STEREOCHEMISTRY - spcmc.ac.in

STEREOCHEMISTRY II

PART-1, PPT-1, SEM-2, CC-3

Dr. Kalyan Kumar Mandal

Associate Professor

St. Paul’s C. M. College

Kolkata

Stereochemistry II

Part-1: Axial Chirality ICONTENTS

❖ Chirality arising out of Stereoaxis

❖ Concept of Chiral Axis

❖ Stereoisomerism of Substituted Allenes

❖ Optically Active Allenes

❖ Nature of Stereoisomerism in Substituted

Cumulenes

Chirality arising out of Stereoaxis

• A necessary and sufficient condition for a molecule to be chiral is

that it not be superposable with its mirror image. The presence of a

(single, configurationally stable) chiral centre in the molecule

(central chirality) is a sufficient condition for the existence of

chirality but not a necessary one.

• There are, in fact, numerous chiral molecules exist which are

devoid of chiral centres. Appropriately substituted allenes,

appropriately substituted cumulenes with even number of double

bonds, alkylidenecycloalkanes, spiranes, biaryls, etc., exhibit

stereoisomerism which is attributed to the presence of chiral axis

(Cahn, Ingold, and Prelog, 1956).

This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata


• Axial chirality is a special case of chirality in which a molecule

does not possess a stereogenic centre (the most common form of

chirality in organic compounds) but an axis of chirality, an axis

about which a set of substituents is held in a spatial arrangement

that is not superposable on its mirror image.

• The enantiomers of axially chiral compounds are usually given the

stereochemical labels Ra and Sa. The designations are based on the

same Cahn-Ingold-Prelog priority rules used for tetrahedral

stereocentres. The chiral axis is viewed end-on and the two “near”

and two “far” substituents on the axial unit are ranked, but with the

additional rule that the two near substituents have higher priority

than the far ones.


Concept of Chiral Axis

• Molecules like Ca4 represents a regular tetrahedron with Td

symmetry where the carbon occupies the centre of gravity of the

tetrahedron. In this case, carbon atom is (pro)3-chiral and

nonstereogenic.

• In case of molecules of the type Ca2b2, the carbon atom also

occupies the centre of a tetrahedron, but the carbon atom is (pro)2-

chiral and nonstereogenic having a C2v point group of symmetry.

• When in the molecule Ca2b2, the carbon centre is replaced by a

rigid axis like C-C, without any free rotation, then two types of

three-dimensional rigid molecules Ca2-Cb2 and Cab-Cab are

obtained. The original tetrahedron becomes elongated where

substituents on each carbon lie in a perpendicular plane with

respect to the other. Structures I and II (Figure 1) represent such

elongated molecules with rigid axis.


Concept of Chiral Axis

• If a tetrahedron is stretched along its S4 axis, it is desymmetrized to

a framework of D2d symmetry (Figure 2). With proper substitution,

the long axis of this framework constitutes the chiral axis. Because

of the intrinsically lower symmetry of the framework shown in

Figure 2 compared to a tetrahedron, it no longer takes four different

substituents to make the framework chiral.

• A necessary and sufficient condition for chirality is that a ≠ b and

c ≠ d. Thus, even when a = c and/or b = d, the framework retains

chirality, for example, in abC=C=Cab.



• A regular tetrahedron with four distinguishable vertices (structure

I in Figure 2) represents a three-dimensional chiral simplex. The

centre of the tetrahedron which is usually occupied by a

tetracoordinated atom, e.g., in Cabcd is a stereocentre. If this

centre is replaced by a linear grouping such as C-C or C=C, the

tetrahedron becomes elongated (extended), along the axis of the

grouping as shown in II and illustrated by an allene, abC=C=Cab.


Chirality arising out of Stereoaxis• Such an elongated tetrahedron (Structure II in Figure 2) with

a = b = c = d (CH2=C=CH2; D2d point group with 3C2 axes and 2σ

planes) has lesser symmetry than a regular tetrahedron (Td) and the

condition for its desymmetrization is less stringent. Instead of all

the four vertices being distinguishable, only pairs of vertices around

the two ends of the axis need to be distinguished (i.e., a ≠ b). The

structure (I in Figure 3) thus becomes three-dimensionally chiral

and is enantiomorphous with its mirror image (II).


• The axis along which the tetrahedron is elongated (shown by the

dotted lines in Figure 2) is called the chiral axis or the stereoaxis.

This is because exchange of ligands at either of the terminal atoms

across the axis reverses the chirality as shown in Figure 4.

• According to Brewster, the stereogenecity rests on both C-1 and

C-3 interdependently, i.e., C-1 is stereogenic because C-3 is and

vice versa. Together they form a stereogenic dyad.

• Actually, the elongated tetrahedron II (C2) in Figure 2 is a

desymmetrized tetrahedron of type Caabb (C2v).


Stereoisomerism of Substituted Allenes

• Allene itself does not exhibit enantiomerism. It has two σ planes

(actually σv), three C2 simple axes and one S4 axis. It belongs to the

point group D2d (Symmetry number = 4; Order = 8). The

symmetry elements present in allene (or molecules of this type) for

any are shown in Figure 5.



• There are several possibilities of differently substituted allenes of

the types aaC=C=Cbb, aaC=C=Cab, abC=C=Cab, abC=C=Cac

and abC=C=Ccd. These are shown in Figure 6.



• Of the five types shown in Figure 6, structures I and II are achiral

because both of them have σ-planes and therefore, they do not exist

as enantiomers. These molecules are superposable with their

mirror images. But structures III, IV, and V are chiral because of

the absence of σ, i and Sn (n > 1).

• Molecules representing structure III has a C2 axis, and therefore,

represents a dissymmetric molecule. In structures IV and V, the Cn

(n > 1) axis is absent and these type of molecules belong to the

point groups C1. Therefore, allenes of the types IV and V are

asymmetric molecules.

• Allenes of the types III, IV, and V are optically active and exist as

a pair of enantiomers.


Optically Active Allenes (Dissymmetric)

• Appropriately substituted allenes are optically active. Depending

upon the nature of substituents attached to the terminal carbon

atoms, the molecule is either asymmetric or dissymmetric. Figure 7

shows a pair of enantiomers of an axially chiral molecule.


Optically Active Allenes• I and II in Figure 7 are nonsuperimposable mirror images

(enantiomers) of 1,3-dimethylallene (penta-2,3-diene). It has a C2

proper axis that passes diagonally through the central sp-carbon. Its

chiral axis is along the axis joining the three carbon atoms in the

unit C=C=C.

• The molecule is dissymmetric with a symmetry point group C2. The

Newman projections of structures I and II are III and IV,

respectively as shown in Figure 7. The Newman projection clearly

shows the C2 axis of the molecule. This axis bisects the angle

between the two perpendicular planes passing through the two

terminal sp2 carbon atoms.

• The pair of enantiomers of pentane-2,3-dienoic acid are shown in

Figure 8.


Optically Active Allenes (Asymmetric)

• Since the structures I and II in Figure 8 devoid of any symmetry

elements except the trivial C1 axis, they are asymmetric allenes.

Absence of symmetry elements is clearly evident from the Newman

projection formula of the molecules.


Nature of Stereoisomerism in Substituted Cumulenes

• The following facts are to be considered in case of stereoisomerism

of substituted cumulenes:

1. Cumulenes with even number of double bonds exhibit

enantiomerism, if each of the terminal sp2 carbon atoms contain

non-identical substituents.

2. Cumulenes with even number of double bonds cannot exhibit

cis-trans isomerism, because interchange of groups on any

terminal sp2 carbon does not produce a diastereoisomer with

different relative positions and dihedral angles among the

substituents. For example, the allenes (I and II) shown in Figure 8

are not cis-trans isomers (diastereoisomers) but enantiomers.



3. Cumulenes with odd number of double bonds never exhibit

enantiomerism irrespective of the nature of substituents on the

terminal carbons. This is because of the fact that in cumulenes with

odd number of double bonds, the terminal carbon atoms along with

their substituents lie in the same plane (II; Figure 9) and such

cumulenes have always σ planes irrespective of the nature of the

substituents.



• Both structures I and II in Figure 10 are achiral because they

possess plane of symmetry [I: C2v = C2 + 2σv and II: C2h= C2 + σh

+ S2 (≡ i)]. The orbital picture of such a cumulene is shown in

Figure 9 (diagram at the right). However, cumulenes with odd

number of double bonds show cis-trans isomerism. For example, I

and II in Figure 10 are cis and trans, respectively.

4. Terminal carbon atoms of cumulenes with even number of double

bonds are chirotopic and stereogenic, provided each of these

terminal carbon atoms contain non-identical atoms or achiral

groups. Figure 11 provides the examples.


Topicity in Cumulenes

• According to Brewster, the stereogenecity of each terminal carbon

is interdependent on the stereogenecity of the other terminal. Thus,

together they form a stereogenic dyad.

5. Terminal carbon atoms of cumulenes with odd number of double

bonds also belong to a stereogenic dyad system but they are

achirotopic.


Nature of stereoisomerism exhibited by Cumulenes

• The nature of stereoisomerism in appropriately substituted

cumulenes depends on two factors:

1. The value of ‘n’: When n is zero or even number, the structure and

hence the molecules which represent this structure show cis-trans

isomerism. When n is zero, the molecules belong to the substituted

alkenes and when n is 1, the molecules belong to the substituted

allenes.

2. The nature of the substituents attached to the terminal carbon

atoms: Here a ≠ b and c ≠ d, but ligands ‘a’ and ‘c’ (or d) as well

as ‘b’ and ‘c’ (or d) may be same or different.


Glossary

• Chirality axis: An axis about which a set of ligands is held so thatit results in a spatial arrangement which is not superposable on itsmirror image.

• For example with an allene abC=C=Ccd the chiral axis is definedby the C=C=C bonds; and with an ortho-substituted biphenyl theatoms C-1, C-1', C-4 and C-4' lie on the chiral axis.

• Axial Chirality: This term is used to refer stereoisomerismresulting from the non-planar arrangement of four groups in pairsabout a chirality axis.

• It is exemplified by allenes abC=C=Ccd (or abC=C=Cab) and bythe atropisomerism of ortho-substituted biphenyls. Theconfiguration in molecular entities possessing axial chirality isspecified by the stereodescriptors Ra and Sa (or by P or M).


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