Cis-Trans Isomers (HL) (HL IB Chemistry)

• Isomers can be grouped into various categories, as shown: 

Flow chart of the various isomers

The flow chart guides you through all the relevant points of the different isomers

• We have already encountered three types of structural isomers, in our revision note on Structural Isomers:
  • Functional group isomers, e.g. propanal and propanone
  • Position isomers, e.g. propan-1-ol and propan-2-ol
  • Branched chain isomers, e.g. butane and methylpropane
• If the atoms within an isomer are arranged in the same order then we are dealing with stereoisomers
• Stereoisomers can be conformational or configurational

Conformational Isomers

• Conformational isomers, or conformers, occur due to free rotation about a single σ-bond and can be described as:
  • Staggered
  • Eclipsed
• One of the simplest examples of conformational isomerism is ethane, CH₃CH₃ 

Structural formula of ethane

The central carbon-carbon single bond is identified as the bond for conformational isomerism

• By looking along the C-C bond highlighted in the diagram we can draw the two Newman projections, staggered and eclipsed

The staggered and eclipsed conformers of ethane

The different geometries possible due to the free rotation about the carbon-carbon bond causes the staggered and eclipsed conformers

• The staggered conformer has angles between hydrogen atoms on adjacent carbons of 60o, as shown
  • It is also more stable / lower energy than the eclipsed conformer because the C-H bonds are as far apart as possible to minimise the repulsion between the electrons in the C-H bonds
• The eclipsed conformer has angles between hydrogen atoms on adjacent carbons of 0o, this is not shown in the diagrams so that the conformation can be seen
  • The eclipsed conformer is less stable / higher energy due to the repulsion between the electrons in the C-H bonds that are closer together
• The free rotation that causes these conformers means that it is easy to interconvert from one conformer to the other and back
  • This is also the reason that it is almost impossible to isolate a single conformer

Conformational Isomerism in Cyclic Structures

• Conformational isomerism can also be seen in cyclic structures
• A common example of this is cyclohexane, C₆H₁₂
  • Cyclohexane isomers exist in boat and chair forms:

Skeletal structures showing the boat and chair forms of cyclohexane

Due to free rotation about the single bonds, cyclohexane can adopt two configurations commonly called boat and chair

• The boat form is less stable / higher energy as there are four eclipsed bonds causing strain on the overall structure
  • There is also repulsion of the hydrogen atoms on the end of the boat structure
• It is possible to "flip" between the boat and chair forms which explains the difficulty in isolating just one of the forms
  • During the interconversions, it is also possible to get other structures commonly called the half chair and the twisted boat

Configurational Isomers

• Configurational isomerism can be seen in unsaturated compounds, cyclic structures or compounds that contain at least one asymmetric carbon (sometimes called a chiral centre)
  • These structures have the same molecular formula and order of atoms (the atoms are connected similarly to each other) but different shapes
• Interconversion of configurational isomers can only occur by breaking bonds or rearranging stereocentres
• Configurational isomers can be divided into:
  • Cis / trans isomers and E / Z isomers
  • Optical isomers - see our revision note on Enantiomers

Cis / trans isomers

• In saturated compounds, the atoms / functional groups attached to the single, σ-bonded carbons are not fixed in their position due to the free rotation about the C-C σ-bond
  • This causes conformational isomers
• In unsaturated compounds, the groups attached to the C=C carbons remain fixed in their position
  • This is because free rotation of the bonds about the C=C bond is not possible due to the presence of a π bond
• Cis / trans nomenclature can be used to distinguish between the isomers
  • Cis isomers have two functional groups on the same side of the double bond / carbon ring, i.e. both above the C=C bond or both below the C=C bond
  • Trans isomers have two functional groups on opposite sides of the double bond / carbon ring, i.e. one above and one below the C=C bond

Comparing 1,2-ethanediol with cis and trans isomers of 1,2-ethenediol

The presence of a π bond in unsaturated compounds restricts rotation about the C=C bond forcing the groups to remain fixed in their position and giving rise to the formation of certain configurational isomers

Naming cis / trans isomers

• For cis / trans isomers to exist, we need two different atoms or groups of atoms on either side (above and / or below) of the C=C bond
  • This means that 2-methylpropene cannot have cis / trans isomers as the methyl groups are both on the same side of the C=C bond:

Full structural formula of 2-methylpropene

2-methylpropene molecules do not have cis / trans isomers

• However, moving one of the methyl groups to the other side of the C=C bond causes cis / trans isomerism:

Full structural formula of the cis and trans isomers of but-2-ene

But-2-ene does have cis / trans isomers

• The atoms or groups of atoms on either side of the C=C bond do not have to be the same for cis / trans isomers:

Full structural formula of the cis and trans isomers of 1-chloroprop-1-ene

1-chloroprop-1-ene also shows cis / trans isomerism

Limitations of cis / trans nomenclature 

• The cis / trans naming system starts to fail once we have more than one atom or group of atoms on either side of the C=C bond
• The cis / trans naming system can still be used with three atoms / groups of atoms but only if:
  • Two of the three atoms or groups of atoms are the same
  • These two atoms or groups of atoms are on opposite sides of the double bond

Full structural formula of the cis and trans isomers of 1,2-dichloropropene

1,2-dichloropropene can be named using cis / trans nomenclature

• The cis / trans naming system cannot be used with three atoms / groups of atoms when they are all different

Full structural formula of 1-bromo-2-chloropropene

1-bromo-2-chloropropene cannot be named using cis / trans nomenclature

Cyclic cis / trans isomers

• Cis / trans isomerism can also occur in cyclic structures
• Even though cyclic alkanes contain single carbon-carbon bonds, the rigid structure of the ring system does not allow for free rotation
  • Therefore, cis isomers can occur when the atoms (or groups of atoms) are on the same side of the ring, i.e. both above or both below
  • While trans isomers can occur when the atoms (or groups of atoms) are on the opposite side of the ring, i.e. one above and one below

Cis / trans isomerism in cyclic compounds

Cis isomers have both groups above or both groups below, while trans isomers have one group above and one group below