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Stereochemistry

Stereochemistry is a very important sub-branch of organic chemistry. Study of stereochemistry has helped in understanding the organic reactions and the products formed under such reactions very clearly.

 

Stereochemistry Definition

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Stereo chemistry is the branch of chemistry which deals with the three-dimensional structures of molecules and their effect on physical and chemical properties. Stereo chemistry is a study of stereo isomers.

One aspect of stereo chemistry is stereo isomerism.
  1. When isomerism is caused by the different arrangements of atoms or groups in space, the phenomenon is called stereo isomerism.
  2. Isomers having the same constitution but different spatial arrangements of their atoms are known as stereo isomers.
  3. The different spatial arrangements of atoms in a molecule which are readily inter convertible by rotation about single bonds are called conformations, if not, configurations.
  4. Stereo isomers have different configurations and are also called as configurational isomers. A molecule can have only one configuration, that is, a different configuration is a different molecule, whereas a molecule could have an infinite number of conformations.

Stereo isomerism is further divided into enantiomers and diasteromers. Isomers which are non superimposable mirror images of each other are called enantiomers (or optical antipodes or enantiomorphs or mirror-image isomers) and the isomerism exhibited by them is known as Enantiomerism.

All the optically active compounds exhibit enantiomerism. Stereo isomers which are mirror images of each other are called as enantiomers and those which are not mirror images of each other are called as diastereomers.

Properties of Enantiomers


Optical isomers that are mirror images are called Enantiomers. These always exist as discrete pairs. Enatiomers are stable, isolable compounds that differ from one another in three-dimensional spatial arrangements. Enantiomers cannot be inter converted under ordinary conditions.

Enantiomers have identical properties in all respects except in their interaction with plane of polarized light. Enantiomers have the same melting point, density, solubility, color and reactivity toward acids and bases. They differ, however, in the direction in which they rotate the plane polarized light. Both rotate the plane of polarized light to exactly the same extent (same angle) but one rotates the plane to the right (clockwise:called dextrorotatory), while the other rotates the plane to the left (anticlockwise: called levorotatory).

Properties of Diastereomers


In general, each asymmetric carbon atom in a molecule doubles the number of theoretically possible isomers. Hence, molecule with n asymmetric carbon atoms should have 2n stereo isomers.

For example,

Properties of Diastereomers

Notice that a is the mirror image of b, and C is the mirror image of d. Thus, four isomers are two pairs of enantiomers. Now, compare a and c. They are neither super imposable nor are they mirror images they are called as diastereomers. Thus, a and C and b and d are diastereomers. So, stereo isomers, which are not mirror images are diastereomers.

Addition and Elimination Reactions

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Elimination Reaction


Few organic reactions give a 100% yield o a single product. The reason for this is that it is uncommon to find a set of reaction conditions which will permit one reaction to occur to the total exclusion of another.

Nucleophilic substitution reactions are no exception. They compete for starting material with elimination reactions. Indeed proper selection of reagents and the reaction conditions may actually result in more starting material undergoing elimination than substitution. The most common type of elimination reaction is one in which two fragments are removed from a substrate to produce a modified substrate and two small units.

CH3-CH2-CH2-L CH3 - CH=CH2 + H+ + L-

Addition Reaction


Addition reactions are those in which atoms or groups of atoms are simply added to a double or triple bond without elimination of any atom or other molecules. In these reactions, at least one π bond is lost while two new σ bonds are formed. Double bonds become saturated and triple bonds are converted into double bonds or may become saturated by further addition to single bonds.

Example:
CH2 = CH2 + Br2 CH2Br - CH2Br

Syn Stereochemistry

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Stereo chemistry of syn-addition


In an addition reaction, if the groups added to the reacting molecule are added in the same faces of the molecule, the addition is referred to as "syn addition". Addition reactions of this kind are stereospecific additions.

Example: Hydrogenation of maleic acid using Wilkinson's catalyst

Syn Stereochemistry

Syn- elimination


In an E2 reaction, to remove two substituents, they should be in the same plane. If the two entities to be removed, say H and X are in the same plane and are in the same sides of the molecule, then, they are said to be "syn- periplanar."

If the elimination reaction removes two substituents from the same side of the molecule, the reaction is a syn-elimination. Syn elimination is a very slow reaction and is not a much preferred one.

A molecule should have an eclipsed conformation for syn-elimination to occur. And the eclipsed form usually requires more energy than the staggered form. Thus, syn-elimination reaction is very rare.

Syn-Elimination Reaction

Anti Stereochemistry

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Anti - Addition


In an addition reaction, if the added groups gets attached to the different faces of a molecule, or different planes of a molecule, then that type of addition is called as anti-addition.

Addition of bromine to the 2-butenes involves anti-addition.

Anti-Addition

Anti-addition is the general rule for the reaction of bromine or chlorine with simple alkenes.

Anti - Elimination

  1. In E2 reaction, if the substituents are removed from the opposite sides of a molecule, the reaction is an anti-elimination reaction.
  2. Anti-elimination is favored in an E2 reaction. The reason is that, anti-elimination requires a staggered conformation of the starting molecule. Staggered conformation is a much stable one, because of less energy. Consequently, anti-elimination occurs more rapidly.
  3. Another reason can be explained by Sawhorse conformation. In syn-elimination, the electrons of the departing hydrogen move to the front side of the carbon bonded to X.
  4. In anti-elimination, the electrons move to the backside of the carbon bonded to X. And, displacement reaction involves backside attack.
  5. Also, anti-elimination avoids the repulsion experienced by the electron-rich base when it is on the same side of the molecule as the electron-rich halogen.

Anti-Elimination

Stereochemistry Problems

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Solved Example

Question: Which among the following pairs are diastereomers?


Solution:
a and b are diastereomers. The pairs are not mirror image of each other.

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