optical isomers
(Stereoisomers) √ √ X
CH3 CH2 C*H(CI)
│
CH3
CH3 C*H(NH2)COOH H3CH2C*H(OH)CH2OH CH3C*H(OH)CH2OH
Optical Isomerism
Which of the carbon has a chiral center?
H H │ │ C = C
│ │ Br CH(OH)CH3
Draw all stereoisomers for CHBr =CHCH(OH)CH3
H CH(OH)CH3 │ │ C = C
│ │ Br H
CHBr = CH-CH(OH)CH3
H │ CHBr = CH - C – CH3
│
OH
H │ CH3 - C – CH=CHBr
│
OH
Optical isomers Geometric isomers
Chiral carbon with 4 different groups
Double bond prevent bond rotation
Cis Trans Enantiomer Enantiomer
C2 H2 CI2
H H │ │ C = C │ │
CI CI
CI H │ │ C = C │ │
CI H
Draw the structural formula of isomers having MF C2H2CI2, and state the type of isomerism
Both are Structural formula
Geometric Isomers
Cis/Tans isomerism
H H │ │ C = C │ │
CI CI
H CI │ │ C = C │ │
CI H
Trans Cis
Questions on Structural and Geometrical Isomerism
Draw the structural formula of cyclic isomers having MF C3H4CI2, and state the type of isomerism
C3H4CI2
Both are Structural formula
Geometric Isomers
Cis/Tans isomerism
Cis Trans
Questions on Structural and Geometrical Isomerism
Which of the following exhibit cis/trans isomerism ?
Br H │ │ C = C │ │
Br H
CH3 CH3 │ │ C = C │ │
H H
H CH3 │ │ C = C │ │
CI CI
CH3CH=CH-CH3 CH3CH=CH2 CH(CI)=CH(CI)-CH3
Which of the following is an structural isomer of CH3COCH2CH3?
CH2 = CH- CH-CH3
│
OH
CH3 –CH2-CH2-CH
║
O
CH3 –CH2- O- CH2-CH3
Draw the geometrical isomer for the following
CH3CH=CHCH2CH3 F-CH = CH-F
H H │ │ C = C │ │
CH3 CH2CH3
CH3 H │ │ C = C │ │
H CH2CH3
Cis pent-2-ene Trans pent-2-ene
F F │ │ C = C │ │
H H
Cis 1, 2 difluoroethene
F H │ │ C = C │ │
H F
Trans 1, 2 difluoroethene
A B C
A B C
A B C
√ √
√ √
√ √
X
X
X
Same atoms on carbon
Same atoms on carbon
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