5.14 Dehydrohalogenation of Alkyl Halides β α β...

5.145.14

Dehydrohalogenation of Dehydrohalogenation of

Alkyl Halides Alkyl Halides XX YY

dehydrohalogenation dehydrohalogenation of alkyl halides:of alkyl halides:X = H; Y = Br, etc.X = H; Y = Br, etc.

ααββCC CCCC CC ++ XX YY

ββ-Elimination Reactions-Elimination Reactions

XX YY

dehydrohalogenation dehydrohalogenation of alkyl halides:of alkyl halides:X = H; Y = Br, etc.X = H; Y = Br, etc.

ααββCC CCCC CC ++ XX YY

ββ-Elimination Reactions-Elimination Reactions

requires baserequires base

(100 %)(100 %)

likewise, NaOCHlikewise, NaOCH33 in methanol, or KOH in ethanol in methanol, or KOH in ethanol

NaOCHNaOCH22CHCH33

ethanol, 55ethanol, 55°°CC

DehydrohalogenationDehydrohalogenation

ClCl

CHCH33(CH(CH22))1515CHCH22CHCH22ClClKOC(CHKOC(CH33))33

dimethyl sulfoxidedimethyl sulfoxide

(86%)(86%)

CHCH22CHCH33(CH(CH22))1515CHCH

DehydrohalogenationDehydrohalogenation

When the alkyl halide is When the alkyl halide is primaryprimary, potassium, potassium

terttert--butoxide butoxide inin dimethyl sulfoxide dimethyl sulfoxide (DMSO), a(DMSO), a

strong non-strong non-protic protic polar solvent is thepolar solvent is the

base/solvent system that is normally used.base/solvent system that is normally used.

BrBr

29 %29 % 71 %71 %

++

RegioselectivityRegioselectivity

follows Zaitsev's rulefollows Zaitsev's rule

more highly substituted double bond predominatesmore highly substituted double bond predominates

KOCHKOCH22CHCH33

ethanol, 70ethanol, 70°°CC

more stable configurationmore stable configurationof double bond predominatesof double bond predominates

StereoselectivityStereoselectivity

KOCHKOCH22CHCH33

ethanolethanol

BrBr

++

(23%)(23%) (77%)(77%)

QuestionQuestion

How many alkenes would you expect to beHow many alkenes would you expect to beformed from the E2 elimination offormed from the E2 elimination of

3-bromo-2-methylpentane?3-bromo-2-methylpentane?

A)A) 22

B)B) 33

C)C) 44

D)D) 55

more stable configurationmore stable configurationof double bond predominatesof double bond predominates

StereoselectivityStereoselectivity

KOCHKOCH22CHCH33

ethanolethanol

++

(85%)(85%) (15%)(15%)

BrBr

5.155.15 The E2 Mechanism of The E2 Mechanism of

Dehydrohalogenation of Alkyl HalidesDehydrohalogenation of Alkyl Halides

FactsFacts

(1)(1) Dehydrohalogenation of alkyl halides Dehydrohalogenation of alkyl halides exhibits second-order kineticsexhibits second-order kinetics

first order in alkyl halidefirst order in alkyl halidefirst order in basefirst order in baserate = rate = kk[alkyl halide][base][alkyl halide][base]

implies that rate-determining stepimplies that rate-determining stepinvolves both base and alkyl halide;involves both base and alkyl halide;i.e., it is bimoleculari.e., it is bimolecular

QuestionQuestion

The reaction of 2-bromobutane withThe reaction of 2-bromobutane withKOCHKOCH22CHCH33 in ethanol produces trans-2- in ethanol produces trans-2-butene. If the concentration of bothbutene. If the concentration of bothreactants is doubled, what would be thereactants is doubled, what would be theeffect on the rate of the reaction?effect on the rate of the reaction?

A)A) halve the ratehalve the rate

B)B) double the ratedouble the rate

C)C) quadruple the ratequadruple the rate

D)D) no effect on the rateno effect on the rate

FactsFacts

(2)(2) Rate of elimination depends on halogenRate of elimination depends on halogen

weaker Cweaker C——X bond; faster rateX bond; faster raterate: RI > RBr > RCl > RFrate: RI > RBr > RCl > RF

implies that carbon-halogen bond breaks in implies that carbon-halogen bond breaks in the rate-determining stepthe rate-determining step

concerted (one-step) bimolecular processconcerted (one-step) bimolecular process

single transition statesingle transition state

CC——H bond breaksH bond breaks

ππ component of double bond forms component of double bond forms

CC——X bond breaksX bond breaks

The E2 MechanismThe E2 Mechanism

––

OORR..

.... ::

CC CC

HH

XX....::::

ReactantsReactants


CC CC

δδ––

OORR..

.... HH

XX....::::δδ––

Transition stateTransition state


OORR....

.... HH

CC CC

––XX....

::::....

ProductsProducts


QuestionQuestion

Which one of the following best describes a mechanisticWhich one of the following best describes a mechanisticfeature of the reaction of 3-bromopentane with sodiumfeature of the reaction of 3-bromopentane with sodiumethoxideethoxide??A)A) The reaction occurs in a single step which isThe reaction occurs in a single step which isbimolecular.bimolecular.B)B) The reaction occurs in two steps, both of which areThe reaction occurs in two steps, both of which areunimolecularunimolecular..C)C) The rate-determining step involves the The rate-determining step involves the formationformationof the of the carbocation carbocation (CH(CH33CHCH22))22CHCH++..D)D) The carbon-halogen bond breaks in a rapid The carbon-halogen bond breaks in a rapid stepstepthat follows the rate-determining step.that follows the rate-determining step.

Stereoelectronic EffectsStereoelectronic Effects

5.165.16

Stereochemistry:Stereochemistry:

Anti Elimination in E2 ReactionsAnti Elimination in E2 Reactions

(CH(CH33))33CC

(CH(CH33))33CC

BrBr

KOC(CHKOC(CH33))33

(CH(CH33))33COHCOH

ciscis-1-Bromo-4--1-Bromo-4-tert-tert-butylcyclohexanebutylcyclohexane

Stereoelectronic EffectStereoelectronic Effect

(CH(CH33))33CC

(CH(CH33))33CCBrBr KOC(CHKOC(CH33))33

(CH(CH33))33COHCOH

transtrans-1-Bromo-4--1-Bromo-4-tert-tert-butylcyclohexanebutylcyclohexane


(CH(CH33))33CC

(CH(CH33))33CC

BrBr

(CH(CH33))33CCBrBr

KOC(CHKOC(CH33))33

(CH(CH33))33COHCOH

KOC(CHKOC(CH33))33

(CH(CH33))33COHCOH

ciscis

transtrans

Rate constant forRate constant fordehydrohalogenationdehydrohalogenationof 1,4- cis is >500of 1,4- cis is >500times than that of 1,4-times than that of 1,4-transtrans


(CH(CH33))33CC

(CH(CH33))33CC

BrBr

KOC(CHKOC(CH33))33

(CH(CH33))33COHCOH

ciscis

H that is removed by base must be antiH that is removed by base must be anticoplanar to Brcoplanar to Br

Two anti coplanar H atoms in cisTwo anti coplanar H atoms in cisstereoisomerstereoisomer

HHHH


(CH(CH33))33CC

KOC(CHKOC(CH33))33

(CH(CH33))33COHCOH

transtrans

H that is removed by base must be antiH that is removed by base must be anticoplanar to Brcoplanar to Br

No anti coplanar H atoms in trans stereoisomer;No anti coplanar H atoms in trans stereoisomer;all vicinal H atoms are gauche to Br; thereforeall vicinal H atoms are gauche to Br; thereforeinfinitesimal or no product is formedinfinitesimal or no product is formed

HHHH

(CH(CH33))33CCBrBr

HH

HH


1,4- cis1,4- cis

more reactivemore reactive

1,4- trans1,4- trans

much less reactivemuch less reactive


QuestionQuestion

Which would react with KOC(CHWhich would react with KOC(CH33))33/(CH/(CH33))33COHCOHfaster?faster?

A)A) ciscis-3--3-terttert-butylcyclohexyl bromide-butylcyclohexyl bromide

B)B) transtrans-3--3-terttert-butylcyclohexyl bromide-butylcyclohexyl bromide

QuestionQuestion

Which would react with KOCHWhich would react with KOCH22CHCH33 in ethanol in ethanolfaster?faster?

A)A) ciscis-2--2-terttert-butylcyclohexyl bromide-butylcyclohexyl bromide

B)B) transtrans-2--2-terttert-butylcyclohexyl bromide-butylcyclohexyl bromide


An effect on reactivity that has its origin inAn effect on reactivity that has its origin inthe spatial arrangement of orbitals or bonds isthe spatial arrangement of orbitals or bonds iscalled a stereoelectronic effect.called a stereoelectronic effect.

The preference for an anti coplanarThe preference for an anti coplanararrangement of H and Br in the transitionarrangement of H and Br in the transitionstate for E2 dehydrohalogenation is anstate for E2 dehydrohalogenation is anexample of a stereoelectronic effectexample of a stereoelectronic effect..

5.175.17

Isotopes Effects And The E2Isotopes Effects And The E2

MechanismMechanism

A C-D bond is A C-D bond is ≈≈12 kJ/mol stronger than a C-H bond.12 kJ/mol stronger than a C-H bond.

The activation energy for breaking a C-D bond is The activation energy for breaking a C-D bond is greater than for breaking a C-H bond. greater than for breaking a C-H bond. The rate constant k for an elementary step whereThe rate constant k for an elementary step where C-D breaks is smallerC-D breaks is smaller than for a C-H bond.than for a C-H bond. The difference in rate is expressed as a ratio k The difference in rate is expressed as a ratio kHH/k/kDD,, and is a and is a kinetic isotope effectkinetic isotope effect.. Because it compares Because it compares 22H to H to 11H, it is called a H, it is called a deuterium deuterium isotope effect isotope effect..

The Isotope EffectThe Isotope Effect

The rate is slower in the reaction below for deuterium, The rate is slower in the reaction below for deuterium, 22H, vs. H, vs. 11H.H.Since in the rate determining step of the E2 mechanism, Since in the rate determining step of the E2 mechanism, a base removes a proton from a a base removes a proton from a ββ carbon. carbon.

The mechanism accounts for the observed deuterium isotopeThe mechanism accounts for the observed deuterium isotopeeffect.effect.

The Isotope EffectThe Isotope Effect

5.185.18

The E1 Mechanism ofThe E1 Mechanism of

Dehydrohalogenation of AlkylDehydrohalogenation of Alkyl

HalidesHalides

CHCH33 CHCH22CHCH33

BrBr

CHCH33

CC

Ethanol, heatEthanol, heat

++

(25%)(25%) (75%)(75%)

HH33CC

CHCH33

CC CC

HH33CC

HH

CHCH22CHCH33

CHCH33

CCHH22CC

ExampleExample

1. Alkyl halides can undergo elimination in1. Alkyl halides can undergo elimination in protic protic solvents in the absence of base. solvents in the absence of base.

2. Carbocation is intermediate.2. Carbocation is intermediate.

3.3. Rate-determining step is unimolecular Rate-determining step is unimolecular ionization of alkyl halide.ionization of alkyl halide.

The E1 MechanismThe E1 MechanismCHCH33 CHCH22CHCH33

BrBr

CHCH33

CC

::....::

CCCHCH22CHCH33CHCH33

CHCH33

++

::....:: BrBr.... ––

slow, slow, unimolecularunimolecular

Step 1Step 1


CHCH33

++


CHCH22

++ CCCHCHCHCH33CHCH33

CHCH33

–– H H++

Step 2Step 2 QuestionQuestion

Which reaction would be most likely to proceedWhich reaction would be most likely to proceedby an E1 mechanism?by an E1 mechanism?

A)A) 2-chloro-2-methylbutane + NaOCH2-chloro-2-methylbutane + NaOCH22CHCH33 in inethanol (heat)ethanol (heat)

B)B) 1-bromo-2-methylbutane + KOC(CH1-bromo-2-methylbutane + KOC(CH33))33 in inDMSODMSO

C)C) 2-bromo-2-methylbutane in ethanol 2-bromo-2-methylbutane in ethanol (heat)(heat)

D)D) 2-methyl-2-butanol + KOH2-methyl-2-butanol + KOH

5.14 Dehydrohalogenation of Alkyl Halides β α β...

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