Cycloalkanes and their Stereochemistry

1. Cycloalkanes and their Stereochemistry Chapter 4

2. Naming Cycloalkanes • Saturated cyclic hydrocarbons are defined as a carbon ring which has hydrogen in every position except the attachment point. These are also called alicyclic compounds. Cyclopropane Cyclo = the cyclic ring propane = the number of carbons

3. Examples of Cycloalkanes

4. Basic Examples of Cycloalkanes Number of Carbons Example: Cyclohexane = a ring of six carbons

5. Naming Cycloalkanes • 1) Find the parent: Count the number carbons in the chain and the substituents. If the substituents is longer then the ring then the parent name is the substituent. If the ring is longer then the ring is the parent.

6. Examples of Rule 1 • If we look at the molecule on the left. A pentane ring has 5 carbons compared to its substituent (1 carbon) a methyl. So the parent is the cyclopentane. However on the molecules on the right butane (4 carbons) is larger than cyclopropane (3 carbons) so the butane is the parent name. 1-Methylcyclopentane 1-Cyclopropylbutane

7. Rule 2 • Number the substituents, and write the name. • Make sure that you follow the ring so that the substituents have the lowest numbers. • If two or more alkyl chains can be the same number then number alphabetically. • Treat halogens as alkyl chains (F, Cl, Br, I).

8. Rule 2 Examples

9. Stereoisomers • Unlike Constitutional Isomers which have the same molecular formula but a different orientation. Stereoisomers have the same order of connection but differ in spatial (or three dimensional) orientation.

10. Cis-Trans Isomerisms • Because ring don’t rotate freely in space like linear chains two different possibilities exist. Cis Isomers are when two atoms are on the same side of a ring. Trans Isomers are when two atoms are on different side of a ring. Trans Isomers Cis Isomers

11. Types of Strain • Angle Strain – also called Ring Strain is the strain due to expansion or compression of bond angles. • Torsional Strain – the strain due to eclipsing of bonds on neighboring atoms. • Steric Strain – the strain due to repulsive interactions when atoms approach each other too closely.

12. Angle (Ring) Strain • Cyclopropane has angles of 90o when we know that SP3 carbons like to bond at 109.5o. In cyclopropane there is angle strain. Example cyclopropane and cyclohexane:

13. Torsional Strain • Is the strain due to eclipsing of bonds on neighboring atoms. • When atoms are eclipsed then there is a natural tendency to move away from each other this is called torsional strain.

14. Steric Strain • Steric strain focuses on the strain of functional groups bound due to size onto the cyclic ring. Because rings don’t rotate freely in space like linear chains then energy is higher when they are on the same side. For example: cyclopropane Higher Energy Lower Energy

15. Stability of Cycloalkanes • Because we know that SP3 carbons like to have bond angles of 109o when the angles are smaller or larger than 109o the energy increases. It does not like bond angles. The greater the angle difference the more it is ready to explode to alleviate the strain.

16. Conformation (Cyclopropane) • The most ring strained alkane ring 60o instead of 109.5o. • The two hydrogens eclipse each other. • Bonds are weaker and more reactive than any other cycloalkanes.

17. Conformation (Cyclobutane) • The most torsional strained alkane ring due to the increase number of hydrogens. • Because of the increased torsional strain the total energy of cyclopropane and cyclobutane are the same.

18. Confirmation (Cyclopentane) • Cyclopentane has no angle (ring) strain but has a lot of torsional strain so it twists to a puckered non-planar configuration.

19. Conformation (Cyclohexane) • Cyclohexane is strain free which means that it is stable. • Cyclohexane is widely used in nature. • Adopt 2 three-dimensional structures or conformations (Chair and Boat).

20. Chair and Boat • There are two chair forms and one boat form. The boat form is a transition between one chair form and the other. If you look closely then you will see that if the red carbons bend up you will have the boat. If the blue carbons bend down then you will have the second chair form.

21. Cyclohexane Bonding Stability • - Steric Strain occurs when substituents are either both axial (higher energy) or equatorial (lower energy). • Mono substituted cyclohexanes are more stable when they are in the equatorial position. • To get the lowest energy isomer always place the largest group in the equatorial position.

22. Cyclohexane (Axial) There are six axial positions that are perpendicular to the ring in the chair configuration.

23. Cyclohexane (equatorial) There are six axial positions that are roughly in the plane of the ring, around the equator.

24. Cis-Trans Isomers of Cyclohexane Regardless of the position (axial and equatorial) the if two atoms are bound to the top position it is the Cis isomer. Alternatively, if the positions are on the opposite side then it is in the Trans position. Cis Isomer Trans Isomer

25. Conformations of Monosubstituted Cyclohexanes • Since the equatorial position is the most stable it is preferred over the axial position when flipping. Always look for the Mono-substituted in the equatorial position.

26. Conformation of Polycyclic Molecules • Polycyclic Molecules are the joining of two individual cycloalkane rings. • There are commonly 2 types of polycyclic molecules: • The joining of two ring structures • Bicyclic alkanes Trans-Decalin Norbornane

27. Take Home Message • Know how to name cycloalkanes. • Know the difference between cis and trans. • Know Angle Strain, Torsional Strain, Steric Strain. • Know why is cyclopropane so high energy. • Know the conformations of cyclohexanes (boat and chair) and what is axial and equatorial. • Know the properties of cyclohexanes.