4.4- dihaloalkanal 2-alkenal

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... 

The first reported electroorganic synthesis of a sizeable amount of material at a modified electrode, in 1982, was the reduction of 1,2-dihaloalkanes at p-nitrostyrene coated platinum electrodes to give alkenes. The preparation of stilbene was conducted on a 20 pmol scale with reported turnover numbers approaching 1 x 10. The idea of mediated electrochemistry has more frequently been pursued for inorganic electrode reactions, notably the reduction of oxygen which is of eminent importance for fuel cell cathodes Almost 20 contributions on oxygen reduction at modified... 

H202 (30%, 6 ml) is added with stirring to the alkene (50 mmol) and TEBA-Cl (0.1 g, 0.44 mmol) in CC14 (10 ml) and CaCl2 or CaBr2 (50 mmol) in cone. HC1 or HBr (10 ml) at 0°C. The mixture js allowed to come to room temperature and is stirred for a further 20 min. Petroleum ether (50 ml) is added and the mixture is washed well with H20. The dried (Na2S04) organic phase is evaporated to yield the dihaloalkane. 

Method 4 Na2S.9H20 (2.65 g, 11 mmol) in H20 (10 ml) is added the dibromoalkane (5 mmol) and Aliquat or TEBA-C1 (0.5 mmol) in PhH (10 ml). The initially exothermic reaction is stirred for ca. 1 h at room temperature and the organic phase is then separated, washed with H20 (2 x 20 ml), dried (MgS04), and evaporated to yield the alkene. Gaseous alkenes are collected using a gas burette ( the solvent can be omitted with liquid dihaloalkanes). 

Alkenes undergo addition reactions with hydrogen to form alkanes, with halogens to form dihaloalkanes, with hydrogen halides to form monohaloalkanes and with water to form alcohols. For example ... 

In general, the preparation of gem-dizincioalkane via the treatment of 1,1-dihaloalkane with zinc is more difficult than the preparation of gewt-dizinciomethane. The reduction of gem-dihaloalkanes carrying fi-hydrogens may suffer from /J-hydride elimination and result in the formation of alkenes. In addition, depending on the substrate, the intermediary a-haloalkylzinc compound is less stable than a-halomethylzinc towards the a-elimination (Scheme 2). 

A variety of sulfur reagents have been used for the dehydrohalogenation of vic-dihaloalkanes to alkenes. Aqueous sodium trithiocarbonate in the presence of a phase transfer catalyst was reported to give high yields under very mild conditions [217]. A radical mechanism involving a one-electron transfer was proposed as the first step in this reductive elimination. 

We may seem to have contradicted ourselves because Equation 10-1 shows a carbocation to be formed in bromine addition, but Equation 10-5 suggests a bromonium ion. Actually, the formulation of intermediates in alkene addition reactions as open ions or as cyclic ions is a controversial matter, even after many years of study. Unfortunately, it is not possible to determine the structure of the intermediate ions by any direct physical method because, under the conditions of the reaction, the ions are so reactive that they form products more rapidly than they can be observed. However, it is possible to generate stable bromonium ions, as well as the corresponding chloronium and iodonium ions. The technique is to use low temperatures in the absence of any strong nucleophiles and to start with a 1,2-dihaloalkane and antimony penta-fluoride in liquid sulfur dioxide ... 

The method of greatest applicability is the reaction of alkali metal phosphides with dihalo-alkanes, -alkenes, etc. (equation 4). Generally this reaction proceeds well and in high yield excess dihaloalkane should be avoided as this leads to the production of phosphonium salts which can contaminate the product. 

Geminal dihaloalkanes are very commonly used reagents for the addition of alkylidene groups to alkenes. The simpler dihalides, especially the dihalomethanes, are readily available and are certainly much less hazardous to use than the corresponding diazoalkanes. Nevertheless, any reactive alkylating agent should be regarded as a toxic substance. 

Addition of Cl2 and Br2 Alkenes react with the halogens Cl2 and Br2 to give dihaloalkane addition products, a process called halogenation. For example,... 

The other major dehalogenation pathway involves elimination of two halogens, leaving behind a pair of electrons that usually goes to form a carbon-carbon double bond. Where the pathway involves halogens on adjacent carbons, it is known as vicinal dehalogenation or reductive / -elimination. The fine dashed arrows in Fig. 4 illustrate this process for PCE. Note that this pathway can produce alkynes from vicinal dihaloalkenes [88,94,95], as well as producing alkenes from vicinal dihaloalkanes [96,97],... 

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