Chapter 10.1 & 20.1: Organic Reactions ALKENES

Alkenes Unsaturated hydrocarbons with at least one double bond present. Ex: Methene Ethene Propene But-1-ene Pent-1-ene Reactivity is higher than that of alkanes C=C double bond is less energetically stable than C-C single bond C=C has a bond energy of 612 kJ/mol C-C has bond energy of 348 kJ/mol Energy of two single bonds > the energy of one double bond Activation energy for alkene addition reactions is relatively low (i.e. they are more reactive than alkanes)

Alkenes undergo electrophilic addition RXNs The 120° trigonal planar structure of Carbon in C=C makes it available for attack  bonds are weaker & not on plane of bond, so easier to break  bond is area of e- density so electrophiles are attracted Electrophile: species that is e- deficient or becomes e- deficient in presence of a  bond (has partial or full + charge)

Alkenes undergo electrophilic addition RXNs Types of electrophilic addition RXNs Addition of: Hydrogen (hydrogenation) = alkene  alkane Halogens = alkene  dihalogenoalkane Hydrogen halide = alkene  halogenoalkane Water (hydration) = alkene  alcohol Alkene + alkene (polymerization) = alkenes  polythene

Alkenes: Addition RXNs – hydrogenation Addition of hydrogen to make an unsaturated molecule into a saturated molecule Requires: a nickel catalyst and heat

Alkenes: Addition RXNs – hydrogenation RXN is the basis for production of margarine from vegetable oils Vegetable oils often long chain polyunsaturated (multiple double bonds) fats As hydrogen is added to oils, # of double bonds decreases and the chain takes on more regular shape Increases melting point = solid

Alkenes: Addition RXNs - Halogenation Double bond of an alkene is broken and two new bonds are formed Ex: bromination of ethene Ethene is bubbled through Br2 @ Room Temp Ethene is colorless Bromine is brown 1,2-dibromoethane is colorless RXN = brown + colorless  colorless

Alkenes: Addition RXNs - Halogenation Bromine (non-polar) becomes polarized when gets close to e- rich double bond Br becomes electrophile as gets + Heterolytic split of Br2 forms Br+ and Br- Slow step: carbocation intermediate

Alkenes: Addition RXNs - Halogenation Carbocation is unstable Reacts quickly with Br- forming 1,2-dibromoethane If reaction with C2H4 and Br2 also includes Cl- Products formed are BrH2CCH2Br and BrH2CCH2Cl Shows that the initial Br+ mechanism is correct No ClH2CCH2Cl forms

Alkenes: Addition RXNs - Halogenation Both carbons in the double bond get one bromine This reaction is simple test for the presence of unsaturated hydrocarbons Bromine (dark brown color) will add “across” double bonds and will be consumed Thus, the brown color gets lighter. If bromine is added to an unknown substance and the color fades, it indicates the presence of unsaturated hydrocarbons

Alkenes: Addition RXNs – hydrogen halides Halogen halide reativity: HI> HBr > HCl Due to decreasing strength of H—X bond When ethene is bubbled through conc. aqueous solution of HBr at RT  bromoethane

Alkenes: Addition RXNs – hydrogen halides Reaction mechanism is similar to halogen H—Br splits heterolytically to form H+ and Br- Slow step: H+ is electrophile attacks double bond Br- reacts with carbocation to form bromoethane

Alkenes: Addition RXNs – hydrogen halides Unsymmetric addition Propene + hydrogen bromide Theoretically results in 2 products: which one is true? The answer comes from which carbocation is most stable

Alkenes: Addition RXNs – hydrogen halides Unsymmetric addition Any asymmetric hydrogen halide will result in predictable product Markovnikov’s Rule: the Hydrogen will attach to the carbon that is already bonded to the greater number of hydrogens More general: the more electropositive part of the reacting species bonds to the least highly substituted carbon atom in the alkene

Alkenes: Addition RXNs – hydration Addition of water to an alkene to form an alcohol Requires: heat with steam and conc. sulfuric acid catalyst Slow step: H+ and HSO4- ions are added across the double bond Hydrolysis (“cutting of water”) where OH- replaces HSO4- and H2SO4 is reformed

Alkenes: Addition RXNs - polymerization Addition polymerization Polymers are long chain molecules formed by the joining together of a large number of repeating units, called monomers. Addition polymerization involves the joining together of monomers containing double bonds:  

Alkenes: Addition RXNs - polymerization Two other common examples of addition polymerization PVC: Polychloroethene (also called polyvinyl chloride) Polypropene

Alkenes: Addition RXNs - polymerization Two other common examples of addition polymerization PTFE: Poly(tetrafluoroethene) (also called Teflon)

Alkenes: Addition RXNs - polymerization

Alkenes undergo electrophilic addition RXNs Page 488 2 videos