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Organic Chemistry
…Alkenes…
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Alkenes Hydrocarbons with one or more double bonds “Unsaturated”
Have fewer than the maximum amount of hydrogens
• Have the general formula of CnH2n
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Structure of Alkenes Unlike alkanes, alkenes cannot rotate
freely about the double bond. The side-to-side overlap in the -bond makes
this impossible without breaking the -bond.
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Structures of Alkenes This creates
geometric isomers, which differ from each other in the spatial arrangement of groups about the double bond.
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Stereoisomers Geometric isomers
Occur as a result of a C=C Cis : identical groups appearon adjacent carbons on the same side of the
doublebond Trans: if those groups appear on opposite
sides
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Alkenes Draw C4H8
Structure of alkenes affects their properties
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Nomenclature of Alkenes Alkenes end with “-ene” The chain is numbered so the double bond gets
the smallest possible number. cis-Alkenes have the carbons in the chain on the
same side of the molecule. trans-Alkenes have the carbons in the chain on
opposite sides of the molecule.
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Nomenclature of Alkenes If there is more than one double bond, use
“di, tri, or tetra” before the ending “-ene” …. Just like with carbon groups (dimethyl)
Name this compound:
Buta-1,3-diene
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Reactions of Alkenes Alkenes are “unsaturated”
More reactive than alkanes Undergo addition reactions Undergo oxidation reactions Starting materials for many polymers
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Reactions of Alkenes and Alkynes Hydrogenation
An addition reaction where the reagent being added is hydrogen (H2)
Essentially turns alkenes into alkanes, etc
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Reactions of Alkenes and Alkynes Halogenation
An addition reaction where the reagent being added is a halogen (Cl2, Br2, HCl)
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Reactions of Alkenes and Alkynes Hydration (addition of water)
Alkenes and water, in the presences of a catalyst (phosphoric acid, sulfuric acid, etc) form an alcohol
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Reaction of Alkenes and Alkynes
Mechanism of an Addition Reaction• In the first step, the -bond breaks and the new C—H bond and a cation form.• In the second step, a new bond forms between the negative bromide ion and the positive carbon.
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Reactions of Alkenes and Alkynes 1860s Vladimir Markovnikov studied
alkene addition reactions that created isomers Markovnikov’s Rule
When a reagent (HX) adds to an unsymmetrical alkene, the hydrogen atom in the reagents will add to the carbon that already has the higher number of hydrogens
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Reactions of Alkenes and Alkynes
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Reactions of Alkenes and Alkynes Oxidation
Alkenes undergo an oxidation reaction with acidified potassium permanganate, KMnO4/H+ (not simply KMnO4)
Forms a “diol”