Cycloalkanes

1. Cycloalkanes Organic Chem I: lecture 9 by Doba Jackson, Ph.D.

2. Lecture • Alkanes • Naming alkanes con’t • Newman projections con’t • Cycloalkanes • Naming cycloalkanes • Cis-Trans isomerism • Cyclopropane • Cyclobutane

3. Introduction to Cycloalkanes

4. Cycloalkanes, common names

5. Naming Cycloalkanes • Step 1:Find the Parent • If the ring has equal or more carbons, it is the parent.

6. Step 2:Number the substituents with the lowest numbers and write the name.

7. Substituents should have the lowest possible numbers

8. When two or more substituents can have the same number, they are numbered by alphabetical priority

9. When two or more substituents can have the same number, they are numbered by alphabetical priority

10. Some Examples

11. Problem 3.1-Give IUPAC names for the following compounds 3-cyclobutyl pentane 1-methyl 3-propyl cyclopentane 1,4 dimethyl cyclohexane 1-isopropyl 2-methyl cyclohexane or 1-(1-methylethyl) 2-methyl cyclohexane 4-bromo 2-methyl 1-tert-butyl cycloheptane 1-bromo 4-ethyl cyclodecane

12. Stability of Cycloalkanes: Ring Strain Rings larger than 3 atoms are not flat Cyclic molecules can assume nonplanar conformations to minimize strains by ring-puckering Larger rings have many more possible conformations than smaller rings and are more difficult to analyze.

13. Summary: Types of Strain Angle strain- expansion or compression of bond angles away from most stable Torsional strain -eclipsing of bonds on neighboring atoms Steric strain -repulsive interactions between nonbonded atoms in close proximity

14. Conformations of Cycloalkanes Cyclopropane 3-membered ring must have planar structure Symmetrical with C–C–C bond angles of 60° Requires that sp3based bonds are bent (and weakened) All C-H bonds are eclipsed

15. Bent Bonds of Cyclopropane In cyclopropane, the C-C bond is displaced outward from internuclear axis

16. Cyclobutane Cyclobutane has less angle strain than cyclopropane Cyclobutane is slightly bent out of plane - one carbon atom is about 25° above - The bend relieves the torsional strain

17. Cyclopentane Planar cyclopentane would have no angle strain but very high torsional strain Four carbon atoms are in a plane The fifth carbon atom is above or below the plane – Puckered

18. Conformations of Cyclohexane The cyclohexane ring is free of angle strain and torsional strain Each carbon is in the staggered conformation This is called a chair conformation

19. How to Draw Cyclohexane Step 1:Draw two parallel lines slanted downward Step 2:Draw two lines starting from the parallel lines slanting upward and intersecting at a point. Step 3:Draw two lines downward starting from the other end of the parallel lines and intersecting at another point.

20. Axial and Equatorial Bonds in Cyclohexane The chair conformation has two kinds of positions for substituents on the ring: axial positions and equatorial positions Chair cyclohexane has six axial hydrogens perpendicular to the ring (parallel to the ring axis) and six equatorial hydrogens near the plane of the ring

21. Axial and Equatorial Positions Each carbon atom in cyclohexane has one axial and one equatorial hydrogen Each face of the ring has three axial and three equatorial hydrogens in an alternating arrangement

22. Drawing the Axial and Equatorial Hydrogens Axial hydrogens are perpendicular to the ring Equatorial hydrogens are paralell to the ring Completed Cyclohexane

23. Conformational Mobility of Cyclohexane Chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions by a ring-flip

25. Conformations of Monosubstituted Cyclohexanes

27. Conformation of Butane

28. Relationship to Gauche Butane Interactions Gauche butane is less stable than anti butane by 3.8 kJ/mol because of steric interference between hydrogen atoms on the two methyl groups The four-carbon fragment of axial methylcyclohexane and gauche butane have the same steric interaction In general, equatorial positions give more stable isomer

29. Conformational Analysis of Disubstituted Cyclohexanes

30. In disubstituted cyclohexanes the steric effects of both substituents must be taken into account. We can denote cis and trans to disubstituted cyclohexanes. In the cis isomer, both methyl groups are on the same face of the ring Conformational Analysis of Disubstituted Cyclohexanes

31. Trans-1,2-Dimethylcyclohexane

32. Methyl groups are on opposite faces of the ring One trans conformation has both methyl groups equatorial and only a gauche interaction between methyls (3.8 kJ/mol). Steric strain of 15.2 kJ/mol makes the diaxial conformation 11.4 kJ/mol less favorable than the diequatorial conformation trans-1,2-dimethylcyclohexane will exist almost exclusively (>99%) in the diequatorial conformation Trans-1,2-Dimethylcyclohexane

33. Conformations of Polycyclic Molecules Decalin consists of two cyclohexane rings joined to share two carbon atoms and a common bond. Two isomeric forms of decalin: trans fused or cis fused In cis-decalin hydrogen atoms at the bridgehead carbons are on the same face of the rings In trans-decalin, the bridgehead hydrogens are on opposite faces Flips and rotations do not interconvert cis and trans

35. Problem 4.13:Draw two conformations of trans 1,4 dimethylcyclohexane