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

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 H3COCH3 or H3CCH2OH dimethyelther ethanol CHEM171 – Lecture Series Six : 2012/02

Another example, C3H8O : ethylmethyl ether propan-1-ol propan-2-ol CHEM171 – Lecture Series Six : 2012/03

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

CONFORMATIONAL ISOMERS CHEM171 – Lecture Series Six : 2012/05

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

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

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

Compounds with double and triple bonds are called unsaturated compounds. E-4-methylhept-2-ene 3-propylhex-1-yne CHEM171 – Lecture Series Six : 2012/09

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

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

-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

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