Axial chirality refers to stereoisomerism of a special case of chirality in which a molecule does no — Analytical Chemistry Chemistry Question
Stereoisomers without stereocenter
Axial chirality refers to stereoisomerism of a special case of chirality in which a molecule does not possess a stereogenic center but an axis of chirality. An axis of chirality is defined as an axis about which a set of substituents is held in a spatial arrangement that is not superimposable on its mirror image.
Two necessary preconditions for axial chirality are:
i. A rotationally stable axis
ii. Presence of different substituents on both sides of the axis
Axial chirality is most commonly observed in atropisomeric biaryl compounds wherein the rotation about the aryl-aryl bond is restricted, for example, biphenyl, binaphthyls. Certain allene compounds also display axial chirality.
From the given structures, draw 3-D structure and the corresponding mirror image of the given compounds. Use plane of symmetry to determine whether they are chiral or achiral.
(i) [VISUAL]
(ii) [VISUAL]
(iii) [VISUAL]
(iv) [VISUAL]
Model Answer
Hint: Student is encouraged to use chemistry model to figure out the plane of symmetry and the C2–symmetry in each molecule.
(i) From the possible enantiomeric pair,
The molecule has a plane of symmetry, therefore, the structure is achiral.
(ii) From the possible enantiomeric pair,
There is no plane of symmetry in these two molecules. Therefore, these two mirror image structures are non-superimposable. The structure is chiral.
(iii) From the structure of cumulene,
This molecule is flat. As a result, there is a plane of symmetry in this molecule. Consequently, its mirror image is achiral.
(iv) From the possible enantiomeric pair,
There is no plane of symmetry in these two molecules. Therefore, these two mirror image structures are non-superimposable. The structure is chiral.
Enantiomers of medium ring trans-cycloalkenes exist. For example, trans-cyclooctene can be resolved and its enantiomers are stable at room temperature. On the other hand, trans-cyclononene has also been resolved but it racemizes with a half-life of 4 min. at 0 °C.
(i) Draw mirror images of the given structures of trans-cyclooctene and trans-cyclononene. [VISUAL]
Model Answer
Enantiomers of trans-cyclooctene and trans-cyclononene are shown above. The two different structures are mirror images of each other and they are non-superimposable.
Why does trans-cyclononone racemize faster than trans-cyclooctene?
Model Answer
The enantiomers of both cycloalkanes are configurational isomers. The enantiomers can be interconverted via the ring flipping, similar to that of the chair cyclohexane. The trans double bond adds a considerable degree of rigidity to the ring. Since the trans-cyclononene has more carbon atoms so it is more flexible and can undergo the configuration inter-conversion more readily.