All About Alkenes

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Alkene

Alkenes are unsaturated hydrocarbons with general formula; CnH2n and isomeric with cycloalkenes. Alkenes can be prepared from dehydration of alcohols and dehydrohalogenation of alkyl halide. To form the root of the IUPAC (International Union and Applied Chemistry ) names for alkenes, simply change the -an- infix of the parent to -en-. For example, CH3-CH3 is the alkane therefore ethene. The name of CH2=CH2 is therefore ethene. Alkenes are hydrocarbons with carbon-carbon double bonds. In contrast to the easy rotation of sigma bonds, pi bonds cannot be rotated. This rigidity gives rise to cis-trans isomerism, also called geometric isomerism. Heats of hydrogenation can be used to compare the stabilities of alkenes. In general, more highly substituted double bonds are more stable, and trans isomers are slightly more stable than cis. Therefore, most eliminations give the most highly substituted alkene as their major product (Saytzeff’s rule). Alkenes undergo mainly electrophilic addition. The major products determined using Markovnikov’s Rule and stability of carbocation.

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January 28, 2010 - Posted by | Uncategorized

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