CEM 494 Special Topics in Chemistry Illinois at Chicago CEM 494 - Lecture 7 Prof. Duncan Wardrop ctober 22, 2012
CEM 494 Special Topics in Chemistry Illinois at Chicago Preparation of Alkenes Elimination Chapter 19
Dehydration can be Coupled with ther Chemical Transformation N Cl F N Cl F N Cl N N N Loratidine (Claritin ) Two-step, one-pot transformation involves a Friedel-Crafts reaction (see, Chapter 12) and dehydration of the resulting 3 alcohol CEM 494, Spring 2010 Slide 3
Rate of Alcohol Dehydration Mirrors Ease of Carbocation Formation rate of dehydration = 3º > 2º > 1º alcohol tertiary alcohol (3º) tertiary cation (3º) secondary alcohol (2º) Reactivity Stability secondary cation (2 ) primary alcohol (1º) primary cation (1 ) CEM 494, Spring 2010 Slide 4
Self Test Question Predict the product for the following reaction scheme. 2 S 4 140 ºC? A. B. - β + β β - 2 β β C. D. β β β β E. no reaction CEM 494, Fall 2012 Slide 5
Predict the product for the following reaction scheme. Self Test Question 2 S 4 140 ºC? A. B. - β + or - β + - 2 β β β β β β C. D. β E. no reaction CEM 494, Fall 2012 Slide 6
CEM 494 Special Topics in Chemistry Illinois at Chicago Regioselectivity & Stereoselectivity of Dehydration Chapter 19
Self Test Question What is the product(s) of the following reaction? A. 2 S 4 80 ºC? B. C. D. E. 2 S CEM 494, Fall 2012 Slide 8
Types of Selectivity in rganic Chemistry There are three forms of selectivity to consider.... Chemoselectivity: which functional group will react Regioselectivity: where it will react Stereoselectivity: how it will react with regards to stereochemical outcome... for each transformation, always question which of these are factors are at play. CEM 494, Spring 2010 Slide 9
Regioselectivity of Elimination Regioselectivity: Where Will It React? Preferential reaction at one site of a single functional group over other sites that could undergo the same reaction CEM 232 Definition, 2009 2 S 4 80 ºC + + 3 C C 3 C 3 10% (identical) 90% CEM 494, Spring 2010 Slide 10
Regioselectivity of Elimination Regioselectivity: Where Will It React? Preferential reaction at one site of a single functional group over other sites that could undergo the same reaction CEM 232 Definition, 2009 C 3 3 P C 4 3 + C 3 heat 84% 16% β β C3 β 2 different leaving group/ β relationships CEM 494, Spring 2010 Slide 11
Greek Lettering & Elimination Reactions Nomenclature The α-carbon is the one to which the leaving group is initially bonded, and the carbon chain from this may be labelled β (beta), γ (gamma), δ (delta) etc, following Greek alphabet. Use primed letters for chains branching at α-carbon δ 3 C γ 2 C Cl C 3 C C 2 β α 2 β' γ' CEM 494, Spring 2010 Slide 12
Regioselectivity of Elimination Zaitsev Rule Na + - S 2 s on this β carbon C C 3 3 C KS 4 3 C 3 hydrosulfate β α heat β C 3 β C 3 3 s on this β carbon 1 s on this β carbon C 3 + C 3 C + 3 87% 13% 0% Zaitsev Rule When elimination can occur in more than one direction, the major alkene is the one formed by loss of a atom from the β carbon having the fewest hydrogens CEM 494, Spring 2010 Slide 13
Considering Stereo & Regioselectivity Combine Zaitsev s Rule and observations about stereoselectivity to predict the major products of dehydration (elimination) 2 S 4 80 ºC major product trisubstituted trisubstituted disubstituted disubstituted most stable alkenes have largest groups on each carbon trans to each other CEM 494, Spring 2010 Slide 14
Self Test Question What is the major product expected for the reaction scheme below? A. B. 2 S 4 80 ºC? C. D. E. CEM 494, Fall 2012 Slide 15
CEM 494 Special Topics in Chemistry Illinois at Chicago E1 & E2 chanisms of Alcohol Dehydration Chapter 19
rganic chanisms (S N 1) Cl fast & reversible alkyloxonium ion Cl slow Cl fast Cl carbocation (t-butyl cation) 2 t-butyl chloride CEM 494, Spring 2010 Slide 17
Remember Curved Arrow Notation? curved arrows show the movement of electrons; never atoms electrons atoms 3 C N C 3 resonance: electrons in a covalent bond moving out to an atom C 3 3 + bond making: lone pair of electrons forming a new bond to another atom 3 C N C 3 C 3 resonance: lone pair of electrons moving in between two atoms to form a new covalent bond 3 C 3 C + bond breaking: electrons in a bond leaving to most electronegative atom CEM 494, Spring 2010 Slide 18
chanism of Dehydration (E1) Step ne Proton Transfer (Protonation) S pka = -3.0 fast & reversible alkyloxonium ion S CEM 494, Spring 2010 Slide 19
Step Two Dissociation chanism of Dehydration (E1) slow 2 carbocation (t-butyl cation) CEM 494, Spring 2010 Slide 20
chanism of Dehydration (E1) Step Three Carbocation Capture β-deprotonation! C 2 S fast S carbocation (t-butyl cation) alkene (2-methylpropene) sulfuric acid (regenerated) S alkyl hydrogen sulfate (product of S N 1) CEM 494, Spring 2010 Slide 21
ughes-ingold Nomenclature slow carbocation (t-butyl cation) 2 E1 elimination unimolecular overall reaction = β-elimination rate determining step (RDS) involves on species = unimolecular rate = k[alkyl oxonium ion] = first order CEM 494, Spring 2010 Slide 22
Each Step of E1 chanism is Reversible S fast & reversible S slow & reversible C 2 S fast & reversible 2 If all steps in E1 are reversible, what drives the reaction forward? CEM 494, Spring 2010 Slide 23
Alkenes Isolated from Dehydration Reactions by Distillation 2 S 4 + + + + 4-methyl-2-pentanol bp = 132 ºC 2-methyl-1-pentene bp = 62 ºC 2-methyl-2-pentene bp = 67 ºC trans-4-methyl- 2-pentene bp = 59 ºC cis-4-methyl- 2-pentene bp = 58 ºC 4-methyl-1-pentene bp = 54 ºC alkenes have much lower boiling points than alcohols alcohols have higher boiling points (b.p.) because of larger van der Waals forces, including strong hydrogen-bonding by removing alkenes through distillation (boiling), equilibrium is shifted toward products (LeChatlier Principle) until no more reactants remain CEM 494, Spring 2010 Slide 24
Why Can t ydrogen alides Be Used for Elimination Reactions? slow & reversible Cl C 2 fast & reversible fast & reversible Cl Cl N u c l e o p h i l i c a d d i t i o n o f chloride (Cl ) to a carbocation is not reversible Cl fast & irreversible nucleophilic addition Cl alkyl chloride (product of S N 1) CEM 494, Spring 2010 Slide
Reactivity Explained R 2 R 1 = C R 2 R 3 R 2 = C R 3 = 2º Carbocation R 2 R 1 = C R 1 R 3 R 2 = C R 3 = C 3º Carbocation R 3 R 2 R 1 3 + R 3 R 2 R 1 2 2 3º carbocations are more stable than 2º = 3º lower in energy smaller activation energy leading to 3º carbocation results in faster reaction CEM 494, Spring 2010 Slide 26
Bimolecular Substitution - S N 2 chanism (from Lecture 8) 3 C Br fast 3 C Br 3 C slow (rate-determining) Step 1 Protonation Step 2 Nucleophilic Attack δ- Br C 3 C δ+ 3 C + Br C- bond breaks at the same time the nucleophile (Br) forms the C-X bond RDS is nucleophilic attack; bimolecular, therefore Ingold notation = S N 2 fewer steps does not mean faster reaction CEM 494, Spring 2010 Slide 27
Dehydration of Primary Alcohols Proceeds via E2 chanism 3 C S fast & reversible 3 C 3 C C 1 Cation S Step 1 Protonation Step 2 β-deprotonation (elimination) S β 2 C slow C- bond breaks at the same time the nucleophile (Br) forms the C-X bond RDS is nucleophilic attack; bimolecular, therefore Ingold notation = S N 2 fewer steps does not mean faster reaction CEM 494, Spring 2010 Slide 28
CEM 494 Special Topics in Chemistry Illinois at Chicago Regioselectivity & Stereoselectivity of Dehydration
Self Test Question What is the product(s) of the following reaction? A. 2 S 4 80 ºC? B. C. D. E. 2 S CEM 494, Fall 2012 Slide 30
Types of Selectivity in rganic Chemistry There are three forms of selectivity to consider.... Chemoselectivity: which functional group will react Regioselectivity: where it will react Stereoselectivity: how it will react with regards to stereochemical outcome... for each transformation, always question which of these are factors are at play. CEM 494, Spring 2010 Slide 31
Regioselectivity of Elimination Regioselectivity: Where Will It React? Preferential reaction at one site of a single functional group over other sites that could undergo the same reaction CEM 232 Definition, 2009 2 S 4 80 ºC + + 3 C C 3 C 3 10% (identical) 90% CEM 494, Spring 2010 Slide 32
Regioselectivity of Elimination Regioselectivity: Where Will It React? Preferential reaction at one site of a single functional group over other sites that could undergo the same reaction CEM 232 Definition, 2009 C 3 3 P C 4 3 + C 3 heat 84% 16% β β C3 β 2 different leaving group/ β relationships CEM 494, Spring 2010 Slide 33
Greek Lettering & Elimination Reactions Nomenclature The α-carbon is the one to which the leaving group is initially bonded, and the carbon chain from this may be labelled β (beta), γ (gamma), δ (delta) etc, following Greek alphabet. Use primed letters for chains branching at α-carbon δ 3 C γ 2 C Cl C 3 C C 2 β α 2 β' γ' CEM 494, Spring 2010 Slide 34
Regioselectivity of Elimination Zaitsev Rule Na + - S 2 s on this β carbon C C 3 3 C KS 4 3 C 3 hydrosulfate β α heat β C 3 β C 3 3 s on this β carbon 1 s on this β carbon C 3 + C 3 C + 3 87% 13% 0% Zaitsev Rule When elimination can occur in more than one direction, the major alkene is the one formed by loss of a atom from the β carbon having the fewest hydrogens CEM 494, Spring 2010 Slide 35
Considering Stereo & Regioselectivity Combine Zaitsev s Rule and observations about stereoselectivity to predict the major products of dehydration (elimination) 2 S 4 80 ºC major product trisubstituted trisubstituted disubstituted disubstituted most stable alkenes have largest groups on each carbon trans to each other CEM 494, Spring 2010 Slide 36
Self Test Question What is the major product expected for the reaction scheme below? A. B. 2 S 4 80 ºC? C. D. E. CEM 494, Fall 2012 Slide 37
CEM 494 Special Topics in Chemistry Illinois at Chicago E1 & E2 chanisms of Alcohol Dehydration Section: 5.12
rganic chanisms (S N 1) Cl fast & reversible alkyloxonium ion Cl slow Cl fast Cl carbocation (t-butyl cation) 2 t-butyl chloride CEM 494, Spring 2010 Slide 39
Remember Curved Arrow Notation? curved arrows show the movement of electrons; never atoms electrons atoms 3 C N C 3 resonance: electrons in a covalent bond moving out to an atom C 3 3 + bond making: lone pair of electrons forming a new bond to another atom 3 C N C 3 C 3 resonance: lone pair of electrons moving in between two atoms to form a new covalent bond 3 C 3 C + bond breaking: electrons in a bond leaving to most electronegative atom CEM 494, Spring 2010 Slide 40
chanism of Dehydration (E1) Step ne Proton Transfer (Protonation) S pka = -3.0 fast & reversible alkyloxonium ion S CEM 494, Spring 2010 Slide 41
Step Two Dissociation chanism of Dehydration (E1) slow 2 carbocation (t-butyl cation) CEM 494, Spring 2010 Slide 42
chanism of Dehydration (E1) Step Three Carbocation Capture β-deprotonation! C 2 S fast S carbocation (t-butyl cation) alkene (2-methylpropene) sulfuric acid (regenerated) S alkyl hydrogen sulfate (product of S N 1) CEM 494, Spring 2010 Slide 43
ughes-ingold Nomenclature slow carbocation (t-butyl cation) 2 E1 elimination unimolecular overall reaction = β-elimination rate determining step (RDS) involves on species = unimolecular rate = k[alkyl oxonium ion] = first order CEM 494, Spring 2010 Slide 44
Each Step of E1 chanism is Reversible S fast & reversible S slow & reversible C 2 S fast & reversible 2 If all steps in E1 are reversible, what drives the reaction forward? CEM 494, Spring 2010 Slide 45
Alkenes Isolated from Dehydration Reactions by Distillation 2 S 4 + + + + 4-methyl-2-pentanol bp = 132 ºC 2-methyl-1-pentene bp = 62 ºC 2-methyl-2-pentene bp = 67 ºC trans-4-methyl- 2-pentene bp = 59 ºC cis-4-methyl- 2-pentene bp = 58 ºC 4-methyl-1-pentene bp = 54 ºC alkenes have much lower boiling points than alcohols alcohols have higher boiling points (b.p.) because of larger van der Waals forces, including strong hydrogen-bonding by removing alkenes through distillation (boiling), equilibrium is shifted toward products (LeChatlier Principle) until no more reactants remain CEM 494, Spring 2010 Slide 46
Why Can t ydrogen alides Be Used for Elimination Reactions? slow & reversible Cl C 2 fast & reversible fast & reversible Cl Cl N u c l e o p h i l i c a d d i t i o n o f chloride (Cl ) to a carbocation is not reversible Cl fast & irreversible nucleophilic addition Cl alkyl chloride (product of S N 1) CEM 494, Spring 2010 Slide
Reactivity Explained R 2 R 1 = C R 2 R 3 R 2 = C R 3 = 2º Carbocation R 2 R 1 = C R 1 R 3 R 2 = C R 3 = C 3º Carbocation R 3 R 2 R 1 3 + R 3 R 2 R 1 2 2 3º carbocations are more stable than 2º = 3º lower in energy smaller activation energy leading to 3º carbocation results in faster reaction CEM 494, Spring 2010 Slide 48
Bimolecular Substitution - S N 2 chanism (from Lecture 8) 3 C Br fast 3 C Br 3 C slow (rate-determining) Step 1 Protonation Step 2 Nucleophilic Attack δ- Br C 3 C δ+ 3 C + Br C- bond breaks at the same time the nucleophile (Br) forms the C-X bond RDS is nucleophilic attack; bimolecular, therefore Ingold notation = S N 2 fewer steps does not mean faster reaction CEM 494, Spring 2010 Slide 49
Dehydration of Primary Alcohols Proceeds via E2 chanism 3 C S fast & reversible 3 C 3 C C 1 Cation S Step 1 Protonation Step 2 β-deprotonation (elimination) S β 2 C slow C- bond breaks at the same time the nucleophile (Br) forms the C-X bond RDS is nucleophilic attack; bimolecular, therefore Ingold notation = S N 2 fewer steps does not mean faster reaction CEM 494, Spring 2010 Slide 50
CEM 494 Special Topics in Chemistry Illinois at Chicago Addition Reactions of Alkenes
Addition Reactions of Alkenes C C + X Y addition X C C Y C C + hydrogenation C C C C + X hydrogen halide addition C C X C C + Br free radical bromine addition C C Br C C + S sulfuric acid addition C C S 3 C C + hydration C C CEM 494, Spring 2010 Slide 52
ydrogenation Pd/C, C3C2 exothermic reaction (- º), but high E act - catalyst required catalysts are heterogeneous transition metals (Pd, R solvent is typically an alcohol (e.g. ethanol, C 3 C2) metals are insoluble (heterogeneous mixture) heat of hydrogenation = - º CEM 494, Spring 2010 Slide 53
eat of ydrogenation (- º) CEM 494, Spring 2010 Slide 54
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 CEM 494, Spring 2010 Slide 55
General chanism 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 CEM 494, Spring 2010 Slide 56
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 CEM 494, Spring 2010 Slide 57
Step 2: Coordination 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 CEM 494, Spring 2010 Slide 58
Step 3: Insertion two carbon atoms inserted between Pd- formation of a weak metal-carbon σ-bond formation of a strong C- σ-bond break a weak metal- σ-bond Pd II Pd II catalyst surface M II insertion catalyst surface M II CEM 494, Spring 2010 Slide 59
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 C- -bond is formed from the same face of alkene as previous metal is a catalyst; it is regenerated Pd II CEM 494, Spring 2010 Slide 60
Complete chanism reductive elimination Pd 0 oxidative addition Pd II Pd II insertion Pd II coordination CEM 494, Spring 2010 Slide 61
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) CEM 494, Spring 2010 Slide 62
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 CEM 494, Spring 2010 Slide 63
Example of Syn Addition C 3 C 3 C 8 12 C 3 C 3 C 8 12 C 3 C 3 C 8 12 1. 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. C 3 C 3 C 8 12 C 3 C 3 C 8 12 C 3 C 3 C 8 12 4. ne hydrogen atom is transferred to alkene, forming C-catalyst bond. 5. Second hydrogen atom is transferred, breaking substrate-catalyst bond. Alkane diffuses away from catalyst. 6. Steps 1-5 are repeated. CEM 494, Spring 2010 Slide 64
Self Test Question What is the major product of the following hydrogenation reaction? D 2, Pd/C A. D C 3 C 2 B. D D M II catalyst surface C. D catalyst surface M II D D. D CEM 494, Fall 2012 Slide 65
Addition of Electrophiles to Alkene C C + hydrogenation C C C C + X hydrogen halide addition C C X C C + Br free radical bromine addition C C Br C C + S sulfuric acid addition C C S 3 C C + hydration C C CEM 494, Spring 2010 Slide 66
Electrophilic Addition of X + δ + δ C C X hydrogen halide addition C C X nucleophile electrophile CEM 494, Spring 2010 Slide 67
Reaction Conditions Br Br CCl 3, -30 ºC hydrogen halide: X common solvents: chloroform (CCl 3 ),dichloromethane (C 2 Cl 2 ), pentane, acetic acid generally performed at low temperature (below 0 C) generally a fast reaction CEM 494, Spring 2010 Slide 68
Electrophilic Addition (Ad E ) chanism Br Protonation (slow) Br Br Cation Capture (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 CEM 494, Spring 2010 Slide 69
X Addition is Regioselective Regioselectivity Preferential reaction at one site of a single functional group over other sites that could undergo the same reaction CEM 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 CEM 494, Spring 2010 Slide 70
Markovnikov s Rule 3º C 3 2º Br 3 C Br C 2 Cl 2-40 ºC 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 71 CEM 494, Spring 2010 Slide
Self Test Question Predict the product when 2,4-dimethyl-2-pentene is treated with Cl? A. 3-chloro-2,4-dimethylpentane Cl Cl B. 2-chloroohexane C. 2,3-dichloro-2,4-dimethylpentane D. 2-chloro-2,4-dimethylpentane E. 1-chloro-2,4-dimethylpentane CEM 494, Fall 2012 Slide 72
chanistic 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 CEM 494, Spring 2010 Slide 73
chanistic 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 CEM 494, Spring 2010 Slide 74