ib chemistry on structural isomers, geometric isomers and stereoisomerism
Post on 11-May-2015
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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
CH3 -CH –CH2 –CH3 │
OH
CH3 -CH –CH –CH3 │
OH
CH3 –CH2–CH2 – O – CH3
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
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
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
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
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
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)
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
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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