Chapter 1 Chemical Bonding and Chemical Structure

Chapter 1 Chemical Bonding and Chemical Structure

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 7.1 Relative Stabilities of the Monocyclic Alkanes

2 Cycloalkane Heats of Formation per CH2 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 sp3 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 13

The Chair Conformation 14 Drawing Cyclohexane

15 Problems Practice drawing both chair conformations of cyclohexane 16

17 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 20

21 22 Relative Enthalpies

7.2 Conformations of Cyclohexane 23

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