Stereochemistry

Stereoisomers Family Tree

What are structural isomers?

Different compounds having the same molecular formula

What type of isomer is this?

• Structural isomers

• Butane

• 2-methylpropane

• They are different compounds, so will have different physical properties

What does stereoisomer mean?

Same constitution, same formula, but different spatial arrangement.

Structural isomer Vs Stereoisomer

Structural isomer - Molecules with the same molecular formula, but different connectivity of atoms (i.e. the atoms are joined in different ways).

Stereoisomer - Molecules with the same molecular formula and the same connectivity, but different spatial (3D) arrangements of atoms.

So all atoms are connected in the same way, but positioned differently in space.

Name the 2 different types of stereoisomers:

Conformational isomers

Configurational isomers

What are conformational isomers?

Different arrangements of atoms from bond rotation (NOT bond breaking)

What is a Newman projection?

A Newman projection helps you visualize the conformation of a molecule by looking straight down a C–C bond.

What is this conformation called?

Staggered conformation

What is this conformation called?

Eclipsed conformation

Conformational Isomers of ethane:

As you rotate around the C-C bond, the molecular passes through staggered conformation (lowest energy) to eclipsed conformation (highest energy because of electron repulsion)

Does cyclohexane experience conformational isomerism?

Yes, it adopts a chair conformation to minimize strain

Axial and equatorial positions in cyclohexane

In cyclohexane, each C has 2 types of H atoms

• Axial = H atoms that point up or down, parallel to the ring’s axis

• Equatorial = H atoms that point outwards, to the equator of the ring

Are axial or equatorial positions more stable?

Equatorial positions are more stable for bulky groups (like CH₃) because they avoid 1,3-diaxial interactions (steric hindrance with other axial hydrogens on the same side).

Which is higher in energy, axial or equatorial positions?

Axial substituents experience steric strain, specifically called 1,3-diaxial interactions.

• These are repulsions between the axial substituent on carbon 1 and the two other axial hydrogens (or groups) on carbons 3 and 5 — both on the same side of the ring. It’s like two groups bumping into each other in a crowded space.

In contrast:

• An equatorial substituent points outwards and avoids those clashes, so it’s more stable (lower energy).

Name the 2 different types of configurational isomers:

• Geometric isomers

• Optical isomers

Geometric isomers

Can be classed as E/Z or cis/trans

Name these geometric isomers

What makes a molecule chiral?

If it does not have a plane of symmetry within

What is the only achiral amino acid?

Glycine