8Chapter 05

8Chapter 05 - Chapter 5 ALKENES 5.1 5.1 Alkene Nomenclature...

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Unformatted text preview: Chapter 5 ALKENES 5.1 5.1 Alkene Nomenclature Alkenes Alkenes Alkenes Alkenes are hydrocarbons that contain a Alkenes carbon-carbon double bond carbon-carbon also called "olefins" characterized by molecular formula CnH2n said to be "unsaturated" said Alkene Nomenclature Alkene Nomenclature Alkene H2C CH2 Ethene or Ethylene (both are acceptable IUPAC names) H2C CHCH3 Propene (Propylene is sometimes used sometimes but is not an acceptable but IUPAC name) Alkene Nomenclature Alkene Nomenclature Alkene H2C CHCH2CH3 1-Butene 1) Find the longest continuous chain that 1) includes the double bond. includes 2) Replace the -ane ending of the unbranched alkane having the same number of carbons by -ene. by 3) Number the chain in the direction that gives 3) the lowest number to the doubly bonded carbon. carbon. Alkene Nomenclature Alkene Nomenclature Alkene H2CCHCHCH2Br CH3 4) If a substituent is present, identify its position 4) by number. The double bond takes precedence over alkyl groups and halogens when the chain is numbered. when The compound shown above is 4-bromo-3-methyl-1-butene. Alkene Nomenclature Alkene Nomenclature Alkene H2C CHCHCH2OH CH3 4) If a substituent is present, identify its position 4) by number. Hydroxyl groups take Hydroxyl precedence over the double bond when the chain is numbered. chain The compound shown above is 2-methyl-3-buten-1-ol. Alkenyl groups Alkenyl groups Alkenyl methylene H2C vinyl H2C CH allyl H2C CHCH2 isopropenyl H2C CCH3 Cycloalkene Nomenclature Cycloalkene Nomenclature Cycloalkene Cyclohexene 1) Replace the -ane ending of the cycloalkane having the same number of carbons by -ene. having Cycloalkene Nomenclature Cycloalkene Nomenclature Cycloalkene CH3 CH2CH3 1) Replace the -ane ending of the cycloalkane having the same number of carbons by -ene. having 2) Number through the double bond in the 2) through direction that gives the lower number to the direction first-appearing substituent. first-appearing Cycloalkene Nomenclature Cycloalkene Nomenclature Cycloalkene CH3 6-Ethyl-1-methylcyclohexene CH2CH3 1) Replace the -ane ending of the cycloalkane having the same number of carbons by -ene. having 2) Number through the double bond in the 2) through direction that gives the lower number to the direction first-appearing substituent. first-appearing 5.2 5.2 Structure and Bonding in Structure Alkenes Alkenes Structure of Ethylene Structure of Ethylene Structure bond angles: bond H-C-H = 117° H-C-H H-C-C = 121° bond distances: bond pm pm C—H = 110 C=C = 134 pm planar Bonding in Ethylene σ σ σ σ σ • Framework of σ bonds • Each carbon is sp2 hybridized Bonding in Ethylene • Each carbon has a half-filled p orbital Bonding in Ethylene • Side-by-side overlap of halffilled p orbitals gives a π bond 5.3 5.3 Isomerism in Alkenes Isomers Isomers Isomers Isomers are different compounds that have the same molecular formula. Isomers Isomers Isomers Constitutional isomers Constitutional isomers Stereoisomers Stereoisomers Isomers Isomers Isomers Constitutional isomers Constitutional isomers different connectivity Stereoisomers Stereoisomers same connectivity; different arrangement of atoms in space Isomers Isomers Isomers Constitutional isomers Constitutional isomers Stereoisomers Stereoisomers consider the isomeric alkenes of consider molecular formula C4H8 molecular H CH2CH3 C H 1-Butene H3C H H C C H C H3C H 2-Methylpropene 2-Methylpropene CH3 C H3C H3C H C C H cis-2-Butene H C CH3 trans-2-Butene H CH2CH3 C H 1-Butene H3C H C C H H3C C H 2-Methylpropene 2-Methylpropene CH3 C H H3C C Constitutional isomers H cis-2-Butene H CH2CH3 C H3C C C H H H 1-Butene C H3C H 2-Methylpropene 2-Methylpropene H3C H C Constitutional isomers H C CH3 trans-2-Butene Stereoisomers H3C CH3 C H H3C H C C H cis-2-Butene cis- H C CH3 trans-2-Butene Stereochemical Notation Stereochemical Notation Stereochemical cis (identical or cis analogous substituents on same side) on trans (identical or trans analogous substitutents on opposite sides) on Figure 5.2 Figure 5.2 Figure Interconversion of stereoisomeric alkenes does not normally occur. Requires that π component of double Requires component bond be broken. cis trans Figure 5.2 Figure 5.2 Figure cis trans 5.4 5.4 Naming Stereoisomeric Naming Alkenes by the E-Z Notational E-Z System System Stereochemical Notation Stereochemical Notation Stereochemical CH2(CH2)6CO2H CH3(CH2)6CH2 C H C Oleic acid H cis and trans are useful when substituents are cis identical or analogous (oleic acid has a cis double bond) double cis and trans are ambiguous when analogies cis are not obvious are Cl Example Example Example Br C H C F What is needed: 1) 1) 2) systematic body of rules for ranking substituents substituents new set of stereochemical symbols other new than cis and trans The E-Z Notational System The E-Z Notational System E : higher ranked substituents on opposite sides higher opposite Z : higher ranked substituents on same side higher same higher higher C lower C The E-Z Notational System The E-Z Notational System E : higher ranked substituents on opposite sides higher opposite Z : higher ranked substituents on same side higher same lower C C higher higher The E-Z Notational System The E-Z Notational System E : higher ranked substituents on opposite sides higher opposite Z : higher ranked substituents on same side higher same higher C lower lower C higher Entgegen Entgegen The E-Z Notational System The E-Z Notational System E : higher ranked substituents on opposite sides higher opposite Z : higher ranked substituents on same side higher same higher C lower lower C higher Entgegen Entgegen higher C lower higher C lower Zusammen The E-Z Notational System The E-Z Notational System Question: How are substituents ranked? Answer: Answer: higher C lower They are ranked in order of decreasing atomic number. lower C higher Entgegen higher C lower higher C lower Zusammen The Cahn-Ingold-Prelog (CIP) System The Cahn-Ingold-Prelog (CIP) System The The system that we use was devised by R. S. Cahn Sir Christopher Ingold Vladimir Prelog Their rules for ranking groups were devised in Their connection with a different kind of stereochemistry—one that we will discuss in Chapter 7—but have been adapted to alkene stereochemistry. stereochemistry. Table 5.1 CIP Rules Table 5.1 CIP Rules (1) Higher atomic number outranks lower (1) Higher atomic number atomic Br > F Cl > H Cl higher Br C lower F higher H lower C Table 5.1 CIP Rules Table 5.1 CIP Rules (1) Higher atomic number outranks lower (1) Higher atomic number atomic Br > F Cl > H Cl higher Br C lower F higher H lower C (Z )-1-Bromo-2-chloro-1-fluoroethene )-1-Bromo-2-chloro-1-fluoroethene Table 5.1 CIP Rules Table 5.1 CIP Rules (2) When two atoms are identical, compare the atoms attached to them on the basis of their atomic numbers. Precedence is established at the first point of difference. —CH2CH3 outranks —CH3 —C(C,H,H) —C(H,H,H) Table 5.1 CIP Rules Table 5.1 CIP Rules (3) Work outward from the point of attachment, comparing all the atoms attached to a particular atom before proceeding further particular along the chain. along —CH(CH3)2 outranks —CH2CH2OH —C —C(C,C,H) —C(C,H,H) Table 5.1 CIP Rules Table 5.1 CIP Rules (4) Evaluate substituents one by one. Don't add atomic numbers within groups. Don't —CH2OH outranks —C(CH3)3 outranks —C —C(O,H,H) —C(C,C,C) Table 5.1 CIP Rules Table 5.1 CIP Rules (5) An atom that is multiply bonded to another (5) An atom is considered to be replicated as a substituent on that atom. substituent —CH=O outranks —CH2OH —C —C(O,O,H) —C(O,H,H) (A table of commonly encountered substituents ranked according to (A precedence is given on the inside back cover of the text.) precedence 5.5 5.5 Physical Properties of Alkenes Dipole moments Dipole moments H What is direction of What dipole moment? dipole Does a methyl group Does donate electrons to the double bond, or does it withdraw them? withdraw H C C H H µ=0D H3C H C H C H µ = 0.3 D 0.3 µ = 1.4 D 1.4 H H C H H C C H Cl C H H µ=0D H3C H C H Dipole moments Dipole moments C H µ = 0.3 D 0.3 Chlorine is Chlorine electronegative and attracts electrons. electrons. µ = 1.4 D 1.4 H Dipole moment H Dipole of 1of CC chloropropene chloropropene is equal to the H Cl sum of the dipole moments of vinyl chloride H C H 3 and propene. and CC H H µ = 0.3 D 0.3 Dipole moments Dipole moments H3C H C C Cl H µ = 1.7 D µ = 1.4 D 1.4 H H C Therefore, a Therefore, methyl group donates electrons to the double bond. bond. Dipole moments Dipole moments C H Cl H3C H C H H3C H C H C C H µ = 0.3 D 0.3 µ = 1.7 D 1.7 Cl Alkyl groups stabilize spp2hybridized Alkyl groups stabilizess 2 hybridized p Alkyl sp ccarbonby releasing electrons arbon by releasing electrons R—C+ is more stable than H—C+ R—C . is more stable than H—C . R—C is more stable than H—C 5.6 5.6 Relative Stabilities of Alkenes Double bonds are classified according to Double bonds are classified according to Double the number of carbons attached to them. the number of carbons attached to them. H R C C H H H R C R' monosubstituted C H disubstituted R' R C H H R C C H disubstituted H C R' disubstituted Double bonds are classified according to Double bonds are classified according to Double the number of carbons attached to them. the number of carbons attached to them. R" R C R' R" R C C H trisubstituted R' C R"' tetrasubstituted Substituent effects on alkene stability Substituent effects on alkene stability Substituent Electronic disubstituted alkenes are more stable disubstituted than monosubstituted alkenes than Steric trans alkenes are more stable than cis alkenes cis Fig. 5.4 Heats of Fig. combustion of C4H8 combustion 8 isomers. 2717 kJ/mol 2717 kJ/mol + 6O2 6O 2710 kJ/mol 2710 kJ/mol 2707 kJ/mol 2707 kJ/mol 2700 kJ/mol 2700 kJ/mol 4CO2 + 8H2O Substituent effects on alkene stability Substituent effects on alkene stability Substituent Electronic alkyl groups stabilize double bonds more than alkyl H more highly substituted double bonds are more stable than less highly substituted ones. Problem 5.8 Problem 5.8 Problem Give the structure or make a molecular model of Give the most stable C6H12 alkene. the C C Problem 5.8 Problem 5.8 Problem Give the structure or make a molecular model of Give the most stable C6H12 alkene. the H3C CH3 C H3C C CH3 Substituent effects on alkene stability Substituent effects on alkene stability Substituent Steric effects trans alkenes are more stable than cis alkenes cis cis alkenes are destabilized by van der Waals strain strain van der Waals strain due to crowding of cis-methyl groups cis-2-butene cis Figure 5.5 Figure cis and trans-2-Butene cis trans-2-butene van der Waals strain due to crowding of cis-methyl groups cis-2-butene Fig. 5.5 Fig. 5.5 Fig. cis and trans-2-butene cis and trans-2-butene trans-2-butene Van der Waals Strain Van der Waals Strain Van Steric effect causes a large difference in stability between cis and trans-(CH3)3CCH=CHC(CH3)3 between cis trans cis is 44 kJ/mol less stable than trans trans CH3 H3C H3C H3C C C C H CH3 C H CH3 5.7 5.7 Cycloalkenes Cycloalkenes Cycloalkenes Cycloalkenes Cyclopropene and cyclobutene have angle strain. strain. Larger cycloalkenes, such as cyclopentene and cyclohexene, can incorporate a double and bond into the ring with little or no angle strain. Stereoisomeric cycloalkenes Stereoisomeric cycloalkenes cis-cyclooctene and trans-cyclooctene cis-cyclooctene trans are stereoisomers cis-cyclooctene is 39 kJ/ mol more stable than trans-cyclooctene than trans H H cis-Cyclooctene cis H H trans-Cyclooctene Stereoisomeric cycloalkenes Stereoisomeric cycloalkenes trans-cyclooctene is smallest trans-cycloalkene trans-cyclooctene trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans trans through C11 cycloalkenes 11 cycloalkenes cis and trans-cyclododecene are approximately trans-cyclododecene equal in stability H H trans-Cyclooctene Stereoisomeric cycloalkenes Stereoisomeric cycloalkenes Stereoisomeric trans-cyclooctene is smallest trans-cycloalkene trans-cyclooctene trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans trans through C11 cycloalkenes 11 cycloalkenes cis and trans-cyclododecene are approximately trans-cyclododecene equal in stability cis-Cyclododecene trans-Cyclododecene Stereoisomeric cycloalkenes Stereoisomeric cycloalkenes trans-cyclooctene is smallest trans-cycloalkene trans-cyclooctene trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans trans through C11 cycloalkenes 11 cycloalkenes cis and trans-cyclododecene are approximately trans-cyclododecene equal in stability When there are more than 12 carbons in the ring, trans-cycloalkenes are more stable than cis. ring, trans-cycloalkenes cis The ring is large enough so the cycloalkene behaves much like a noncyclic one. 5.8 Preparation of Alkenes: Elimination Reactions β-Elimination Reactions Overview β-Elimination Reactions Overview •dehydrogenation of alkanes: β H; Y = H •dehydration of alcohols: β H; Y = OH •dehydrohalogenation of alkyl halides: β H; Y = Br, etc. H β C C α Y C C +H Y Dehydrogenation Dehydrogenation Dehydrogenation • limited to industrial syntheses of ethylene, propene, 1,3-butadiene, and styrene • important economically, but rarely used in laboratory-scale syntheses CH3CH3 CH3CH2CH3 750°C 750°C H2C CH2 + H2 H2C CHCH3 + H2 5.9 5.9 Dehydration of Alcohols Dehydration of Alcohols Dehydration of Alcohols Dehydration CH3CH2OH OH H2SO4 160°C CH2 + H2O + H2C H2O H2SO4 140°C (79-87%) CH3 H3C C CH3 OH H2SO4 heat H3C C H3C CH2 (82%) + H2O R' Relative Reactivity R C OH tertiary: most reactive R" R' R C OH H H R C H OH primary: least reactive 5.10 5.10 Regioselectivity in Alcohol Dehydration: The Zaitsev Rule Regioselectivity Regioselectivity H2SO4 HO HO + 80°C 10 % 10 • 90 % A reaction that can proceed in more than one direction, but in which one direction predominates, is said to be regioselective. Regioselectivity Regioselectivity CH3 CH CH3 OH H3PO4 CH3 CH + heat 84 % • 16 % A reaction that can proceed in more than one direction, but in which one direction predominates, is said to be regioselective. The Zaitsev Rule The Zaitsev Rule The • When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the β carbon having the R OH fewest hydrogens. R CH2R C C H CH3 three protons on this β carbon three The Zaitsev Rule The Zaitsev Rule The • When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the β carbon having the R OH fewest hydrogens. R CH2R C C H CH3 two protons on this β carbon two The Zaitsev Rule The Zaitsev Rule The • When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the β carbon having the R OH fewest hydrogens. R CH2R C C H CH3 only one proton on this β carbon only The Zaitsev Rule The Zaitsev Rule The • When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the β carbon having the R CH2R R OH fewest hydrogens. R C CH2R C C H CH3 R C CH3 only one proton on this β carbon only The Zaitsev Rule The Zaitsev Rule The Zaitsev Rule states that the elimination Zaitsev reaction yields the more highly substituted reaction alkene as the major product. The more stable alkene product The predominates. predominates. 5.11 5.11 Stereoselectivity in Alcohol Dehydration Alcohol Stereoselectivity Stereoselectivity Stereoselectivity H2SO4 + heat OH (25%) (25%) (75%) • A stereoselective reaction is one in which a single starting material can yield two or more stereoisomeric products, but gives one of 5.12 5.12 The Mechanism of the The Acid-Catalyzed Dehydration of Alcohols Alcohols A connecting point... A connecting point... • The dehydration of alcohols and the reaction of alcohols with hydrogen halides share the following common features: • 1) Both reactions are promoted by acids • 2) The relative reactivity decreases in the order tertiary > secondary > primary These similarities suggest that carbocations are intermediates in the acid-catalyzed dehydration of alcohols, just as they are in Dehydration of tert-Butyl Alcohol Dehydration of tert-Butyl Alcohol CH3 H3C C CH3 OH H2SO4 heat H3C C CH2 + H2O H3C •first two steps of mechanism are identical to those for the reaction of tert-butyl alcohol with hydrogen halides Mechanism Mechanism Step 1: Proton transfer to tert-butyl alcohol Step tert-butyl H .. (CH3)3C O : + H O + .. H H fast, bimolecular fast, H + (CH3)3C O : H + H tert-Butyloxonium ion :O: H Mechanism Mechanism Step 2: Dissociation of tert-butyloxonium ion Step tert to carbocation to H + (CH3)3C O : H slow, unimolecular H (CH3)3C + (CH + tert-Butyl cation :O: H Mechanism Mechanism Step 3: Deprotonation of tert-butyl cation. Step tert H H3C +C H + :O: H CH2 H3C fast, bimolecular H H3C C H3C CH2 + H + O: H Carbocations Carbocations Carbocations are intermediates in the acid-catalyzed dehydration of tertiary and secondary alcohols Carbocations can: •react with nucleophiles •lose a β-proton to form an alkene (Called an E1 mechanism) Dehydration of primary alcohols Dehydration of primary alcohols Dehydration CH3CH2OH H2SO4 160°C H2C CH2 + H2O •A different mechanism from 3 o or 2 o alcohols •avoids carbocation because primary carbocations are too unstable •oxonium ion loses water and a proton in a bimolecular step Mechanism Step 1: Proton transfer from acid to ethanol Proton H .. CH3CH2 O : + H O .. .. H H fast, bimolecular H + CH3CH2 O : H Ethyloxonium ion H + :O: H Mechanism Step 2: Oxonium ion loses both a proton and a water molecule in the same step. water H H + : O : + H CH2 CH2 O : H H slow, bimolecular H + :O H H H + H2C CH2 + :O: H Mechanism Step 2: Oxonium ion loses both a proton and a water molecule in the same step. water H H + : O : + H CH2 CH2 O : H + :O H H H Because rate-determining step is bimolecular, this slow, bimolecular is called the E2 mechanism. H H + H2C CH2 + :O: H 5.13 5.13 Rearrangements in Alcohol Rearrangements Dehydration Dehydration Sometimes the alkene product does not have the same carbon skeleton as the starting alcohol. Example Example OH OH H3PO4, heat + 3% + 64% 33% Rearrangement involves alkyl group migration Rearrangement involves alkyl group migration CH3 CH3 C CHCH3 + CH3 CH 3% 3% • carbocation can lose a proton as shown • or it can undergo a methyl migration • CH3 group migrates with its pair of electrons to adjacent positively charged carbon Rearrangement involves alkyl group migration Rearrangement involves alkyl group migration CH3 CH3 CH3 C CHCH3 + 97% CH3 + C CHCH3 CH3 CH3 CH 3% 3% • tertiary carbocation; more stable Rearrangement involves alkyl group migration Rearrangement involves alkyl group migration CH3 CH3 CH3 C CHCH3 + 97% CH3 + C CH3 CH3 CH 3% 3% CHCH3 Another rearrangement Another rearrangement Another CH3CH2CH2CH2O H H3PO4, heat CH3CH2CH 12% CH2 + CH3CH CHCH3 mixture of cis (32%) mixture cis and trans-2-butene (56%) and trans Rearrangement involves hydride shift Rearrangement involves hydride shift Rearrangement CH3CH2CH2CH2 H + O: H CH3CH2CH CH2 oxonium ion can lose water and a proton (from C-2) to give1-butene doesn't give a carbocation directly because primary carbocations are too unstable Rearrangement involves hydride shift Rearrangement involves hydride shift Rearrangement CH3CH2CH2CH2 CH H + O: H CH3CH2CH CH2 CH3CH2CHCH3 + hydrogen migrates with its pair of electrons from C-2 to C-1 as water is lost carbocation formed by hydride shift is secondary Hydride shift Hydride shift Hydride H CH3CH2CHCH2 + O: H H + CH3CH2CHCH2 + H H : O: H Rearrangement involves hydride shift Rearrangement involves hydride shift Rearrangement CH3CH2CH2CH2 H + O: H CH3CH2CH CH2 CH3CH2CHCH3 + CH3CH CHCH3 mixture of cis mixture cis and trans-2-butene and trans Carbocations can... Carbocations can... Carbocations •react with nucleophiles •lose a proton from the β-carbon to form an alkene •rearrange (less stable to more stable) (alkyl shift or hydride shift) 5.14 Dehydrohalogenation of Alkyl Halides β-Elimination Reactions Overview β-Elimination Reactions Overview •dehydrogenation of alkanes: β H; Y = H •dehydration of alcohols: β H; Y = OH •dehydrohalogenation of alkyl halides: β H; Y = Br, etc. H C β CY α C C +H Y β-Elimination Reactions Overview β-Elimination Reactions Overview •dehydrogenation of alkanes: industrial process; not regioselective •dehydration of alcohols: acid-catalyzed •dehydrohalogenation of alkyl halides: consumes base H C β CY α C C +H Y Dehydrohalogenation Dehydrohalogenation Dehydrohalogenation A useful method for the preparation of alkenes Cl NaOCH2CH3 ethanol, 55°C (100 %) likewise, NaOCH3 in methanol, or KOH in ethanol Dehydrohalogenation Dehydrohalogenation When the alkyl halide is primary, potassium tert-butoxide in dimethyl sulfoxide is the base/solvent system that is normally used. KOC(CH3)3 KOC(CH CH3(CH2)15CH2CH2Cl CH dimethyl sulfoxide CH3(CH2)15CH (86%) CH2 Regioselectivity Regioselectivity KOCH2CH3 Br Br + ethanol, 70°C 29 % 71 % follows Zaitsev's rule: more highly substituted double bond predominates Stereoselectivity Stereoselectivity KOCH2CH3 KOCH ethanol Br Br + (23%) (77%) •more stable configuration of double bond predominates Stereoselectivity Stereoselectivity Br KOCH2CH3 ethanol + (85%) (15%) •more stable configuration of double bond 5.15 5.15 Mechanism of the Mechanism Dehydrohalogenation of Alkyl Halides: The E2 Mechanism The Facts Facts • (1) Dehydrohalogenation of alkyl halides exhibits second-order kinetics first order in alkyl halide first order in base rate = k[alkyl halide][base] implies that rate-determining step involves both base and alkyl halide; i.e., it is bimolecular (second-order) Facts Facts • (2) Rate of elimination depends on halogen weaker C—X bond; faster rate rate: RI > RBr > RCl > RF implies that carbon-halogen bond breaks in the rate-determining step The E2 Mechanism The E2 Mechanism The •concerted (one-step) bimolecular process •single transition state C—H bond breaks π component of double bond forms C—X bond breaks The E2 Mechanism The E2 Mechanism The R .. – O: .. H C C : X: .. Reactants The E2 Mechanism The E2 Mechanism The R .. – O: .. H C C : X: .. Reactants Reactants The E2 Mechanism The E2 Mechanism R Transition state δ– .. O .. H C C δ– : X: .. The E2 Mechanism The E2 Mechanism The R .. O .. H C C .. – : X: .. Products 5.16 & 5.17 5.16 Anti Elimination in E2 Anti Reactions Reactions Stereoelectronic Effects Isotope Effects Stereoelectronic effect Br KOC(CH3)3 (CH3)3COH (CH3)3C (CH cis-1-Bromo-4-tertbutylcyclohexane butylcyclohexane (CH3)3C (CH Stereoelectronic effect (CH3)3C (CH trans-1-Bromo-4-tertbutylcyclohexane butylcyclohexane Br (CH3)3C (CH KOC(CH3)3 (CH3)3COH Stereoelectronic effect cis Br KOC(CH3)3 (CH3)3COH (CH3)3C (CH Rate constant for dehydrohalogenation of cis is 500 times greater than that of trans (CH3)3C (CH Br (CH3)3C (CH trans KOC(CH3)3 (CH3)3COH Stereoelectronic effect cis Br KOC(CH3)3 (CH3)3COH (CH3)3C (CH HH (CH3)3C (CH H that is removed by base must be anti periplanar to Br Two anti periplanar H atoms in cis stereoisomer Stereoelectronic effect trans H Br H (CH3)3C (CH KOC(CH3)3 (CH3)3COH HH (CH3)3C (CH H that is removed by base must be anti periplanar to Br No anti periplanar H atoms in trans stereoisomer; all vicinal H atoms are gauche to Stereoelectronic effect cis cis more reactive trans less reactive Stereoelectronic effect Stereoelectronic An effect on reactivity that has its origin in the spatial arrangement of orbitals or bonds is called a stereoelectronic effect. The preference for an anti periplanar arrangement of H and Br in the transition state for E2 dehydrohalogenation is an example of a stereoelectronic effect. Isotope effect Isotope Deuterium,D, is a heavy isotope of hydrogen but will undergo the same reactions. But the C-D bond is stronger so a reaction where the rate involves breaking a C-H (C-D) bond, the deuterated sample will have a reaction rate 3-8 times slower. In other words comparing the two rates, i.e. kH/kD = 3-8 WHEN the rate determining step involves breaking the C-H bond. 5.18 5.18 A Different Mechanism for Different Alkyl Halide Elimination: Alkyl The E1 Mechanism Example Example Example CH3 CH3 CH2CH3 C Br Ethanol, heat H3C CH3 H2C + C CH2CH3 (25%) H C C CH3 H3C (75%) The E1 Mechanism The E1 Mechanism 1. Alkyl halides can undergo elimination in absence of base. 2. Carbocation is intermediate 3. Rate-determining step is unimolecular ionization of alkyl halide. 4. Generally with tertiary halide, base is weak and at low concentration. CH3 Step 1 Step 1 Step CH3 CH2CH3 C : Br: .. slow, unimolecular CH3 C CH3 + CH2CH3 .. – : Br : .. CH3 Step 2 Step 2 Step CH3 C + CH2CH3 – H+ CH3 CH2 CH3 C + CH2CH3 CH3 C CHCH3 End of Chapter 5 ...
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