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28-408: Several pathways exist for the halogenation of organic compounds, including free radical halogenation , ketone halogenation , electrophilic halogenation , and halogen addition reaction . The nature of the substrate determines the pathway. The facility of halogenation is influenced by the halogen. Fluorine and chlorine are more electrophilic and are more aggressive halogenating agents. Bromine

56-519: A Lewis acidic catalyst is used, such as ferric chloride . Many detailed procedures are available. Because fluorine is so reactive , other methods, such as the Balz–Schiemann reaction , are used to prepare fluorinated aromatic compounds. In the Hunsdiecker reaction , carboxylic acids are converted to organic halide , whose carbon chain is shortened by one carbon atom with respect to

84-458: A free-radical chain mechanism. The chain mechanism is as follows, using the chlorination of methane as an example: The net reaction is: The steady-state approximation implies that this process has rate law k [CH 4 ][Cl 2 ] . As a radical reaction , the process is halted or severely slowed by radical traps , such as oxygen . The relative rates at which different halogens react vary considerably: Radical fluorination with

112-403: A few unusual cases, free-radical halogenation can regioselect . Phenylic hydrogens have extremely strong bonds and are rarely displaced by halogens. Non- enolizable aldehydes oxidize to the acyl halide , but enolizable aldehydes typically halogenate at the α position instead. Indeed, allylic and benzylic hydrogens have bonds much weaker than alkanes , and are selectively replaced in

140-647: A pedagogical demonstration of the reactivity–selectivity principle and the Hammond postulate . A bromine radical is not very reactive and the transition state for hydrogen abstraction has much radical character and is reached late. The reactive chlorine radical develops a transition state resembling the reactant with little radical character. When the alkyl radical is fully formed in the transition state, it can benefit fully from any resonance stabilization present thereby maximizing selectivity. Bond dissociation energies strongly influence any radical process and in

168-495: A relatively large scale is for the production of phosphorus trichloride and disulfur dichloride . Free radical halogenation In organic chemistry , free-radical halogenation is a type of halogenation . This chemical reaction is typical of alkanes and alkyl -substituted aromatics under application of UV light . The reaction is used for the industrial synthesis of chloroform (CHCl 3 ), dichloromethane (CH 2 Cl 2 ), and hexachlorobutadiene . It proceeds by

196-440: A so-called statistical product distribution . Butane (CH 3 −CH 2 −CH 2 −CH 3 ), for example, can be chlorinated at the "1" position to give 1-chlorobutane (CH 3 −CH 2 −CH 2 −CH 2 Cl) or at the "2" position to give 2-chlorobutane (CH 3 −CH 2 −CHCl−CH 3 ). The latter occurs faster, and is the major product. The experimental relative chlorination rates at primary , secondary , and tertiary positions match

224-504: A spark, flame, or flash), electricity (e.g. a battery), or sound (e.g. explosion heard when burning hydrogen). The term exothermic was first coined by 19th-century French chemist Marcellin Berthelot . The opposite of an exothermic process is an endothermic process, one that absorbs energy, usually in the form of heat. The concept is frequently applied in the physical sciences to chemical reactions where chemical bond energy

252-413: A system that does not exchange heat with the surroundings), an otherwise exothermic process results in an increase in temperature of the system. In exothermic chemical reactions, the heat that is released by the reaction takes the form of electromagnetic energy or kinetic energy of molecules. The transition of electrons from one quantum energy level to another causes light to be released. This light

280-405: A transformation in which a closed system releases energy (heat) to the surroundings, expressed by When the transformation occurs at constant pressure and without exchange of electrical energy , heat Q is equal to the enthalpy change, i.e. while at constant volume , according to the first law of thermodynamics it equals internal energy ( U ) change, i.e. In an adiabatic system (i.e.

308-421: Is a common iodine source for radical iodination. Exothermic In thermodynamics , an exothermic process (from Ancient Greek έξω ( éxō )  'outward' and θερμικός ( thermikós )  'thermal') is a thermodynamic process or reaction that releases energy from the system to its surroundings , usually in the form of heat , but also in a form of light (e.g.

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336-436: Is a weaker halogenating agent than both fluorine and chlorine, while iodine is the 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. Halogenation of saturated hydrocarbons is a substitution reaction . The reaction typically involves free radical pathways. The regiochemistry of

364-514: Is converted to thermal energy (heat). Exothermic and endothermic describe two types of chemical reactions or systems found in nature, as follows: An exothermic reaction occurs when heat is released to the surroundings. According to the IUPAC , an exothermic reaction is "a reaction for which the overall standard enthalpy change Δ H ⚬ is negative". Some examples of exothermic process are fuel combustion , condensation and nuclear fission , which

392-401: Is equivalent in energy to some of the stabilization energy of the energy for the chemical reaction, i.e. the bond energy . This light that is released can be absorbed by other molecules in solution to give rise to molecular translations and rotations, which gives rise to the classical understanding of heat. In an exothermic reaction, the activation energy (energy needed to start the reaction)

420-656: Is nearly as likely to chlorinate as the 6 hydrogens terminating the branches, despite their much greater abundance. Many mixtures of radical initiators , oxidants , and halogen compounds can generate the necessary halogen radicals. For example, consider radical bromination of toluene : This reaction takes place on water instead of an organic solvent and the bromine is obtained from oxidation of hydrobromic acid with hydrogen peroxide . An incandescent light bulb suffices to radicalize. Other sources include alkyl hypohalites or single-electron oxidation -capable transition metals . In particular, tert-butyl hypoiodite

448-436: Is notoriously unselective. Chlorination rarely stops at monosubstitution: depending on reaction conditions, methane chlorination yields varying proportions of chloromethane , dichloromethane , chloroform and carbon tetrachloride . For asymmetric substrates, the reaction produces all possible isomers , but not equally. Radical halogenations are generally indifferent amongst equi-substituted potential radicals and effect

476-586: Is the route to the anesthetic halothane from trichloroethylene : Iodination and bromination can be effected by the addition of iodine and bromine to alkenes. The reaction, which conveniently proceeds with the discharge of the color of I 2 and Br 2 , is the basis of the analytical method . The iodine number and bromine number are measures of the degree of unsaturation for fats and other organic compounds. Aromatic compounds are subject to electrophilic halogenation : This kind of reaction typically works well for chlorine and bromine . Often

504-402: Is used commercially for the production of perfluorinated compounds . It generates small amounts of elemental fluorine in situ from hydrogen fluoride . The method avoids the hazards of handling fluorine gas. Many commercially important organic compounds are fluorinated using this technology. Unsaturated compounds , especially alkenes and alkynes , add halogens: In oxychlorination ,

532-406: Is used in nuclear power plants to release large amounts of energy. In an endothermic reaction or system, energy is taken from the surroundings in the course of the reaction, usually driven by a favorable entropy increase in the system. An example of an endothermic reaction is a first aid cold pack, in which the reaction of two chemicals, or dissolving of one in another, requires calories from

560-509: The Wohl-Ziegler reaction . Generally, N-haloamines in sulfuric acid (but not other haloradical sources) halogenate alkane chains at penultimate carbons (e.g. pentane to 2-halopentane ), chains terminating in only carboxylic acids at the center, and bridged compounds at the bridgehead. As of 2020 , the reasons for the latter N-haloimide selectivities remained unclear. Aside from those few exceptions, free-radical halogenation

588-454: The carbon chain of the particular carboxylic acid. The carboxylic acid is first converted to its silver salt, which is then oxidized with halogen : Many organometallic compounds react with halogens to give the organic halide: All elements aside from argon , neon , and helium form fluorides by direct reaction with fluorine . Chlorine is slightly more selective, but still reacts with most metals and heavier nonmetals . Following

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616-425: The combination of hydrogen chloride and oxygen serves as the equivalent of chlorine , as illustrated by this route to 1,2-dichloroethane : The addition of halogens to alkenes proceeds via intermediate halonium ions . In special cases, such intermediates have been isolated. Bromination is more selective than chlorination because the reaction is less exothermic . Illustrative of the bromination of an alkene

644-406: The corresponding radical species' stability: Thus any single chlorination step slightly favors substitution at the carbon already most substituted. The rates are generally constant across reactions and predict product distributions with relatively high accuracy. For example, 2-methyl butane ((CH 3 ) 2 CHCH 2 CH 3 ) exhibits the following results: Note that the sole tertiary hydrogen

672-673: The free radical iodination. Because of its extreme reactivity, fluorine ( F 2 ) represents a special category with respect to halogenation. Most organic compounds, saturated or otherwise, burn upon contact with F 2 , ultimately yielding carbon tetrafluoride . By contrast, the heavier halogens are far less reactive toward saturated hydrocarbons. Highly specialised conditions and apparatus are required for fluorinations with elemental fluorine . Commonly, fluorination reagents are employed instead of F 2 . Such reagents include cobalt trifluoride , chlorine trifluoride , and iodine pentafluoride . The method electrochemical fluorination

700-737: The halogenation of alkanes is largely determined by the relative weakness of the C–H bonds . This trend is reflected by the faster reaction at tertiary and secondary positions. Free radical chlorination is used for the industrial production of some solvents : Naturally-occurring organobromine compounds are usually produced by free radical pathway catalyzed by the enzyme bromoperoxidase . The reaction requires bromide in combination with oxygen as an oxidant . The oceans are estimated to release 1–2 million tons of bromoform and 56,000 tons of bromomethane annually. The iodoform reaction , which involves degradation of methyl ketones , proceeds by

728-455: The pure element is difficult to control and highly exothermic ; care must be taken to prevent an explosion or a runaway reaction. With chlorine the reaction is moderate to fast; with bromine, slow and requires intense UV irradiation ; and with iodine, it is practically nonexistent and thermodynamically unfavored. However, radical iodination can be completed with other iodine sources (see § Variants ). The different rates are often

756-525: The surroundings, and the reaction cools the pouch and surroundings by absorbing heat from them. Photosynthesis , the process that allows plants to convert carbon dioxide and water to sugar and oxygen, is an endothermic process: plants absorb radiant energy from the sun and use it in an endothermic, otherwise non-spontaneous process. The chemical energy stored can be freed by the inverse (spontaneous) process: combustion of sugar, which gives carbon dioxide, water and heat (radiant energy). Exothermic refers to

784-405: The usual trend, bromine is less reactive and iodine least of all. Of the many reactions possible, illustrative is the formation of gold(III) chloride by the chlorination of gold . The chlorination of metals is usually not very important industrially since the chlorides are more easily made from the oxides and hydrogen chloride . Where chlorination of inorganic compounds is practiced on

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