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Alkenes (Reactions of alkenes (• Alkenes much more reactive than…
Alkenes
Properties of alkenes
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• For each carbon atom of double bond, 3 of 4 outer electrons used in sigma bonds, while the singled remaining electron is used in a Pi bond
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• The π-electron density is concentrated above and below the line joining the nuclei of the bonded atoms
• The π bond locks the two carbon atoms in position and prevents them from rotating around the double bond
• The shape around each of the carbon atoms is trigonal planar as there are three regions of electron density around each of the carbon atoms which repel as far as possible so 120 degree bond angle, and all the atoms are in the same plane
Stereoisomerism
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E/Z isomerism
• Stereoisomerism around a double bond occurs because rotation about the double bond is restricted and the groups attached to each carbon atom are therefore fixed relative to each other
• If a molecule has a C=C bond and different groups attached to each carbon atom of the double bond then it will have E/Z isomerism
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Cis-trans isomerism
• In cis-trans isomerism one of the attached groups on each of the carbon atoms on the double bond must be the same
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Reactions of alkenes
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• The π electrons are more exposed than the electrons in the σ bond so a π bond readily breaks and the σ remains intact when alkenes react
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• Hydrogenation of alkenes - alkene is mixed hydrogen and passed over nickel catalyst an addition reaction takes place to form an alkane
• Halogenation - alkenes undergo rapid addition reaction with halogens Cl and Br at room temp to form dihaloalkane
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Markownikoff's rule - when a hydrogen halide reacts with an unsymmetrical alkene the hydrogen of the hydrogen halide attaches itself to the carbon atom with the greater number of hydrogen atoms and smaller number of carbon atoms
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