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)