Chapter 3 Alkanes and Cycloalkanes: Conformations and cis-trans Stereoisomers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

eclipsed conformation Ethane

eclipsed conformation

Ethane staggered conformation

Ethane staggered conformation

Projection Formulas of the Staggered Conformation of Ethane Newman Projection Sawhorse Projection H H HH HH H H H H H H Model

H H HH HH H H H H H H 180° Anti Relationships in Ethane Two bonds are anti when the angle between them is 180°.

H H HH HH H H H H H H 60° Gauche Relationships in Ethane Two bonds are gauche when the angle between them is 60°. 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/mol Energy Relationships in Ethane

The eclipsed conformation of ethane is 12 kJ/mol less stable than the staggered. The eclipsed conformation is destabilized by torsional strain. Torsional strain is the destabilization that results from eclipsed or partially eclipsed bonds. Torsional Strain

3.2 Conformational Analysis of Butane

0° 60° 120° 180° 240°300°360° 3 kJ/mol 14 kJ/mol

The gauche conformation of butane is 3 kJ/mol less stable than the anti. The gauche conformation in butane is destabilized by van 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 two methyl groups are eclipsed with each other is the least stable of all the conformations. It is destabilized by both torsional strain (eclipsed bonds) and van der Waals strain (close together). van der Waals Strain

3.3 Conformations of Higher Alkanes

The most stable conformation of unbranched alkanes has anti relationships between carbons. Hexane Unbranched Alkanes

3.4 The Shapes of Cycloalkanes: Planar or Nonplanar ?

Adolf von Baeyer (19th century) Baeyer assumed cycloalkanes are planar polygons. Also, that distortion of bond angles from 109.5° gives angle strain to cycloalkanes with rings either smaller or larger than cyclopentane. Baeyer deserves credit for advancing the idea of angle strain as a destabilizing factor. But Baeyer was incorrect in his belief that cycloalkanes were planar.

Torsional strain strain that results from eclipsed bonds. van der Waals strain (steric strain) strain that results from atoms being too close together. Angle strain strain that results from distortion of bond angles from normal values (lack of colinear orbital overlap). Types of Strain

Measuring Strain in Cycloalkanes Heats of combustion can be used to compare stabilities of isomers. However cyclopropane, cyclobutane, etc. are not isomers. Heats of combustion of compounds increase as the number of carbon atoms increase. Therefore, the heats of combustion are divided by number of carbons and compared on a "per CH 2 group" basis.

CycloalkanekJ/molper CH 2 Cyclopropane2, Cyclobutane2, Cyclopentane3, Cyclohexane3, Cycloheptane4, Cyclooctane5, Cyclononane5, Cyclodecane6, Heats of Combustion in Cycloalkanes

According to Baeyer, cyclopentane should have less angle strain than cyclohexane. kJ/molper CH 2 Cyclopentane3, Cyclohexane3, The heat of combustion per CH 2 group is less for cyclohexane than for cyclopentane. Therefore, cyclohexane has less strain than cyclopentane. Heats of Combustion in Cycloalkanes

3.5 Small Rings: Cyclopropane Cyclobutane

Sources of strain: torsional strain, angle strain Cyclopropane

Nonplanar conformation relieves some torsional strain but angle strain is present. Cyclobutane

3.6 Cyclopentane

In a planar conformation, all bonds are eclipsed. The planar conformation destabilized by torsional strain. Planar Representation of Cyclopentane

EnvelopeHalf-chair These relieve some, but not all, of the torsional strain. Envelope and half-chair are of similar stability and interconvert rapidly. Nonplanar Conformations of Cyclopentane

3.7 Conformations of Cyclohexane Heat of combustion suggests that angle strain is unimportant in cyclohexane. Tetrahedral bond angles require nonplanar geometries.

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 tetrahedral but close contact between flagpole hydrogens causes van der Waals strain in boat. 180 pm Boat conformation is less stable than the chair

Eclipsed bonds bonds gives torsional strain to boat. 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

3.8 Axial and Equitorial Bonds in Cyclohexane

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

Axial bonds point “up” and “down”. 6 Bonds are axial

Equatorial bonds lie along the “equator”. 6 Bonds are equatorial

3.9 Conformational Inversion in Cyclohexane

Chair-chair interconversion (ring-flipping). A rapid process (activation energy = 45 kJ/mol). All axial bonds become equatorial and vice versa. Conformational Inversion

Half- chair Skew boat

45 kJ/mol 23 kJ/mol

3.10 Conformational Analysis of Monosubstituted Cyclohexanes The most stable conformation is chair. A substituent is more stable when equatorial.

5%95% Chair chair interconversion occurs, but at any instant 95% of the molecules have their methyl group equatorial. Equilibrium favors equitorial. Axial methyl group is more crowded than an equatorial one. Methylcyclohexane CH 3

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(CH 3 ) 3 t-Butylcyclohexane

van der Waals strain due to 1,3-diaxial repulsions t-Butylcyclohexane

3.11 Disubstituted Cyclohexanes: Cis-trans Stereoisomers Stereoisomers are isomers that have same constitution but different arrangement of atoms in space.

Isomers Constitutional isomers Stereoisomers

There are two stereoisomers of 1,2-dimethylcyclopropane. They differ in spatial arrangement of atoms. 1,2-Dimethylcyclopropane

cis-1,2-Dimethylcyclopropane has methyl groups on same side of ring. trans-1,2-Dimethylcyclopropane has methyl groups on opposite sides. 1,2-Dimethylcyclopropane

3371 kJ/mol 3366 kJ/mol Van der Waals strain makes cis stereoisomer less stable than trans. Relative stabilities of stereoisomers may be determined from heats of combustion.

3.12 Conformational Analysis of Disubstituted Cyclohexanes

cistrans CH 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. H3CH3C H H H3CH3C CH 3 H H 1,4-Dimethylcyclohexane Stereoisomers

CH 3 H3CH3C H H Two equivalent conformations; each has one axial methyl group and one equatorial methyl group H CH 3 H H H3CH3C H Conformational analysis of cis-1,4-dimethylcyclohexane

CH 3 H3CH3C H H Two conformations are not equivalent; most stable conformation has both methyl groups equatorial. H H3CH3C H CH 3 H H3CH3C H Conformational analysis of trans-1,4-dimethylcyclohexane

cistrans 5223 kJ/mol 5217 kJ/mol less stable more stable Analogous to 1,4 in that trans is more stable than cis. CH 3 H H H3CH3C H H 1,2-Dimethylcyclohexane Stereoisomers

CH 3 H H Two equivalent conformations; each has one axial methyl group and one equatorial methyl group. H CH 3 H H H Conformational analysis of cis-1,2-dimethylcyclohexane

CH 3 H3CH3C H H Two conformations are not equivalent; most stable conformation has both methyl groups equatorial. H CH 3 H H H3CH3C H Conformational analysis of trans-1,2-dimethylcyclohexane

cistrans 5212 kJ/mol 5219 kJ/mol more stable less stable Unlike 1,2 and 1,4; cis-1,3 is more stable than trans. H3CH3C CH 3 H H H3CH3C H H 1,3-Dimethylcyclohexane Stereoisomers

CH 3 H3CH3C H H Two conformations are not equivalent; most stable conformation has both methyl groups equatorial. H3CH3C H H CH 3 H H Conformational analysis of cis-1,3-dimethylcyclohexane

Two equivalent conformations; each has one axial and one equatorial methyl group. H3CH3C H H CH 3 H H3CH3C H H3CH3C H H Conformational analysis of trans-1,3-dimethylcyclohexane

CompoundOrientation-  H° (kJ/mol) cis-1,2-dimethylax-eq5223 trans-1,2-dimethyleq-eq5217* cis-1,3-dimethyleq-eq5212* trans-1,3-dimethylax-eq5219 cis-1,4-dimethylax-eq5219 trans-1,4-dimethyleq-eq5212* *more stable stereoisomer of pair Table 3.2 Heats of Combustion of Isomeric Dimethylcyclohexanes

3.13 Medium 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

3.14 Polycyclic Ring Systems contain more than one ring: bicyclic tricyclic tetracyclic etc

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

Have adjacent atoms common to two rings and two rings share a common side Fused Ring

Has nonadjacent atoms common to two rings Bridged Ring

The carbon skeleton is tetracyclic. Steroids

3.15 Heterocyclic Compounds Contain an atom other than C.

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

Pyrrolidine Piperidine Nitrogen-containing Heterocycles

Lipoic acid Lenthionine CH 2 CH 2 CH 2 CH 2 COH O Sulfur-containing Heterocycles

End of Chapter 3 Alkanes and Cycloalkanes: Conformations and cis-trans Stereoisomers