Halogenation-double dehydrohalogenation

A Halogenation-Double Dehydrohalogenation Used in Alkyne Synthesis... 

Illustrate the use of halogenation-double dehydrohalogenation in the synthesis of the alkynes... 

But-l-en-3-yne ( vinylacetylene ), F, is made industrially by the CuCl-catalyzed dimerization of ethyne. How would you make it We learned that alkenes can be converted to alkynes by halogenation-double dehydrohalogenation (Section 13-4). This approach suggests 1,3-butadiene as a starting material. We have seen, however, that halogenation of 1,3-butadiene, such as bromination, is not as straightforward as that of a monoalkene 1,2- and 1,4-addition occurs (Section 14-6). Is that a problem Answer No. The 1,2-dihalobutene will eliminate normally, first to 2-bromobutadiene (via deprotonation at the more acidic allylic position) and then to F ... 

Just as It IS possible to prepare alkenes by dehydrohalogenation of alkyl halides so may alkynes be prepared by a double dehydrohalogenation of dihaloalkanes The dihalide may be a geminal dihalide, one m which both halogens are on the same carbon or it may be a vicinal dihalide, one m which the halogens are on adjacent carbons... 

Double dehydrohalogenation of vicinal dihalides (Section 9 7) Dihalides in which the halogens are on adjacent carbons undergo two elimination processes analogous to those of gemmal dihalides... 

Double dehydrohalogenation (Section 9 7) Reaction in which a geminal dihahde or vicinal dihahde on being treated with a very strong base such as sodium amide is converted to an alkyne by loss of two protons and the two halogen substituents... 

P Elimination (Section 5.8) Reaction in which a double or triple bond is formed by loss of atoms or groups from adjacent atoms. (See dehydration, dehydrogenation, dehydro-halogenation, and double dehydrohalogenation.)... 

Halogen additions to olefins followed by double dehydrohalogenation is a classical sequence which has been widely used for making carbon-carbon triple bonds 2). 

A vicinal dihalide (abbreviated rTtc-dihalide) is a compound bearing the halogens on adjacent carbons vicinus, Latin adjacent). Vicinal dihalides are also called 1,2-dihalides. A vicinal dibromide, for example, can be synthesized by addition of bromine to an alkene (Section 8.1). The w c-dibromide can then be subjected to a double dehydrohalogenation reaction with a strong base to yield an alkyne. 

The most often applied and most reliable procedure to obtain a cycloalkyne (Fig. 2, bottom), as originally developed by Brandsma in the Netherlands, [11], involves the stepwise double dehydrohalogenation of a vicinal dihalogenide (typically bromide), which can be readily obtained from a cyclic alkene upon treatment with elemental halogen. In this case, a first E2-elimination may take place under mildly basic conditions, forming an intermediate alkene, which in turn will undergo a second elimination upon treatment with a strong base like sodium amide or potassium ferf-butoxide. More conveniently even, in many cases both... 

The double dehydrohalogenation reaction is usually run in the presence of a strong base, such as KOH or NaNH2, and proceeds in two stages. In the first, an intermediate bromoalkene is formed, which can be isolated under more mildly basic conditions. In fact, this reaction is a valuable route to vinyl halides. The mechanism of elimination involves the abstraction of the proton on the carbon atom p to the halogen. The E2 mechanism, which operates under these strongjy basic conditions, is fastest when it involves removal of a proton, antiperipla-... 

Because vicinal dihaloalkanes are readily available from alkenes by halogenation, this sequence, called halr enation-double dehydrohalogenation, is a ready means of converting alkenes into the corresponding alkynes. 

Addition to the Double Bond. Chlorine, bromine, and iodine react with aHyl chloride at temperatures below the inception of the substitution reaction to produce the 1,2,3-trihaLides. High temperature halogenation by a free-radical mechanism leads to unsaturated dihalides CH2=CHCHC1X. Hypochlorous and hypobromous acids add to form glycerol dihalohydrins, principally the 2,3-dihalo isomer. Dehydrohalogenation with alkah to epicbl orobydrin [106-89-8] is ofgreat industrial importance. 

Disubstituted pyridazine-3,6(l//,2//)-diones add halogens to the 4,5-double bond, followed by dehydrohalogenation to give 4-halo derivatives. 1,2-Disubstituted 5-bromopyridazine-3,6(l//,2F0 diones react with bromine to give the corresponding 4,5-dibromo derivative. The Mannich reaction with 2-arylpyridazin-3(2//)-one occurs at position 4. 

In a /3-elimination (dehydrohalogenation) reaction a halogen and a hydrogen atom are removed from adjacent carbon atoms to form a double bond between the two C s. The reagent commonly used to remove HX is the strong base KOH in ethanol (cf. Section 6.2). 

When subjected to strong bases, gem-dihalocyclopropanes undergo dehydro-halogenations, and cyclopropenes are formed. These are generally unstable under the reaction conditions and participate in further transformations. The most common of these processes is the isomerization of the newly formed double bond from the endo- to the exo-orientation, followed by a second dehydrohalogenation step. The methylenecyclopropenes thus generated are still not stable, and subsequently tend to rearrange to less strained systems. 

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]... 

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