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

Conformational AnalysisConformational Analysis

Conformations are different spatial arrangements of a

molecule that are generated by rotation about single

bonds.

Conformational analysis is the study of how conformational

factors affect the structure of a molecule and its properties.

Representing ConformationsRepresenting Conformations

These are common ways to show conformations.

Newman ProjectionsNewman Projections

In Newman projections we sight down a C C bond. ⎯The front carbon by a point and the back carbon by a circle.

Each carbon has three other bonds that are placed

symmetrically around it.

The bonds on the back carbon are shown sticking out from

the circle.

H

H

H

H

HH

Newman Projections of EthaneNewman Projections of Ethane

Newman projections differ with respect to the rotation

of the front and back carbon atoms relative to each

other. The angles H-C-C-H angle is the torsional or

dihedral angle.

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.

Relative Stability of Newman ProjectionsRelative Stability of Newman Projections

The eclipsed conformation of ethane is the highest

energy conformation. Repulsion between bonds

destabilizes the eclipsed conformation.

The staggered conformation is the most stable. Better

electron delocalization stabilizes the staggered

conformation.

Conformations that are not staggered are said to have

torsional strain.

Relative Stability of Ethane ConformationsRelative Stability of Ethane Conformations

Ethane has infinite conformations corresponding to changes

in the H-C-C-H torsional angle. Follow the “red” hydrogen

atoms.

eclipsed

staggered

Relative Stability of Newman ProjectionsRelative Stability of Newman Projections

At any instant, almost all of the molecules are in

staggered conformations; hardly any are in eclipsed

conformations.

The difference between these two conformations is 12

kJ/mol.

Propane Conformations

•Propane is shown here as a perspective drawing and as Propane is shown here as a perspective drawing and as

a Newman projection looking down the C1a Newman projection looking down the C1—C2 bond.—C2 bond.

•Chapter 3Chapter 3 •1010

•The staggered conformations of propane is lower in energy than the eclipsed The staggered conformations of propane is lower in energy than the eclipsed

conformations. Since the methyl group occupies more space than a hydrogen, the conformations. Since the methyl group occupies more space than a hydrogen, the

torsional strain will be 0.3 kcal/mol higher for propane than for ethane.torsional strain will be 0.3 kcal/mol higher for propane than for ethane.

Propane Conformations

•Chapter 3Chapter 3 •1111

Conformations of ButaneConformations of Butane

There are two different staggered conformations for butane.

The anti conformation is the most stable and the gauche

conformation is higher in energy because the larger CH3

groups are closer. This is called steric strain.

Strain in Newman ProjectionsStrain in Newman Projections

Torsional strain is the strain that results from eclipsed

bonds.

Steric hindrance results when two atoms are too close

together. Also called van der Waals strain.

Steric strain is the combination of both of these.

Newman Projections of ButaneNewman Projections of Butane

The eclipsed conformation with the CH3 groups eclipsed

has the most steric strain and is the highest energy

conformation.

Conformations of Higher AlkanesConformations of Higher Alkanes

The lowest energy conformation of alkanes has all bonds

staggered. With simple alkanes this has a zig-zag

arrangement of carbon atoms.

The Shapes of Cycloalkanes:The Shapes of Cycloalkanes:Planar or Nonplanar?Planar or Nonplanar?

© 2013 Pearson Education, Inc.

Stabilities of Cycloalkanes

• Five- and six-membered rings are the most common in nature.

• Carbons of cycloalkanes are sp3 hybridized and thus require an angle of 109.5º.

• When a cycloalkane carbon has an angle other than 109.5º, there will not be optimum overlap and the compound will have angle strain.

• Angle strain is sometimes called Baeyer strain in honor of Adolf von Baeyer, who first explained this phenomenon.

• Torsional strain arises when all the bonds are eclipsed.

• Chapter 3Chapter 3 • 1717

• 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

Angle strain in CycloalkanesAngle strain in Cycloalkanes

Tetrahedral carbons prefer bond angles of 109.5o and

angle strain refers to the strain molecules have when

this bond angle cannot be matched.

Cyclopropane has the highest angle strain since its

bond angles are about 60o.

Other cycloalkanes try to minimize the angle strain and

are therefore not planar.

Angle strain in CycloalkanesAngle strain in Cycloalkanes

Heat of combustion gives a way to measure the relative

stability of cycloalkanes. The lowest heat of combustion per CH2 group corresponds to the most stable cycloalkane.

CyclopropaneCyclopropane

Strong sp3-sp3 s-bonds cannot be formed because of

60o bond angle.

The bonds are called bent bonds.

CyclopropaneCyclopropane

There is also high torsional strain because the C-H

bonds are all eclipsed.

© 2013 Pearson Education, Inc.

Nonplanar Cyclobutane

• Cyclic compound with four carbons or more adopt nonplanar conformations to relieve ring strain.

• Cyclobutane adopts the folded conformation (“envelope”) to decrease the torsional strain caused by eclipsing hydrogens.

• Chapter 3Chapter 3 • 2323

CyclopentaneCyclopentane

Planar cyclopentane has low angle strain since the natural

bond angle is 108o. Torsional strain is significant because

the C-H bonds are eclipsed.

CyclopentaneCyclopentane

The envelope and half-chair conformations have similar

lower energy and rapidly interconvert. They relieve some,

but not all, torsional strain.

ConformationsConformations of Cyclohexane of Cyclohexane

CyclohexaneCyclohexane

The most stable conformation of cyclohexane is known as

the chair conformation.

The side view shows the bonds across from each other

are parallel. One end carbon (1) is up and the other (4) is

down.

Solid wedges show bonds projecting towards the viewer.


The Newman projection shows that all bonds are

staggered minimizing torsional strain.


Other conformations include the skew boat.

Most molecules are in the chair conformation and less

than 5 molecules in 100,000 are in the skew boat

conformation at any point in time at 25 oC.

Therefore we concentrate on the chair conformation

Axial and Equatorial Bonds Axial and Equatorial Bonds in Cyclohexanein Cyclohexane

DefinitionsDefinitions

The 12 hydrogens of cyclohexane can be divided into two

groups: axial and equatorial.

Equatorial hydrogens lie around the equator of the molecule.

Axial hydrogens are directed alternately up and down.

Conformational InversionConformational Inversion in Cyclohexane in Cyclohexane

Ring InversionRing Inversion

There are two chair conformations of cyclohexane that

rapidly interconvert.

An axial group in the original chair conformation becomes

equatorial in the ring-inverted form and vice versa.

Ring InversionRing Inversion

Ring inversion procedes through highest energy half-chair

conformations and the twist boat and boat conformations.

Conformational Analysis of Conformational Analysis of Monosubstituted CyclohexanesMonosubstituted Cyclohexanes

Interconversion between the two chair conformations occurs rapidly and the methyl group is either equatorial or axial. The conformation with an equatorial methyl is favored.

MethylcyclohexaneMethylcyclohexane

Van der Waals strain between the methyl and the axial hydrogen atoms on the same side of the molecule destabilize the conformation with an axial methyl.

MethylcyclohexaneMethylcyclohexane

Crowding is less pronounced with a "small" substituent such as fluorine so the difference in energy is lower than that observed for the methyl substitutent.

Chair Inversion of FluorocyclohexaneChair Inversion of Fluorocyclohexane

Crowding is more pronounced with a “larger" substituent such as the tertiary butyl group so the difference in energy is much higher than that observed for the methyl substitutent.

Chair Inversion of t-butylcyclohexaneChair Inversion of t-butylcyclohexane

DisubstitutedDisubstituted Cycloalkanes: Cycloalkanes:cis-trans Stereoisomerscis-trans Stereoisomers

Isomers are different compounds that have the same molecular formula.

Constitutional isomers differ in connectivity.

Stereoisomers have the same connectivity but a different arrangement of the atoms in space.

Definitions of IsomersDefinitions of Isomers

A cycloalkane with two substituents on different carbons in the ring may have two orientations.

If the substituents are on the same side of the ring we say they are cis to each other.

If the substituents are on the opposite sides of the ring we say they are trans to each other.

ciscis and and transtrans Substituents Substituents

The cis-stereoisomer is higher in energy due to van der Waals strain. The difference in energy is determined by measuring the heat of combustion.

Relative Energies of StereoisomersRelative Energies of Stereoisomers

Conformational AnalysisConformational Analysisof 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


less stable more stable

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

CH3

CH3H

HH3C

CH3

H

H


CH3

CH3H

H

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

HCH3

H

CH3 H

CH3

H

CH3


dimethylcyclohexane

CH3

H3C H

H


H

CH3

H

CH3

H

H3C

H

CH3


dimethylcyclohexane

cis trans


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


CH3

H3C

H H


H3C

HH

CH3

H

CH3

H

CH3


dimethylcyclohexane

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

H3C H

H CH3

H

H3C

HCH3

H3C

CH3

H

H


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

With two different substituents the lowest conformation of

a particular isomer will have the larger substituent in the

equatorial position. Consider for example the two chair

conformations of cis-1-tert-butyl-2-methylcyclohexane

Other Disubstituted CyclohexanesOther Disubstituted Cyclohexanes

More stable

Most stable conformation minimizes total strain.

This is more complicated than cyclohexane because

there are many conformations.

Furthermore several conformations may be of similar

energy.

Cycloheptane and Larger RingsCycloheptane and Larger Rings

Polycyclic Ring SystemsPolycyclic Ring Systems

Spirocyclic SystemsSpirocyclic Systems

Spirocyclic compounds have two rings with one common atom.

Named as spiro[number.number]alkane. The alkane suffix corresponds to the number of carbons in the two rings.The numbers of carbons in each ring not including the common atom are given in increasing order. This is spiro[3.4]octane.

Two nonadjacent atoms common to two, or more, rings.

Bridged CompoundsBridged Compounds

Named as bicyclo[number.number.number]alkane. The parent alkane corresponds to the total number of carbons in the bicyclic skeleton. The numbers correspond to the number of carbon atoms between the bridgehead atoms in descending order.

This is bicyclo[3.2.1]octane.

Compounds where two rings share a common side. Named as bridged bicyclic systems with one bridge with 0 carbons.

Fused Ring CompoundsFused Ring Compounds

cis-Bicyclo[4.4.0]decane

Steroids have a tetracyclic carbon skeleton with

fused rings. SteroidsSteroids

Cyclic compounds that contains an atom other than

carbon in the ring (these are called heteroatoms).

Oxygen containing heterocycles:

Heterocyclic CompoundsHeterocyclic Compounds

ethylene oxide and tetrahydrofuran

Nitrogen containing heterocycles:

Heterocyclic CompoundsHeterocyclic Compounds

pyrrolidine and piperidine

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

Documents

eclipsed conformations

propane conformations

infinite conformations

conformations of butanethere

eclipsed bonds

steric strain

gauche conformation

anti conformation