2017-04-14     10:59 来源: 来自互联网
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The homologous series of alkenes has the general formula CnH2n. Alkenes are unsaturated hydrocarbons with one carbon-carbon double bond consisting of a σbond and a πbond.
烯烃同系物通式为:CnH2n。烯烃是具有一个碳碳双键的不饱和碳氢化合物,碳碳双键由一个σ键和一个 π键组成。

Ethene is a planar molecule, other alkenes are planar of the double bond and the four adjacent atom. Many alkenes have cis-trans isomers which arise because rotation about the double bond is prevented.


Alkenes are more reactive than alkanes because they contain aπbond. The characteristic reaction of the alkene functional group is addition, which occurs across the πbond. For example ethene produces ethane with hydrogen over a nickel catalyst; 1,2-dibromoethane with bromine at room temperature; chloroethane with hydrogen chloride at room temperature; ethanol with steam in the prescence of H3PO4.


The mechanism of the reaction of bromine with ethene is electrophilic addition. Electrophiles accept a pair of electrons from an electron-rich atom or centre, in this case theπbond. A carbocation intermediate is formed after the addition of the first bromine atom. This rapidly reacts with a bromide ion to form 1,2-dibromoethane


Alkenes produce many useful polymers by addition polymerisation. For example, poly(ethene) from CH2=CH2, poly(propene) from CH3CH=CH2, poly(chloroethene) from CH2=CHCl and poly(tetrafluoroethene) from CF2=CF2.


The disposal of polymers is difficult as they are chemically inert and non-biodegradable. When burnt, they may produce toxic products such as hydrogen chloride from PVC(poly(chloroethene)). Whilst much manufacturing wast plastic is recycled, the costs of collecting and sorting most domestic plastic are too high to make recycling worthwhile. Use of the energy released on combustion (for heating buildings) is a better option for domestic waste, but treatment of flue gases is required to remove toxic pollutants. A third option is feedstock recycling where the polymers are separated and used as feedstock to make new polymers.