Dr. Wolf's CHM 201 & Chapter 5 ALKENES

Dr. Wolf's CHM 201 & Alkene Nomenclature

Dr. Wolf's CHM 201 & AlkenesAlkenes Alkenes are hydrocarbons that contain a carbon-carbon double bond also called "olefins" characterized by molecular formula C n H 2n said to be "unsaturated"

Dr. Wolf's CHM 201 & Alkene Nomenclature H2CH2CH2CH2C CH 2 H2CH2CH2CH2C CHCH 3 Etheneor Ethylene (both are acceptable IUPAC names) Propene (Propylene is sometimes used but is not an acceptable IUPAC name)

Dr. Wolf's CHM 201 & Alkene Nomenclature 1) Find the longest continuous chain that includes the double bond. 2) Replace the -ane ending of the unbranched alkane having the same number of carbons by -ene. 3) Number the chain in the direction that gives the lowest number to the doubly bonded carbon. H2CH2CH2CH2C CHCH 2 CH 3 1-Butene

Dr. Wolf's CHM 201 & Alkene Nomenclature 4) If a substituent is present, identify its position by number. The double bond takes precedence over alkyl groups and halogens when the chain is numbered. The compound shown above is 4-bromo-3-methyl-1-butene. H2CH2CH2CH2C CHCHCH 2 Br CH 3

Dr. Wolf's CHM 201 & Alkene Nomenclature 4) If a substituent is present, identify its position by number. Hydroxyl groups take precedence over the double bond when the chain is numbered. The compound shown above is 2-methyl-3-buten-1-ol. H2CH2CH2CH2C CHCHCH 2 OH CH 3

Dr. Wolf's CHM 201 & Alkenyl groups methylenevinylallylisopropenyl CH H2CH2CH2CH2C H2CH2CH2CH2C CHCH 2 H2CH2CH2CH2C CCH 3 H2CH2CH2CH2C

Dr. Wolf's CHM 201 & Cycloalkene Nomenclature 1) Replace the -ane ending of the cycloalkane having the same number of carbons by -ene. Cyclohexene

Dr. Wolf's CHM 201 & Cycloalkene Nomenclature 1) Replace the -ane ending of the cycloalkane having the same number of carbons by -ene. 2) Number through the double bond in the direction that gives the lower number to the first-appearing substituent. CH 3 CH 2 CH 3

Dr. Wolf's CHM 201 & Cycloalkene Nomenclature 1) Replace the -ane ending of the cycloalkane having the same number of carbons by -ene. 2) Number through the double bond in the direction that gives the lower number to the first-appearing substituent. 6-Ethyl-1-methylcyclohexene CH 3 CH 2 CH 3

Dr. Wolf's CHM 201 & Structure and Bonding in Alkenes

Dr. Wolf's CHM 201 & Structure of Ethylene bond angles: H-C-H = 117° H-C-C = 121° bond distances: C—H = 110 pm C=C = 134 pm planar

Dr. Wolf's CHM 201 &      Framework of  bonds Each carbon is sp 2 hybridized Bonding in Ethylene

Dr. Wolf's CHM 201 & Each carbon has a half-filled p orbital Bonding in Ethylene

Dr. Wolf's CHM 201 & Side-by-side overlap of half- filled p orbitals gives a  bond Bonding in Ethylene

Dr. Wolf's CHM 201 & Isomerism in Alkenes

Dr. Wolf's CHM 201 & IsomersIsomers Isomers are different compounds that have the same molecular formula.

Dr. Wolf's CHM 201 & IsomersIsomers StereoisomersStereoisomers Constitutional isomers

Dr. Wolf's CHM 201 & IsomersIsomers StereoisomersStereoisomers Constitutional isomers different connectivity same connectivity; different arrangement of atoms in space

Dr. Wolf's CHM 201 & IsomersIsomers StereoisomersStereoisomers Constitutional isomers consider the isomeric alkenes of molecular formula C 4 H 8

Dr. Wolf's CHM 201 & Methylpropene 1-Butene cis-2-Butene trans-2-Butene C CHH H CH 2 CH 3 H3CH3CH3CH3C C C CH 3 HH H C C H3CH3CH3CH3CH C C H H H3CH3CH3CH3C H3CH3CH3CH3C

Dr. Wolf's CHM 201 & Methylpropene 1-Butene cis-2-Butene C CHH H CH 2 CH 3 H CH 3 C C H3CH3CH3CH3CH C C H H H3CH3CH3CH3C H3CH3CH3CH3C Constitutional isomers

Dr. Wolf's CHM 201 & Methylpropene 1-Butene trans-2-Butene C CHH H CH 2 CH 3 H3CH3CH3CH3C C C CH 3 HH C C H H H3CH3CH3CH3C H3CH3CH3CH3C Constitutional isomers

Dr. Wolf's CHM 201 & cis-2-Butene trans-2-Butene H3CH3CH3CH3C C C CH 3 HH H C C H3CH3CH3CH3CH Stereoisomers

Dr. Wolf's CHM 201 & Stereochemical Notation cis (identical or analogous substituents on same side) trans (identical or analogous substitutents on opposite sides)

Dr. Wolf's CHM 201 & Figure 5.2 trans cis Interconversion of stereoisomeric alkenes does not normally occur. Requires that  component of double bond be broken.

Dr. Wolf's CHM 201 & Figure 5.2 trans cis

Dr. Wolf's CHM 201 & Naming Stereoisomeric Alkenes by the E-Z Notational System

Dr. Wolf's CHM 201 & Stereochemical Notation cis and trans are useful when substituents are identical or analogous (oleic acid has a cis double bond) cis and trans are ambiguous when analogies are not obvious C C CH 3 (CH 2 ) 6 CH 2 CH 2 (CH 2 ) 6 CO 2 H H H Oleic acid

Dr. Wolf's CHM 201 & ExampleExample What is needed: 1) systematic body of rules for ranking substituents 2) new set of stereochemical symbols other than cis and trans C C H FClBr

Dr. Wolf's CHM 201 & C C The E-Z Notational System E :higher ranked substituents on opposite sides Z :higher ranked substituents on same side higher lower

Dr. Wolf's CHM 201 & C C The E-Z Notational System E :higher ranked substituents on opposite sides Z :higher ranked substituents on same side higher lower

Dr. Wolf's CHM 201 & C C The E-Z Notational System E :higher ranked substituents on opposite sides Z :higher ranked substituents on same side Entgegen higher higherlower lower

Dr. Wolf's CHM 201 & C C C C The E-Z Notational System E :higher ranked substituents on opposite sides Z :higher ranked substituents on same side EntgegenZusammen higher higherlower lower lower higher lower higher

Dr. Wolf's CHM 201 & C C C C The E-Z Notational System Answer: They are ranked in order of decreasing atomic number. EntgegenZusammen higher higherlower lower lower higher lower higher Question: How are substituents ranked?

Dr. Wolf's CHM 201 & The Cahn-Ingold-Prelog (CIP) System The system that we use was devised by R. S. Cahn Sir Christopher Ingold Vladimir Prelog Their rules for ranking groups were devised in connection with a different kind of stereochemistry—one that we will discuss in Chapter 7—but have been adapted to alkene stereochemistry.

Dr. Wolf's CHM 201 & (1)Higher atomic number outranks lower atomic number Br > FCl > H higher lower Br F Cl Hhigherlower C C Table 5.1 CIP Rules

Dr. Wolf's CHM 201 & (1)Higher atomic number outranks lower atomic number Br > FCl > H (Z )-1-Bromo-2-chloro-1-fluoroethene higher lower Br F Cl Hhigherlower C C Table 5.1 CIP Rules

Dr. Wolf's CHM 201 & (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. —CH 2 CH 3 outranks —CH 3 —C(C,H,H) Table 5.1 CIP Rules —C(H,H,H)

Dr. Wolf's CHM 201 & (3) Work outward from the point of attachment, comparing all the atoms attached to a particular atom before proceeding further along the chain. —CH(CH 3 ) 2 outranks —CH 2 CH 2 OH —C(C,C,H) —C(C,H,H) Table 5.1 CIP Rules

Dr. Wolf's CHM 201 & (4) Evaluate substituents one by one. Don't add atomic numbers within groups. —CH 2 OH outranks —C(CH 3 ) 3 —C(O,H,H) —C(C,C,C) Table 5.1 CIP Rules

Dr. Wolf's CHM 201 & (5)An atom that is multiply bonded to another atom is considered to be replicated as a substituent on that atom. —CH=O outranks —CH 2 OH —C(O,O,H) —C(O,H,H) Table 5.1 CIP Rules (A table of commonly encountered substituents ranked according to precedence is given on the inside back cover of the text.)

Dr. Wolf's CHM 201 & Physical Properties of Alkenes

Dr. Wolf's CHM 201 &  = 0 D C CHHHH  = 0.3 D H H H C C H3CH3CH3CH3C Dipole moments What is direction of dipole moment? Does a methyl group donate electrons to the double bond, or does it withdraw them?

Dr. Wolf's CHM 201 &  = 0 D C CHHHH  = 1.4 D C C HH ClH  = 0.3 D H H H C C H3CH3CH3CH3C Dipole moments Chlorine is electronegative and attracts electrons.

Dr. Wolf's CHM 201 &  = 1.4 D C C HH ClH  = 0.3 D H H H C C H3CH3CH3CH3C = 1.7 D H H Cl C C H3CH3CH3CH3C Dipole moments Dipole moment of 1- chloropropene is equal to the sum of the dipole moments of vinyl chloride and propene. 

Dr. Wolf's CHM 201 &  = 1.7 D  = 1.4 D C C HH ClH  = 0.3 D H H H C C H3CH3CH3CH3C H H Cl C C H3CH3CH3CH3C Dipole moments Therefore, a methyl group donates electrons to the double bond.

Dr. Wolf's CHM 201 & Alkyl groups stabilize sp 2 hybridized carbon by releasing electrons.. R—C+ H—C+ is more stable than R—C H—C is more stable than R—C is more stable than H—C

Dr. Wolf's CHM 201 & Relative Stabilities of Alkenes

Dr. Wolf's CHM 201 & Double bonds are classified according to the number of carbons attached to them. H C C R HHmonosubstituted R' C C R HHdisubstituted H C C R H R' disubstituted H C C R H R' disubstituted

Dr. Wolf's CHM 201 & Double bonds are classified according to the number of carbons attached to them. R' C C R HR"trisubstituted R' C C R R"'R"tetrasubstituted

Dr. Wolf's CHM 201 & Substituent effects on alkene stability Electronic disubstituted alkenes are more stable than monosubstituted alkenes Steric trans alkenes are more stable than cis alkenes

Dr. Wolf's CHM 201 & O 2 4CO 2 + 8H 2 O 2700 kJ/mol 2707 kJ/mol 2717 kJ/mol 2710 kJ/mol Fig. 5.4 Heats of combustion of C 4 H 8 isomers.

Dr. Wolf's CHM 201 & Substituent effects on alkene stability Electronic alkyl groups stabilize double bonds more than H more highly substituted double bonds are more stable than less highly substituted ones.

Dr. Wolf's CHM 201 & Problem 5.8 Give the structure or make a molecular model of the most stable C 6 H 12 alkene. C C

Dr. Wolf's CHM 201 & Problem 5.8 Give the structure or make a molecular model of the most stable C 6 H 12 alkene. C C H3CH3CH3CH3C H3CH3CH3CH3C CH 3

Dr. Wolf's CHM 201 & Substituent effects on alkene stability Steric effects trans alkenes are more stable than cis alkenes cis alkenes are destabilized by van der Waals strain

Dr. Wolf's CHM 201 & cis-2-butene trans-2-butene van der Waals strain due to crowding of cis-methyl groups Figure 5.5 cis and trans-2-Butene

Dr. Wolf's CHM 201 & Fig. 5.5 cis and trans-2-butene cis-2-butene trans-2-butene van der Waals strain due to crowding of cis-methyl groups

Dr. Wolf's CHM 201 & Van der Waals Strain Steric effect causes a large difference in stability between cis and trans-(CH 3 ) 3 CCH=CHC(CH 3 ) 3 cis is 44 kJ/mol less stable than trans C C HH CC CH 3 H3CH3CH3CH3C H3CH3CH3CH3C H3CH3CH3CH3C

Dr. Wolf's CHM 201 & Cycloalkenes

Dr. Wolf's CHM 201 & Cyclopropene and cyclobutene have angle strain. Larger cycloalkenes, such as cyclopentene and cyclohexene, can incorporate a double bond into the ring with little or no angle strain. CycloalkenesCycloalkenes

Dr. Wolf's CHM 201 & cis-cyclooctene and trans-cyclooctene are stereoisomers cis-cyclooctene is 39 kJ/ mol more stable than trans-cyclooctene Stereoisomeric cycloalkenes cis-Cyclooctene trans-Cyclooctene H HHH

Dr. Wolf's CHM 201 & trans-cyclooctene is smallest trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans through C 11 cycloalkenes cis and trans-cyclododecene are approximately equal in stability Stereoisomeric cycloalkenes trans-Cyclooctene HH

Dr. Wolf's CHM 201 & Stereoisomeric cycloalkenes trans-Cyclododecene cis-Cyclododecene trans-cyclooctene is smallest trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans through C 11 cycloalkenes cis and trans-cyclododecene are approximately equal in stability

Dr. Wolf's CHM 201 & trans-cyclooctene is smallest trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans through C 11 cycloalkenes cis and trans-cyclododecene are approximately equal in stability When there are more than 12 carbons in the ring, trans-cycloalkenes are more stable than cis. The ring is large enough so the cycloalkene behaves much like a noncyclic one. Stereoisomeric cycloalkenes

Dr. Wolf's CHM 201 & Preparation of Alkenes: Elimination Reactions

Dr. Wolf's CHM 201 & H Y dehydrogenation of alkanes:  H; Y = H dehydration of alcohols:  H; Y = OH dehydrohalogenation of alkyl halides:  H; Y = Br, etc.    -Elimination Reactions Overview C C C C +HY

Dr. Wolf's CHM 201 & DehydrogenationDehydrogenation limited to industrial syntheses of ethylene, propene, 1,3-butadiene, and styrene important economically, but rarely used in laboratory-scale syntheses 750°C CH 3 CH 3 750°C CH 3 CH 2 CH 3 H2CH2CH2CH2C CH 2 + H2H2H2H2 H2CH2CH2CH2C CHCH 3 + H2H2H2H2

Dr. Wolf's CHM 201 & Dehydration of Alcohols

Dr. Wolf's CHM 201 & Dehydration of Alcohols (79-87%) (82%) H 2 SO 4 160°C CH 3 CH 2 OH H2CH2CH2CH2C CH 2 + H2OH2OH2OH2O OH H 2 SO 4 140°C + H2OH2OH2OH2O C OH CH 3 H3CH3CH3CH3C H 2 SO 4 heat CH 2 H3CH3CH3CH3CC H3CH3CH3CH3C + H2OH2OH2OH2O

Dr. Wolf's CHM 201 & RR'R" OH C RR'H OH C RHH OH C Relative Reactivity tertiary: most reactive primary: least reactive

Dr. Wolf's CHM 201 & Regioselectivity in Alcohol Dehydration: The Zaitsev Rule

Dr. Wolf's CHM 201 & % 90 % HO H 2 SO 4 80°C + Regioselectivity A reaction that can proceed in more than one direction, but in which one direction predominates, is said to be regioselective.

Dr. Wolf's CHM 201 & Regioselectivity A reaction that can proceed in more than one direction, but in which one direction predominates, is said to be regioselective. 84 % 16 % H 3 PO 4 heat CH 3 OH +

Dr. Wolf's CHM 201 & The Zaitsev Rule When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the  carbon having the fewest hydrogens. ROH CH 3 C C H R CH 2 R three protons on this  carbon

Dr. Wolf's CHM 201 & The Zaitsev Rule When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the  carbon having the fewest hydrogens. ROH CH 3 C C H R CH 2 R two protons on this  carbon

Dr. Wolf's CHM 201 & The Zaitsev Rule When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the  carbon having the fewest hydrogens. ROH CH 3 C C H R CH 2 R only one proton on this  carbon

Dr. Wolf's CHM 201 & The Zaitsev Rule When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the  carbon having the fewest hydrogens. R R CH 2 R CH 3 C C ROH C C H R CH 2 R only one proton on this  carbon

Dr. Wolf's CHM 201 & The Zaitsev Rule Zaitsev Rule states that the elimination reaction yields the more highly substituted alkene as the major product. The more stable alkene product predominates.

Dr. Wolf's CHM 201 & Stereoselectivity in Alcohol Dehydration

Dr. Wolf's CHM 201 & StereoselectivityStereoselectivity A stereoselective reaction is one in which a single starting material can yield two or more stereoisomeric products, but gives one of them in greater amounts than any other. (25%) (75%) + OH H 2 SO 4 heat

Dr. Wolf's CHM 201 & The Mechanism of the Acid-Catalyzed Dehydration of Alcohols

Dr. Wolf's CHM 201 & 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 the reaction of alcohols with hydrogen halides. A connecting point...

Dr. Wolf's CHM 201 & first two steps of mechanism are identical to those for the reaction of tert-butyl alcohol with hydrogen halides Dehydration of tert-Butyl Alcohol C OH CH 3 H3CH3CH3CH3C H 2 SO 4 heat CH 2 H3CH3CH3CH3CC H3CH3CH3CH3C + H2OH2OH2OH2O

Dr. Wolf's CHM 201 & Mechanism Step 1: Proton transfer to tert-butyl alcohol (CH 3 ) 3 C O H.. :HO + H O : + H+ fast, bimolecular tert-Butyloxonium ion..HH + O H :H:

Dr. Wolf's CHM 201 & Mechanism Step 2: Dissociation of tert-butyloxonium ion to carbocation + (CH 3 ) 3 C O H :H+ slow, unimolecular (CH 3 ) 3 C O H :H: tert-Butyl cation +

Dr. Wolf's CHM 201 & Mechanism Step 3: Deprotonation of tert-butyl cation. fast, bimolecular + O H :H: CH 2 + H3CH3CH3CH3C C H3CH3CH3CH3CH H3CH3CH3CH3CC H3CH3CH3CH3C + O H :HH +

Dr. Wolf's CHM 201 & CarbocationsCarbocations are intermediates in the acid-catalyzed dehydration of tertiary and secondary alcohols Carbocations can: react with nucleophiles lose a  -proton to form an alkene (Called an E1 mechanism)

Dr. Wolf's CHM 201 & Dehydration of primary alcohols 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 H 2 SO 4 160°C CH 3 CH 2 OH H2CH2CH2CH2C CH 2 + H2OH2OH2OH2O

Dr. Wolf's CHM 201 & Step 1: Proton transfer from acid to ethanol H.. :HO + O CH 3 CH 2..HH H O : + H + fast, bimolecular Ethyloxonium ion CH 3 CH 2 O H : H : Mechanism

Dr. Wolf's CHM 201 & Step 2: Oxonium ion loses both a proton and a water molecule in the same step. + H O :H+ CH 2 H O H :H: slow, bimolecular + O H : H : O H H : H + H2CH2CH2CH2C CH 2 + Mechanism

Dr. Wolf's CHM 201 & Step 2: Oxonium ion loses both a proton and a water molecule in the same step. + H O : H + CH 2 H O H : H : slow, bimolecular + O H : H : O H H : H + H2CH2CH2CH2C CH 2 + Mechanism Because rate-determining step is bimolecular, this is called the E2 mechanism.

Dr. Wolf's CHM 201 & Rearrangements in Alcohol Dehydration Sometimes the alkene product does not have the same carbon skeleton as the starting alcohol.

Dr. Wolf's CHM 201 & ExampleExample OH H 3 PO 4, heat 3% 64% 33% ++

Dr. Wolf's CHM 201 & Rearrangement involves alkyl group migration 3% CH 3 CHCH 3 CH 3 + C carbocation can lose a proton as shown or it can undergo a methyl migration CH 3 group migrates with its pair of electrons to adjacent positively charged carbon

Dr. Wolf's CHM 201 & Rearrangement involves alkyl group migration 3% CH 3 CHCH 3 CH % CHCH 3 CH 3 + C C tertiary carbocation; more stable

Dr. Wolf's CHM 201 & Rearrangement involves alkyl group migration 3% CH 3 CHCH 3 CH % CHCH 3 CH 3 + C C

Dr. Wolf's CHM 201 & Another rearrangement CH 3 CH 2 CH 2 CH 2 OH H 3 PO 4, heat 12% + mixture of cis (32%) and trans-2-butene (56%) CH 2 CH 3 CH 2 CH CHCH 3 CH 3 CH

Dr. Wolf's CHM 201 & Rearrangement involves hydride shift 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 CH 3 CH 2 CH 2 CH 2 OHH + : CH 2 CH 3 CH 2 CH

Dr. Wolf's CHM 201 & Rearrangement involves hydride shift hydrogen migrates with its pair of electrons from C-2 to C-1 as water is lost carbocation formed by hydride shift is secondary CH 3 CH 2 CH 2 CH 2 OHH + : CH 2 CH 3 CH 2 CH CH 3 CH 2 CHCH 3 +

Dr. Wolf's CHM 201 & Hydride shift H H CH 3 CH 2 CHCH 2 O H+: + + H OHH : :

Dr. Wolf's CHM 201 & Rearrangement involves hydride shift CH 3 CH 2 CH 2 CH 2 OHH + : CH 2 CH 3 CH 2 CH CH 3 CH 2 CHCH 3 + mixture of cis and trans-2-butene CHCH 3 CH 3 CH

Dr. Wolf's CHM 201 & Carbocations can... react with nucleophiles lose a proton from the  -carbon to form an alkene rearrange (less stable to more stable) (alkyl shift or hydride shift)

Dr. Wolf's CHM 201 & Dehydrohalogenation of Alkyl Halides

Dr. Wolf's CHM 201 & H Y dehydrogenation of alkanes:  H; Y = H dehydration of alcohols:  H; Y = OH dehydrohalogenation of alkyl halides:  H; Y = Br, etc.    -Elimination Reactions Overview C C C C +HY

Dr. Wolf's CHM 201 & H Y dehydrogenation of alkanes: industrial process; not regioselective dehydration of alcohols: acid-catalyzed dehydrohalogenation of alkyl halides: consumes base    -Elimination Reactions Overview C C C C +HY

Dr. Wolf's CHM 201 & DehydrohalogenationDehydrohalogenation A useful method for the preparation of alkenes Cl (100 %) likewise, NaOCH 3 in methanol, or KOH in ethanol NaOCH 2 CH 3 ethanol, 55°C

Dr. Wolf's CHM 201 & CH 3 (CH 2 ) 15 CH 2 CH 2 Cl When the alkyl halide is primary, potassium tert-butoxide in dimethyl sulfoxide is the base/solvent system that is normally used. KOC(CH 3 ) 3 dimethyl sulfoxide (86%) DehydrohalogenationDehydrohalogenation CH 2 CH 3 (CH 2 ) 15 CH

Dr. Wolf's CHM 201 & Br 29 % 71 % + Regioselectivity follows Zaitsev's rule: more highly substituted double bond predominates KOCH 2 CH 3 ethanol, 70°C

Dr. Wolf's CHM 201 & more stable configuration of double bond predominates Stereoselectivity KOCH 2 CH 3 ethanol Br + (23%) (77%)

Dr. Wolf's CHM 201 & more stable configuration of double bond predominates Stereoselectivity KOCH 2 CH 3 ethanol + (85%) (15%)Br

Dr. Wolf's CHM 201 & Mechanism of the Dehydrohalogenation of Alkyl Halides: The E2 Mechanism

Dr. Wolf's CHM 201 & 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)

Dr. Wolf's CHM 201 & 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

Dr. Wolf's CHM 201 & The E2 Mechanism concerted (one-step) bimolecular process single transition state C—H bond breaks  component of double bond forms C—X bond breaks

Dr. Wolf's CHM 201 & The E2 Mechanism – O R.... : CCCCCCCCHX.. :: Reactants

Dr. Wolf's CHM 201 & The E2 Mechanism – O R.... : CCCCCCCCHX.. :: Reactants

Dr. Wolf's CHM 201 & CCCCCCCC The E2 Mechanism –––– OR.... H X..:: –––– Transition state

Dr. Wolf's CHM 201 & The E2 Mechanism OR.... H CCCCCCCC–X.. ::.. Products

Dr. Wolf's CHM 201 & Anti Elimination in E2 Reactions Stereoelectronic Effects Isotope Effects

Dr. Wolf's CHM 201 & (CH 3 ) 3 C Br KOC(CH 3 ) 3 (CH 3 ) 3 COH cis-1-Bromo-4-tert- butylcyclohexane Stereoelectronic effect

Dr. Wolf's CHM 201 & (CH 3 ) 3 C Br KOC(CH 3 ) 3 (CH 3 ) 3 COH trans-1-Bromo-4-tert- butylcyclohexane Stereoelectronic effect

Dr. Wolf's CHM 201 & (CH 3 ) 3 C Br Br KOC(CH 3 ) 3 (CH 3 ) 3 COH cis trans Rate constant for dehydrohalogenation of cis is 500 times greater than that of trans Stereoelectronic effect

Dr. Wolf's CHM 201 & (CH 3 ) 3 C Br KOC(CH 3 ) 3 (CH 3 ) 3 COH cis H that is removed by base must be anti periplanar to Br Two anti periplanar H atoms in cis stereoisomer H H Stereoelectronic effect

Dr. Wolf's CHM 201 & (CH 3 ) 3 C KOC(CH 3 ) 3 (CH 3 ) 3 COH trans 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 Br H H (CH 3 ) 3 C BrHH Stereoelectronic effect

Dr. Wolf's CHM 201 & cis more reactive trans less reactive Stereoelectronic effect

Dr. Wolf's CHM 201 & Stereoelectronic effect 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.

Dr. Wolf's CHM 201 & Isotope effect 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. k H /k D = 3-8 WHEN the rate determining step involves breaking the C-H bond.

Dr. Wolf's CHM 201 & A Different Mechanism for Alkyl Halide Elimination: The E1 Mechanism

Dr. Wolf's CHM 201 & ExampleExample CH 3 CH 2 CH 3 Br CH 3 Ethanol, heat + (25%) (75%) C H3CH3CH3CH3C CH 3 C C H3CH3CH3CH3C H CH 2 CH 3 CH 3 C H2CH2CH2CH2C

Dr. Wolf's CHM 201 & 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. The E1 Mechanism

Dr. Wolf's CHM 201 & Step 1 slow, unimolecular C CH 2 CH 3 CH 3 + CH 2 CH 3 Br CH 3 C :.. : :.. : Br..–

Dr. Wolf's CHM 201 & Step 2 C CH 2 CH 3 CH 3 + C CH 2 CH 3 CH 3 CH 2 + C CHCH 3 CH 3 – H +

Dr. Wolf's CHM 201 & End of Chapter 5