Stereochemistry - An Introduction is the introduction about the Stereochemistry it includes, Axial Chirality, Atropisomers, Planar Chirality, Helical Chirality, Cyclosteroisomerism and Reteroenantio Isomerism.
CONTENTS
1 AXIAL CHIRALITY
2 ATROPISOMERS
3 PLANAR CHIRALITY
4 HELICAL CHIRALITY
5 CYCLOSTEROISOMERISM
6 RETEROENANTIO ISOMERISM
AXIAL CHIRALITY
- The regular tetrahedron represent a three dimensional chiral centre.
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- In which centre is occupied by tetra ordinate atom in which C is chiral centre.
- If this centre is replaced by linear grouping such as C-C or C=C=C, the tetrahedron becomes elongated along the axis as shown below.
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- Such an elongated tetrahedron has lesser symmetry than the regular tetrahedron and conditions for its dissymmertrisation is less.
- Here structure-II becomes three dimensionally chiral will give entionmer shown below.
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- The axis along which the tetrahedron is elongated shown by doted line is called chiral axis or stereoaxis.
- Some of the examples are:- (1) Allenes
(2) Biphenyls.
- Allenes:-
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- Here two end C1 and C3 in allenes are SP2 hybridized and central carbon atom is SP- hybridized.
- In the orbital picture the shaded p-orbital and unshaded p-orbital separately overlape with each other forming π –Bonds making the two end groups non-planer.
- The structure can projected to Newman formula the allene posseses C2 axis, if 3 and 4 or substituents are different, the molecule is totally asymmetric.
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- Here, first example 1,3-diphenyl 1,3-di-α-napthalenyl allene having R-configuration.
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- Spirens, alkylidenecycloalkanes and admentane appropriately substituted can be chiral in the same way as allenes.
- Alkylidenecycloalkanes (hemispiranes) are compounds in which one of the double bond of allenes is replaced by ring shown in fig-1, and thus it will exhibit enantiomerism.
- Optically active Spirans:- The centre atom C-5 common to both the ring is chiral. Four group attached to C-5 of this compound is sequenced as C1>C6>C4>C9.
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- When molecule exhibit both central and axial chirality the formal has configurational nomenclature.
- When this diene is reduced to diol, three diastereomers diols are produced and each of these is resolvable.
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- Admantane:-
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- It satistify the condition of axial chirality in which C2 and C6 methylene are non-planer and in two enantiomeric form.
- Chiral axis passes through C1 and C6.
- Catenanes (optically active):-
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- A catenan with two dissimilar rings interlink with each other may give rise to chirality due to secondary structure.
- If the two rings are held with their planes perpendicular to each other in a following structure with 4 different groups in the chain.
- Like other axially chiral molecule configurational nomenclature will be given from the projection formula.
- Thus particular enantiomer has “R” configuration.
- Biphenyl:-
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- Biphenyl is represented by the structure in which two phenyl groups are joined by single bond is called pivotal bond. Configuration of both carbons is SP2.
- The distance between ortho hydrogen in planer conformation is 0.29nm which is greater than Vander walls radius of Hydrogen. i.e. 2*0.12=0.24nm. So rotation about the pivotal bond is not impeded by stearic factor.
- The phenyl group which is dissymmetrically substituted is two dimensionally chiral and affords cis and trans diastereomer. So, this is planar combination. If a non-planer combination would give enantiomer and such molecules are axially chiral. Thus, biphenyl has resolved properties.
- This can be done by introducing bulky groups in the ortho positions, so the planar conformations are dis-stabilised by stearic repulsion.
- The energy diagram is shown in fig-a.
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- Essential conditions of chirality of biphenyls:-
- Biphenyls with 4 bulky groups in ortho position are not free to rotate about the central bond. So, two rings are must be in perpendicular plane with each other.
- When either or both the rings are symmetrical molecule will have plane of symmetry and will be Achiral.
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- When neither ring is symmetric compound is chiral as shown in structure.
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- It is not always necessary that 4 large ortho group should be present for rotation to be prevented. The presence of lesser no. of group, if large enough can prevent rotation which exist enantiomer in the following case.
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- The hydrogen-atom is quite small optically active compound exist with 2 or 3 ortho positions of the biphenyls occupied by hydrogens. In this case benzene ring is unsubstitued at ortho position but substituted at meta position, m-substitution has no influence on rotation. But it creates necessary chirality in the molecule as in “3-bromo-biphenyl-2-trimethyl Aarsonium iodide.”
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