1. Isomers isomersisomers are different compounds that have the same molecular formula. isomers

3. Isomerism Steroisomerism Structural isomerism Geometrical Isomerism Geometrical Isomerism(cis, trans isomers) Optical Isomerism Functionl Ex.ethanol, diethylether Chain Ex. N. and isobutane Position Ex. Ortho, meta, para

4. constitutional isomers C 4 H 8 hydrocarbons

5. Structural isomers. * A fifth possible isomer of formula C 4 H 8 is CH 3 CH=CHCH 3. This would be named 2-butene according * to the IUPAC rules. This would be named 2-butene according * to the IUPAC rules. * These isomers may be isolated as distinct compounds, having characteristic and different properties.

6. Conformational Energy Profile of Ethane

7. Potential Energy Profile for Ethane Conformers Dihedral Angle

8. stereoisomers. * The only difference being the relative orientation or configuration of the two methyl groups.* The only difference being the relative orientation or configuration of the two methyl groups. * They are shown here with the designations cis and trans.* They are shown here with the designations cis and trans.

9. Configurational Stereoisomers of Alkenes

11. The Sequence Rule for Assignment of Alkene Configurations Assign priorities to double bond substituents by looking at the atoms attached directly to the double bond carbons. Assign priorities to double bond substituents by looking at the atoms attached directly to the double bond carbons.

12. 1.The higher the atomic number of the immediate substituent atom, the higher the priority.1. The higher the atomic number of the immediate substituent atom, the higher the priority. For example,H– < C– < N– < O– < Cl–. (priority increases left to right)For example,H– < C– < N– < O– < Cl–. (priority increases left to right) (Different isotopes of the same element are assigned a priority according to their atomic mass.)(Different isotopes of the same element are assigned a priority according to their atomic mass.)

13. –2. If two substituents have the same immediate substituent atom, move to the next atom (away from the double bond) until a difference is found. –For example, CH 3 – < C 2 H 5 – < ClCH 2 – < BrCH 2 – < CH 3 O–.

15. Conformational Isomerism –Extended ChainCoiled Chain –Extended Chain Coiled Chain

16. Ethane Conformations Name of Conformer Wedge-Hatched Bond Structure Sawhorse Structure Newman Projection

17. Bond Repulsions in Ethane

18. Four Conformers of Butane

19. Potential Energy Profile for Butane Conformers

20. summarize some important aspects of conformational stereoisomerism at this time. (i) Most conformational interconversions in simple molecules occur rapidly at room temperature. Consequently, isolation of pure conformers is usually not possible. (ii) Specific conformers require special nomenclature terms such as staggered, eclipsed, gauche and anti when they are designated. (ii) Specific conformers require special nomenclature terms such as staggered, eclipsed, gauche and anti when they are designated.

21. (iii) Specific conformers may also be designated by dihedral angles. In the butane conformers shown above, the dihedral angles formed by the two methyl groups about the central double bond are: A 180º, B 120º, C 60º & D 0º. (iii) Specific conformers may also be designated by dihedral angles. In the butane conformers shown above, the dihedral angles formed by the two methyl groups about the central double bond are: A 180º, B 120º, C 60º & D 0º. (iv) Staggered conformations about carbon- carbon single bonds are more stable (have a lower potential energy) than the corresponding eclipsed conformations. The higher energy of eclipsed bonds is known as eclipsing strain.

22. (v) In butane the gauche-conformer is less stable than the anti-conformer by about 0.9 kcal/mol. This is due to a crowding of the two methyl groups in the gauche structure, and is called steric strain or steric hindrance. (vi) Butane conformers B and C have non- identical mirror image structures in which the clockwise dihedral angles are 300º & 240º respectively. These pairs are energetically the same, and have not been distinguished in the potential energy diagram shown here.

23. Cycloalkane Stereoisomers Configurational Stereoisomers of Cycloalkanes

25. Ring Conformations

26. Some Conformations of Cyclohexane Rings

27. Conformational Energy Profile of Cyclohexane TC = twist chair B = boat TB = twist boat C = chair

28. Conformational Structures of Disubstituted Cyclohexanes 1,1-dimethylcyclohexane 1-t-butyl-1-methylcyclohexane cis-1,2-dimethylcyclohexane trans-1,2-dimethylcyclohexane

29. cis-1,3-dimethylcyclohexane trans-1,3-dimethylcyclohexane cis-1,4-dimethylcyclohexane trans-1,4-dimethylcyclohexane