d polymeationproduce highly 12-16 Ethene:An Important Industrial Feedstock of polyethen ththpodufmanyth …“岁 12-17 Alkenes in Nature:Insect Pheromones Iegeacgohevaskwommohs10-tans12-cs HH -H 12 Important Concepts Important Concepts Hydroeborationeiropiea2bewee um on carb : on-( f Carbenes and Carbenoids-usefulor sythe nikov's Rule) ox8gponenao mium 12 12 Metal-catalyzed polymerizations produce highly regular chains. Ziegler-Natta catalysts are important initiators for metal￾catalyzed polymerizations. They are typically made from titanium tetrachloride and a trialkylaluminum such as Al(CH2CH3)3. Polymers produced using a Ziegler-Natta catalyst are characterized by regularity of construction and high linearity. This results in much higher density and strength than similar polymers obtained from radical polymerization. 12-16 Ethene: An Important Industrial Feedstock Ethene is the basis for the production of polyethene (polyethylene). The major source of ethene is the pyrolysis of petroleum, or hydrocarbons derived from natural gas. Ethene is the starting material for the production of many other industrial chemicals: 12-17 Alkenes in Nature: Insect Pheromones Pheromones are chemical substances used for communication within a living species. Pheromones are used for sex, trail, alarm, and defense signaling, to name a few. The sex attractant for the male silkworm moth is 10-trans-12-cis￾hexadecadien-1-ol (bombykol). The natural pheromone is 10 billion times more active in eliciting a response than is the 10-cis-12-trans isomer, and 10 trillion times more active than the trans, trans isomer. 12 Important Concepts 1. Double Bond Reactivity – exothermic addition reactions leading to saturated products 2. Hydrogenation of Alkenes – immeasurably slow unless a catalyst is used • Palladium on carbon, PtO2, Raney nickel • H2 preferentially added to the least hindered face of the double bond 3. π Bond – attacked by acid and electrophiles • If the initial intermediate is a carbocation, the more highly substituted carbocation is formed (Markovnikov’s Rule). • If the initial intermediate is cyclic onium ion, nucleophilic ring opening is at the more substituted carbon (control of both regio- and stereochemistry). 12 Important Concepts 4. Hydroboration – mechanistically between hydrogenation and electrophilic addition • Step 1: π complexation to boron • Step 2: concerted transfer of hydrogen to carbon • Hydroboration-oxidation: anti-Markovnikov hydration of alkenes 5. Carbenes and Carbenoids – useful for synthesis of cyclopropanes from alkenes 6. Peroxycarboxylic Acids – contains oxygen atom transferable to alkenes to give oxacyclopropanes (epoxidation) 7. Osmium Tetroxide – addition to alkenes in a concerted syn manner to give vicinal diols