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Reactions of Chiral Molecules

The configurations of one compound can be compared to those of another using these reactions.
There are a number of ways that chiral molecules react with reagents, according to which reactions can be divided into:
  • The bond with the chiral center is not broken in these reactions.
  • These reactions generate chiral centers.
  • Reactions with optically active molecules in chiral compounds.
  • Interactions between compounds with chiral centers.
In chiral reactions, bonds with chiral centers remain intact:

The configurations of one compound can be compared to those of another using these reactions. If a bond to a chiral center is not broken in the reaction, configuration is maintained.

Due to the fact that the bond to the chiral center is not broken, the 'S' configuration is maintained. That is, -CH2–Cl occupies the same relative position as did -CH2OH in the reactant. By converting optically active compounds into each other through reactions which do not break the bonds with chiral centers, one can determine the configurational relationship between them. It is impossible to assign absolute configurations instead.

Chemical reactions of this type are used to create optically pure compounds with specific rotations. For example, 2-methyl-1-butanol from fusel oil, for instance, is optically pure and rotational specific to –5.90°. The specific rotation of 1-chloro 2-methyl butane is +1.67° after being treated with hydrogen chloride. Hence, a compound is said to be pure if its rotation is equal to this value. A compound that is only 50% optically pure is said to rotate about +0.8°.

Chiral centers are generated by the following reactions:

An enantiomer is typically formed in equal quantities by the formation of the first chiral center in a compound, but due to the formation of the second chiral center, the amount of diastereomers in the mixture varies depending on which side of the reaction is acted upon.

The configuration(s) are retained because the chiral center is not bonded to the molecule. The chiral center forms diastereomers, but in unequal quantities, depending on whether the attack comes from the same or opposite side. Because the intermediate radical contains a chiral center and lacks symmetry, it is characterized by its chirality. Two faces of the molecule for an attack have different properties. SS and meso compounds yielded by S isomer in 29:71 ratio.

Even if both configurations aren't generated, there is a probability of occurrence. In a similar manner, the R isomer would result in the RR and meso compounds in a ratio of 29:71. It yields optically inactive products if the reactant is optically inactive.

A chiral compound reacts with an optically active reagent:

A racemic mixture/modification is commonly resolved or separated into its enantiomers by such a reaction. There is no way to separate enantiomers by fractional distillation or crystallization because they have similar physical properties (other than optical rotation). It is necessary to use optically active reagents to obtain pure enantiomers during racemic modification. Natural sources or reagents can be used to prepare such optically active reagents.

Acids and bases are commonly used to form salts as reactions.

By fractional distillation or crystallization, formed diastereomers can easily be separated by their physical properties. A mineral acid may then be added to the solution in order to recover the resolved enantiomers.

Brucine, quinine, strychnine, and others are all alkaloid bases that are commonly used.

Acid reagents like (-) malic acid can also be used to separate racemic bases. Bases are not the only compounds that can be separated. Since alcohol is weakly ionized and is neither particularly acidic nor basic, attaching them to a handle that is acidic can facilitate their subsequent removal.

Bonds are broken between the chiral center and a reaction product:

Such reactions exhibit stereochemistry based on their mechanism. A particular mechanism can thus be determined from stereochemistry.

A racemic mixture and the fact that the product is optically inactive imply that second chlorine may attach to either side of the intermediate, leading to a free alkyl radical losing its chirality. A simultaneous attack by chlorine and displacement of hydrogen would have yielded only the optically inactive product, as opposed to a chlorine backside attack. Therefore, the mechanism involving free alkyl radicals is correct.
  • Stereospecificity occurs when a reaction involves stereoisomers, and each stereoisomeric reactant yields a different stereospecific product.
  • Stereoisomerism is observed in a reaction when no matter what stereoisomerism is present, a particular stereoisomeric form of the product dominates or dominates other possible forms.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of pharmaguideline.com, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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