Electrophilic Addition to Alkenes with Formation of Optical Isomers

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This Demonstration shows a type of chemical reactivity typical of alkenes. A double bond (very often ) attracts an electrophile species (designated by E), which bonds to the carbon atom in accordance with Markovnikov's rules. In the phase that follows, a nucleophile species, for example, designated by a green sphere labeled , bonds to the second carbon atom, which acquires a positive charge. This carbon atom, in our example, becomes a chiral center, since it is bonded to four different entities [1].

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As a final result, we can obtain an - or an -enantiomer, depending on which side the nucleophile attacks. When they are equally likely, a racemic mixture results.

In the illustration of phase 1, the white spheres represent hydrogen atoms, the black spheres represent carbon atoms and the red sphere represents an ethyl group. Rotation cannot occur about a double bond, but as it turns into a single bond, rotation becomes possible, and the rotation angle control becomes active.

Phase 2 shows the nucleophilic attack, Using the "enantiomer" control it is possible to choose, as final product, the - or -enantiomer. Using the "chiral center viewpoint" checkbox will show the priority rules for the attached group on the chiral center and determine the sense of rotation. A center with a clockwise rotation is an or rectus center, while a center with a counterclockwise rotation is an or sinister center. The blue arrow indicates the direction of observation, which can be positioned in the chiral center viewpoint. The numbers above the attached groups represent priorities according to the CIP convention.

By selecting "CIP convention," you can show the two-dimensional projection of -enantiomers according to the classic convention, with the less significant substituent at the top.

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Contributed by: D. Meliga and S. Z. Lavagnino (January 2018)
Open content licensed under CC BY-NC-SA


Snapshots


Details

Snapshot 1: in the first phase, the alkene undergoes electrophilic attack, by , and follows Markovnikov's rules

Snapshot 2: in the second phase, the nucleophile species, for example , attacks the second carbon atom; the attack site determines the resulting or isomer

Snapshot 3: optical isomers are represented in two dimensions and classified according to Cahn's priority rules

Reference

[1] H. Hart, L. E. Craine and D. J. Hart, Organic Chemistry: A Short Course, 10th ed., Boston: Houghton Mifflin, Co., 1999.



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