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IB Chemistry on Structural Isomers, Geometric Isomers and Stereoisomerism

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IB Chemistry on Structural Isomers, Geometric Isomers and Stereoisomerisms
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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 CH 3 -CH CH 2 CH 3 OH CH 3 -CH CH CH 3 OH CH 3 CH 2 CH 2 O CH 3
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Page 1: IB Chemistry on Structural Isomers, Geometric Isomers and Stereoisomerism

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

Page 2: IB Chemistry on Structural Isomers, Geometric Isomers and Stereoisomerism

Isomers – Organic molecules with same molecular formula but different arrangement of atoms

Structural Isomerism

Structural Isomerism for C4H9OH • 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 - CH – CH2 – OH │

CH3

CH3 │ CH3 - C – OH │

CH3

CH3 –CH2 –CH2 –CH3 │

OH

CH3 –CH2–CH2 – O – CH3

CH3 –CH2 – O – CH2 -CH3

Butan – 1-ol 2- Methylpropan-1-ol

2- Methylpropan-2-ol

Butan – 1-ol Butan – 2- ol

Butan – 1-ol Methoxypropane Ethoxyethane

Structural Isomerism

CH3 –CH2 –CH2 –CH3 │

OH

Page 3: IB Chemistry on Structural Isomers, Geometric Isomers and Stereoisomerism

H │ CH3 –CH2– C=O

O ║ CH3 – C– CH3

Structural Isomers for C4H9OH – All have same Molecular Formula but different Structural Formula

Isomers – Organic molecules with same molecular formula but different arrangement of atoms

Structural Isomerism

CH3 –CH2 –CH2 –CH2 │

OH

CH3 –CH2 –CH –CH3 │

OH

CH3 - CH – CH2 – OH │

CH3

CH3 │ CH3 - C – OH │

CH3

CH3 –CH2–CH2 –O – CH3

CH3 –CH2 –O– CH2 -CH3

C4H9OH C4H9OH C4H9OH

C4H9OH

C4H9OH C4H9OH

Structural Isomers for C4H9Br – All have same Molecular Formula but different Structural Formula

CH3 –CH2 –CH2 –CH2 │

Br

CH3 –CH2 –CH –CH3 │

Br

CH3 - CH – CH2 – Br

CH3

CH3 │ CH3 - C – Br

CH3

Structural Isomers for C3H6O – All have same Molecular Formula but different Structural Formula

OH │ CH3 –CH = C-H

OH │ CH2 =CH - C-H2

CH3 – O – CH = CH2

C4H9Br C4H9Br C4H9Br

C4H9Br

C3H6O C3H6O C3H6O C3H6O C3H6O

Page 4: IB Chemistry on Structural Isomers, Geometric Isomers and Stereoisomerism

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

Page 5: IB Chemistry on Structural Isomers, Geometric Isomers and Stereoisomerism

Geometric Isomerism Geometric Isomers – Organic molecules with same molecular/structural formula but different spatial arrangement

Cis Isomers Atoms are located on the same side

Trans Isomers Atoms are located on the different side

CI –CH=CH –CI CI CI │ │ C = C │ │

H H

CI H │ │ C = C │ │

H CI

Cis 1, 2 dichloroethene

m/p : -80C ↓

Trans 1, 2 dichloroethene

m/p : -50C ↑

CH2 –CH = CH–CH2 CH3 CH3 │ │ C = C │ │

H H

CH3 H │ │ C = C │ │

H CH3

Cis but-2-ene

m/p : -139C ↓

Trans but-2-ene

m/p: -106C ↑

Cis melting point is lower ↓ • Bend structure (kink) prevent molecules from interacting /pack together • Less surface area due to bend/spherical structure • Lower intermolecular forces of attraction – lower melting point ↓

Trans melting point higher ↑ • Linear structure – molecules interacting/ pack together • More surface area due to linear structure • Greater intermolecular forces of attraction – higher melting point ↑

Cis

Isom

er

Tra

ns Is

om

er

Geometric Stereoisomerism • Same molecular formula • Same structural formula • Different spatial arrangement of atoms

Page 6: IB Chemistry on Structural Isomers, Geometric Isomers and Stereoisomerism

Geometric Isomerism Geometric Isomers – Organic molecules with same molecular/structural formula but different spatial arrangement

Cis Isomers Atoms are located on the same side

Trans Isomers Atoms are located on the different side

CI –CH=CH –CI CI CI │ │ C = C │ │

H H

CI H │ │ C = C │ │

H CI

CH2 –CH = CH–CH2 CH3 CH3 │ │ C = C │ │

H H

CH3 H │ │ C = C │ │

H CH3

Cis

Isom

er

Tra

ns Is

om

er

Cis boiling point is higher ↑ • CI groups are on same side- Bond polarity- Net dipole moment • Cis molecule is polar – Dipole interaction • Stronger intermolecular forces of attraction – Higher boiling point ↑

Trans boiling point lower ↓ • CI groups on diff side – Bond polarity cancel – No net dipole moment • Trans molecule is non polar – No Dipole interaction • Weaker intermolecular forces attraction – Lower boiling point ↓

Cis 1, 2 dichloroethene

b/p : 60C ↑

Cis but-2-ene

b/p : 4C ↑

Trans 1, 2 dichloroethene

b/p : 48C ↓

Trans but-2-ene

b/p : 1C ↓

Geometric Stereoisomerism • Same molecular formula • Same structural formula • Different spatial arrangement of atoms

Page 7: IB Chemistry on Structural Isomers, Geometric Isomers and Stereoisomerism

HOOC–CH = CH–COOH

HOOC COOH │ │ C = C │ │

H H

Cis butene dioic acid

m/p: 130 ↓

Decomposes at 130C

HOOC H │ │ C = C │ │

H COOH

Trans butene dioic acid

m/p: 286 ↑

Decomposes at 280

Cis melting point lower ↓ • COOH same side – intramolecular hydrogen bonding • Less H atoms available for hydrogen bonding between molecules • Lower intermolecular forces attraction- m/p lower ↓

Trans melting point higher ↑ • COOH opposite side – intermolecular hydrogen bonding • More H atoms available for hydrogen bonding between molecules • Higher intermolecular forces attraction – m/p higher ↑

Cis

Isom

er

Tra

ns Is

om

er

Cis decomposes at lower temperature ↓ • COOH same side – undergo condensation when heated • Loss of water molecule forming anhydride

Trans decomposes at higher temperature ↑ • COOH opposite side – condensation of water molecule happens at higher temperature

O O= C C = O │ │ C = C │ │

H H

Geometric Isomerism Geometric Isomers – Organic molecules with same molecular/structural formula but different spatial arrangement

Cis Isomers Atoms are located on the same side

Trans Isomers Atoms are located on the different side

HOOC COOH │ │ C = C │ │

H H

heated HOOC H │ │ C = C │ │

H COOH

heated

Geometric Stereoisomerism • Same molecular formula • Same structural formula • Different spatial arrangement of atoms

Stable cause COOH at different sides,

(no condensation)

Page 8: IB Chemistry on Structural Isomers, Geometric Isomers and Stereoisomerism

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

C3 H4 CI2

Both are Structural formula

Geometric Isomers

Cis/Tans isomerism

Cis Trans

Page 9: IB Chemistry on Structural Isomers, Geometric Isomers and Stereoisomerism

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 structural 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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