L> Chapter 4 notesChemistry 331 - Fall 1996 Elements of Organic Chemistry I Professor Carl C. WamserChapter 4 - Alkenes & AlkynesAlkene Addition Reactions pi bonds undergo addition reactions CH2=CH2 + HCl --> CH3CH2Cl in general, C=C + HX --> H-C-C-X alkenes react with hydrogen halides to khung alkyl halides Addition of HX to Alkenes cyclohexene + HBr --> bromocyclohexane 1-methylcyclohexene + HBr --> 1-bromo-1-methylcyclohexane (not 1-bromo-2-methylcyclohexane) Reaction Notation reactants -------> products focus on the organic reactants and products show reagents over the arrow show solvent và conditions under the arrow (or show full balanced reaction) Orientation of Addition regiochemistry: specific orientation of addition (which C gets H, which gets X?) alkene additions are regioselective: one direction of addition is usually preferred Markovnikov"s Rule the original: showroom H to lớn the C with more H"s (or to the C with fewer alkyl groups) the reason: showroom H+ to size the more stable cation CH3CH=CH2 + HCl ---> CH3CH+CH3 (not CH3CH2CH2+) ---> CH3CHClCH3 (not CH3CH2CH2Cl) Tues, Feb. 13Carbocations structure: trigonal (sp2) stability: 3° > 2° > 1° more alkyl groups stabilize a cation by electron donation to lớn the electron-deficient (6-electron) carbocation Markovnikov AdditionHydration of Alkenes alkene + water --> alcohol CH2=CH2 + H2O --(H+)--> CH3CH2OH mechanism: step 1: addition of H+ electrophile khổng lồ pi bond step 2: addition of H2O nucleophile khổng lồ cation Hydration MechanismHalogenation of Alkenes CH2=CH2 + Cl2 ---> Cl-CH2-CH2-Cl mechanism: Cl2 is an electrophile (adds Cl+) then Cl- is a nucleophile Anti Addition anti stereochemistry: two new groups are added lớn opposite sides of the original pi bond cyclopentene + Br2 ---> trans-1,2-dibromocyclopentane (no cis) anti - describes the process trans - describes the sản phẩm Bromonium Ion carbocations can be stabilized by bonding to a neighboring Br (also works with Cl, but less favorable) Reduction of Alkenes reduction - addition of H2 (or removal of O) CH2=CH2 + H2 ---> CH3-CH3 R-O-H + H2 ---> R-H + H2O Catalytic Hydrogenation CH2=CH2 + H2 ---> CH3-CH3 requires an active catalyst, typically Pt, Pd, Ni, PtO2 reaction occurs on the surface both Hs are delivered lớn the same side of the pi bond Syn Addition syn stereochemistry: two new groups are added to lớn the same side of the original pi bond 1,2-dimethylcyclohexene + H2 --(cat)-->cis-1,2-dimethylcyclohexane(no trans) syn - describes the process cis - describes the hàng hóa Oxidation of Alkenes oxidation - addition of O (or removal of H2) RCH2OH ---> RCH=O ---> RCOOH there are a wide variety of oxidizing agents: O2, O3, KMnO4, CrO3, Na2Cr2O7 metals in high positive oxidation states Hydroxylation alkene + KMnO4 --(base)--> 1,2-diol addition of two OH groups is syn cyclopentene --> cis-1,2-cyclopentanediol Oxidative Cleavage C=C --> C=O + O=C acidic KMnO4 causes cleavage ozone (O3) causes cleavage sometimes useful degradation method lớn identify unknown compounds Polymers long chains of repeating units (monomers) n CH2=CH2 --(init)--> (init)-(CH2-CH2)n- n=100-10,000 polyethylene has properties like a very long alkane many polyalkenes are commercially important materials and plastics e.g., PVC, Teflon, Orlon Chain Reactions polymerization occurs by a không tính phí radical chain mechanism initiation - generation of the first free radical from an initiator R-O-O-R --(heat)--> 2 R-O· (initiators have one weak bond) Chain Reactions propagation - radical adds to a phường bond RO· + CH2=CH2 ---> RO-CH2-CH2· note that the hàng hóa is also a radical RO-CH2-CH2· + CH2=CH2 ---> RO-CH2-CH2-CH2-CH2· ---> etc. typically this occurs hundreds or thousands of times (until radicals recombine - termination) Substituted Monomers radical additions follow the Markovnikov Rule: showroom radicals to form the more stable radical intermediate radical stability is lượt thích cation stability: 3° > 2° > 1° this leads khổng lồ polymers with alternating substituents Vinyl Polymers polyvinyl chloride polypropylene polystyrene Elimination Reactions alkenes are typically prepared by elimination reactions loss of HX from alkyl halides (promoted by strong base) loss of H2O from alcohols (promoted by strong acid) eliminations are the reverse of additions DehydrohalogenationDehydrationZaitsev Rule predicts regiochemistry the major product in an elimination reaction is the more substituted alkene (generally more stable) Conjugated Dienes two double bonds separated by one single bond overlap their phường orbitals into an extended (conjugated) molecular orbital more stable than separate pi bonds e.g., 1,3-butadiene 1,2- and 1,4-Additions electrophilic additions (HX, X2, etc.) often địa chỉ at opposite ends (1,4) of a conjugated diene Allylic Carbocations initial electrophilic addition occurs at end (not middle) of a conjugated diene the resultant cation retains three overlapping phường orbitals (stabilized) allyl - position next khổng lồ a C=C bond (vinyl - position on a C=C bond) Allylic Resonance allylic cation has two Lewis structures (resonance forms) the actual structure is a hybrid the allyl cation is more stable than normal alkyl cations (due khổng lồ resonance) Resonance Forms each resonance form is a correct Lewis structure no atoms change (only electrons) equivalent resonance forms are most important for stabilization nonequivalent resonance forms may also contribute actual structure is a resonance hybrid Alkynes - Structure carbon-carbon triple bond sp hybridization (linear) no cis-trans possibilities the two pi bonds are perpendicular high electron mật độ trùng lặp từ khóa (usually more reactive than alkenes) Alkynes - Nomenclature -yne suffix (with number) rules similar to lớn alkenes with both -enes and -ynes, suffix is -enyne and numbering is from the end closer khổng lồ a multiple bond (E)-4-hexen-1-yne Alkyne Additions similar to lớn alkenes but more reactive Markovnikov Rule is followed excess reagent gives double addition single addition is usually possible single addition gives alkene product, which may be cis (syn addition) or trans (anti addition) or nonspecific Reduction of Alkynes excess H2 + catalyst gives alkanes Lindlar catalyst gives cis-alkenes Halogenation of Alkynes first addition of X2 is anti product is trans-dibromoalkene Hydration of Alkynes initial sản phẩm is an enol, which is typically unstable an enol isomerizes to a ketone (tautomers - a special kind of isomer, where the only difference is the placement of one hydrogen) Alkyne Acidity terminal alkynes have a C-H bond which is slightly acidic pKa ~ 25 (for CH2=CH2 , pKa ~44) NaNH2 is a strong enough base lớn deprotonate 1-alkynes anions are called acetylide ions, strong bases và good nucleophiles Alkyne Syntheses acetylide anions are useful in preparing larger alkynes