1. CHAPTER 6: ISOMERISM
Isomers are compounds that have the same numbers and kinds of atoms but differ in the way they are arranged.
ISOMERS
constitutional
stereoisomers
CHEM171 – Lecture Series Six : 2012/01

2. CONSTITUTIONAL ISOMERS
Atoms are connected differently
n- butane
isobutane
Also, when a molecular formula can give rise to different functional groups.
For compounds of molecular formula of C2H6O
H3COCH or H3CCH2OH
dimethyelther ethanol
CHEM171 – Lecture Series Six : 2012/02

3. Another example, C3H8O : ethylmethyl ether propan-1-ol propan-2-ol
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4. STEREOISOMERS
Stereoisomers are compounds where the atoms are connected in the same order but the molecules have different geometries.
STEREOISOMERS
CONFORMATIONAL GEOMETRIC ENANTIOMERS
CHEM171 – Lecture Series Six : 2012/04

5. CONFORMATIONAL ISOMERS
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6. Complicated with higher alkanes, e.g., butane
Newman Projection
eclipsed
staggered
Eclipsed and staggered forms of ethane are said to be conformational isomers or conformers.
Complicated with higher alkanes, e.g., butane
CHEM171 – Lecture Series Six : 2012/06

7. Consider the molecule, 2-butene:
GEOMETRIC ISOMERS
Consider the molecule, 2-butene:
(I)
(II)
Molecule (I) has the two methyl groups on the same side of the double bond, called the the cis– isomer or the Z– isomer.
The other isomer has the methyl groups on the opposite sides of the double bond, known as the trans– isomer or the E– isomer.
CHEM171 – Lecture Series Six : 2012/07

8. EXAMPLE SOLUTION Name the following compound:2 2 1 1
Z-3-chloro-2-hexene OR Z-3-chlorohex-2-ene
CHEM171 – Lecture Series Six : 2012/08

9. Compounds with double and triple bonds are called unsaturated compounds.
E-4-methylhept-2-ene
3-propylhex-1-yne
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10. Stereoisomerism arises from molecules which contain a chiral centre.
ENANTIOMERS
Stereoisomerism arises from molecules which contain a chiral centre.
A chiral centre is a carbon centre which is attached to four different substituents.
1,1-chloroiodoethane OR 1-chloro-1-iodoethane
CHEM171 – Lecture Series Six : 2012/10

11. Structures are mirror images of each other and are not superimposable – called enantiomers.
The Cahn Ingold Prelog R, S notation is employed to distinguish between two enantiomeric forms.
RULES
Locate the chiral centre.
Assign priority to each substituent 1 (highest) to 4 (lowest) based on the atomic number of the 4 atoms directly attached to the chiral centre.
CHEM171 – Lecture Series Six : 2012/11

12. -CH2-H -CH2-CH3 -CH2-NH2 -CH2-OH
-H -CH3 -NH2 -OH -Cl -Br
Increasing priority
-CH2-H -CH2-CH3 -CH2-NH2 -CH2-OH
Increasing priority
Orient the molecule so that the group with the lowest priority (usually hydrogen) is directed away from you.
If the numbers increase from 1-3 in a clockwise direction , the configuration is R, anticlockwise direction, S.
CHEM171 – Lecture Series Six : 2012/12

13. EXAMPLE
Assign the absolute configuration, R or S to the following compound:
SOLUTION
(R)-2-butanol
CHEM171 – Lecture Series Six : 2012/13