Alkene Br2

The Electrophilic Addition of Bromine to Alkenes: A Deep Dive into Alkene-Br₂ Reactions

This article delves into the fascinating reaction between alkenes and bromine (Br₂), a classic example of electrophilic addition in organic chemistry. We will explore the mechanism, stereochemistry, regioselectivity (where applicable), and practical applications of this crucial transformation. Understanding this reaction is fundamental to grasping a broader understanding of alkene reactivity and its importance in synthesis.

I. The Nature of Alkenes and Bromine

Alkenes, also known as olefins, are hydrocarbons containing a carbon-carbon double bond (C=C). This double bond comprises a strong sigma (σ) bond and a weaker pi (π) bond, which is crucial for the reactivity of alkenes. The π electrons are relatively loosely held and are susceptible to attack by electrophilic reagents.

Bromine (Br₂), a diatomic molecule, is a nonpolar molecule with a relatively high electronegativity. Its relatively weak Br-Br bond makes it susceptible to heterolytic cleavage, resulting in the formation of electrophilic bromine species.

II. The Mechanism of Electrophilic Addition

The reaction between an alkene and bromine proceeds via a concerted electrophilic addition mechanism. This means the breaking of bonds and formation of new bonds occur simultaneously in a single step. The process can be broken down as follows:

1. Electrophilic Attack: The π electrons of the alkene act as a nucleophile, attacking one of the bromine atoms. This initiates a heterolytic cleavage of the Br-Br bond. One bromine atom becomes positively charged (a bromonium ion), and the other becomes negatively charged (bromide ion).

2. Bromonium Ion Formation: A three-membered cyclic bromonium ion intermediate is formed. The positive charge is delocalized over both carbons initially involved in the double bond.

3. Nucleophilic Attack: The negatively charged bromide ion acts as a nucleophile, attacking one of the carbon atoms in the bromonium ion. This leads to the opening of the three-membered ring and the formation of a vicinal dibromide (a molecule with two bromine atoms on adjacent carbons).

This process is shown schematically below:

```
CH2=CH2 + Br2 -----> [CH2-CH2]<sup>+</sup>Br<sup>-</sup> ----> CH2Br-CH2Br
Alkene Bromine Bromonium Ion Intermediate Vicinal Dibromide
```

III. Stereochemistry of the Reaction

The addition of bromine to an alkene is anti stereospecific. This means that the two bromine atoms add to opposite faces of the double bond. This is a direct consequence of the mechanism; the bromide ion attacks the bromonium ion from the backside, leading to a trans configuration of the vicinal dibromide. For example, the addition of bromine to cis-2-butene yields meso-2,3-dibromobutane, while the addition to trans-2-butene yields a racemic mixture of (+)- and (-)-2,3-dibromobutane.

IV. Regioselectivity and Applications

In the case of symmetrical alkenes (like ethene), regioselectivity isn't a factor as both carbons are identical. However, with unsymmetrical alkenes, the more substituted carbon will be favored in some instances. This reaction is primarily used for the synthesis of vicinal dibromides, which are important intermediates in various organic syntheses. They can be further functionalized through various reactions, leading to a wide range of products.

V. Practical Examples

The synthesis of 1,2-dibromopropane: The reaction of propene with bromine yields 1,2-dibromopropane.
Qualitative test for unsaturation: The decolorization of bromine water (an orange-brown solution) upon addition of an alkene is a classic test for the presence of a carbon-carbon double bond. The bromine reacts with the alkene, resulting in a colorless solution.

VI. Conclusion

The addition of bromine to alkenes is a fundamental reaction in organic chemistry, offering a clear example of electrophilic addition. Understanding its mechanism, stereochemistry, and applications is essential for any student or professional working in organic synthesis. The anti-stereospecificity and the formation of vicinal dibromides are key characteristics of this reaction, making it a versatile tool in organic synthesis.

FAQs

1. Is this reaction exothermic or endothermic? The reaction is exothermic, meaning it releases heat.
2. What are the common solvents used in this reaction? Common solvents include dichloromethane (DCM), chloroform, and carbon tetrachloride.
3. Can this reaction be used with alkynes? Yes, but the reaction mechanism and product are different. It typically leads to the formation of a tetrabromide.
4. What happens if I use excess bromine? Excess bromine does not significantly alter the product but can lead to further reactions if other reactive sites are present in the molecule.
5. Are there any safety precautions I should take when performing this reaction? Bromine is a corrosive and toxic substance. Appropriate safety measures, including the use of a fume hood and personal protective equipment (PPE), are essential.

Search Results:

Reactions of Alkenes with Bromine - Chemistry LibreTexts 23 Jan 2023 · This page gives you the facts and a simple, uncluttered mechanism for the electrophilic addition reactions between bromine (and the other halogens) and alkenes like …

electrophilic addition mechanism steps addition bromine to alkenes ... Electrophilic addition reaction of bromine, electrophilic addition to alkenes with pure bromine or in non-polar solvent (non-aqueous Br2 (l/solvent)) to give dibromoalkanes or electrophilic …

3.4 Alkenes - chemrevise.org 3 Feb 2017 · approaches the alkene, the pi bond electrons repel the electron pair in the Br-Br bond. This INDUCES a DIPOLE. Br2 becomes polar and ELECTROPHILIC (Brδ+). The …

Alkene + Br2 + H2O | Video Summary and Q&A | Glasp 5 Jul 2020 · This video explains the reaction between an alkene and bromine in water, resulting in the formation of a halo hydrant with selective product distribution based on the primary and …

Organic Chemistry Revision Sheets REACTANTS: Alkene and Bromine (Br 2) CONDITIONS: Non-polar organic solvent (or pure liquid bromine) PRODUCT: Dibromoalkane REACTION TYPE: Electrophilic Addition Reaction Br 2 …

electrophilic addition - symmetrical alkenes and bromine Alkenes react in the cold with pure liquid bromine, or with a solution of bromine in an organic solvent like tetrachloromethane. The double bond breaks, and a bromine atom becomes …

Reactions of Alkenes 26 Oct 2024 · Br2 is a non-polar molecule however when placed close to an area of high electron density it can get polarised. Example of an electrophilic addition reaction of Br2 and ethene to …

EXPLAINING THE REACTION BETWEEN SYMMETRICAL ALKENES … This page guides you through the mechanism for the electrophilic addition of bromine to symmetrical alkenes like ethene or cyclohexene. Unsymmetrical alkenes are covered …

Bromination of Alkenes - The Mechanism – Master Organic … 15 Mar 2013 · Bromination, Chlorination, and Halohydrin Formation from Alkenes. Alkenes undergo halogenation when treated with Cl 2, Br 2 and (less commonly) I 2 to give vicinal …

EXPLAINING THE REACTION BETWEEN UNSYMMETRICAL ALKENES … This page guides you through the mechanism for the electrophilic addition of bromine to unsymmetrical alkenes like propene.

Alkene Reactions: Dibromide Formation using Br2 and Alkenes Bromine (Br2) in the presence of alkenes reacts to form dibromides: Bromine reacts with the alkene molecule to form a vicinal dibromide (1,2-dibromide) where bromines are added to the …

O615: Bromination and Oxidation – Alkanes and Alkenes The rapid disappearance of the amber color of the bromine at room temperature is characteristic of reaction with alkenes. Alkenes: Oxidation (reaction with KMnO4) RHC = CHR + KMnO4 → …

10.4 Reactions of Alkenes: Addition of Bromine and Chlorine to Alkenes ... The reaction between a C=C double bond and bromine (Br 2) can be used as a test for the presence of alkene in an unknown sample. The bromine reagent is in a reddish color, and the …

5.3 Radical Bromination of Alkenes Part II: Allylic Bromination with ... 13 Oct 2023 · Hydrogen halides (HCl, HBr, and HI) react with alkenes in an electrophilic addition reaction to yield alkyl halides as products. Also, Bromine (Br 2) and chlorine (Cl 2) can react …

Electrophilic Addition of Alkenes | Cambridge (CIE) A Level … 4 Jan 2025 · In an addition reaction, the H atom acts as an electrophile and accepts a pair of electrons from the C-C bond in the alkene. The H-Br bond breaks heterolytically, forming a Br …

10.4: Reactions of Alkenes- Addition of Bromine and Chlorine to Alkenes ... 16 Dec 2021 · The reaction between C=C double bond and bromine (Br 2) can be used as a test for the presence of alkene in an unknown sample. The bromine reagent is in reddish color, …

8.2: Halogenation of Alkenes - Addition of X₂ - Chemistry LibreTexts A halogen molecule, for example Br 2, approaches a double bond of the alkene, electrons in the double bond repel electrons in the bromine molecule causing polarization of the halogen …

Bromination of alkenes with Br2 to give dibromides Description: Treatment of alkenes with bromine (Br2) gives vicinal dibromides (1,2-dibromides). Notes: The bromines add to opposite faces of the double bond (“anti addition”). Sometimes the …

More organic chemistry - AQA Reactions of alkenes - BBC CH 2 =CH 2 + Br 2 → CH 2 BrCH 2 Br. covalent bondA bond between atoms formed when atoms share electrons to achieve a full outer shell of electrons. Alkanes cannot take part in addition...

Alkene Reactions: Bromohydrin Formation using Br2 and H2O Bromine (Br2) or N-bromosuccinate (NBS) in the presence of alkenes and water, react together form bromohydrins: Bromine reacts first with the alkene molecule, and ultimately resides on …