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How many ways can you make an alkene?

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Introduction

Year: 2nd (first semester) Date: 2nd Dec 2002 How many ways can you make an alkene? 1 Introducing the alkene: Alkenes (or olefins, olefines in old-fashioned parlance) - unsaturated hydrocarbons that contain one or more carbon-carbon double bonds. In IUPAC nomenclature alkenes have the -ene suffix. The mono-unsaturated alkenes ethene, propene, butene . . form a homologous series with the general formula CnH2n. As an example of ethylene, each of the carbon atoms is sp2-hybridized, and the double bond possesses a ? component and a ? component. Homologues with n greater than or equal to 4 exhibit geometric isomerism (E-Z national system). The physical properties of alkenes: the dipole moments of most alkenes are quite small, a methyl group releases electrons to a double bond in much the same way that it releases electrons to the positively charged carbon of a carbocation-by an inductive effect and by hyperconjugation. In general, alkenes with more highly substituted double bonds are more stable than isomers with less substituted double bonds. Van der Waals strain. Alkenes are more stable when large substituents are trans to each other than when they are cis. ...read more.

Middle

The mechanism of acid-catalysed dehydration of alcohols. The overall reaction: (CH3)3COH (CH3)3C=CH2 + H2O Step 1: protonation of tert-butyl alcohol: Step 2: Dissociation of tert-butyloxonium ion: Step 3: Deprotation of tert-butyl cation: (ii) Dehydrohalogenation of alkyl Halides: Dehydrohalogenation is loss of a hydrogen and a halogen from an alkyl halide. It is one of the most useful methods for preparing alkenes by � elimination. When applied to the preparation of alkenes, the reaction is carried out in the presence of a strong base, such as NaOCH2CH3 in ethyl alcohol as solvent. Mechanism of the dehydrohalogenation of alkyl halides: The E2 mechanism 1.The dehydrohalogenation reaction exhibits second-order kinetics; it is first-order in alkyl halide and first-order in base 2. The rate of elimination depends on the halogen, the reactivity of alkyl halides increasing with decreasing strength of the carbon-carbon bond. Increasing rate of dehaydrohalogenation RF < RCl< RBr< RI. <2> The Witting reaction: Alkenes may be formed by dehydrating alcohols with conc. sulphuric acid or by passing over hot Al2O3. A powerful method of synthesizing alkenes is the Wittig reaction. the reaction of an ?-phosphorus-stabilised anion with a carbonyl derivative, is undoubtedly one of the most useful methods for selectively constructing carbon-carbon double bonds. ...read more.

Conclusion

HC CH HgCl2 CH2=CHCl <5> Polymerization of ethyne: The Ni(CN)2 is the catalyst of the reaction of polymerization, at the 80-120?, 1.5 MPa. 4 HC CH ( cycloctene) (Chinese universities' textbook, alkene, polymerization, 1999) <6> Form the petroleum In industry producing, people usually use the petroleum to generate the alkenes by cracking. 2 Conclusion: For making the alkene, there are hundreds of methods. In spite of the fact that some of the methods are very complex, whatever they are all according to the properties of alkenes to discover. Generally speaking, there are two main hints for making the alkene that generates the carbon-carbon double bond from the original carbon frame and synthesis the carbon-carbon double bond as well as construct a new carbons frame. Usually the elimination reactions belong to the first hint of making alkene. As above of the methods of making alkene, dehydration is the most common method. But the petroleum cracking is the most useful in industry. Further the ethene and propylene are the most important in industry. Nowadays, the standard of the ethene manufacture is used to scale the development of the petroleum chemistry industry for a country. Therefore people are making great efforts to investigate and attempt their new ideas which are the advanced. Then making the progress from the exploring. ...read more.

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