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Chapter 03Alkanes and Cycloalkanes:

Conformations and cis-trans Stereoisomers

CHEM 341: Spring 2012

Prof. Greg Cook

cook.chem.ndsu.nodak.edu/chem341

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Conformations

• All single bonds freely rotate at room temperature (unless constrained by a ring).

• Thus, linear alkanes are in constant motion.

• If the molecules were frozen to absolute zero you could see different arrangements of the groups depending on the state of the bond rotations.

• Conformers: Different rotational isomers (conformations) of a molecule.

• Conformational Analysis: Study of how conformations affect a molecule.

2

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Representation of 3D Structures

• Sawhorse Projection: A view of a molecule showing wedges and dashes for bonds coming out or going into the plane of the paper - resembles a sawhorse.

3

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Representation of 3D Structures

• Newman Projection: A view of a molecule looking straight down one C-C single bond (see below).

4

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Ethane Conformations

5

C CH

H H

HH

H

Ethane Conformations

sawhorse

view down this axis to see Newman Projection

H

H

H

H

H

H

Newman Projectionstaggered

60° rotate front C

by 60° H

H

HHH

HNewman Projection

eclipsed

0°

higher in energy by 2.9 kcal/mol

E

rotation0° 60° 120° 180° 240° 300° 360°

Staggered Staggered Staggered Staggered

Eclipsed Eclipsed Eclipsed

2.9 kcal/mol

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A few more terms

• Staggered Conformation: one in which the relationship of the groups on one carbon versus an adjacent carbon (front and back on a Newman projection) are aligned 60° apart.

• Eclipsed Conformation: one in which the relationship of the groups on one carbon versus an adjacent carbon are aligned 0° apart.

6

H

H

H

H

H

H

Newman Projectionstaggered

60°

H

H

HHH

HNewman Projection

eclipsed

0°

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A few more terms

• Torsional Strain: The strain introduced by electron repulsion of bonds on adjacent carbons. This is highest when the bonds are eclipsed and lowest when staggered.

• Steric Strain: The strain introduced when atoms are forced to become close to each other (they bump into each other).

7

H

H

H

H

H

H

Newman Projectionstaggered

60°

H

H

HHH

HNewman Projection

eclipsed

0°

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8

anti Butane Conformations

CCH3C

CH3H

HH

H

Butane Conformations

sawhorse

view down this axis to see Newman Projection

H

CH3

H

H

CH3

H

Newman Projectionanti-staggered

anti-methyls - 180°

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anti-Butane

9

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eclipsed Butane ConformationsButane Conformations

H

CH3

H

H

CH3

H

Newman Projectionanti-staggered

rotate front Cby 60°

H

CH3

HH3CH

HCH3 and H

higher in energy by 3.8 kcal/mol

CH3 and H

Newman Projectioneclipsed

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Eclipsed Butane

11

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12

gauche Butane Conformations

Butane Conformations

H

CH3

H

H3C

H

H

Newman Projectiongauche-staggered

rotate front Cby 60°

H

CH3

HH3CH

H 0.9 kcal/mol higher in energy than the anti-staggered (lowest energy) conformation

Newman Projectioneclipsed

60°

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Gauche Butane

13

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eclipsed Butane ConformationsButane Conformations

H

CH3

H

H3C

H

H

Newman Projectiongauche-staggered

rotate front Cby 60°

CH3 and CH3

4.5 kcal/mol higher in energy than the anti-staggered conformationH

CH3

HH H

CH3

Newman Projectioneclipsed

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syn-eclipsed Butane

15

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Butane Energy Profile

16

E

rotation0° 60° 120° 180° 240° 300° 360°

Staggeredanti

Staggeredgauche

Staggeredgauche

Staggeredanti

Eclipsed

EclipsedMethyls Aligned

Eclipsed

3.8 kcal/mol 4.5 kcal/mol 0.9 kcal/mol

H

CH3

H

H

CH3

H

H

CH3

HH3CH

H

H

CH3

H

H3C

H

H

60°

H

CH3

HH H

CH3

H

CH3

H

H3C

H

H

60°

H

CH3

HH3CH

H

H

CH3

H

H

CH3

H

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A few more terms

• Anti Conformation: one in which the two groups on adjacent carbons are as far apart as possible (180°) in a staggered conformation.

• Gauche Conformation: one in which the two groups on adjacent carbons are still staggered, but closer (60° apart).

17

H

CH3

H

H

CH3

H

Newman Projectionanti-staggered

H

CH3

H

H3C

H

H

Newman Projectiongauche-staggered

60°

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Higher Alkanes

• The most stable conformation in longer chain alkanes is the all-anti conformation

18

CC

CC

CCH

HH

H

H

H

H

H

H

H

H

H

H

H

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Cyclic Compounds

• Ring Strain

• Angle Strain: the strain due to bond angles being forced to expand or contract from their ideal.

• Torsional Strain: the strain due to electron repulsion of eclipsing bonds.

• Steric Strain: the strain due to atoms coming too close.

19

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Cyclic Compounds

• Heat of Combustion: the amount of heat (energy) released when a molecule burns completely with oxygen.

20

3 4 5 6 7 8 9 10 11 12 13 14Ring Size

Rin

g St

rain

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Cyclopropane

• Highest amount of angle strain

21

60°

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Cyclopropane

22

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Cyclobutane

23

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Cyclopentane

24

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Cyclohexane

25

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Axial and Equatorial Positions of Cyclohexane

26

Pink - AxialBlue - Equatorial

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Ring Flips in Cyclohexane

27

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Energy of Chair Flip Conformations

28

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Cyclohexane Axial Positions More Crowded

29

H

CH3

H

HH

HH

CH3

H

H

H

Haxial methyl equatorial methyl

1,3-diaxial interaction

1.8 kcal/mol more stable conformation

ring flip

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axial Methyl Cyclohexane

30

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equatorial Methyl Cyclohexane

31

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methylcyclohexane Ring Flips

32

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A few more terms

• Stereoisomers: Isomers (different compounds) that have all the same number and kind of atoms that are all connected the same, but differ in their arrangement in three dimensions.

• Due to the restricted rotation in cycloalkanes, molecules with more than one substituent could have the groups either on the same side (cis) or opposite sides (trans) of the plane of the ring. These are stereoisomers.

33

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cis and trans Cycloalkanes

34

Br Br

Br

H

Br

Hcis-1,2-dibromocyclopropane

Br

H

H

Br

trans-1,2-dibromocyclopropane

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cis and trans dimethylcyclopentane

35

=

cis-1,3-dimethylcyclopentane

H3C

H

CH3

HH3C CH3

=

trans-1,3-dimethylcyclopentane

H3C

H

H

CH3H3C CH3

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dimethylcyclohexane

36

CH3

HCH3

H

ax

eq

cis-1,2-dimethylcyclohexane

same interactions in both conformations -- equal in energy

ring flip

CH3

H

CH3

Heq

ax

ring flip

1

21

221

CH3

CH3

CH3

HCH3

H

ax

eq

trans-1,2-dimethylcyclohexane

different interactions in both conformations -- NOT equal in energy

H

CH3

H

CH3eqax

1

21

221

CH3

CH3lower in energy

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cis-1,2-dimethylcyclohexane

37

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cis-1,2-dimethylcyclohexane

38

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trans-1,2-dimethylcyclohexane

39

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trans-1,2-dimethylcyclohexane

40

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trans-1,2-dimethylcyclohexane

41

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cis-1,3-dimethylcyclohexane

42

CH3

HCH3

H

ax

eq

cis-1,3-dimethylcyclohexane

same interactions in both conformations -- equal in energy

ring flip

axring flip1 1

3

1CH3

CH3different interactions in both conformations -- NOT equal in energy

lower in energy

3

CH3

H

H3C

H

3eq

CH3

HCH3

H

ax

eq

trans-1,3-dimethylcyclohexane

ax

1 1

3

1CH3

CH3

3

H

H3C

H

CH3

3

eq

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cis-1,3-dimethylcyclohexane

43

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1,4-dimethylcyclohexane

44

CH3

HCH3

H

ax

eq

cis-1,4-dimethylcyclohexane

same interactions in both conformations -- equal in energy

ring flip

ax

ring flip

1 11CH3

different interactions in both conformations -- NOT equal in energy

lower in energy

eq

CH3

HCH3

H

ax

eq

trans-1,4-dimethylcyclohexane

ax

1 14

1CH3

eq

H3C

H3C

4 4

4

4

4

H3C

H

H

CH3

H

H3C

CH3

H

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1,4-dimethylcyclohexane

45

ax

ring flip

The larger group would prefer to be in the equatorial position -- NOT equal in energy

lower in energy

eq

CH

HCH

H

ax

eq

cis-1-isopropyl-4-methylcyclohexane

1 14

1CH

H3C4

4

H3C

H

CH3

H

H3C CH3

CH3

CH3CH3

CH3

YH

1,3-diaxial interaction

Y kcal/mol strain

-F 0.12

-Cl 0.25

-Br 0.25

-OH 0.5

Y kcal/mol strain

-CH3 0.9

-CH2CH3 0.95

-CH(CH3)2 1.1

-C(CH3)3 2.7

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Boat Conformations

46

boat cyclohexane

H

H

H

HH H

H H

norbornane

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47

Sugar Structure

• Glucose

OHHHHOOHHOHH

CH2OH

OH

HO

H

HO

H

HOHH OH

OH

OH

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48

Polysaccharides

• Starch

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Polycyclic Molecules

49

O

OR

R

CO2R

RH

H

H HH H

fused spiro bridged

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Heterocyclic Molecules

50

ethyleneoxide

OHN

aziridine

OHN

OHN

tetrahydrofuran pyrrolidine tetrahydropyran piperidine

OHN

O N

furan pyrrole pyran pyridineO

HN

morpholine

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