Chapter 7 Cyclic Compounds Stereochemistry of Reactions Monocyclic

Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

Monocyclic Compounds • Compounds containing a single ring • Relative stabilities are determined from heats of formation (DHf°) • All have the same empirical formula: CH 2 • Thus, stabilities can be readily compared on a per carbon basis 7. 1 Relative Stabilities of the Monocyclic Alkanes 2

Cycloalkane Heats of Formation per CH 2 7. 1 Relative Stabilities of the Monocyclic Alkanes 3

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

Angle Strain • Angle strain: strain due to expansion or compression of bond angles – Recall: tetrahedral C likes bond angles of 109. 5° 5

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

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

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

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

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

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

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

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The Chair Conformation 14

Drawing Cyclohexane 15

Problems • Practice drawing both chair conformations of cyclohexane 16

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Adding the Hydrogens 18

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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Relative Enthalpies 7. 2 Conformations of Cyclohexane 23