Electrophilic addition mechanism consists of two steps. Before constructing the mechanism let us summarize conditions for this reaction. We will use Br2 in our example for halogenation of ethylene. | Nucleophile | Double bond in alkene | | Electrophile | Br2, Cl2 | | Regiochemistry | not relevant | | Stereochemistry | ANTI | Step 1: In the first step of the addition the Br-Br bond polarizes, heterolytic cleavage occurs and Br with the positive charge forms a intermediate cycle with the double bond. Step 2: In the second step, bromide anion attacks any carbon of the bridged bromonium ion from the back side of the cycle. Cycle opens up and two halogens are in the position anti. Summary Hallogens can act as electrophiles due to polarizability of their covalent bond.Addition of halogens is stereospecific and produces vicinial dihalides with anti addition.Cis starting material will give mixture of enantiomers and trans produces a meso compound.

The carbocation intermediate forms a positive charge on the left carbon after the hydrogen was added to the carbon with the most hydrogen substituents. The Bromine, which has a negative charge, attacks the positively charged carbocation forming the final product with the nucleophile on the more substituted carbon. Addition due to excess HBr present Reaction: Halogenation of Alkynes Summary: - Stereoslectivity: anti addition - Reaction proceeds via cyclic halonium ion Addition of Br2 - The addition of Br2 to an alkyne is analogous to adding Br2 to an alkene. - Once Br2 approaches the nucleophilic alkyne, it becomes polarized. - The \(\pi\) electrons, from the triple bond, can now attack the polarized bromine forming a C-Br bond and displacing the bromine ion. - Now, you will get an intermediate electrophilic carbocation, which will immediately react with the bromine ion giving you the dibromo product. First, you see the polarized Br2 being attacked by the \(\pi\) electrons. Once you form the C-Br bond, the other bromine is released as a bromine ion. The intermediate here is a bromonium ion, which is electrophilic and reacts with the bromine ion giving you the dibromo product.

passes through a short-lived strongly electrophilic bromonium intermediate. This is an example of a halogen addition reaction. Bromine is significantly less Bromine is a chemical element; it has symbol Br and atomic number 35. It is a volatile red-brown liquid at room temperature that evaporates readily to form a similarly coloured vapour. Its properties are intermediate between those of chlorine and iodine. Isolated independently by two chemists, Carl Jacob Löwig (in 1825) and Antoine Jérôme Balard (in 1826), its name was derived from Ancient Greek Bromine is a chemical element; it has symbol Br and atomic number 35. It is a volatile red-brown liquid at room temperature that evaporates readily to form a similarly coloured vapour. Its properties are intermediate between those of chlorine and iodine. Isolated independently by two chemists, Carl Jacob Löwig (in 1825) and Antoine Jérôme Balard (in 1826), its name was derived from Ancient Greek βρῶμος (bromos) 'stench', referring to its sharp and pungent smell. Elemental bromine is very reactive and thus does not occur as a free element in nature. Instead, it can be isolated from colourless soluble crystalline mineral halide salts analogous to table salt, a property it shares with the other halogens. While it is rather rare in the Earth's crust, the high solubility of the bromide ion (Br−) has caused its accumulation in the oceans. Commercially the element is easily extracted from brine evaporation ponds, mostly in the United States and Israel. The mass of bromine in the oceans is about one three-hundredth that of chlorine. At standard conditions for temperature and pressure it is a liquid; the only other element that is liquid under these conditions is mercury. At high temperatures, organobromine compounds readily dissociate to yield free bromine atoms, a process that stops free radical chemical chain reactions. This effect makes organobromine compounds useful as fire retardants, with more than half the bromine produced worldwide each year put to this purpose. The same property causes ultraviolet sunlight to dissociate volatile organobromine compounds in the atmosphere to yield free bromine atoms, causing ozone depletion. As a result, some organobromine compounds—such as the pesticide methyl bromide—are no longer used. Bromine compounds are still used in well drilling fluids, in photographic film, and as an intermediate… L… An old qualitative test for the presence of the alkene functional group is that alkenes turn brown aqueous bromine solutions colourless, forming a bromohydrin with some of the dibromoalkane also produced. The reaction passes through a short-lived strongly electrophilic bromonium intermediate. This is an example of a halogen addition reaction.

This chapter evaluates electrophilic addition to alkenes. Electrophilic addition to double bonds gives three-membered ring intermediates with bromine (Br 2 ), with Hg 2+ , and with peroxy-acids (in which case the three-membered rings are stable and are called epoxides). Alkenes decolourize bromine water: alkenes react with bromine. The product of the reaction is a dibromoalkane. All three classes of three-membered rings react with nucleophiles to give 1,2-difunctionalized products with control over regioselectivity and stereoselectivity. Meanwhile, protonation of a double bond gives a cation, which also traps nucleophiles, and this reaction can be used to make alkyl halides. The chapter also looks at dihydroxylation, periodate cleavage, and ozonolysis.

This chapter gives an overview of the structure and reactivity of alkenes and alkynes and highlights the most important methods of preparing these compounds. Alkenes are an important class of organic compounds that contain a C=C bond, while the C≡C bond in alkynes reacts with electrophiles in electrophilic addition reactions. The chapter describes how alkenes are prepared from halogenoalkanes, alcohols, alkynes, and aldehydes/ketones and how C=C and C≡C bonds react in electrophilic addition reactions. It shows how alkynes are prepared from 1,2-dibromoalkanes and how substituted alkynes are prepared by alkylation of alkynyl anions with halogenoalkanes. It also outlines reaction mechanisms in order to explain how C=C bonds undergo the addition reactions.