CH221 CLASS 7CHAPTER 4: STEREOCHEMISTRY OF ALKANES AND CYCLOALKANES

Synopsis. Class 7 introduces the conformational isomerism of alkanes and cycloalkanes as another kind of stereoisomerism and discusses conformer stability in terms of balance between different kinds of strain.

Stereoisomerism: An Introduction

It has been shown in earlier classes (see especially class 6) that alkanes, such as ethane, exist as a number of rapidly interconverting isomers known as conformers: interconversion occurs via C-C bond rotation.

The conformers of a particular alkane, although they have the same connectivity, differ in their spatial arrangements of atoms (they differ in their 3-dimensional shape). Conformational isomerism is thus one form of stereoisomerism and conformers are particular kinds of stereoisomers.

Also, it can be seen be seen that the cis-trans isomers of substituted cycloalkanes (class 6) are also stereoisomers, so that cis-trans isomerism is another kind of stereoisomerism. Both conformational isomerism and cis-trans isomerism will be described in more detail next, and a third type of stereoisomerism – optical isomerism – will be discussed in later classes.

Conformational Isomerism of Ethane

This has been discussed in previous classes (especially class 6), where it can be seen that the two extreme conformers – the staggered and eclipsed conformers – are separated by 12 kJ mol-1 in energy, the former being the more stable of the two. If we represent the eclipsed conformer of ethane in two ways, as shown on the next page, we can see that the 12 kJ mol-1 of extra energy associated with this conformer comes from the presence of three eclipsed C-H bonds (i.e. 4 kJ mol-1 per eclipsed C-H bond. Thus, this extra energy results from the greater repulsion between electron pairs of eclipsed bonds, compared with staggered bonds, where the inter-bond distances are some 25% greater. This fundamental

repulsion between electron pairs in different bonds is sometimes called Pitzer strain or torsional strain.

H

H

HH

HH

The eclipsed conformer of ethane

H

H H

Sawhorse formula Newman formula

H

H H

The inter-bond angle, called the dihedral angle (), can be seen to be 0o (360o), 120o and 240o for the eclipsed conformer and 60o, 180o and 300o for the staggered conformer, as shown below.

The conformational isomerism of ethane can be represented by an energy versus bond rotation (displacement dihedral angle) diagram, as shown overleaf. Propane, the next member of the alkane family, can be considered in a similar manner, except that the torsional energy barrier is slightly higher, at 14 KJ mol -1. By comparison with ethane, we can conclude that an eclipsed CH3-C and C-H bond accounts for 6 kJ mol-1 of the extra energy of the eclipsed conformer. The addition 2 kJ mol-1 can be considered to arise from steric hindrance (steric strain) between the eclipsed methyl group and hydrogen atom.

Butane, the next member of the family, gives a more complex picture and the molecule is best considered as an ethane derivative (“1,2-dimethylethane”). It can be seen from the diagram overleaf that butane has four extreme conformer types, which are named according to the dihedral angle between the methyl groups.

HH

CH3

HH

CH3

HH

CH3

H H

H

H CH3H

HCH3

H H H

HCH3

HH

HH

CH3

H H

H

H

CH3

CH3

CH3

H H

CH3CH3

HH

CH3

CH3

Dihedralangle 0o 60o 120o 180o 240o

300o 360o

eclipsedgauche 1staggered

synperiplanar eclipsed 1 antiperiplanarstaggered

eclipsed 2

gauche 2 synperiplanar

The most stable arrangement is the staggered anti conformer, where the torsional strain is minimal and the steric strain is also minimal, because the two largest groups (methyl) are as far apart as possible. The least stable arrangement is the syn conformer, which is not only eclipsed, but has the largest steric strain, because the methyl groups are closest in this arrangement. The difference in energy between the two extremes is known to be 19 kJ mol -1. Since we know that the extra energy associated with C-H eclipsed by C-H is 4 kJ mol -1, it can be concluded that the extra energy involved with C-CH3 eclipsed with C-CH3 must be 11 kJ mol-1, accounting for both torsional strain and steric strain. It is possible to calculate the energies of the eclipsed conformers 1 and 2 as 16 kJ mol-1 and it is known that the gauche conformers 1 and 2 have energies of about 4 kJ mol-1 (all with respect to the energy of the anti conformer). We now have all the information to draw a conformational energy diagram for butane (overleaf).Although the energy barriers are high enough to allow separate existence of the conformers, they are far too low to prevent rapid interconversion at room temperature. However, at any given instant, the majority of molecules will be in the more stable staggered conformations.

Stability of Cycloalkanes: the Baeyer Strain Theory

By the late 1800s, many cyclic organic compounds had been prepared, but all of them contained either 5- or 6-membered rings. Attempts to prepare smaller or larger rings failed and so it appeared that 5- and 6-membered rings were the only stable ones. Adolf von Baeyer (1885) was the first to try to explain the different stabilities of ring systems of various sizes: especially the apparent instability of small and large rings. He argued that because the bond angles of small rings (60o for cyclopropane and 90o for cyclobutane) are significantly lower than the tetrahedral angle (109.5o), the rings suffer from strain (angle strain) and so are

highly reactive, readily undergoing strain-relieving ring opening reactions. Baeyer’s arguments for larger alicyclic were that, beyond cyclopentane and cyclohexane, the bond angles became significantly higher than the tetrahedral angle, leading to the opposite kind of ring strain and instability. This argument was erroneous, especially for the larger rings, because Baeyer assumed planar structures. In reality, only cyclopropane is truly planar, the rest having puckered structures. Despite this error, Baeyer’s ideas on angle strain are still very useful. It is now known that rings larger than 6-membered were not prepared in Baeyer’s time because of difficulty in ring closure and that the small rings proved elusive because of their (ring-opening) reactivity: chemical techniques in Baeyer’s time were simply not sufficiently advanced.

Heats of Combustion and Relative Stabilities of Cycloalkanes

Stability, according to heats of combustion, reaches a maximum at cyclohexane and then decreases slightly until the ring size reaches 12, as shown in the table below.

Ring size Heat of combustion per CH2/kJ mol-1

3 696.44 685.55 663.46 657.97 661.78 662.912 658.817 657.1

The heat of combustion per CH2 for alkanes is 657.9 kJ mol-1.From the above data, it is possible to calculate ring strain energy by subtracting the value 657.9 kJ mol-1 from the figures in the table and multiplying by the number of CH2 units in the rings (see textbook, figure 4.8). It can be seen that cyclopropane and cyclobutane are indeed strained, but cyclopentane is more strained than predicted by Baeyer’s theory and for larger rings, there is no regular increase in strain. Furthermore, rings of more than 14 carbon atoms are

actually strain-free.

The Nature of Ring Strain

The complex picture presented above can be explained only by supposing that cycloalkane minimum-energy conformations result from the balancing of three kinds of strain:

Angle strain – the strain caused by distortion of bond anglesTorsional strain – the strain caused by eclipsing of bonds on adjacent atomsSteric strain – the strain caused by repulsive interactions when nonbonding atoms are too close to each other

Cyclopropane and cyclobutane, with ring angles of 60o and 90o respectively, suffer from the most angle strain and even the two main conformers of cyclopentane have a small amount of angle strain (see later). Apart from angle strain, the single planar conformer of cyclopropane has eclipsed C-H bonds, with maximum torsional strain:

C

H

H

H

HH

H

It will be shown later, and in the next class, that larger rings can minimize torsional strain by adopting puckered (nonplanar) conformations. Also, it will be shown in the next class that steric interactions play a part in the determination of minimum-energy conformations of medium-size cycloalkanes (C6 – C11).

Cyclopropane: an Orbital Picture of Ring Strain

In normal saturated compounds, the carbon atoms are sp3-hybridized, with C-C-C and H-C-H bond angles close to 109.5o. Overlap between the atomic orbitals involved in these bonds is a maximum.

In cyclopropane, the C-C-C bond angle cannot be 109.5o, so there is poor overlap making the C-C bonds weaker than in open chain equivalents.On the basis of quantum mechanical calculations, Coulson and Moffitt have proposed a different model for cyclopropane, in which the C-H bonds have more s character (i.e. sp2, like alkenes) and the C-C bonds have more p character (e.g. sp4): their character is somewhere between a (sigma) and a (pi) bond:

The Conformations of Cyclobutane and Cyclopentane

Like cyclopropane, the planar conformation of cyclobutane has considerable bond strain and torsional strain and hence the more favorable conformer of cyclobutane is puckered, with one carbon atom lying some 25o out of the plane formed by the other three carbon atoms. This conformer has rather more angle strain but considerably less torsional strain, because the C-H bonds are not fully eclipsed.

MAJORMINOR

puckered conformer:angle strain more, butless torsional strain

angle C-C-C < 90o

CH2

H

H

H

H

H

Hplanar conformer: lessangle strain but moretorsional strain (eclipsedC-H bonds)

angle C-C-C = 90o

CH2CH2

HH

H

CC

H

A similar, but more complex situation arises in cyclopentane, the two puckered

forms being (about equally) more favorable than the planar conformer.

Class Questions

1. Draw Newman formulas for the most and least stable conformers of 1-chloropropane.

Cl

H H

CH3

HH

LEAST STABLE

Cl

H H

CH3

H H

MOST STABLE

2. Suggest what might be the most stable conformer of 2-flouroethanol and draw its Newman formula.

H H

OH

H

FH

H

FH

H HH

O

Gauche conformer, stabilized by intramolecular hydrogen bonding