EpoxidesEpoxide: A three-membered ring made oftwo carbons and one oxygen.Very reactive towards nucleophiles.benzo[a]pyrene oxidecytochrome P450benzo[a]pyreneO2a polyaromatichydrocarbon (PAH)byproduct of burning tobacco, charring meatreacts with nucleophiles in DNA to generate DNA “lesions”, which can produce cancer-causing mutationsExample from biology:Synthesis of EpoxidesVia isotope labeling studies, we know which oxygen from peroxyacid adds.a peroxyacide.g.,meta-chloroperoxybenzoic acid(mCPBA)Stereochemistry of alkene starting material is retained in epoxide product.So, acis-alkene…mCPBA…gives acis-epoxide.Ring-Opening Reactions of Epoxides111Example:In acid, nucleophile adds to site of most stable carbocation.In base, nucleophile adds to least hindered carbon.(hydrazoicacid)(sodiumazide)HN3NaN3Nucleophile, acidNucleophile, baseDihydroxylation of Alkenes(+ enantiomer)trans-diolIn non-nucleophilic solvents (e.g., CH2Cl2), reaction stops at epoxide.mCPBAH2Ospontaneouslyanti-Dihydroxylation via epoxides:syn-Dihydroxylation with permanganate:KMnO4H2O/-OH-OHcleavagecis-diolCatalytic Reduction (Hydrogenation) of AlkenesAddition of 2 H’s is syn, because they replace C-Pt bonds at the catalyst surface.PtPt/H2 (g)Adds two H atoms to the same face of an alkene (to yield an alkane).PtAlkenes in Unsaturated Fatsa polyunsaturatedfat/triglyceridean unsaturatedfat/triglycerideester oflinoleic acid(omega-6)esters of linolenic acid(omega-3)Associated with heart health.“Essential”—we need them, but our bodies don’t make them.Critical for cell membrane fluidity.Rancidity in Unsaturated FatsH2OO2(from atmosphere)Allylic C-H’s in unsaturated fats are readily abstracted by atmospheric free radicals.This leads to decomposition of fatty acid chain (and to rancid flavor).free radical chain reactiona hydroperoxide(reactive!)In Theory, Hydrogenation of Unsaturated Fats Yields Fully Saturated Fatstristearin(a saturated fat/triglyceride)solid, mp = 72 °Ctriolein(an unsaturated fat/triglyceride)liquid, mp = -4 °CNi/H2 (g)Hydrogenation of unsaturated, liquid soybean oil leads to solid, saturated fat—less rancid, easier to process.Nickel typically used instead of platinum to lower cost. Requires heat, pressure.150 °C, 2000 psiIn Practice, Partial Hydrogenation Can Yieldtrans-FatsNi/H2 (g)150 °C, 2000 psitrans-alkeneAt high temperatures, reactions between Ni catalyst and alkene are reversible.NiNiNiNiNiNiReducing trans-FatsThe problem:Omega-3/6 fats are essential, and both saturated fats and cis-fats are used in the body, but trans-fats are not.New York Cityrestaurant postertrans-fat-containingfood productsa
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