chapter 3 alkanes and cycloalkanes: conformations and cis-trans stereoisomers

Chapter 3Chapter 3Alkanes and Cycloalkanes: Alkanes and Cycloalkanes:

Conformations and cis-trans Conformations and cis-trans StereoisomersStereoisomers

3.13.1

Conformational Analysis of EthaneConformational Analysis of Ethane

Conformations are different spatial Conformations are different spatial arrangements of a molecule that are arrangements of a molecule that are generated by rotation about single bonds.generated by rotation about single bonds.

eclipsed conformation

Ethane

Ethane

eclipsed conformation

Ethane

staggered conformation

Ethane

staggered conformation

Projection Formulas of the Staggered Conformation of Ethane

Newman Sawhorse

H

H

H H

H H

H

H HH

H

H

H

H

H H

H H

H

H HH

H

H180°

Anti Relationships

Two bonds are anti when the angle between them is 180°.

H

H

H H

H H

H

H HH

H

H

60°

Gauche Relationships

Two bonds are gauche when the angle between them is 60°.

An Important Point:

The terms anti and gauche apply only to bonds (or groups) on adjacent carbons, and only to staggered conformations.

The terms anti and gauche apply only to bonds (or groups) on adjacent carbons, and only to staggered conformations.

0° 60° 120° 180° 240° 300° 360°

12 kJ/mol12 kJ/mol

The eclipsed conformation of ethane is 12 kJ/mol less stable than the staggered.

The eclipsed conformation is destabilized bytorsional strain.

Torsional strain is the destabilization that resultsfrom eclipsed or partially eclipsed bonds.

Torsional Strain

3.23.2Conformational Analysis of ButaneConformational Analysis of Butane

0° 60° 120° 180° 240° 300° 360°

3 kJ/mol3 kJ/mol

14 kJ/mol14 kJ/mol

The gauche conformation of butane is 3 kJ/molless stable than the anti.

The gauche conformation is destabilized byvan der Waals strain (also called steric strain).

van der Waals strain is the destabilization that results from atoms being too close together.

van der Waals Strain

The conformation of butane in which the twomethyl groups are eclipsed with each other isthe least stable of all the conformations.

It is destabilized by both torsional strain(eclipsed bonds) and van der Waals strain.

van der Waals Strain

3.33.3

Conformations of Higher AlkanesConformations of Higher Alkanes

The most stable conformation of unbranchedalkanes has anti relationships between carbons.

Hexane

Unbranched Alkanes

3.4The Shapes of Cycloalkanes:

Planar or Nonplanar?

Baeyer assumed cycloalkanes are planar polygons,

and that distortion of bond angles from 109.5° givesangle strain to cycloalkanes with rings eithersmaller or larger than cyclopentane.

Baeyer deserves credit for advancing the ideaof angle strain as a destabilizing factor.

But Baeyer was incorrect in his belief that cycloalkanes were planar.

Adolf von Baeyer (19th century)

• Torsional strain strain that results from eclipsed bonds

• van der Waals strain (steric strain)strain that results from atoms being too close together

• angle strainstrain that results from distortion of bondangles from normal values

Types of Strain

Measuring Strain in Cycloalkanes

Heats of combustion can be used to comparestabilities of isomers.

But cyclopropane, cyclobutane, etc. are not isomers.

All heats of combustion increase as the numberof carbon atoms increase.

Measuring Strain in Cycloalkanes

Therefore, divide heats of combustion by number of carbons and compare heats of combustion on a "per CH2 group" basis.

Cycloalkane kJ/mol Per CH2

Cyclopropane 2,091 697

Cyclobutane 2,721 681

Cyclopentane 3,291 658

Cyclohexane 3,920 653

Cycloheptane 4,599 657

Cyclooctane 5,267 658

Cyclononane 5,933 659

Cyclodecane 6,587 659

Heats of Combustion in Cycloalkanes

Cycloalkane kJ/mol Per CH2

According to Baeyer, cyclopentane should

have less angle strain than cyclohexane.

Cyclopentane 3,291 658

Cyclohexane 3,920 653

The heat of combustion per CH2 group is

less for cyclohexane than for cyclopentane.

Therefore, cyclohexane has less strain than

cyclopentane.

Heats of Combustion in Cycloalkanes

Adolf von Baeyer (19th century)

assumed cycloalkanes are planar polygons

distortion of bond angles from 109.5° givesangle strain to cycloalkanes with rings eithersmaller or larger than cyclopentane

Baeyer deserves credit for advancing the ideaof angle strain as a destabilizing factor.

But Baeyer was incorrect in his belief that cycloalkanes were planar.

Cyclopropane Cyclopropane

Cyclobutane Cyclobutane

3.53.5Small RingsSmall Rings

sources of strain

torsional strain

angle strain

Cyclopropane

nonplanar conformation relieves some torsional strain

angle strain present

Cyclobutane

3.63.6CyclopentaneCyclopentane

all bonds are eclipsed in planar conformation

planar conformation destabilizedby torsional strain

Cyclopentane

Envelope Half-chair

Relieve some, but not all, of the torsional strain.

Envelope and half-chair are of similar stabilityand interconvert rapidly.

Nonplanar Conformations of Cyclopentane

heat of combustion suggests that angle strain is unimportant in cyclohexane

tetrahedral bond angles require nonplanar geometries

3.73.7Conformations of CyclohexaneConformations of Cyclohexane

All of the bonds are staggered and the bond angles at carbon are close to tetrahedral.

Chair is the most stable conformation of cyclohexane

All of the bond angles are close to tetrahedralbut close contact between flagpole hydrogenscauses van der Waals strain in boat.

180 pm

Boat conformation is less stable than the chair

Eclipsed bonds bonds gives torsional strain toboat.

Boat conformation is less stable than the chair

Less van der Waals strain and less torsional strain in skew boat.

Boat Skew boat

Skew boat is slightly more stable than boat

The chair conformation of cyclohexane is themost stable conformation and derivativesof cyclohexane almost always exist in the chair conformation.

Generalization

3.83.8Axial and Equatorial Bonds in Axial and Equatorial Bonds in

CyclohexaneCyclohexane

The 12 bonds to the ring can be divided into two sets of 6.

Axial bonds point "north” and “south"

6 Bonds are axial

The 12 bonds to the ring can be divided into two sets of 6.

Equatorial bonds lie along the “equator.”

6 Bonds are equatorial

3.93.9Conformational InversionConformational Inversion

in Cyclohexane in Cyclohexane

chair-chair interconversion (ring-flipping)

rapid process (activation energy = 45 kJ/mol)

all axial bonds become equatorial and vice versa

Conformational Inversion

Half-chair

Skewboat

45 45 kJ/molkJ/mol

45 45 kJ/molkJ/mol

23 kJ/mol

most stable conformation is chairmost stable conformation is chair

substituent is more stable when equatorialsubstituent is more stable when equatorial

3.103.10Conformational Analysis of Conformational Analysis of

Monosubstituted CyclohexanesMonosubstituted Cyclohexanes

5% 95%

Chair chair interconversion occurs, but at any instant 95% of the molecules have their methyl group equatorial.

Axial methyl group is more crowded than an equatorial one.

Methylcyclohexane

CH3 CH3

5% 95%

Source of crowding is close approach to axial hydrogens on same side of ring.

Crowding is called a "1,3-diaxial repulsion" and is a type of van der Waals strain.

Methylcyclohexane

40% 60%

Crowding is less pronounced with a "small" substituent such as fluorine.

Size of substituent is related to its branching.

F

F

Fluorocyclohexane

Less than 0.01% Greater than 99.99%

Crowding is more pronounced with a "bulky" substituent such as tert-butyl.

tert-Butyl is highly branched.

C(CH3)3 C(CH3)3

tert-Butylcyclohexane

van der Waalsstrain due to1,3-diaxialrepulsions

tert-Butylcyclohexane

3.113.11Disubstituted Cycloalkanes:Disubstituted Cycloalkanes:

cis-trans Stereoisomerscis-trans Stereoisomers

Stereoisomers are isomers that have Stereoisomers are isomers that have same constitution but different same constitution but different arrangement of atoms in spacearrangement of atoms in space

Isomers

Constitutional isomersConstitutional isomers StereoisomersStereoisomers

1,2-Dimethylcyclopropane

There are two stereoisomers of 1,2-dimethylcyclopropane.

They differ in spatial arrangement of atoms.

1,2-Dimethylcyclopropane

cis-1,2-Dimethylcyclopropane has methyl groupson same side of ring.

trans-1,2-Dimethylcyclopropane has methyl groupson opposite sides.

Relative stabilities of stereoisomers may bedetermined from heats of combustion.

3371 kJ/mol

3366 kJ/mol

Van der Waals strain makes cis stereoisomer less stable than trans.

3.123.12Conformational AnalysisConformational Analysis

of Disubstituted Cyclohexanesof Disubstituted Cyclohexanes

cis trans

CH3

5219 kJ/mol 5212 kJ/mol

less stable more stable

Trans stereoisomer is more stable than cis, but methyl groups are too far apart to crowd each other.

H3C

H H

H3C

CH3H

H

1,4-Dimethylcyclohexane Stereoisomers

CH3H3C

H H

Two equivalent conformations; each has one axial methyl group and one equatorial methyl group

H

CH3

HCH3

H

H3C

H

CH3

Conformational analysis ofcis-1,4-

dimethylcyclohexane

CH3

H3C

H

H

Two conformations are not equivalent; most stableconformation has both methyl groups equatorial.

H

H3C

H

CH3

H

H3C

H

CH3

Conformational analysis oftrans-1,4-

dimethylcyclohexane

cis trans

5223 kJ/mol 5217 kJ/mol

less stable more stable

Analogous to 1,4 in that trans is more stablethan cis.

CH3

CH3H

HH3C

CH3

H

H

1,2-Dimethylcyclohexane Stereoisomers

CH3

CH3H

H

Two equivalent conformations; each has one axial methyl group and one equatorial methyl group

HCH3

H

CH3 H

CH3

H

CH3

Conformational analysis ofcis-1,2-

dimethylcyclohexane

CH3

H3C H

H

Two conformations are not equivalent; most stableconformation has both methyl groups equatorial.

H

CH3

H

CH3

H

H3C

H

CH3

Conformational analysis oftrans-1,2-

dimethylcyclohexane

cis trans

5212 kJ/mol 5219 kJ/mol

more stable less stable

Unlike 1,2 and 1,4; cis-1,3 is more stable than trans.

H3C

CH3

H

H

CH3

H3C

H H

1,3-Dimethylcyclohexane Stereoisomers

CH3

H3C

H H

Two conformations are not equivalent; most stableconformation has both methyl groups equatorial.

H3C

HH

CH3

H

CH3

H

CH3

Conformational analysis ofcis-1,3-

dimethylcyclohexane

Two equivalent conformations; each has one axialand one equatorial methyl group.

H3C H

H CH3

H

H3C

HCH3

H3C

CH3

H

H

Conformational analysis oftrans-1,3-

dimethylcyclohexane

Compound Orientation -H° (kJ/mol)

cis-1,2-dimethyl ax-eq 5223trans-1,2-dimethyl eq-eq 5217*

cis-1,3-dimethyl eq-eq 5212*trans-1,3-dimethyl ax-eq 5219

cis-1,4-dimethyl ax-eq 5219trans-1,4-dimethyl eq-eq 5212*

*more stable stereoisomer of pair

Table 3.2 Heats of Combustion ofIsomeric Dimethylcyclohexanes

3.133.13Medium and Large RingsMedium and Large Rings

More complicated than cyclohexane.

Common for several conformations to be of similar energy.

Principles are the same, however:minimize total strain.

Cycloheptane and Larger Rings

contain more than one ring

bicyclic

tricyclic

tetracyclic

etc

3.143.14

Polycyclic Ring SystemsPolycyclic Ring Systems

spirocyclic

fused ring

bridged ring

Types of Ring Systems

Adamantane: A Tricyclic Compound

Three bond cleavages are needed to create an open-chain structure.

one atom common to two rings Spirocyclic

adjacent atoms common to two rings

two rings share a common side Fused Ring

nonadjacent atoms common to two rings

Bridged Ring

equals minimum number of bond disconnectionsrequired to give a noncyclic species

Number of Rings

requires one bond disconnection Monocyclic

requires two bond disconnections Bicyclic

requires two bond disconnections Bridged Bicyclic

carbon skeleton is tetracyclic Steroids

3.153.15

Heterocyclic CompoundsHeterocyclic Compounds

a cyclic compound that contains an atom other

than carbon in the ring

(such atoms are called heteroatoms)

typical heteroatoms are N, O, and S

Heterocyclic Compound

Ethylene oxide

Tetrahydrofuran

Tetrahydropyran

Oxygen-containing Heterocycles O

O

O

O

Pyrrolidine Piperidine

Nitrogen-containing Heterocycles N H

N

H

Lipoic acid

Lenthionine

CH2CH2CH2CH2COH

O

SS SS

SS

S

Sulfur-containing Heterocycles