Chirality of molecules without stereogenic (chiral) center

1. Presented by
Mr. Sunil Pandurang Gawali
Asst. Prof.
Sundarro More Arts, Commerce and Science College Poladpur
Raigad
Chirality of molecules without stereogenic
(chiral) centre
Subject- Organic Chemistry TYBSC Stereochemistry- I

2. Chirality of molecules without stereogenic (chiral) centre
Molecular Chirality
 A molecule which is non- superimposable on its mirror image is a called a
‘chiral’ molecule. a ‘chiral’ molecule displays optical activity and exist as
enantiomers.
 When a molecule contains one chiral (asymmetric) carbon atom, it exits in two
enantiomeric forms, i.e, in two optically active (dextro and laevo) forms.
 For. example of 2 – Chlorobutane.
(I) (II)
The above molecule has one stereogenic centre. Its mirror images are non-
superimposable. Thus the molecule is ‘chiral’ and molecules shows optical activity.
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Gawali S.P. TYBSC Sem -V Stereochemistry- I

3. In case molecules which contain two or more chiral
centres, it is found that under certain condition, they
exist in optically inactive (non-resolvable) forms,
e.g., meso- Tartaric acid.
At the same time, there are certain compounds which do not contain any chiral centre,
but still they exist in optically active forms,
e.g., biphenyl compounds and allene compounds.
If the molecule and its mirror image are non-superimposable, then the molecule is chiral or
dissymmetric and optically active. 3
Gawali S.P. TYBSC Sem -V Stereochemistry- I

4. The chirality of a molecule can be determined by finding whether the molecule is
superimposable on its mirror image or not.
The most satisfactory way to do this would be to construct a model of the molecule as
well as its mirror image. Then the two models are placed on each other; if all the parts
are superimposable, the molecule is optically inactive, if they are non-superimposable
then the compound is optically active.
A more convenient way to detect chirality is to find if any of the elements of
symmetry, such as a plane of symmetry, a centre of symmetry or an axis of
symmetry is present in the molecule.
If anyone of these is present in the molecule, then the molecule is achiral or
symmetrical, i.e., it will be superimposable on its mirror image and hence it will
be optically inactive.
If all the elements of symmetry are absent, then the molecule will be non
superimposable on its mirror image, the molecule will be chiral and hence it will
exist in optically active forms.
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Gawali S.P. TYBSC Sem -V Stereochemistry- I

5. Elements of Symmetry
(i) Plane of Symmetry (Mirror Plane):
A plane of symmetry is a imaginary plane that passes through a molecule such that atoms or groups on
One side of plane form a mirror image of those on the other side. e.g. meso- Tartaric acid.
The three element of symmetry are-
i) Plane of symmetry
ii) Centre of Symmetry
iii) Alternating Axis of Symmetry
Plane of symmetry
Meso- Tartaric acid
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Gawali S.P. TYBSC Sem -V Stereochemistry- I

6. ii) Centre of Symmetry (Inversion Centre):
A centre of symmetry defined as, ‘a point (or atom) in a molecule from which lines drawn on
one side and then produced to equal distance on the opposite side, meet exactly similar points
(groups or atoms) in the molecule.
For example, an isomer of 2,4-dimethylcyclobutane-1, 3-dicarboxylic acid
Another example of a molecule having a centre of symmetry is
dimethyl diketopiprazine. Dimethyl diketopiprazine exist in two
geometrical isomeric form i.e. cis- and trans -forms.
The trans-isomer has a centre of symmetry and hence optically
inactive.
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Gawali S.P. TYBSC Sem -V Stereochemistry- I

7. (iii) Alternating Axis of Symmetry (Improper Rotation Axis, Sn):
A molecule is said to possess an n-fold alternating axis of symmetry if, on
rotating through an angle of 360/n about this axis and followed by a
reflection of the resulting molecule in a plane perpendicular to the axis,
then the mirror is exactly identical to the original molecule. It is also called
rotation-reflection axis.
For. e .g. 1, 2, 3, 4-tetramethylcyclobutane (I).
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Gawali S.P. TYBSC Sem -V Stereochemistry- I

8. (III) which is exactly
identical to (I)
Therefore, the molecule is said to
possess four-fold alternating axis
a symmetry.
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Gawali S.P. TYBSC Sem -V Stereochemistry- I

9. B) Chirality of molecules without stereogenic (chiral) centre
Stereochemistry of Cumulenes
Molecules like allenes, biphenyls and spirans do not have chiral carbon atoms but, show optical
isomerism due to molecular chirality.
Cumulenes are compounds having two or more than two cumulated double bonds.
Cumulenes having odd number of double bonds exhibit geometrical isomerism but if
there are even number of double bonds then they exhibit optical isomerism. There
should be unsymmetrical groups on terminal carbon atoms.
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Gawali S.P. TYBSC Sem -V Stereochemistry- I

10. Allenes
 Allenes are compounds having cumulated system of double bonds.
 The simplest allene is propa-1,2-diene (CH2=C=CH2).
 The allenes with unsymmetrical substitution at the term carbon atoms show
optical activity and isomerism. However, all the four groups need not be
different.
 The terminal carbon atoms should have unsymmetrical substitution. Hence,
allenes of the following type show isomerism.
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Gawali S.P. TYBSC Sem -V Stereochemistry- I

11. In allenes, the two end carbon atoms are sp2 hybridised and the centre carbon atom is sp
hybridised. Thus, the centre carbon atom forms two π-bonds which are perpendicular to each
other.
Hence, the allene with unsymmetrical substitution will not
have any element of symmetry. Under this condition, a
molecule will not be superimposable on its own mirror
image. Thus, it possesses molecular chirality.
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Gawali S.P. TYBSC Sem -V Stereochemistry- I

12. Van't Hoff in 1875 predicted that allene compounds of the above type should be
resolvable
1935 when Mills and Maitland provided the first experimental verification and
subjected 1, 3-di-1-naphthyl-1, 3-diphenylprop-2-enol (I) to catalytic asymmetric
dehydration to 1,3-dinaphthyl-1, 3-diphenylpropadiene (II).
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Gawali S.P. TYBSC Sem -V Stereochemistry- I

13. Kohler et al. resolved the glycolic ester of 4-(1-naphthyl)-2, 4-diphenylbuta-2, 3-dienoic acid
(III) (by using brucine).
Similarly, the allenic acid (IV) was resolved, by Wotiz et al. (1951), by using strychnine.
Jacob et al. (1957), when they rearranged 1,3-diphenylpropyne (V) to active 1,3-diphenylpropadiene
(VI), by adsorption over alumina catalyst impregnated with brucine (which gives the laevorotatory
allene) or quinine (which gives the dextrorotatory allene).
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Gawali S.P. TYBSC Sem -V Stereochemistry- I

14. Landor et al. (1959) synthesised (+)1-chloro-3,4, 4-trimethyl-1,2 pentadiene (VIII) by the
action of thionyl chloride on (+) 3,4, 4-trimethyl-1 pentyne-3-ol (VII)
Antibiotic mycomycin (IX) owes its optical
activity due to the presence of allene
grouping.
Perkin, Pope and Wallach (1909),
resolved 4-methyl-cyclohexylidene
ethanoic acid (X).
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Gawali S.P. TYBSC Sem -V Stereochemistry- I

15. Thank You
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Gawali S.P. TYBSC Sem -V Stereochemistry- I