INTRODUCTION

Halogenation is a chemical reaction that involves the reaction of a compound, usually an organic compound, with a halogen. The pathway and stoichiometry of halogenation depends on the structural features and functional groups of the organic substrate as well as the halogen. Example: Alkenes readily react with halogens under standard laboratory conditions.

Halogens Facts

1. Chlorine, fluorine, bromine, iodine are all halogens. 2. All these halogens have seven electrons in their outer orbit, i.e. they are monovalent. 3. Halogens have lower melting and boiling points compared to other non metals. 4. All halogens are poor thermal conductors and also poor conductors of electricity. 5. Except fluorine, all other halogen elements can expand their shell to include valence electrons. They can hold around fourteen valence electrons in their outer shell.

In this addition reaction, halogens atoms are added across the double bond of the alkene.In an alkene containing only one double bond, the double bond is broken, the halogen atoms are added, and, the only product of the reaction will be a dihaloalkane. In a bromination reaction, if excess alkene is present, then the reaction mixture will change from a red-brown colour to colourless.

Ethene + Bromine Water → (UV) Bromoethane + Hydrogen bromide

There are several processes for the halogenation of organic compounds, including free radical halogenation, ketone halogenation, electrophilic halogenation, and halogen addition reaction. The determining factors are the functional groups. Saturated hydrocarbons typically do not add halogens but undergo free radical halogenation, involving substitution of hydrogen atoms by halogen. Unsaturated compounds, especially alkenes and alkynes, add halogens. Aromatic compounds are subject to electrophilic halogenation.

The facility of halogenation is influenced by the halogen. Fluorine and chlorine are more electronegative and are more aggressive halogenating agents. Bromine is a weaker halogenating agent than both fluorine and chlorine, while iodine is least reactive of them all. The facility of dehydrohalogenation follows the reverse trend: iodine is most easily removed from organic compounds and organofluorine compounds are highly stable. Bromination and iodination are more likely to substitute at the beta carbon.

Halogenation reactions with elemental chlorine, bromine, and iodine are of considerable importance. Because of high exothermocities, fluorinations with elemental fluorine tend to have high levels of side reactions. Consequently, elemental fluorine is generally not suitable for direct fluorination. Two types of reactions are possible with thesehalogen elements, substitution and addition. Substitution halogenation is characterized by the substitution of a halogen atom for another atom (often a hydrogen atom) or group of atoms (or functional group) on paraffinic, olefinic, aromatic, and other hydrocarbons. A chlorination reaction of importance that involves substitution is that between methane and chlorine.

Addition halogenation involves a halogen reacting with an unsaturated hydrocarbon. Chlorine, bromine, and iodine react readily with most olefins; the reaction between ethylene and chlorine to form 1,2-dichloroethane is of considerable commercial importance, since it is used in the manufacture of vinyl chloride. Addition reactions with bromine or iodine are frequently used to measure quantitatively the number of CH&dbnd;CH(or ethylenic-type) bonds in organic compounds. Bromine numbers or iodine values are measures of the degree of unsaturation of the hydrocarbons. Substitution halogenation on the aromatic ring can be made to occur via ionic reactions. The chlorination reactions with elemental chlorine are similar to those used for addition chlorination of olefins. Chlorinated paraffins have been widely used in industry throughout the world since their introduction nearly 40 years ago. Chlorinated paraffins are needed where chemical stability is highly desired. The primary application of Chlorinated paraffins is in industrial cutting fluids, particularly in the manufacture of automobiles and automobile parts. In addition to their use in cutting oils, Chlorinated paraffins are also used in a lot of commercial paints, adhesives, sealant and caulks. Chlorinated paraffins are a family of complex substances representing more than 200 commercial products. Use applications for chlorinated paraffins range from extreme pressure additives in lubricants, to secondary plasticizers in paints and plastics, to flame retardants in various plastics and textiles.Chlorinated paraffins are produced through chlorination of straight- chain paraffin fractions which are typically subdivided into three categories based on their carbon chain lengths.The properties of the different chlorinated paraffins can vary significantly depending on their carbon chain length and degree of chlorination.

The conventional methods of halogenation of organic compounds caused serious Environmental problems. Further these methods are expensive too. New reagent system may have commercial importance by saving resources including time, health, and environment and may be cost effective. Keeping in mind these facts the study is planned with following objectives. 1. Halogenation of organic compounds in aqueous medium 2. Halogenation of organic compounds in solid state 3. Halogenation of organic compounds using surfactants. 4. Halogenation of organic compounds using molecular bromine.