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