From Halides

Because a neutral molecule is eliminate4 rather than an anion, there is no electrostatic attraction (ion pairing) between the products of the dissociation step. As a result, the carbocations generated by diazonium-ion decomposition frequently exhibit somewhat different behavior from those generated from halides or sulfonates under solvolytic conditions. ... 

Whereas simple olefins are not usually made by elimination from halides, conjugated systems are frequently obtained in this way. The cases of a- and j5-halo ketones and their vinylogues have already been covered. Allylic halides may also be eliminated to form dienes, for example, the 2,4-diene (109)... 

The reaction of chlorodifluorotnethane with alkoxide ions generated in low concentration from halide ions and epoxides [28, 29] is an interesting, higher temperature method that gives good to excellent yields of gem-difluorocyclo-propanes from just moderately nucleophilic olefins (equation 9)... 

Apart from halide and protic impurities, ionic liquids can also be contaminated with other ionic impurities from the metathesis reaction. This is especially likely if the alkali salt used in the metathesis reaction shows significant solubility in the... 

Most of the reactions of the halogens are of the oxidation-reduction type. The halogens are so reactive that they do not occur uncombined in nature and they must be made from halide compounds (salts). We shall consider briefly the preparation of the elements and then explore some of the very interesting chemistry of this family. 

J. K. Kochi, D. M. Singleton u. L. J. Andrews, Alkenes from Halides and Epoxides by Reductive Eliminations with Cr(lI)-Complexes, Tetrahedron 24, 3503 (1968). 

Some preparations of alkene and acetylenic compounds from alkene and acetylenic starting materials can, in principle, be classified in either the monofunctional or difunctional sections for example, the transformation RCH=CHBr — RCH=CHCOOH could be considered as preparing carboxylic acids from halides (Section 25, monofunctional compounds) or preparing a carboxylic acid-alkene (Section 322, difunctional compounds). The choice usually depends on the focus of the particular paper where this reaction was found. In such cases both sections should be consulted. 

SECTION 25 ACID DERIVATIVES FROM HALIDES AND SULFONATES... 

The replacement of halogen by alkyl or aryl groups is included in this section. For the conversion of RX RH (X = halogen) see Section 160 (Hydrides from Halides and Sulfonates). 

Related Methods Section 25 (Acid Derivatives from Halides). 

Aldehyde (25) is piperonal (see p T 9) which we made from halide (31), the precursor of ylid (30), so either route is suitable. The route via (28) has been used. 

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