Not all chiral molecules remain locked in a single stereochemical form. In some molecular systems, the three-dimensional arrangement can change with time because of rotation, inversion, or another structural motion.
When such motion leads to interconversion between stereochemically distinct forms, the chirality of the system becomes dynamic.
Dynamic chirality is therefore concerned not only with what stereochemical form a molecule has, but also with how readily that form can change into another form.
Dynamic chirality describes chiral systems in which molecular motion can lead to interconversion between stereochemically different forms.
The important questions are:
These questions connect molecular structure with molecular dynamics.
Rotation about a single bond is often assumed to be rapid and unrestricted. In some molecular structures, steric, electronic, or conformational factors can make rotation sufficiently difficult that distinct conformations become configurationally stable.
When restricted rotation gives rise to isolable stereoisomeric forms, the resulting stereoisomers are known as atropisomers.
Configurational stability describes the persistence of a particular stereochemical arrangement against interconversion.
A stereochemical structure may be highly stable and readily isolated, moderately stable and condition-dependent, or sufficiently labile to undergo rapid interconversion.
A useful distinction is:
Configuration describes the stereochemical state; configurational stability describes how long that state persists.
When two enantiomeric forms can interconvert, a chiral sample may progressively lose its enantiomeric imbalance.
This process is known as racemization when it results in conversion toward an equal mixture of the two enantiomers.
Dynamic chirality therefore provides a molecular explanation for why a stereochemically pure material may not remain stereochemically pure under particular conditions.
The Enantiomers & Racemates section introduces enantiomeric composition and racemic mixtures.
This section focuses on the dynamic process by which one stereochemical form can become another.
The behaviour of a dynamically chiral system depends strongly on the energy barrier for interconversion.
A low barrier generally allows rapid interconversion, so individual stereochemical forms may not be distinguishable under ordinary observation.
A higher barrier slows interconversion and may allow distinct stereochemical forms to persist long enough to be observed or isolated.
The relevant timescale is therefore crucial.
A pair of stereochemical forms may behave as:
Rapidly interconverting forms
→ one observed species under the conditions used
or
Slowly interconverting forms
→ distinct stereoisomers that can potentially be
observed separately.
Thus, dynamic chirality lies at the boundary between structure and molecular motion.
Whether two interconverting forms can be distinguished depends on the relationship between:
rate of interconversion
and
timescale of observation.
If interconversion is much faster than the measurement or experimental process, the system may appear as a single averaged species.
If interconversion is sufficiently slow, the individual forms may be distinguishable and, under suitable conditions, isolable.If interconversion is sufficiently slow, the individual forms may be distinguishable and, under suitable conditions, isolable.
Atropisomerism is one of the best-known examples of dynamic stereochemistry, but the broader concept extends to other systems in which molecular motion can alter stereochemical identity.
Dynamic stereochemistry can involve:
The defining feature is stereochemical change associated with molecular motion.
As Chiralpedia develops, individual dynamic systems can be added here without changing the basic structure of this section.
