When we talk about two chiral diastereoisomers, we have no choice but to draw the structure of one enantiomer of each diastereoisomer because we need to include the stereochemical information to distinguish them, even if we’re talking about a racemic mixture of the two enantiomers. To avoid confusion, it’s best to write something definite under the structure, such as ‘ ± ’ (meaning racemic) under a structure if it means ‘this diastereoisomer’ but not ‘this enantiomer of this diastereoisomer’. So, in this case we should say that the chemists were able to separate these two diastereoisomers, but wanted only one enantiomer of the trans diastereoisomer and that this could not be separated by physical means.

When the stereochemistry drawn on a molecule means ‘this diastereoisomer’, we say that we are representing relative stereochemistry; when it means ‘this enantiomer of this diastereoisomer’ we say we are representing its absolute stereochemistry. Relative stereochemistry tells us only how the stereogenic centres within a molecule relate to each other.

● Enantiomers and diastereoisomers

• Enantiomers are stereoisomers that are mirror images. A pair of enantiomers are mirror-image forms of the same compound and have opposite absolute stereochemistry.

 • Diastereoisomers are stereoisomers that are not mirror images. Two diastereoisomers are different compounds, and have different relative stereochemistry.

• Diastereoisomers may be achiral (have a plane of symmetry) or they may be chiral (have no plane of symmetry).

مواضيع ذات صلة

Diastereoisomers can be chiral or achiral

Dia stereoisomers are stereoisomers that are not enantiomers

The rotation of plane-polarized light is known as optical activity

R and S can be used to describe the configuration of a chiral centre

Many chiral molecules are present in nature as single enantiomers

Stereogenic centres

Chiral molecules have no plane of symmetry

Some compounds can exist as a pair of mirror-image forms

Coupling in the proton NMR spectrum Nearby hydrogen nuclei interact and give multiple peaks

Exchange of acidic protons is revealed in proton NMR spectra

Protons on heteroatoms have more variable shifts than protons on carbon

Non-aldehyde protons in the aldehyde region: pyridines