Chapter 4 Lect. 2 Finish Cycloalkanes

I. Hydrogens on CyclohexaneA. Two types of H on cyclohexane chair conformations

1) 6 C-H bonds perpendicular to the ring axis = Equatorial2) 6 C-H bonds parallel to the ring axis = Axial

B. Drawing chair conformations1) 3 sets of 2 parallel lines give the ring2) Axial bonds straight up and down on alternating carbons3) Equatorial bonds are parallel to one C-C bond away

C. Ring Flipping1) Ea for interconverting between chair conformations = 10.8 kcal/mol

2) Happens about 100,000 times per second for cyclohexane3) Ring inverts to give the other conformation

4) Every equatorial H becomes axial, every axial H become equatorialII. Substituted Cyclohexanes

A. Equatorial vs. Axial Positions: Methylcyclohexane1) Two possibilities for where CH3 group can go

2) Axial position has methyl group close to 2 axial H’sa) 1,3-diaxial interactionb) Like gauche butane

3) Equatorial position has methyl group out away from other H’s: like anti-4) Go = -1.7 kcal/mol going axial to equatorial methyl: 95% equatorial5) Energy difference increases as the substituent gets larger: 99.99%

equatorial for (1,1-dimethylethyl) substituent (tertiary butyl)

B. Multiple Substituents1) Predict which substituent “wins” equatorial position based on size

H3C

CH3

CH3

H3C CH3

CH3CH3

CH3

2) Same substituent on one carbon: no change when ring flips

3) Same substituent cis on two different carbons: no change with ring flip

4) Same substituent trans on two different carbons: diaxial vs. diequatorial

III. Polycyclic AlkanesA. Fused rings: when 2 or more cycloalkanes share the same 2 adjacent carbons

1) Decalin = 2 fused cyclohexanesa) Fused Bicyclicb) 2 shared carbons are called ring fusion carbons

CH3

CH3 CH3

CH3

CH3

CH3

H3C

CH3

CH3

CH3

H3CCH3

2) Cis and trans isomers of decalin have different properties

B. Bridged Bicyclics: 2 rings share any 2 carbons1) Norbornane

2) Shared carbons are called Bridgehead Carbons

C. Cool structures from Fused/Bridged Cycloalkanes

H

H

H

H

H

H

H

H

+

Tetrahedrane(C4H4)

CubaneC8H8

IV. Natural Product CycloalkanesA. Natural Products = organic compounds produced by plants or animals

1) Classification bya) Structureb) Physiological activityc) Taxonomyd) Biochemical origin

2) Drugs, dyes, raw materials, etc…3) Organic Chemists try to synthesize from simple molecules

a) Expensiveb) Rare

B. Taxol1) Terpene (biochemical origin classifications)2) Comes from Isoprene3) Powerful anticancer drug

C. Steroids1) Hormones that regulate body biochemistry are often steroids2) Birth control pills contain estrogen and progesterone