Dehydrohalogenation An elimination

The 2-methylpropene product results from dehydrohalogenation, an elimination of hydrogen and a halogen atom. Under these first-order conditions (the absence of a strong base), dehydrohalogenation takes place by the El mechanism Ionization of the alkyl halide gives a carbocation intermediate, which loses a proton to give the alkene. Substitution results from nucleophilic attack on the carbocation. Ethanol serves as a base in the elimination and as a nucleophile in the substitution. 

Cheleotropic fragmentation a fragmentation reaction in which two a bonds to a single atom are broken Dehydrohalogenation an elimination reaction in which the elements of a hydrogen halide molecule are eliminated from adjacent atoms leaving a n bond... 

Dehydrohalogenation An elimination reaction in which a hydrogen halide, HX (X = Cl, Br, I), is eliminated from a haloalkane. A C=C double bond is formed. 

This dehydrohalogenation (an elimination reaction) is also not very atom-economical. The percentage atom economy or utilisation is 27% which is even less atom-economical than the Hofinann elimination reaction. 

The Dow Process utilizes an elimination/addition reaction to convert chlorobenzene to phenol. The proposed mechanism for this reaction is shown in Figure 8-3. The high-temperature reaction begins with chlorobenzene and aqueous sodium hydroxide. Note that this mechanism starts with the hydroxide attacking as a base, beginning dehydrohalogenation to form benzyne. The second hydroxide ion attacks as a nucleophile to form a carbanion intermediate, which behaves as a base in the last step to yield the final product. 

A number of competing pathways have also been identified 155> 156>. Indeed the dehydrohalogenation of (228, X = Br, Cl) is reported to lead to (229). A mechanism is proposed which does not in this case involve a cyclopropene, but instead is initiated by an elimination with rearrangement157) ... 

Dehydrohalogenation, another abiotic reaction of importance in the subsurface, is an elimination reaction involving halogenated alkanes in which a halogen is removed from one carbon atom, with removal of a hydrogen atom from an adjacent carbon atom and formation of a double bond, as [25]... 

Dehydrohalogenation (Section 8.1) An elimination reaction in which the elements of hydrogen and halogen are lost from a starting material. 

Methods for the preparation of alkenes generally involve elimination reactions, such as dehydrohalogenation, the elimination of HX from an alkyl halide, and dehydration, the elimination of water from an alcohol. 

An elimination reaction, dehydrohalogenation, can occur for chloro-, bromo- and iodoalkanes. In such a reaction, the halogen, X, from one C atom and a hydrogen from an adjacent C atom are eliminated. A double bond between two carbon atoms is formed the molecule becomes more unsaturated. The net reaction is the transformation of an alkyl halide (or haloalkane) into an alkene. Dehydrohalogenation reactions usually require a strong base such as sodium hydroxide, NaOH. 

In addition to undergoing nucleophilic substitution reactions, alkyl halides undergo J8-elimination reactions The halogen is removed from one carbon and a proton is removed from an adjacent carbon. A double bond is formed between the two carbons from which the atoms are eliminated. Therefore, the product of an elimination reaction is an alkene. Removal of a proton and a halide ion is called dehydrohalogenation. There are two important jS-elimination reactions, El and E2. 

The double dehydrohalogenation (two dehydrohalogenations or eliminations) of vicinal dihalide results in an alkyne. 

Chlorinated solvents are advected, dispersed, and sorbed in ground water systems. They also volatilize although their different components have varying degrees of volatility. Chlorinated solvents additionally hydrolyze and undergo other chemical reactions such as dehydrohalogenation or elimination and oxidation and reduction. These abiotic reactions, as will be seen later in the chapter, are typically not complete and often result in the formation of an intermediate that may be at least as toxic as the original contaminant. 

In the substitution reaction, the nucleophile replaces the halogen X. In the elimination reaction, the nucleophile acts as a base and removes a proton from carbon-2, the carbon next to the one that bears the halogen X. The halogen X and the hydrogen from the adjacent carbon atom are eliminated, and a new bond (a pi bond) is formed between carbons-1 and -2 The symbol E is used to designate an elimination process. Since in this case a hydrogen halide is eliminated, the reaction is called dehydrohalogenation. Elimination reactions provide a useful way to prepare compounds with double or triple bonds. 

Recall that addition of HX to an alkene is called hydrohalogenation removal of HX from a haloalkane is called dehydrohalogenation. The removal of atoms from adjacent carbons to form an alkene is also called an elimination reaction and is discussed fully in Chapter 9. The following example shows the conversion of 2-butene to 2-butyne. 

Eliminations are the opposite of additions. In an elimination one molecule loses the elements of another small molecule. Elimination reactions give us a method for preparing compounds with double and triple bonds. In Chapter 7, for example, we shall study an important elimination ealled dehydrohalogenation, a reaction that is used to prepare alkenes. In dehydrohalogenation, as the word suggests, the elements of a hydrogen halide are eliminated. An alkyl halide becomes an alkene ... 

Dehydration reaction (Sections 7.7 and 7.8) An elimination that involves the loss of a molecule of water from the substrate. Dehydrohalogenation (Sections 6.15A and 7.6) An elimination reaction that results in the loss of HX from adjacent carbons of the substrate and the formation of a 77 bond. 

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