Organic Chemistry …Alkenes…

Alkenes  Hydrocarbons with one or more double bonds  “Unsaturated” Have fewer than the maximum amount of hydrogens Have the general formula of C n H 2n

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.

Structures of Alkenes  This creates geometric isomers, which differ from each other in the spatial arrangement of groups about the double bond.

Stereoisomers  Geometric isomers Occur as a result of a C=C Cis : identical groups appear on adjacent carbons on the same side of the double bond Trans: if those groups appear on opposite sides

Alkenes  Draw C 4 H 8  Structure of alkenes affects their properties

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.

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

Reactions of Alkenes  Alkenes are “unsaturated” More reactive than alkanes  Undergo addition reactions  Undergo oxidation reactions  Starting materials for many polymers

Reactions of Alkenes and Alkynes  Hydrogenation An addition reaction where the reagent being added is hydrogen (H 2 ) Essentially turns alkenes into alkanes, etc

Reactions of Alkenes and Alkynes  Halogenation An addition reaction where the reagent being added is a halogen (Cl 2, Br 2, HCl)

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

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.

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

Reactions of Alkenes and Alkynes

 Oxidation Alkenes undergo an oxidation reaction with acidified potassium permanganate, KMnO 4 /H + (not simply KMnO 4 )  Forms a “diol”