Chapter 7 Cyclic Compounds. Stereochemistry of Reactions

Chapter 7Cyclic 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

Cycloalkane Heats of Formation per CH2

37.1 Relative Stabilities of the Monocyclic Alkanes

Strain Theory

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

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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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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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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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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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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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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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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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Cyclohexane

• Prevalent in many naturally occurring organic compounds

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MentholMorphine

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The Chair Conformation

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Drawing Cyclohexane

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Problems

• Practice drawing both chair conformations of cyclohexane

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Adding the Hydrogens

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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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Relative Enthalpies

237.2 Conformations of Cyclohexane