Chiral Compounds without Asymmetric Atom

Chiral Compounds without Asymmetric Atoms

– Most chiral organic compounds have at least one asymmetric carbon atom.

– Some compounds are chiral because they have another asymmetric atom, such as phosphorus, sulfur, or nitrogen, serving as a chirality center.

– Some compounds are chiral even though they have no asymmetric atoms at all.

– In these types of compounds, special characteristics of the molecules’ shapes lend chirality to the structure.

Conformational Enantiomerism

– Some molecules are so bulky or so highly strained that they cannot easily convert from one chiral conformation to the mirror-image conformation.

– They cannot achieve the most symmetric conformation because it has too much steric strain or ring strain.

– Since these molecules are “locked” into a conformation, we must evaluate the individual locked-in conformation to determine whether the molecule is chiral.

– The following Figure shows three conformations of a sterically crowded derivative of biphenyl.

Chiral Compounds without Asymmetric Atom
Three conformations of a biphenyl. This biphenyl cannot pass through its symmetric conformation because there is too much crowding of the iodine and bromine atoms. The molecule is “locked” into one of the two chiral, enantiomeric, staggered conformations


– The center drawing shows the molecule in its most symmetric conformation. This conformation is planar, and it has a mirror plane of symmetry.

– If the molecule could achieve this conformation, or even pass through it for an instant, it would not be optically active.

– This planar conformation is very high in energy, however, because the iodine and bromine atoms are too large to be forced so close together.

– The molecule is conformationally locked. It can exist only in one of the two staggered conformations shown on the left and right.

– These conformations are nonsuperimposable mirror images, and they do not interconvert. They are enantiomers, and they can be separated and isolated.

– Each of them is optically active, and they have equal and opposite specific rotations.

– Even a simple strained molecule can show conformational enantiomerism. trans- Cyclooctene is the smallest stable trans-cycloalkene, and it is strained.

– If transcyclooctene existed as a planar ring, even for an instant, it could not be chiral.

– Make a molecular model of trans-cyclooctene, however, and you will see that it cannot exist as a planar ring. Its ring is folded into the three-dimensional structure pictured in the following Figure.

Chiral Compounds without Asymmetric Atom
Conformational enantiomerism. trans-Cyclooctene is strained, unable to achieve a symmetric planar conformation. It is locked into one of these two enantiomeric conformations. Either pure enantiomer is optically active, with [α] = ± 430°

– The mirror image of this structure is different, and trans-cyclooctene is a chiral molecule.

– In fact, the enantiomers of trans-cyclooctene have been separated and characterized, and they are optically active.


– Allenes are compounds that contain the unit, with two double bonds meeting at a single carbon atom.

– The parent compound, propadiene, has the common name allene.

– In allene, the central carbon atom is sp hybridized and linear, and the two outer carbon atoms are hybridized and trigonal.

– We might imagine that the whole molecule lies in a plane, but this is not correct.

– The central sp hybrid carbon atom must use different p orbitals to form the pi bonds with the two outer carbon atoms.

– The two unhybridized p orbitals on the sp hybrid carbon atom are perpendicular, so the two pi bonds must also be perpendicular.

– The following Figure shows the bonding and threedimensional structure of allene.

Chiral Compounds without Asymmetric Atom
Structure of allene. The two ends of the allene molecule are perpendicular


– Allene itself is achiral. If you make a model of its mirror image, you will find it identical with the original molecule.

– If we add some substituents to allene, however, the molecule may be chiral. Make a model of the following compound:

– Carbon atom 3 is the sp hybrid allene-type carbon atom.

– Carbons 2 and 4 are both and planar, but their planes are perpendicular to each other.

– None of the carbon atoms is attached to four different atoms, so there is no asymmetric carbon atom.

– Nevertheless, penta-2,3-diene is chiral, as you should see from your models and from the following drawings of the enantiomers.

Chiral Compounds without Asymmetric Atom

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