Gen Organic Chemistry I
Gen Organic Chemistry I CHEM 3311
University of Memphis
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This 23 page Class Notes was uploaded by Ms. Noemie Klein on Friday October 23, 2015. The Class Notes belongs to CHEM 3311 at University of Memphis taught by Staff in Fall. Since its upload, it has received 46 views. For similar materials see /class/228407/chem-3311-university-of-memphis in Chemistry at University of Memphis.
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Date Created: 10/23/15
OxidationReduction Reactions Oxidation States Carbon oxidation states are assigned on the basis of the electronegativity of attached atoms For each bond to a more electronegative atom give 1 For each bond to a less electronegative atom give 1 For each bond to carbon give 0 O 2 bonds to O 2 1 bond to H 1 CH 1bondtoCO 3 Oxidation State 1 CH2 H 1 bond to C O T 3 bonds to H 3 2 bonds to C 2 0 Each H has one bond to C 1 Oxidation State 3 2 bonds to H 2 Oxidation State 1 Oxidation State 2 QSquot39gtP E H Carbon Reduction Reactions Alkyl halides gt alkanes Alkyl halides gt organometallics Alkenes gt alkanes Alkynes gt alkenes Alkynes gt alkanes Carbonyls gt alcohols Carbon Oxidation Reactions 7 Alkcncs gt diols 8 Alkcncs gt carbonyls 9 Alkcncs gt dihalides 10 Alkcncs gt alcohols 11 Alkenes gt cpoxides 12 Alcohols gt carbonyls Problem Demonstrate Why your assigned reaction involves either oxidation or reduction at carbon you may want to draw a speci c example of reaction that is described generically by functional group Decide if another reaction category based on net result also applies to your reaction Reduction of Alkyl Halides 1 Br M Zn HBr gt Zn2 2Br39 Zn is a reducing agent because it has two valence electrons that can easily be lost to provide a more stable electron con guration The carbon attached to bromine is in the zero oxidation state and most reducible While other carbons in the alkyl halide are either 2 or 3 and less reducible An intermediate organometallic R39Zn2Br39 is thought to be formed during the reaction Reduction of Alkyl Halides II These act like M B Br MgBr g r M CH3CHZOCH2CH3 M M Mg gt a quotGrignardquot reagent Br Li L1 CH CH OCH CH 00 2Li Li 3 M LlBr Nucleophilic carbon atoms are extremely useful in substitution reactions that produce more complex carbon skeletons Reactions of alkyl halides with metals are therefore often NOT followed by treatment with acid as in the previous example to produce the alkane Reduction of Alkenes and Alkynes H2 Pt H2 Pt Alkenes and alkynes can be reduced to alkanes A source of hydrogen atoms is required A catalyst is required since neither alkenes nor alkynes are suf ciently polar to provide good handles for reactions to start Pt Ni Pd and many other solid metals can catalyze this reaction Catalyst Function catalyst hypothetical Catalyst present usually multistep AH 120 kJ mol l 1 AGNgtAGQ Catalysts increase reaction RATES and thus must lower the activation energy 7 this does not change the overall favorability of the reaction Catalysm are INVOLVED in the reaction but are not consumed Reduction to Alkanes Mechanism Catalyst Surface Catalyst Surface Catalyst Surface Catalyst Surface Catalyst Surface Hydrogen approaches catalyst Hydrogen reacts with catalyst Pi bond interacts with catalyst Pi bond reacts with nearby H atom Second H atom is rapidly added Mechanism Consequence StereoselectiVity Due to the rapid addition of both hydrogen atoms at the catalyst surface they are added to the same face of the pi bond syn addition CH3 CH3 or H2 L cc CH3 CH3 only cisl 2dimethylcyclohexane is produced 3R4S 3isopropyl 4methylhexane 3 S4R 3isopropy1 4methylhexane no 3R 4R or 3S 4S are produced Alkyne Reduction to Alkenes I The catalysts previously mentioned Ni Pt Pd are highly active and reduce alkynes to alkanes Without stopping at alkenes Less active catalysts NizB P2 catalyst PdCaCO3 Lindlar s catalyst will stop at the Zalkene Problem What products do you predict for the following reactions 2hexyne H2 Nj gt PdCaCO3 2pentyne H2 Anti Hydrogen Addition H3C H 1 Li CHgCHZNHZ 78 degrees 2 NH4C1 H CH3 Why the difference The reaction consumes the metal Li gt Li rather than being catalyzed at a metal surface and is thus called a dissolving metal reduction The two hydrogen atoms are added in solution from the solvent and the thermodynamically more stable product is formed since the alkyne is surrounded by solvent and they can add from any side Alkene Oxidations H H H H OH H O A moi Cold Q l H H OyO OH OH 11 Stereoselective 0111 H20 0 0 SYN addition u u u H1ghly react1ve1ntermed1ate can also use 0804 degrades when heated The cleavage reaction at left was DHO historically used as a means to determine the location of double bonds in complex molecules from natural sources Cleavage of more highly substituted alkenes gives ketones rather than carboxylic acids 00 Problem What initial structure would give the following products upon treatment with hot potassium permanganate in base WO O Halide Addition review Oxidation state of alkene Oxidation state of halide carbons 1 carbons O Alkenes gt Alcohols I H H H CH gtCC H20 4M SO H3CCCCH3 H3C CH3 H3C OH II 0 CH3 H CH3 0 H H H H cc Hg gt g MaBH H20 I C CH3 OH C CH3 H3C CH3 CH3 H3C H3C C 0 multiple stereoisomers Why two reactions to give the same produots Problem Predict the products of the following alkene oxidation reactions H2804 AcO NaBH 0A HOO H J BH3 39OH Alkenes gt Epoxides S P I H0 C gt O Q 0 R H Epoxides are highly strained and are easily attacked by nucleophiles thus useful reagents Peroxy acids are used to transfer an oxygen atom to the alkene Common peroxy acids used Metachloroperbenzoic acid MCPBA Magnesium monoperoxyphthalate MMPP Carbonyls Alcohols 0 reduction OH gt A o 0X1dat10n Reducing agents often used NaBH4 LiAlH4 both act as hydride donors Oxidizing agents often used KMnO4 H2CrO4 pyridinium ohloroohromate C5H6N CrO3Cl39 Alkenes gt Alcohols II H CH3 HCBH2 CH mu HCOH CH cc BH3 H3C C 3 39OH H c C 3 3 H3C CH3 H3C H H3C H multiple stereoisomers First step hydroboration stereoselective SYN addition as we saw previously Second step oxidation of carbon attached to boron oxidation state 2 to an alcohol carbon oxidation state O stereochemical con guration of carbon UNCHANGED Note regioselectivity is opposite previous two reactions Summary of Oxidizing Agents Metals in high oxidation states KMnO4 OsO4 HZCrO4 pyridinium chlorochromate C5H6N CrO3Cl39 Less reactive than permanganate or chromate will oxidize primary alcohols to aldehydes rather than continuing to the carboxylic acid Nonmetals in higher than usual oxidation states 2 Peroxides Peroxy acids Br2