Organic Chemistry Chapter 4 Lecture Notes PDF

Organic Chemistry Chapter 4 ORGANIC COMPOUNDS: CYCLOALKANES AND THEIR STEREOCHEMISTRY Chapter Contents 4.1 Naming Cycloalkanes 4.2 Cis-Trans Isomerism in Cycloalkanes 4.3 Stability of Cycloalkanes: Ring Strain 4.4 Conformations of Cycloalkanes 4.5 Conformations of Cyclohexa...

Organic Chemistry Chapter 4 ORGANIC COMPOUNDS: CYCLOALKANES AND THEIR STEREOCHEMISTRY Chapter Contents 4.1 Naming Cycloalkanes 4.2 Cis-Trans Isomerism in Cycloalkanes 4.3 Stability of Cycloalkanes: Ring Strain 4.4 Conformations of Cycloalkanes 4.5 Conformations of Cyclohexane 4.6 Axial and Equatorial Bonds in Cyclohexane 4.7 Conformations of Monosubstituted Cyclohexanes 4.8 Conformations of Disubstituted Cyclohexanes 4.9 Conformations of Polycyclic Molecules Cycloalkanes – Chrysanthemic Acid Cycloalkanes – Prostaglandin E 1 Cycloalkanes – Cortisone Naming Cycloalkanes Naming Cycloalkanes – Step 1 Naming Cycloalkanes – Step 2 Naming Cycloalkanes – Step 2a Naming Cycloalkanes – Step 2b Naming Cycloalkanes – Additional Examples Figure 4.2 Bond rotation in ethane and cyclopropane. (a) Rotation occurs around the carbon–carbon bond in ethane, but (b) no rotation is possible around the carbon–carbon bonds in cyclopropane without breaking open the ring. Figure 4.3 There are two different 1,2-dimethylcyclopropane isomers, one with the methyl groups on the same face of the ring (cis) and the other with the methyl groups on opposite faces of the ring (trans). The two isomers do not interconvert. Constitutional Isomers and Stereoisomers Cis-Trans Isomers Worked Example 4.1 Naming Cycloalkanes Name the following substances, including the cis- or trans- prefix: Naming Cycloalkanes Naming Cycloalkanes Angle Strain Figure 4.4 Cycloalkane strain energies, as calculated by taking the difference between cycloalkane heat of combustion per CH2 and acyclic alkane heat of combustion per CH 2, and multiplying by the number of CH 2 units in a ring. Small and medium rings are strained, but cyclohexane rings and very large rings are strain-free. Figure 4.5 The structure of cyclopropane, showing the eclipsing of neighboring C−H bonds that gives rise to torsional strain. Part (b) is a Newman projection along a C−C bond. Cycloalkane Conformations – Cyclopropane Figure 4.6 The conformation of cyclobutane. Part (c) is a Newman projection along a C−C bond, showing that neighboring C−H bonds are not quite eclipsed. Figure 4.7 The conformation of cyclopentane. Carbons 1, 2, 3, and 4 are nearly coplanar, but carbon 5 is out of the plane. Part (c) is a Newman projection along the C1–C2 bond, showing that neighboring C−H bonds are nearly staggered. Cyclohexane Conformations – Menthol Figure 4.8 The strain-free chair conformation of cyclohexane. All C−C−C bond angles are 111.5°, close to the ideal 109° tetrahedral angle, and all neighboring C−H bonds are staggered. Cyclohexane Conformation – Chair Cyclohexanes Chair Cyclohexanes – Bond Indication Cyclohexane Conformations – Twist-Boat Conformation Chair Conformations – Cyclohexane and Glucose Figure 4.9 Axial and equatorial positions in chair cyclohexane. The six axial hydrogens are parallel to the ring axis, and the six equatorial hydrogens are in a band around the ring equator. Figure 4.10 Alternating axial and equatorial positions in chair cyclohexane, looking directly down the ring axis. Each carbon atom has one axial and one equatorial position, and each face has alternating axial and equatorial positions. Figure 4.11 A procedure for drawing axial and equatorial bonds in chair cyclohexane. Figure 4.12 A ring-flip in chair cyclohexane interconverts axial and equatorial positions. What is axial in the starting structure becomes equatorial in the ring-flipped structure, and what is equatorial in the starting structure is axial after ring-flip. Axial and Equatorial Bonds – Bromocyclohexane Worked Example 4.2 Drawing the Chair Conformation of a Substituted Cyclohexane Draw 1,1-dimethylcyclohexane in a chair conformation, indicating which methyl group in your drawing is axial and which is equatorial. Figure 4.14 Interconversion of axial and equatorial methylcyclohexane, represented in several formats. The equatorial conformation is more stable than the axial conformation by 7.6 kJ/mol. Figure 4.15 The origin of 1,3-diaxial interactions in methylcyclohexane. The steric strain between an axial methyl group and an axial hydrogen atom three carbons away is identical to the steric strain in gauche butane. (To display clearly the diaxial interactions in methylcyclohexane, two of the equatorial hydrogens are not shown.) Figure 4.16 Conformations of cis-1,2-dimethylcyclohexane. The two chair conformations are equal in energy because each has one axial methyl group and one equatorial methyl group. Figure 4.17 Conformations of trans-1,2-dimethylcyclohexane. The conformation with both methyl groups equatorial (top) is favored by 11.4 kJ/mol (2.7 kcal/mol) over the conformation with both methyl groups axial (bottom). Axial and Equatorial Bonds and Angle Strain – Glucose and Mannose Worked Example 4.3 Drawing the Most Stable Conformation of a Substituted Cyclohexane Draw the more stable chair conformation of cis-1-tert- butyl-4-chlorocyclohexane. By how much is it favored? Cycloalkanes Cycloalkanes Cycloalkanes Cycloalkanes