Name: radical halogenation: chiral products
Uploaded: 2023-04-30T11:22:05.000Z
Duration: 1 min 33 s
Channel: JoVE

The stereochemistry of radical halogenation depends on whether the reacting molecule is chiral or achiral.
For example, achiral n-butane, upon radical chlorination, gives 1- and 2-chlorobutane. The reaction introduces a new chiral center, as one of the products is a racemic mixture.
The formation of a racemic mixture is driven by the generation of an achiral trigonal planar radical intermediate, on which chlorine can attack from either face, yielding equal amounts of R and S enantiomers.
This makes carbon-2 hydrogens enantiotopic in nature.
Similarly, radical halogenation at an existing chiral center also forms a racemic mixture.
Here, the radical intermediate loses the reactant&#39;s configuration and becomes achiral, enabling the halogen attack from either side and producing equal amounts of enantiomers.
In contrast, a chiral compound with a chiral carbon not involved in radical halogenation gives diastereomeric products, introducing a second chiral center.
The existing chiral center makes the trigonal planar radical intermediate chiral. Consequently, chlorine attacks to a greater extent at one face than the other and produces chiral, diastereomeric products of unequal amounts.

radical halogenation: stereochemistry

Radical Halogenation: Stereochemistry

Stereochemistry is the study of the different spatial arrangements of atoms in a given molecule. The stereochemistry of radical halogenations can be understood from three different situations:
Halogenation to form a new chiral center:
<img alt="Free radical chlorination mechanism, showing alkanes reacting with Cl2 under UV light to form racemic mix." src="/files/ftp_upload/12898/PT_12898_Figure%201.svg" style="text-align: center; height: 110px;" data-altupdatedat="1761830439000" data-alt="Figure1" />
For example, radical halogenation of butane forms the products 2-chlorobutane and 1-chlorobutane, where the former has a chiral center. However, 2-chlorobutane is obtained as a racemic mixture due to the trigonal planar structure of the formed radical intermediate.
Halogenation at an existing chiral center: 
<img alt="Alkene bromination process, chemical reaction diagram with Br&#8322;, hv, racemic mixture formation." src="/files/ftp_upload/12898/PT_12898_Figure%202.svg" style="text-align: center; height: 110px;" data-altupdatedat="1761830439000" data-alt="Figure2" />
If the radical halogenation occurs at an existing chiral carbon, a 1:1 ratio of enantiomers is formed. This is also because of the trigonal planar structure of the radical intermediate, where the chirality of the starting material has been lost.
Halogenation to generate a second chiral center: 
<img alt="Allylic bromination reaction diagram forming diastereomers with Br&#8322; and hv." src="/files/ftp_upload/12898/PT_12898_Figure%203.svg" style="text-align: center; height: 125px;" data-altupdatedat="1761830439000" data-alt="Figure3" />
If the radical halogenation of an enantiomer occurs at a position other than the chiral carbon, diastereomers are formed. In this case, also the trigonal planar radical intermediate is formed without affecting the existing chiral center. This chiral center differentiates the two faces of the radical intermediate, so the halogen attacks unequally at two faces leading to the formation of unequal amounts of diastereomers.

Stereochemistry is the study of the different spatial arrangements of atoms in a given molecule. The stereochemistry of radical halogenations can be ...

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Organic Chemistry

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Radical Chemistry

KEY TERMS AND CONCEPTS

radical halogenation: chiral products

Radical halogenation can change the chirality of a molecule, which is the three-dimensional arrangement of its atoms. The products depend on whether the reaction creates a new chiral center, reacts at an existing chiral center, or adds a second chiral center.

When radical halogenation forms a new chiral center, the product can be a racemic mixture. For example, halogenation of butane gives 2-chlorobutane and 1-chlorobutane. The 2-chlorobutane product has a chiral center, but it is formed as a racemic mixture because the radical intermediate is trigonal planar, or flat in shape.

If halogenation happens at an existing chiral carbon, the result is a 1:1 ratio of enantiomers. Enantiomers are mirror-image forms of the same molecule. Here, the trigonal planar radical intermediate removes the original chirality of the starting material.

Radical halogenation can also generate a second chiral center. In that case, diastereomers form. The radical intermediate is still trigonal planar, but the existing chiral center is not changed. That chiral center makes the two faces of the radical intermediate different, so halogen attack is unequal and the diastereomers form in unequal amounts.

Radical Halogenation: Chiral Products

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key terms and concepts

Learn how radical halogenation affects chirality, including racemic mixtures, enantiomers, and diastereomers formed through trigonal planar radical intermediates.

Radical Halogenation: Chiral Products | JoVE K12

organic chemistry

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Radical Halogenation: Chiral Products

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