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STEREOCHEMISTRY:

DR. KARTIK KUMAR NANDI

ASSOCIATE PROFESSOR OF CHEMISTRY

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DEPARTMENT OF CHEMISTRY

BRAHMANANDA KESHAB CHANDRA COLLEGE

KOLKATA – 700108

E-Mail: kknandi@yahoo.com

STEREOISOMERISM

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2. STEREOISOMERISM

  • Isomers which have the same molecular formula and same structural formula but differ in the manner their atoms or groups are arranged in the space are called stereoisomers. It is of two types:
  • Configurational Isomerism
  • Conformational Isomerism

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I. Configurational Isomerism

  • The stereoisomers which cannot be interconverted unless a covalent bond is broken are called configurational isomers. These isomers can be separated under normal conditions.
  • The configurational isomerism is again of two types:

a) Optical Isomerism or Enantiomerism

b) Geometrical Isomerism

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6.1 Cis-trans isomers� GEOMETRICAL ISOMERISM�

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  • Differ in the arrangement of their atoms in space (cannot interconvert)- Alkenes.

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6.1 Cis-trans isomers

  • Cyclic structure.

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STEREOISOMERISM

SAME MOLECULAR FORMULA / CONSTITUTION BUT DIFFERENT IN ARRANGEMENTS OF GROUPS/ATOMS IN SPACE ARE STEREOISOMERS & THE PHENOMENON IS STEREOISOMERISM.

GEOMETRICAL/DIASTEREO ISOMERISM

[Around double bonded Carbon or any Rigid part of molecule]

OLD SYSTEM: CIS/TRANS

IUPAC SYSTEM: E/ Z ISOMERS

Differ in both Physical and Chemical properties;

MP/BP; 1,2-dichloroethene: Z = 60 deg ; E = 48 deg

Dipole Moment; 1,2-dichloroethene: Z = 1.85 D; E = 0 D

Acidity/ Basicity: Z-Maleic acid is more acidic than E-Fumaric acid

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�OPTICAL ISOMERISM; Capable rotating Plane-Polarised Light�

  • Optically active: Non-superimposable mirror image.

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  • Chiral & Chirality: Handedness; Molecule Devoid of any ‘Elements of Symmetry’ is Chiral Molecule.

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  • Property of being chiral is known as chirality.

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  • Chirality is due to lack/ void of any symmetry,
  • i,e., when a molecule does not possess any elements of symmetry.

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a) Optical Isomerism or Enantiomerism

  • The stereoisomers which are related to each other as an object and its non-superimposable mirror image are called optical isomers or enantiomers (Greek: enantion means opposite).
  • The optical isomers can also rotate the plane of polarised light to an equal degree but in opposite direction.
  • The property of rotating plane of polarised light is known as optical activity.
  • The optical isomers have similar physical and chemical properties.

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For example,

  • Molecular formula C3H6O3 represents two enantiomeric lactic acids as shown below:

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Definitions

  • Stereoisomers – compounds with the same connectivity, different arrangement in space
  • Enantiomers – stereoisomers that are non- superimposible mirror images; only properties that differ are direction (+ or -) of optical rotation
  • Diastereomers – stereoisomers that are not mirror images; different compounds with different physical properties

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More Definitions

  • Asymmetric center – sp3 carbon with 4 different groups attached
  • Optical activity – the ability to rotate the plane of plane –polarized light
  • Chiral compound – a compound that is optically active (achiral compound will not rotate light)
  • Polarimeter – device that measures the optical rotation of the chiral compound

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Chirality

  • “Handedness”: Right-hand glove does not fit the left hand.
  • An object is chiral if its mirror image is different from the original object.

Chapter 5

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Achiral

  • Mirror images that can be superposed are achiral (not chiral).

Chapter 5

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Chapter 5

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Chapter 5

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Stereoisomers

Enantiomers: Compounds that are nonsuperimposable mirror images. Any molecule that is chiral must have an enantiomer.�

Chapter 5

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Chiral Carbon Atom

  • Also called asymmetric carbon atom.
  • Carbon atom that is bonded to four different groups is chiral.
  • Its mirror image will be a different compound (enantiomer).

Chapter 5

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Stereocenters

  • An asymmetric carbon atom is the most common example of a chirality center.
  • Chirality centers belong to an even broader group called stereocenters. A stereocenter (or stereogenic atom) is any atom at which the interchange of two groups gives a stereoisomer.
  • Asymmetric carbons and the double-bonded carbon atoms in cis-trans isomers are the most common types of stereocenters.

Chapter 5

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Examples of Chirality Centers

Asymmetric carbon atoms are examples of chirality centers, which are examples of stereocenters.

Chapter 5

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