Halogenation Alkenes
Halogenation Alkenes - Related Videos
02:46
Halogenation of Alkenes
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2025
Halogenation is the addition of chlorine or bromine across the double bond in an alkene to yield a vicinal dihalide. The reaction occurs in the presence of inert and non-nucleophilic solvents, such as methylene chloride, chloroform, or carbon tetrachloride. Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the π electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate. A...
03:01
Halogens
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2020
Group 17 elements, known as halogens, are nonmetals. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine a solid. Astatine is a highly unstable radioactive element, so currently, most of its properties are unknown due to its short half-life. Tennessine is a synthetic element also predicted to be in this group. The halogens are not found as single atoms but exist as diatomic molecules. The atomic radius increases from fluorine to iodine. The valence shell...
06:52
Ozonolysis of Alkenes
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2023
Source: Vy M. Dong and Zhiwei Chen, Department of Chemistry, University of California, Irvine, CA This experiment will demonstrate an example of an ozonolysis reaction to synthesize vanillin from isoeugenol (Figure 1). Ozonolysis of alkenes, an oxidation reaction between ozone and an alkene, is a common method to prepare aldehydes, ketones, and carboxylic acids. This experiment also demonstrates the use of an ozone generator and a low temperature (−78 °C) reaction. Figure 1. Diagram showing...
06:44
From Molecules to Materials: Engineering New Ionic Liquid Crystals Through Halogen Bonding
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Cited by 2 •
2018
A protocol for the synthesis of a new type of mesogens, based on the halogen-bonded supramolecular anion [CnF2n+1-I···I···I-CnF2n+1]−, is reported.
02:29
Nomenclature of Alkenes
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2023
The IUPAC naming system for alkenes replaces -an- with -en- in the corresponding parent alkanes. Accordingly, a simple alkene replaces the -ane suffix of the alkane with -ene. As per the IUPAC rules, the longest carbon chain containing the maximum number of double bonds is identified as the parent chain and is numbered such that the doubly bonded carbon atoms receive the lowest possible numbers. The location of the double bond is indicated by the number of its first carbon atom. In branched...
FAQs
The alkene’s pi bond acts as the electron-rich site and polarizes a nearby Br2 or Cl2 molecule. This polarization creates electrophilic character in one halogen atom, allowing the double bond to form a cyclic halonium ion. That intermediate is central to the reaction because it organizes addition before a halide ion opens the three-membered species.
Anti addition occurs because the halide ion attacks the cyclic halonium ion from the side opposite the halogen bridge. Opening the intermediate places the two halogen atoms on opposite sides of the former double bond. This stereochemical pattern follows directly from the intermediate’s cyclic structure and the order of bond formation.
The standard transformation described here treats the alkene with a halogen such as Br2 or Cl2 in an inert solvent. This solvent condition accompanies formation of the halonium ion and halide-mediated opening, supporting the typical anti-addition outcome. The selected halogen therefore supplies both the electrophilic halogen component and the halide involved in intermediate opening.
A useful workflow begins with polarization of the halogen molecule by the alkene’s pi bond. The double bond then forms a cyclic halonium ion, after which a halide ion opens that intermediate. The sequence converts the original unsaturated site into a neighboring pair of halogen-bearing carbon atoms, while accounting for the observed stereochemical arrangement.
The resulting vicinal dihalides, meaning compounds bearing halogens on neighboring carbon atoms, serve as versatile intermediates in organic synthesis. They can undergo further substitution, elimination, and functional-group transformations. Consequently, this reaction is useful not only for installing halogen functionality, but also for preparing molecules that can participate in later synthetic steps.
Halogenation of alkenes connects several central chemistry ideas in one transformation: electrophilic addition, cyclic-intermediate formation, and stereochemical control. It also demonstrates how an electron-rich pi bond reacts with a polarized reagent and how intermediate geometry influences product arrangement. In research and teaching, the reaction therefore provides both a practical functionalization method and a clear mechanistic model.