This document discusses the stereochemistry of allenes, spiranes, and biphenyls. It explains that allenes with different substituents on the terminal carbons can exhibit chirality and enantiomers. Spiranes can also show chirality and optical isomerism if they have different substituents. Biphenyls become chiral when large substituents in the ortho position prevent free rotation of the phenyl rings, leading to atropisomerism with a chiral axis and restricted rotation.

2. Stereochemistry &
configuration of Allenes ,
Spirans & Biphenyls.

3. CONTENTS
Stereochemistry of allenes
Some Examples
Stereochemistry in spiranes
Stereochemistry in biphenyles

4. Stereochemistry of allenes
• When three or more adjacent carbon atom are present in a molecule &
bounded by the double bonds, the compound is called “CUMULENE” or
is said cumulative double bonds.
• Allene is the simple cumulene.
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5. • In allenes the two carbon-carbon bond that share a single sp carbon. (the
central carbon atom is sp hybridised bond & the remaining two p-orbit
are perpendicular to each other.)
• Thus allene of the type C=C=C (A≠B & C≠D) are chiral i.e. non
superimposable on their mirror image. And exist as a ENANTIOMER.
• Although they have chiral centre.
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6. • Allenes also display chirality only if both the terminal atom are
unsymmetrical substituted.
• When the number of double bonds even, the four ligands lie in two
perpendicular planes as in Allenes and a chiral axis exist thus such
cumulene exhibit ENANTIOMERS.
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7. SOME EXAMPLES
Br COOH COOH Br
C==C==C C==C==C
Me H H Me
R-Allene
S-Allene

8. STEREOCHEMISTRY IN SPIRANES
• Spiranes are the bicyclic compounds in which one carbon atom is
common in both the ring. Or If both the double bonds in allenes are
replaced by ring system the resulting molecule are known as spiranes &
spiro compounds.
• In spiranes the common carbon atom is 𝑠𝑝3
hybridized and hence its
valencies are tetrahedral. Thus two valency of this carbon atom are
perpendicular to the other two valencies.
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9. Hence the rings formed with these valencies are also perpendicular
planes.
• Now if the molecule have different substituents then the molecules will
have no element symmetry hence it will be optically active,& will show
optical isomerism.

10. • But if the substituents are same then there exist plane of symmetry and
the compound becomes optically inactive.
• In spiro the two rings are perpendicular to each other the element of
chirality in spiranes also is the presence of chiral axis.

11. STEREOCHEMISTRY IN BIPHENYLES
• Biphenyls are those compounds in which two benzene ring are jointed to
each other by C-C single bond, free rotation about this C-C single bond
is possible.
• But ,in case when a large substituent is in a ortho position, they prevent
the rotation about a single axis of a phenyl ring. Thus two benzene ring
are perpendicular to each other and a molecule become chiral and
mirror image is not superimposable. Thus it will show optical activity.

12. • Suitable substituted biphenyl exhibits enantiomerism due to the presence
of a chiral axis and there are ATROPISOMERISM because they exhibit
enantiomer due to restricted rotation about the single bond between the
two phenyl rings.
• There is restricted rotation about the bond linking the two phenyl rings due
to steric hinderance between the bulky ortho substituents. This
phenomena is known as ATROPISOMERISM.

13. • Atropisomers are stereoisomers arising because of hindered rotation
about a single bond, where energy differences due to steric strain or other
contributors create a barrier to rotation that is high enough to allow for
isolation of individual conformers.

14. • Because this steric hinderance the two phenyl rings lie in a different plane
which are perpendicular. Thus the molecule become chiral and exhibit
enantiomerism.
• Biphenyls shows enantiomerism when the following thus condition are
fulfilled.
• 1.each ring must be unsymmetrical substituted.
• 2.both rings must be substituted in ortho position and the substituents must
have a large size (e.g. Al,Br,𝑁𝑂2,H, 𝐶𝐻3etc.)
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15. References
• Dr. Jagdamba Singh, Dr. L.D.S. Yadav (eighteenth edition 2020).
Advanced organic chemistry, Published by A.K. Mittal ,Pragati
Prakashan, Pragati Bhawan, 240,W.K. Road,Meerut.