1. Section 2.7 Conformational Isomerism

2. Stereoisomerism- isomer variations in spatial or 3-D orientation of atoms. One type of stereoisomerism is conformational isomerism. This is a more subtle form of isomerism than skeletal and positional isomerism. Remember these differ in actual bonding arrangement of atoms (the carbon skeleton or the position of noncarbon atoms ).

3. In conformational isomerism The bonding arrangement of atoms remains constant The relationship of atoms in space differs as a result of rotation round carbon-carbon single bonds. This rotation occurs readily, with easy interconversion of conformers. Conformational isomers are conformers. Example: ethane- CH 3 CH 3 The 2 C’s are connected by a single, sigma bond. Because sigma molecular orbitals overlap in only one position the rotation of the C’s around the single bond does not affect the degree of overlap.

4. This basically means the rotation is unrestricted. As the C’s in ethane rotate the H’s on the adjacent carbons also change continually. 2 Extreme Forms 1.eclipsed- this is where the H’s on the adjacent C’s are lined up with one another and are therefore as close together as possible. - this is the least stable form - not very abundant. See example drawn on the board.

5. 2. Staggered- the H’s on adjacent C’s are as far apart as possible. - most stable form See example on the board. Conformational isomerism is represented in 2 ways. 1.Sawhorse diagram- stick drawings 2.Newman projections- uses end-on projection of a C-C bond. See board for drawings and explanation.

6. 2.8 Cycloalkanes- Conformational and Geometric Isomerism Structure and Stability Saturated hydrocarbons possessing one or more rings called cycloalkanes. Remember each corner represents a carbon with enough H’s to satisfy valence. Cyclopropane (smallest) and cyclobutane are more unstable then larger rings due to their structures.

7. They are less stable because of the internal angles of the ring. Remember each C is bonded to four things; sp 3 hybridized and should be tetrahedral with 109.5 degree angles. Because cyclopropane only has 3 carbons in the ring it has the geometry of an equilateral tri angle with internal angles of 60 o.

8. Cyclobutane has 4 carbons but takes the geometry of a square with only 90 o angles. In both of these cases this causes a decreased orbital overlap in the sigma bonds and internal angle strain. Cyclopentane is bent out of the plane and is energetically very stable. The bond angles are at 108 o very close to 109.5 o

9. The larger cycloalkanes are large enough to have flexibility through bond rotation to bond, twist, and pucker until each carbon has the stable tetrahedral angle.

10. Conformational Isomerism in Cyclohexane Cyclohexanes actually pucker to form the stable compound with the bond angles of 109.5 o. The two puckering forms are called the boat and the chair. In both of these conformations the carbons are tetrahedral and all bond angles are 109.5 o. However, the chair form is more stable and the predominant conformer of the cyclohexane.

11. You can compare the stability of the structures to see why the chair form is more predominant. The carbons on the opposite ends (C-1 and C-4) are pulled closer to each other causing steric interactions between the hydrogen's. In the chair form these same two carbons are bent away from each other not having this repulsion between the hydrogen's. You can also see this by drawing Newman projections of both the boat form and chair form.

12. In the boat form the C 2 -C 3 and C 5 -C 6 carbons make the eclipsed conformation, and are in the less stable conformation. The chair formation puts the carbons in a staggered conformation. This is the more stable conformation. See Page 58 Figure 2.7.

13. There are two basic orientations of the hydrogen’s on the chair form. They are the axial position and equatorial position. Axial bonds mean that the hydrogen’s go up or down in a straight line from the carbon it is bonded to. The axial positions alternate up then down from carbon to carbon starting with the hydrogen on carbon one being up. Equatorial bonds mean that the hydrogen’s are parallel to the carbon it is bonding off of.

14. Drawing the Cyclohexane Chair You have to learn how to draw the chair form of cyclohexane. You have to practice drawing in the axial and equatorial positions. Starting left to right you draw an upward line for the axial position and then alternate up and down for each consecutive carbon. Next you add the equatorial in the perimeter positions.

15. Look on page 58 to help guide you in your drawings. You will also have to be able to draw the cyclohexane in the opposite manner after doing what is called a ring flip. Please look at page 59 to see a ring flip.

16. Conformational Isomerism in Substituted Cyclohexanes Axial positions in the chair formation are more crowded than the equatorial positions. This is because the hydrogen's protrude directly above or below the ring and are closer to each other than the hydrogen’s in the equatorial positions. A substituent other than hydrogen would prefer to bond in an equatorial position because they are more stable.

17. The compound is more stable if the substituent is bonded in an equatorial position. There is an equilibrium occurring when a substituent bonds in the axial position. This occurs because the cyclohexane ring is constantly flipping between two conformations. This is because all axial positions become equatorial positions during a ring flip (see page 60).

18. Geometric Isomerism in Cyclic Compounds- Geometric isomers (cis and trans isomers) is a type of stereoisomerism in which atoms or groups display orientation differences around a double bond or ring. Cis isomer is a geometric isomer in which groups are on the same side of a ring or double bond. Trans isomer is a geometric isomer in which groups are on the opposite sides of a ring or double bond