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Stereochemistry and Rearrangement of Carbocations

It is usually necessary to introduce briefly some historical background before presenting a systematic treatment of a specific area of science.

Stereochemistry

It is usually necessary to introduce briefly some historical background information before presenting a systematic treatment of a specific area of science and technology. In stereochemistry, atoms and groups are arranged spatially within a molecule. The term stereochemistry comes from the word 'chemistry of space'. It was discovered in 1842 by Louis Pasteur that salts of tartaric acid collected from wine production vessels could rotate plane-polarized light, but salts from other sources were unable to do so.

This discovery led to stereochemistry's development. Optical isomerism explains this phenomenon.
  • Atoms of molecules are arranged in three dimensions as a result of their three-dimensional structure. Another way to modify the arrangement of atoms is stereochemistry.
  • Three-dimensional chemistry is the study of stereoisomers, compounds with different arrangements of spatial coordinates in three dimensions.
  • The study of stereochemistry involves examining molecules whose mirror images cannot superimpose on each other due to chirality, a geometric property of molecules.
  • Dynamic stereochemistry is another branch of 3D chemistry. Different atom arrangements are examined in this study to determine how they affect chemical reaction rates.

Rearrangement of carbocations

In organic chemistry, carbocation rearrangement occurs by reorganizing the molecule structurally to transition from an unstable to a stable state.

Carbocations formed from alkyl halides, alcohols or alkenes, may be rearrangeable. The two types of carbocation rearrangements are hydride shifts and alkyl shifts. The carbocation resulting from the rearranging will react further to form a product with an alkyl skeleton different from the starting material. It is very common to observe both the rearranged and unrearranged carbocations forming a mixture of isomeric products. Beginning with alcohols, we will examine this process in more detail in SN1 substitution reactions. An acid-base reaction converts the OH atom into a leaving group (so that it leaves as water) in the SN1 substitution reaction involving alkyl halides.

Hydride shift

A rearrangement known as a hydride shift can occur in the formed carbocation. A hydrogen is transferred from one carbon to another carbon that is nearby (less substituted). HBr, HCl, and HI often react with alcohol to form a hydride shift. Alcohol reacts with hydrogen chloride the way the following:



In place of the alcohol portion (-OH), the atom of Cl has been introduced. Although the Cl atom replaces the OH atom, it does not directly replace it. This occurs as follows:


Alkyl shift

To rearrange carbon atoms on adjacent carbon atoms, certain carbocations lack suitable hydrogen atoms (secondary or tertiary). Alkyl group migration (or shift of an alkyl group) is a form of rearrangement in this case. Similar to the hydride shift, the alkyl shift also acts by converting one atom to another. An electron pair is carried by the shifting alkyl group to form a bond with the neighboring carbocation.

When an alkyl group and a carbohydrate switch positions on the molecule, their positions are reversed. It is more stable for tertiary carbocations to exist than primaries or secondary ones, so absorption of alkyl changes may transform them into tertiary carbocations.
Example -

 

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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, 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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