CHEM 212 Exam 3 Part 1 Professor Kelly Boebinger 6.7 Alkene Stability Cis alkenes are less stable than trans alkenes Compare heat given off on hydrogenation: Ho Less stable isomer is higher in energy, and gives off more heat Comparing heat given off when C=C is converted to C-C More stable alkene gives off less heat Trans butene generates 5 kJ less heat than cis- butene 2 Alkene Stability Alkenes become more stable with increasing substitution Stability Order: Most to Least stable This order is due to two factors: 1) 2) 3 Hyperconjugation An interaction that results from the overlap of a vacant p orbital with a neighboring bond. The more the substituents present, the more opportunities for hyperconjugation. 4 Bond Strengths sp2 to sp3 is ____________ than sp3-sp3 5 Problem: Which is more stable? Why? 6 Writing Organic Reactions No established convention – __________________ Can be formal _____________________________ Not necessarily ____________________________ ___________________ can be before or on arrow __________ ______________ _________on arrow 7 6.8 Electrophilic Addition of HX to Alkenes ( X = Br, Cl, I) For unsubstituted alkenes or for symmetrically substituted alkenes, the addition of HX across a double bond appears to proceed in an identical fashion as the addition of halogens across a double bond. The reaction is successful with HCl and with HBr. Complete the reaction: CH CH H3C CH3 3 3 ether C C + Br H H3C C C Br H C CH 3 3 H CH3 8 6.8 Electrophilic Addition of HX to Alkenes ( X = Br, Cl, I) Note that HI is generated from KI and phosphoric acid 9 Electrophilic Addition of HX to Alkenes ( X = Br, Cl, I) For unsymmetrical alkenes, the addition of HX across a double bond occurs with regiochemistry (Regiospecific – one product forms where two are possible) 10 Let’s look at background on why this happens 6.9, 6.10 Orientation of Electrophilic Addition: Markovnikov’s Rule In an unsymmetrical alkene, HX reagents can add in two different ways, but one way may be preferred over the other If one orientation predominates, the reaction is regiospecific Markovnikov observed in the 19th century that in the addition of HX to alkene, the H attaches to the carbon with the most H’s and X attaches to the other end (to the one with the most alkyl substituents) Markovnikov’s rule: Hydrogen will add to the side that will produce the more stable carbocation. 11 Carbocation Structure and Stability Hydrogen ________________________. It will add in a way that leaves the most stable carbocation, ready for the halogen to add and complete the reaction. 12 Carbocation Structure and Stability + Stability of carbocations: 3º > 2º > 1º > CH3 13 Energy of Carbocations and Markovnikov’s Rule More stable carbocation _______________________ __________________________ and associated transition states are more stable than primary cations 14 Electrophilic Addition: Markovnikov’s Rule 15 Back to the reactions, now the mechanisms! - H-X X X CH CH 3 CH 3 H3C 3 H3C C C + C C C C H C H C H H C 3 3 3 H H C H H 3 H - H-X X H X H H C CH3 3 + CH3 C C C C C C H C H C H C H 3 3 CH 3 H H C 3 3 H H3C 16 6.11 The Hammond Postulate: Transition State Structures A transition state is the _____________________ in a reaction step By definition, its structure is not stable enough to exist for ______________________ . But the structure controls the rate of reaction So we need to be able to guess about its properties in an informed way We classify them in general ways and look for trends in reactivity – the conclusions are in the Hammond Postulate The Hammond Postulate helps explain Markovnikov’s rule by explaining the ____________________ reaction rate and intermediate stability. “The structure of a transition state resembles the structure of the nearest stable species…” 17 The Hammond Postulate (a) The transition state of an _______________ reaction step structurally resembles the product (b) The transition state of an _______________ reaction step structurally resembles the reactant. 18 Statement of the Hammond Postulate A transition state should be similar to an intermediate that is close in energy In a reaction involving a carbocation, ________________ G ________________ carbocation ________________ ________________ Reaction ________________ 19 6.12 Evidence of Mechanism of Electrophilic Addition: Rearrangements of Carbocations The most likely evidence of these ________________ mechanisms are that some products are formed from rearrangement (shifts), these products cannot be formed if it was a single step reaction. There is _____________ of a _______________ shift in some reactions, and a __________________ shift in others. 20 CHEM 212 Exam 3 Part 2 Professor Kelly Boebinger 7. Alkenes: Reactions and Synthesis X CH2Cl2 C C + X2 C C X X2 = Cl2, Br2 anti addition Diverse Reactions of Alkenes Alkenes react with many electrophiles to give useful products by addition (often through special reagents) alcohols (add H-OH) alkanes (add H-H) halohydrins (add HO-X) 1,2-dihalides (add X-X) halides (add H-X) 1,2-diols (add HO-OH) cyclopropane (add :CH2) carbonyl 23 Diverse Reactions of Alkenes 24 7.1 Preparation of Alkenes: A Preview of Elimination Reactions Alkenes are commonly made by elimination of HX from alkyl halide (dehydrohalogenation) Uses KOH and ethanol H Br KOH C C K Br HO H ethanol C C + + 25 7.1 Preparation of Alkenes: A Preview of Elimination Reactions Alkenes are commonly made by elimination of H-OH from an alcohol (dehydration) require strong acids (sulfuric acid, 50 ºC) H OH + H3O , C C C C + HO H 26 7.2 Addition of Halogens to Alkenes Bromine and chlorine add to alkenes to give 1,2- dihaldes, an industrially important process F2 is too reactive and I2 does not add + - Cl2 reacts as Cl Cl , Br2 is similar X CH2Cl2 C C + X2 C C X X2 = Cl2, Br2 anti addition Antistereochemistry: Anti addition reaction which two ends of a double bond are attacked ___________ ________________________________________ 27 Addition of Br2 to Cyclopentene proves anti addition Addition is exclusively trans Mechanism 28 Problems: Complete the following reactions: A. H OH H O+, 3 H3C H H3C C CH2 C C + H OH CH3 H3C H B. KOH CH CH CH CH CH Br CH CH CH CH=CH K Br 3 2 2 2 2 3 2 2 2 HO H ethanol + + C. Write the bromination reaction of 2-methylpropene 29 7.3 Halohydrin Formation This is formally the addition of HO-X to an alkene (with +Br as the electrophile) to give a 1,2-halo alcohol, called a halohydrin The actual reagent is the dihalogen (Br2 or Cl2 in water in an organic solvent) Anti addition Markonivnikov regiochemistry observed, Br adds first to less substituted carbon 30 Mechanism of Formation of a Bromohydrin Br2 forms bromonium ion, then _______ _______________ (acts as a nucleophile) and opens the ring Aromatic rings do not react 31 An Alternative to Bromine Bromine (as in the previous reaction) is a difficult reagent to use for this reaction since few alkenes are soluble in water N-Bromosuccinimide ______________ produces bromine in organic solvents and is a safer source 32 7.4 Addition of Water to Alkenes: Oxymercuration Hydration of an alkene is the addition of ___________ _____________________________________________ Acid catalysts are used in high temperature __________ processes: ethylene is converted to ethanol Regiochemistry corresponds to Markovnikov addition 33 Oxymercuration Intermediates For ____________________hydration of an alkene Use mercuric acetate in THF followed by sodium borohydride Markovnikov orientation via mercurinium ion 34 Problems: Complete the following reactions: A. Br OH CH 2 3 CH + 3 + HBr H2O Br H B. CH3 CH3 1. Hg(OAc)2, H2O H3C H3C CH3 CH3 2. NaBH4 OH C. Write the hydration reaction of 2-methylpropene using the industrial method. 35 7.5 Addition of Water to Alkenes: Hydroboration Borane (BH3) is electron deficient is a Lewis acid Borane adds to an alkene to give an organoborane Boron attaches to the less highly substituted carbon (steric reasons) General Overall Reaction H OH 1. BH3, THF C C C C - R 2. H2O2, OH R 36 Syn addition, anti Markovnikov Orientation in Hydration via Hydroboration Regiochemistry is opposite to _____________ orientation OH is added to carbon with most H’s H and OH add with syn stereochemistry, to the same face of the alkene (opposite of anti addition) - H2O2, OH inserts OH in place of B Retains syn orientation 37 7.6 Addition of Carbenes to Alkenes The carbene functional group is “half of an alkene” Carbenes are electrically neutral with _____ electrons in the outer shell They _______________________________ across double bonds to form cyclopropanes This reaction happens in a single step ___________ intermediates. 38 7.6 Addition of Carbenes to Alkenes Overall reaction One of the simplest methods for generating a substituted carbene is with treatment of chloroform, with a strong base. The addition is stereospecific. Starting from a cis- alkene, a _____ ____________________ cyclopropane is produced, _________ gives a trans- product. + HOH 39 Simmons-Smith Reaction Best method for preparing nonhalogenated cyclopropanes. Equivalent of addition of CH2: Reaction of diiodomethane with zinc-copper alloy produces a carbenoid species Forms cyclopropanes by cycloaddition Zn(Cu) + CH I + ZnI 2 2 ether 2 40 7.7 Reduction of Alkenes: Hydrogenation Addition of __________ across C=C Reduction in general is addition of H2 or its equivalent Requires Pt or Pd or Ni as powders on carbon and H2 Hydrogen is first ____________________ on catalyst Reaction is heterogeneous (process is not in solution) 41 Mechanism of Catalytic Hydrogenation Heterogeneous – reaction between phases Addition of H-H is syn 42 Problems: Complete the following reactions: A.