1. Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

2. Monocyclic Compounds Compounds containing a single ring Relative stabilities are determined from heats of formation (  H f °) All have the same empirical formula: CH 2 Thus, stabilities can be readily compared on a per carbon basis 2 7.1 Relative Stabilities of the Monocyclic Alkanes

3. Cycloalkane Heats of Formation per CH 2 3 7.1 Relative Stabilities of the Monocyclic Alkanes

4. Strain Theory Cycloalkanes encounter three types of strain: – Angle Strain – Torsional Strain – Steric Strain 4

5. Angle Strain Angle strain: strain due to expansion or compression of bond angles – Recall: tetrahedral C likes bond angles of 109.5° 5

6. Torsional Strain Torsional Strain: strain due to eclipsing bonds on neighboring atom Neighboring C-H bonds eclipse each other in cyclopropane 6

7. Steric Strain Steric Strain: strain due to repulsive interactions due to atoms that approach each other too closely – Occurs between non-bonded atoms 7

8. Cyclopropane Planar structure Bond angles of 60° – Requires that sp 3 based bonds are bent Angle Strain! Bent bonds = weak bonds All C-H bonds are eclipsed – Torstional strain! 8

9. Cyclobutane Less angle strain than cyclopropane More torsional strain – larger number of ring hydrogens Slightly bent out of plane – One carbon atom is about 25° above – Increases angle strain but decreases torsional strain 9

10. Cyclopentane Conformations of cyclopentane are nonplanar – reduces torsional strain – Planar cyclopentane would have almost no angle strain but very high torsional strain Envelope – Four carbon atoms plane – The fifth carbon atom is above or below the plane 10

11. Cyclohexane Adopts a non-planar, puckered conformation – Chair conformation Free of angle strain and torsional strain – Very stable! – Same stability as a typical unbranched alkane 11

12. Cyclohexane Prevalent in many naturally occurring organic compounds 12 Menthol Morphine

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14. The Chair Conformation 14

15. Drawing Cyclohexane 15

16. Problems Practice drawing both chair conformations of cyclohexane 16

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18. Adding the Hydrogens 18

19. Conformational Mobility of Cyclohexane Chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions by a ring-flip 19

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23. Relative Enthalpies 23 7.2 Conformations of Cyclohexane