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Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

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Page 1: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

Chapter 7Cyclic Compounds.

Stereochemistry of Reactions

Page 2: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

Monocyclic Compounds

• Compounds containing a single ring

• Relative stabilities are determined from heats of formation (Hf°)

• All have the same empirical formula: CH2

• Thus, stabilities can be readily compared on a per carbon basis

27.1 Relative Stabilities of the Monocyclic Alkanes

Page 3: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

Cycloalkane Heats of Formation per CH2

37.1 Relative Stabilities of the Monocyclic Alkanes

Page 4: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

Strain Theory

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

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Page 5: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

Angle Strain

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

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Page 6: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

Torsional Strain

• Torsional Strain: strain due to eclipsing bonds on neighboring atom

• Neighboring C-H bonds eclipse each other in cyclopropane

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Page 7: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

Steric Strain

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

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Page 8: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

Cyclopropane• Planar structure• Bond angles of 60°– Requires that sp3

based bonds are bent• Angle Strain!• Bent bonds = weak bonds

• All C-H bonds are eclipsed– Torstional strain!

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Page 9: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

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

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Page 10: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

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

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Page 11: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

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

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Page 12: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

Cyclohexane

• Prevalent in many naturally occurring organic compounds

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MentholMorphine

Page 13: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

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Page 14: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

The Chair Conformation

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Page 15: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

Drawing Cyclohexane

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Page 16: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

Problems

• Practice drawing both chair conformations of cyclohexane

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Page 17: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

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Page 18: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

Adding the Hydrogens

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Page 19: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

Conformational Mobility of Cyclohexane

• Chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions by a ring-flip

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Page 20: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

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Page 23: Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

Relative Enthalpies

237.2 Conformations of Cyclohexane