Lecture 11 Organic Chemistry 1
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1 EM 232 rganic hemistry I at hicago Lecture 11 rganic hemistry 1 Professor Duncan Wardrop February 16,
2 Self Test Question What is the product(s) of the following reaction? 3 K( 3 ) 3 A 3 ( 3 ) 3 3 D Br DMS, 40º B E University of Illinois at hicago EM 232, Spring 2010 Slide 2 2
3 EM 232 rganic hemistry I at hicago Addition Reactions of Alkenes Section You are responsible for Section
4 Addition Reactions of Alkenes + X Y addition X Y + hydrogenation + X hydrogen halide addition X + Br free radical bromine addition Br + S sulfuric acid addition S 3 + hydration at hicago EM 232, Spring 2010 Slide 4 4
5 ydrogenation Pd/, 32 exothermic reaction (- º), but high E act - catalyst required catalysts are heterogeneous transition metals (Pd, R solvent is typically an alcohol (e.g. ethanol, 32) metals are insoluble (heterogeneous mixture) heat of hydrogenation = - º at hicago EM 232, Spring 2010 Slide 5 5
6 eat of ydrogenation (- º) at hicago EM 232, Spring 2010 Slide 6 6
7 eat of ydrogenation (- º) Since only the double bond is undergoing the reaction, heat of hydrogenation is independent of the number of carbon atoms in the molecule Alkene - º (kj/mol) example ethylene 136 monosubstituted 126 cis-disubstituted 119 terminally disubstituted 117 trisubstituted 112 tetrasubstituted 110 at hicago EM 232, Spring 2010 Slide 7 7
8 General Mechanism for eterogenious ydrogenation σ d reductive elimination catalyst surface M II catalyst surface M 0 oxidative addition catalyst surface M II reaction takes places at the surface of the catalyst (many metal atoms combined) insertion coordination catalyst surface M II at hicago EM 232, Spring 2010 Slide 8 8
9 Step 1: xidative Addition d σ hydrogen (2) is added to metal metal is oxidized from M 0 to M II catalyst surface M 0 oxidative addition catalyst surface M II Pd 0 Pd II at hicago EM 232, Spring 2010 Slide 9 9
10 Step 2: oordination metal is a Lewis acid (electron acceptor) π-bond is a lewis base (electron donor) coordination = Lewis acid/base complex Pd II Pd II catalyst surface M II coordination catalyst surface M II at hicago EM 232, Spring 2010 Slide 10 10
11 Step 3: Insertion two carbon atoms inserted between Pd- formation of a weak metal-carbon σ-bond formation of a strong - σ-bond break a weak metal- σ-bond Pd II Pd II catalyst surface M II insertion catalyst surface M II at hicago EM 232, Spring 2010 Slide 11 11
12 Step 4: Reductive Elimination d σ catalyst surface M 0 reductive elimination catalyst surface M II metal is reduced from M II to M 0 last σ - -bond is formed from the same face of alkene as previous metal is a catalyst; it is regenerated Pd II at hicago EM 232, Spring 2010 Slide 12 12
13 omplete Mechanism reductive elimination Pd 0 oxidative addition Pd II Pd II insertion Pd II coordination at hicago EM 232, Spring 2010 Slide 13 13
14 Syn Addition of ydrogen as a consequence of mechanism, both hydrogens are added to the same face of the π-bond: syn addition no anti-addition products are formed (addition of hydrogen to opposite faces) at hicago EM 232, Spring 2010 Slide 14 14
15 ydrogenation is Stereoselective This methyl group sterically hinders hydrogen from approaching the π-bond from the top face both products are arise from syn additions of hydrogen to alkene stereoselective: preference for one stereoisomer when two or more are possible at hicago EM 232, Spring 2010 Slide 15 15
16 Example of Syn Addition Diatomic hydrogen and alkene are present in solution phase. No reaction occurs. 2. ydrogen absorbed on to catalyst surface. - bond cleaved. 3. π System of alkene coordinates catalyst surface ne hydrogen atom is transferred to alkene, forming -catalyst bond. 5. Second hydrogen atom is transferred, breaking substrate-catalyst bond. Alkane diffuses away from catalyst. 6. Steps 1-5 are repeated. at hicago EM 232, Spring 2010 Slide 16 16
17 Self Test Question What is the major product of the following hydrogenation reaction? D 2, Pd/ A. D 3 2 B. D D M II catalyst surface. D catalyst surface M II D D. D University of Illinois at hicago EM 232, Spring 2010 Slide 17 17
18 Addition of Electrophiles to Alkene + hydrogenation + X hydrogen halide addition X + Br free radical bromine addition Br + S sulfuric acid addition S 3 + hydration at hicago EM 232, Spring 2010 Slide 18 18
19 Electrophilic Addition of X + δ + δ X hydrogen halide addition X nucleophile electrophile at hicago EM 232, Spring 2010 Slide 19 19
20 Reaction onditions Br Br l 3, -30 º hydrogen halide: X common solvents: chloroform (l 3 ),dichloromethane ( 2 l 2 ), pentane, acetic acid generally performed at low temperature (below 0 ) generally a fast reaction at hicago EM 232, Spring 2010 Slide 20 20
21 Electrophilic Addition (Ad E ) Mechanism Br Protonation (slow) Br Br ation apture (fast) electrophilic addition: AdE RDS = protonation of carbon rate = k[alkene][hydrogen halide] unlike oxygen and nitrogen, protonation of carbon is slow proceeds through carbocation intermediate at hicago EM 232, Spring 2010 Slide 21 21
22 X Addition is Regioselective Regioselectivity Preferential reaction at one site of a single functional group over other sites that could undergo the same reaction EM 232 Definition, 2010 R X R R rather than X X R R X R R rather than R R X X R R R X R R rather than R R R X R X at hicago EM 232, Spring 2010 Slide 22 22
23 Markovnikov s Rule 3º 3 2º Br 3 Br 2 l 2-40 º addition of X to an unsymmetrically substituted alkene proceeds so that hydrogen () adds to the least substituted carbon and the halide (X) adds to the most substituted carbon atom 23 at hicago EM 232, Spring 2010 Slide 23
24 Self Test Question Predict the product when 2,4-dimethyl-2-pentene is treated with l? A. 3-chloro-2,4-dimethylpentane l l B. 2-chloroohexane. 2,3-dichloro-2,4-dimethylpentane D. 2-chloro-2,4-dimethylpentane E. 1-chloro-2,4-dimethylpentane University of Illinois at hicago EM 232, Spring 2010 Slide 24 24
25 Mechanistic Basis for Markovnikov s Rule X X X 3º carbocation 3º alkyl halide X X + 2º carbocation X 2º alkyl halide curved arrows do not indicate which carbon is protonated fastest protonation leads to more stable (more substituted) carbocation more substituted carbocation = more substituted alkyl halide at hicago EM 232, Spring 2010 Slide 25 25
26 Mechanistic Basis for Markovnikov s Rule ammond Postulate: transition state structure resembles closest energy intermediate transition state resembles carbocation for endothermic RDS (late transition state) what stabilizes carbocation also stabilizes transition state lowest energy transition state leads to more substituted carbocation at hicago EM 232, Spring 2010 Slide 26 26
27 Rearrangement an Precede Addition -Br hydride shift 2 2º carbocation 3º carbocation Br Br Br 2 Br 2 at hicago EM 232, Spring 2010 Slide 27 27
28 Stability of arbocations (Lecture 8) 2. yperconjugation stabilizing interaction that results from the interaction of the electrons in a σ-bond ( or bond ) with an adjacent empty (or partially filled) orbital. Leads to the formation of an extended molecular orbital that increases the stability of the system filled σ orbital 1º cation empty p orbital at hicago EM 232, Spring 2010 stabilization results from σ-donation to empty p orbital of planar carbocation electron donation through σ-bonds toward carbocation delocalizes charge (spreads out) methyl cations cannot be stabilized by hyperconjugation since σ-bonds are perpendicular to the empty p orbital Slide 28 28
29 Stability of arbocations (Lecture 8) 2. yperconjugation filled σ orbital empty 2p Z orbital empty p orbital Energy - bonding (filled) σ 2p Z Stabilization resulting from hyperconjugation at hicago EM 232, Spring 2010 Slide 29 29
30 Rearrangement an Precede Addition -Br 3 1,2-methyl shift 2º carbocation 3º carbocation Br Br Normal Addition Br Abnormal Addition Br at hicago EM 232, Spring 2010 Slide 30 30
31 Reversal of Addition Regioslectivty The Peroxide Effect -Br no peroxide 2 l 2 Br -Br l 2 1-butene 2-bromobutane (90% yield) 1-butene 1-bromobutane (95% yield) R R R peroxide hydroperoxide hydrogen peroxide alkyl peroxides easily fromed from alkanes/ alkenes by reaction with 2 in the air presence of peroxides leads to anti-markovnikov product (least substituted alkyl bromide) peroxide effect only operates when Br at hicago EM 232, Spring 2010 Slide 31 31
32 Mechanistic Rationale for Peroxide Effect peroxides are radical initiators: they undergo homolysis to generate alkoxy radicals, which begin the chain mechanism initiation R R heat ( ) or light (hυ) 2 R alkoxy radical R initiation Br R + Br bromine radical at hicago EM 232, Spring 2010 Slide 32 32
33 Mechanistic Rationale for Peroxide Effect Br bromine radical adds to the least substituted carbon of alkene Br Br propagation via alkene addition Br this generates the most substituted and most stable alkyl radical alkyl radical undergoes hydrogen abstraction from Br to generate a new bromine radical (chain mechanism) at hicago EM 232, Spring 2010 Slide 33 33
34 Addition of Sulfuric Acid to Alkenes + alkene S sulfuric acid addition S 3 alkyl hydrogen sulfate (ester of sulfuric acid) hydrolysis 2 alcohol compare to: alcohol 2 S 4 80 º elimination + alkene(s) at hicago EM 232, Spring 2010 Slide 34 34
35 Sulfuric Acid Addition: Ad E Mechanism 3 3 S Protonation (slow) 3 3 S ation apture (fast) 3 S 3 3 Markovnikov s rule applies protonation occurs to provide most stable (most substituted) carbocation leads to formation of most substituted alkyl hydrogen sulfate at hicago EM 232, Spring 2010 Slide 35 35
36 ydrolysis of Alkyl ydrogen Sulfates S hydrolysis 2 S alkyl hydrogen sulfate (ester of sulfuric acid) alcohol sufuric acid don t worry about mechanism for hydrolysis only requires hot water cleavage of the -S bond substitution of S with at hicago EM 232, Spring 2010 Slide 36 36
37 Examples of Alkene ydration at hicago EM 232, Spring 2010 Slide 37 37
38 ydration of Alkenes (Addition of Water) + hydration alkene alcohol compare to: alcohol 2 S 4 80 º elimination + alkene(s) at hicago EM 232, Spring 2010 Slide 38 38
39 ydration: Ad E Mechanism Protonation 3 (slow) l 3 3 Protonation (fast) 3 3 ation apture (fast) Principle of Microscopic Reversibility in an equilibrium, the forward mechanism is identical to the reverse mechanism at hicago EM 232, Spring 2010 Slide 39 39
40 ydration: Ad E Mechanism S fast & reversible S slow & reversible Me 2 Me S fast & reversible Me Me 2 Principle of Microscopic Reversibility in an equilibrium, the forward mechanism is identical to the reverse mechanism at hicago EM 232, Spring 2010 Slide 40 40
41 Quiz This Next Week Topic = hapter 5 Alkenes Nomenclature Alkenes: Structure, Bonding & Stability Geometrical Isomerism of Alkenes (E vs. Z) Mechanism of E1 Eliminations Zaitsev s Rule (E1 & E2) & Stereoselectivity Rearrangement During Elimination Mechanism of E2 Eliminations at hicago EM 232, Spring 2010 Slide 41 41
42 EM 232 rganic hemistry I at hicago Next Lecture... hapter 6: Sections
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