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6. Alkenes: Structure and Reactivity

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6. Alkenes: Structure and Reactivity 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) Hyperconjugation 2) Bond strengths 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 stronger than sp3-sp3 5 Problem: Which is more stable? Why? More substitution mono- vs di- & sp2-sp3 bonding Trans more stable than cis More substitution tri- vs di- 6 Writing Organic Reactions No established convention – shorthand Can be formal kinetic expression Not necessarily balanced Reactants can be before or on arrow Solvent, temperature, details, 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. H H H H ether C C + Br H H C C Br H H H H Complete the reaction: CHCHCHCH H3CH3C CHC3 H3 3 3 3 3 etheretherether C C C C ++ Br BrH H H3CH3C C C C C Br Br H CH C CHCH 3 3 3 3 H H CHC3 H3 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) H C H CH3 H CH3 H 3 ether C C + HCl H3C C C H and H3C C C H H3C H Cl H H Cl 100 % 0 % H C CH CH3 CH3 CH3 CH3 3 3 ether C C + HCl H3C C C H and H3C C C H H3C H Cl H H Cl mostly this very little Let’s look at background on why this happens 10 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 adds to the double bond first. 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 forms faster Tertiary cations 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 C CH CH3 CH3 CH3 CH3 3 3 ether C C + HCl H3C C C H and H3C C C H H3C H Cl H H Cl mostly this little of this - 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 highest energy species in a reaction step By definition, its structure is not stable enough to exist for one vibration 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 relationship between 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 endergonic reaction step structurally resembles the product (b) The transition state of an exergonic 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, the transition states look G like the intermediate carbocation Reaction 19 6.12 Evidence of Mechanism of Electrophilic Addition: Rearrangements of Carbocations The most likely evidence of these two step mechanisms are that some products are formed from rearrangement (shifts), these products cannot be formed if it was a single step reaction. There is evidence of a hydride shift in some reactions, and a methyl 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 from different sides. 27 Addition of Br2 to Cyclopentene proves anti addition Addition is exclusively trans Mechanism 28 Problems: Complete the following reactions: A. HH OOHH HHOO++, , 33 HH33CC HH HH33CC CC CHCH22 CC CC ++ HH OOHH CCHH33 HH33CC HH 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 CH CH3 Br CH CH Cl Br CH3 CH2Cl2 H2C C CH3 Br2 H C C CH 2 3 + 2 H C C CH3 H Br 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 water adds (acts as a nucleophile) and opens the ring Aromatic rings do not react 31 An Alternative to Bromine Bromine is a difficult reagent to use for this reaction since few alkenes are soluble in water N-Bromosuccinimide (NBS) 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 H-OH to to give an alcohol Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol Regiochemistry corresponds to Markovnikov addition 33 Oxymercuration Intermediates For laboratory-scale 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 OOHH CH Br22 CH33 CH ++ CH33 HBr H O ++ HBr H22O BrBr HH CH B. CH 3 CH 3 CH 3 1. Hg(OAc)2, H2O 3 H C 1. Hg(OAc) , H O H3C H C3 CH 2 2 H C 3 3 3 CH CH3 3 2. NaBH4 CH3 2. NaBH4 OH OH C. Write the hydration reaction of 2-methylpropene using the industrial method. CH 3 H CH H3PO4, catalyst 3 H2C C CH3 + H2O H C C CH 250°C 3 H OH 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.
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