Chap 11A - Isomerism

Describe and define isomers (isomers with different functional groups etc.)

• Isomers (Definition) : Occurrence of two or more compounds with the same molecular formula but having different structures or spatial orientation

• Single bonds can rotate freely 

• Isomers have different IUPAC names -> different molecules 

• Isomers with the same functional groups have similar chemical properties but different physical properties 

• Isomers with different functional groups have different physical and chemical properties

Define CT isomerism

Definition : molecules with the same molecular formula but different structural formula

Describe chain isomers

• Molecules have different atomic arrangements of carbon backbone chain

• Backbone chain: longest series of covalently bonded carbon atoms that creates a continuous chain in the molecule 

• Possess the same functional group or homologous series

• ALWAYS check if the C-C single bonds can be rotated 

Are 1 and 2 the same?

• Both 1 and 2 have the same structure and are the same molecule

• There is free rotation about the C-C single bond. Due to the tetrahedral arrangement of atoms around the sp carbon, notice that the carbon chains are represented in zig-zag arrangements that can be rotated as shown in the diagrams above

• They have the same carbon backbones and are therefore NOT chain isomers 

Describe functional group and positional isomers

1. Positional isomers

• Same functional group but differ in its position along the same carbon chain length

1. Functional group isomers

• Different functional groups consist of the same type of atoms and bonds

Explain how to find degree of saturation in an organic compound

• Can find out how many double bonds, triple bonds or rings are present in a compound by comparing its molecular formula with the general formula of alkanes with same number of C atoms

Rules : 

1. If a compound with n carbon atoms has fewer H atoms than 2n + 2, it must contain double/triple bonds and/or ring

2. Every decrease of two H atoms from the alkane general formula contributes to 1 unsaturation

• 1 unsaturation : either one double bond or one ring present

• 2 unsaturation : either 2 double bonds , 2 rings , 1 double + 1 ring , 1 triple 

3. Halogen , O and S atoms do not affect degree of unsaturation 

4. For every N atom present , there will be one additional H atom compared to the general saturated formula

Find all the isomers of C3H5Br 

• Can vary position of Br group or C=C group 

• Structures 1 and 6 have the same structure

• Structures 2 and 5 have the same structure

• Structures 3 and 4 have the same structure

• Total: 4 CI (3 alkenes, 1 cycloalkane)

Define stereoisomerism

• (Def.): Isomers having the same molecular and constitutional (structural) formulae but differs in the way the atoms are arranged in 3-dimensional space

• They contain the same atoms bonded to each other, the bonding and functional groups are identical and differ only in the way the atoms are arranged in 3-D space

Describe conditions for cis-trans isomerism

1. A covalent bond with restricted rotation Eg. Presence of π (double) bond or ring structure or steric factors AND 

• If a multiple bond is rotated through 90°, lateral p orbital overlap will be reduced to zero which implies the non-existence of the pi bond

  • NOTE: The C=C bond needs more energy to be rotated than a single bond -> this does NOT mean it cannot be rotated at all

• A single bond is a sigma bond in which two orbitals overlap head-on -> rotation about the bond does not reduce the extent of orbital overlap

2. Non-identical groups attached to both (NOT one only) C atom of C=C bond

*MUST show 120 bond angle when drawing alkenes 

State characteristics of cis-trans isomers

1. Restricted rotation about C=C bond prevents spontaneous inter-conversion of cis isomer into trans isomer (they can be isolated as two different compounds)

2. Cis-trans isomers contain the same functional groups and have similar chemical properties

• Chemical properties are not identical as the structures are neither identical nor mirror images -> react with the same reagents but at different rates 

3. Cis-trans isomers have different physical properties

4. Cis isomer has a higher boiling point than the trans isomer as it has higher dipole moment but it has a lower melting point as cis isomers pack less closely together

Describe CT isomerism in alkenes

• Alkenes with two non-identical groups attached to each carbon of the C=C double bond will exhibit CTI

• If there are n such C=C double bonds in a molecule, the maximum number of cis-trans isomers will be 2^n

Describe CT isomerism in cyclic alkene

• Cyclic alkenes with 3 to 7 carbon atoms are fixed in the cis structure

  • C=C bond in a ring does not show cis-trans isomerism due to ring strain in the trans isomer

• A trans structure for a double bond can exist in cyclic compounds only when a large ring is present Eg. cyclodecene

Describe CT isomerism in ring structures

• Can exhibit cis-trans isomerism due to restricted rotation about single bonds in cyclic systems 

• Ring structures with non-identical groups attached to two or more atoms in the ring will exhibit cis-trans isomerism

Explain why this C=C got no CT isomerism

• Ihe C=C bond (left side) is situated in a 6-membered ring -> not large enough to accommodate the geometry required by a trans C=C bond to remain stable at room temperature

Trans-configuration for such a molecule will be too strained to exist at room temperature -> this C=C bond does not exhibit cis-trans isomerism