carey chapter 3 – conformations of alkanes and cycloalkanes figure 3.5
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Carey Chapter 3 – Conformations of Alkanes and Cycloalkanes
Figure 3.5
Conformational Analysis
N
NH2
ON
O
HO
HH
PO
HO
OH
OH
Cytosine (C) (from TGCA alphabet in
DNA
Hemoglobin
Thymidine – incorporated into DNA as “T”
Zidovudine (AZT) – incorporated into DNA instead of T – stops chain growth
O N
N3
HONH
O
O
O N
HO
HONH
O
O
3.1 Conformational analysis of Ethane
Figure 3.1
(use SpartanModel)
Since single bonds can rotate around the bond axis, different conformations are possible -
conformational analysis
3.1 Conformational analysis
H
H
H
HH
H
H
H H
H
HH
H
H HH
HH
Different 3-D depictions of Ethane
Wedge/dash Sawhorse Newman Projection
Rotation around the central C-C bond will cause the hydrogens to interact - rotamers or conformers
Definitions
H
H HH
HH
H
HH
H
HH
H
H HH
HH
Gauche
torsion angle 60o
Eclipsed
torsion angle 0o
Anti
torsion angle 180o
Both gauche and anti conformers are staggered
Eclipsed conformers are destabilized by torsional strain
3.1 Conformational analysis of Ethane
Figure 3.4
3.2 Conformational analysis of Butane
Figure 3.7
3.3 Conformations of higher alkanes
CH3
HHCH3
H HH3C
HHH3C
H
H
CH3
HHH
H3C HH3C
HHH
CH3
H
Anti(staggered)
eclipsed Gauche(staggered)
eclipsed
Applicable for any
acyclic molecule
3.4 Cycloalkanes – not planar
Cyclohexane
3.5 Cyclopropane and Cyclobutane
Figure 3.10
3.6 Cyclopentane
Figure 3.12
3.7 Conformations of Cyclohexane
Conformationally flexible (without breaking bonds)
Chair Boat Chair
3.7-3.8 Cyclohexane – axial and equatorial positions
Figure 3.13-3.14
3.9 Conformational inversion – ring flipping
Figure 3.18
3.10 Analysis of monosubstituted cyclohexanes
CH3
H CH3
H
F
H F
H
C(CH3)3
H C(CH3)3
H
40% 60%
5% 95%
<0.01% >99.99%
Based on unfavourable 1,3-diaxial interactions
3.11 Disubstituted cycloalkanes - Stereoisomers
Cis-1,2-dimethylcyclopropane is less stable than the trans isomer
Cis-1,2-dimethylcyclohexane is less stable than the trans isomer
Cis-1,3-dimethylcyclohexane is more stable than the trans isomer
Cis-1,4-dimethylcyclohexane is less stable than the trans isomer
All based on interactions between substituents and other groups on the ring
3.11 Disubstituted Cyclopropanes
Figure 3.20
3.12 Disubstituted Cyclohexanes
trans-1,2-dimethylcyclohexane is more stable than the cis isomer
cis-1,3-dimethylcyclohexane is more stable than the trans isomer
trans-1,4-dimethylcyclohexane is more stable than the cis isomer
Consider 1,2- ; 1,3- ; and 1,4-disubstituted isomers
3.13 Medium and large rings – not covered
3.14 – Polycyclic compounds – covering bicyclics
Bicyclobutane
Bicyclo[3.2.0]heptane
Bicyclo[2.2.2]octane
3.15 Heterocyclic compounds
OHN
HN
tetrahydrofuran pyrrolidine piperidine
morphine ritilin librium
3.15 Heterocyclic compounds
O
OH
HOHO
OHOH
D-Glucose (dextrose, blood sugar)