Cycloalkanes

1. Cycloalkanes • All C atoms in the ring are sp3. • General formula for cycloalkanes is CnH2n. • Physical properties are similar to those of the compact, branched alkanes.

2. Nomenclature of Cycloalkanes • Count the number of C atoms in the ring and use a prefix of cyclo- . • If there is only one substituent, no numbering is needed. • If there are two or more substituents, number the C’s in the ring to give the lowest possible numbers for the substituents. • A cycloalkane may be named as the main chain or as a substituent.

3. Nomenclature of Cycloalkanes 3-cyclopropyl-1,1-dimethylcyclohexane 3-sec-butyl-1,1-dimethylcyclohexane 1,1-dimethyl-3-(1-methylpropyl)cyclohexane

4. cis-trans Isomerism of Cycloalkanes • Although cycloalkanes do not have double bonds, they do have two “faces.” • Substituents pointing toward the same face are cis. • Substituents pointing toward opposite faces are trans.

5. cis-trans Isomerism of Cycloalkanes trans-1,2-dichlorocyclopentane cis-1,2-dichlorocyclopentane

6. Ring Strain in Cycloalkanes • Five- and six-membered rings are the most stable. • Components of ring strain • angle strain • when bond angles are <109.5° • torsional strain (aka steric strain) • when eclipsed conformations result • Quantified by comparing heats of combustion data.

7. Ring Strain in Cyclopropane • Angle strain is large • bond angle is 60° instead of 109.5° • Torsional (steric) strain is present • 3-membered ring results in an eclipsed conformation that is “locked in.” • Heat of combustion is 38.5 kJ per -CH2- higher than that of a long-chain alkane. • Total ring strain is 115 kJ.

8. Ring Strain in Cyclopropane torsional strain angle strain

9. Ring Strain in Cyclobutane • Angle strain is large • bond angle is 88° instead of 109.5° • Torsional strain is present • 4-membered ring results in an almost-eclipsed conformation. • Heat of combustion is 27.5 kJ per -CH2- higher than that of a long-chain alkane. • Total ring strain is 110 kJ.

10. Ring Strain in Cyclobutane C-C bond angles are 88°, the result of a nonplanar geometry that relieves a little torsional strain.

11. Ring Strain in Cyclopentane • Cyclopentane flips through puckered “envelope” conformations. • Allows bond angles of nearly 109.5°. • Reduces torsional strain by allowing movement away from a totally eclipsed (planar) conformation. • Heat of combustion is only 5.4 kJ per -CH2- higher than that of a long-chain alkane. • Total ring strain is only 27 kJ.

12. Not Much Ring Strain in Cyclopentane

13. Cyclohexane Has NO Ring Strain • Cyclohexane can assume various puckered conformations. • Allows bond angles of 109.5°. • Chair conformation has no eclipsing. • Heat of combustion is the same as that of a long-chain alkane. • Total ring strain is 0 kJ. • The six-membered ring is ubiquitous in nature. • carbohydrates • steroids • plant products

14. Chair Conformation of Cyclohexane

15. Boat Conformation of Cyclohexane The hydrogens in the boat conformation are all eclipsed, leading to steric strain.

16. Twist-Boat Conformation of Cyclohexane The hydrogens are not totally eclipsed. This is the second most stable conformation. It is this conformation that is generally meant when the term “boat” is used.

17. Comparison of Chair and Boat Conformations of Cyclohexane

18. Half-Chair Conformation of Cyclohexane

19. Conformational Energies of Cyclohexane At room temperature, cyclohexane flips through all of these conformations, but spends the most time in the chair conformations.

20. Ring Flipping in Cyclohexane • At room temperature, cyclohexane “flips” from one chair conformation to the other. • When this happens, atoms in axial positions become atoms in equatorial positions, and vice versa. axial equatorial

21. 1,3-diaxial Interaction in Cyclohexane • A substituent in an axial position is gauche to the ring C’s that are two away. This form of steric hindrance is called a 1,3-diaxial interaction. • When the ring flips, the substituent is in an equatorial position and anti to the ring C’s that are two away. • This means the equatorial positions are lower energy and more favored for substituents.

22. 1,3-diaxial Interaction in Cyclohexane • When there are two substituents on the ring in a 1,3 arrangement, the steric hindrance is even more severe. • The cis isomer will be lower in energy than the trans isomer. • When the substituents are not the same, the larger substituent goes to the equatorial position.

23. 1,3-diaxial Interaction in Cyclohexane • The cis isomer will be lower in energy than the trans isomer. cis trans

24. 1,3-diaxial Interaction in Cyclohexane • When the substituents are not the same, the larger substituent goes to the equatorial position.

25. 1,3-diaxial Interaction in Cyclohexane • The t-butyl group is so bulky that it will ALWAYS go to the equatorial position. • t-butyl groups on C1 and C4 will cause the twist-boat conformation to be lower energy than either of the chair conformations.

26. Other Interactions in Cyclohexane • Which is more stable, the cis or trans isomer of 1-ethyl-2-methylcyclohexane?

27. Nomenclature of Bicyclo Compounds • Count the number of C atoms in the rings (ignore substituents). • This compound will be a bicyclodecane.

28. Nomenclature of Bicyclo Compounds • Find the bridge carbons. Here they are numbered 1 and 6. • Count the carbons in the two chains that connect the bridge carbons. Here, there are four carbons in each.

29. Nomenclature of Bicyclo Compounds • Finally, count the carbons in the bridge and between the bridge carbons. Here there are none. • The name of this compound is bicyclo[4.4.0]decane

30. Nomenclature of Bicyclo Compounds bicyclo[2.2.1]heptane bicyclo[3.2.2]nonane