Transcript Ch 4 Lecture 2

Chapter 4 Lect. 2 Finish Cycloalkanes
I.
Hydrogens on Cyclohexane
A.
B.
Two types of H on cyclohexane chair conformations
1) 6 C-H bonds perpendicular to the ring axis = Equatorial
2) 6 C-H bonds parallel to the ring axis = Axial
Drawing chair conformations
1) 3 sets of 2 parallel lines give the ring
2) Axial bonds straight up and down on alternating carbons
3) Equatorial bonds are parallel to one C-C bond away
C.
Ring Flipping
1) Ea for interconverting between chair conformations = 10.8 kcal/mol
2) Happens about 100,000 times per second for cyclohexane
3) Ring inverts to give the other conformation
4)
II.
Every equatorial H becomes axial, every axial H become equatorial
Substituted Cyclohexanes
A.
Equatorial vs. Axial Positions: Methylcyclohexane
1) Two possibilities for where CH3 group can go
2) Axial position has methyl group close to 2 axial H’s
a) 1,3-diaxial interaction
b) Like gauche butane
3) Equatorial position has methyl group out away from other H’s: like anti4) DGo = -1.7 kcal/mol going axial to equatorial methyl: 95% equatorial
5) Energy difference increases as the substituent gets larger: 99.99%
equatorial for (1,1-dimethylethyl) substituent (tertiary butyl)
B.
Multiple Substituents
1) Predict which substituent “wins” equatorial position based on size
CH3
H3C
H3C
CH3
CH3
CH3
CH3
CH3
2)
Same substituent on one carbon: no change when ring flips
CH3
CH3
CH3
CH3
3)
Same substituent cis on two different carbons: no change with ring flip
CH3
CH3
H3C
4)
CH3
Same substituent trans on two different carbons: diaxial vs. diequatorial
CH3
H3C
CH3
CH3
III. Polycyclic Alkanes
A.
Fused rings: when 2 or more cycloalkanes share the same 2 adjacent carbons
1) Decalin = 2 fused cyclohexanes
a) Fused Bicyclic
b) 2 shared carbons are called ring fusion carbons
2)
Cis and trans isomers of decalin have different properties
H
H
H
H
H
H
H
B.
Bridged Bicyclics: 2 rings share any 2 carbons
1) Norbornane
+
2)
C.
Shared carbons are called Bridgehead Carbons
Cool structures from Fused/Bridged Cycloalkanes
Tetrahedrane
(C4H4)
Cubane
C8H8
H
IV. Natural Product Cycloalkanes
A.
B.
C.
Natural Products = organic compounds produced by plants or animals
1) Classification by
a) Structure
b) Physiological activity
c) Taxonomy
d) Biochemical origin
2) Drugs, dyes, raw materials, etc…
3) Organic Chemists try to synthesize from simple molecules
a) Expensive
b) Rare
Taxol
1) Terpene (biochemical origin classifications)
2) Comes from Isoprene
3) Powerful anticancer drug
Steroids
1) Hormones that regulate body biochemistry are often steroids
2) Birth control pills contain estrogen and progesterone