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AS/A-level Chemistry - Stereoisomerism

Organic Chemistry, Stereoisomerism, Geometrical, Optical

· AS Chemistry,A-level Chemistry,organic chemistry,Stereoisomerism,optical isomers

Isomerism

Isomerism are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. That excludes any different arrangements which are simply due to the molecule rotating as a whole, or rotating about particular bonds.

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Stereoisomerism

Stereoisomers are molecules that have the same molecular formula, but the atoms are joined to each other in a different spacial arrangement – they occupy a different position in 3-dimensional space.

 

Geometrical Isomerism

  • An example of stereoisomerism found in some alkenes
  • C=C bonds in alkenes have restricted rotation.
  • while single covalent bonds can rotate easily, therefor alkanes cannot have geometrical isomers
  • Found in two forms CIS and TRANS
  • CIS: Groups/atoms are on the same side of the double bond.
  • TRANS: Groups/atoms are on opposite sides across the double bond.
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Questions regarding geometrical isomerism are often seen in AS/A-level Chemistry exam! 👨‍🏫 Don't mix up CIS and Trans!

  • Trans isomers have higher melting points than cis isomers. 
  • However, cis isomers have higher boiling points than trans isomers.
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Optical Isomerism

  • Another form of stereoisomerism
  • Occurs when compounds have non-superimposable mirror images
  • The two isomers are known as optical isomers or enantiomers
  • They occur when molecules have a chiral centre
  • A chiral centre contains an asymmetric carbon atom
  • An asymmetric carbon atom has four different atoms (or groups) arranged tetrahedrally around it.
  • If a carbon has more than one of any atom/group attached, it cannot be chiral
  • Identical physical properties (except for polarised light)

  • Identical chemical properties (unless reacting with other optical isomers when the orientation of the two molecules could have a bearing on the final result)

In AS/A-level Chemistry exam, they will ask you to draw enantiomers, make sure your know how to draw it as if it's a mirrored image! 👨‍🏫
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Reference:

  • Chirality and Stereoisomers - Chemistry LibreTexts

This is the end of the topic!

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Drafted by Cherry (Chemistry)

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