acid-catalyzed hydration of alkenes -...

6.106.10Acid-Catalyzed Hydration of AlkenesAcid-Catalyzed Hydration of Alkenes

HH——OOHHCC CC ++ OOHHCC CCHH

Acid-Catalyzed Hydration of AlkenesAcid-Catalyzed Hydration of Alkenes

reaction is acid catalyzed; typical hydrationreaction is acid catalyzed; typical hydrationmedium is 50% Hmedium is 50% H22SOSO44-50% H-50% H22OO

Follows Markovnikov's RuleFollows Markovnikov's Rule

(90%)(90%)

50% H50% H22SOSO44

50% H50% H22OO

HH33CC

HH33CC CHCH33

HH

CC CC

OOHH

CC CHCH22CHCH33CHCH33

CHCH33


50% H50% H22SOSO44

50% H50% H22OO


(80%)(80%)

OOHH

CHCH33

CHCH22

+ H+ H22OOHH++

involves a carbocation intermediateinvolves a carbocation intermediate

is the reverse of acid-catalyzed dehydrationis the reverse of acid-catalyzed dehydrationof alcohols to alkenesof alcohols to alkenes

HH33CC

HH33CC

CC CHCH22

OOHH

CC CHCH33CHCH33

CHCH33

MechanismMechanism

Addition of Water to Alkene(alcohols)

Addition of Water to Alkenes(alcohols)

QuestionQuestion

Which Which alkene alkene will undergo acid-catalyzedwill undergo acid-catalyzedhydrolysis at the fastest rate?hydrolysis at the fastest rate?

A)A) B)B)

C)C) D)D)

In an equilibrium process, the same intermediates In an equilibrium process, the same intermediates and transition states are encountered in the and transition states are encountered in the forward direction and the reverse, but in the forward direction and the reverse, but in the opposite order. opposite order.

+ H+ H22OOHH++

HH33CC

HH33CC

CC CHCH22

OOHH

CC CHCH33CHCH33

CHCH33

Principle of Microscopic ReversibilityPrinciple of Microscopic Reversibility

HH22O addition: O addition: Markovnikov's Markovnikov's RuleRule QuestionQuestion

The product isolated from the acid-catalyzedThe product isolated from the acid-catalyzedhydration of (hydration of (Z)Z)-3-methyl-2-pentene is:-3-methyl-2-pentene is:

A)A) 2-ethyl-2-butanol2-ethyl-2-butanol

B)B) 2-ethyl-1-butanol2-ethyl-1-butanol

C)C) 3-methyl-2-pentanol3-methyl-2-pentanol

D)D) 3-methyl-3-pentanol3-methyl-3-pentanol

QuestionQuestion

The product isolated from the acid-catalyzedThe product isolated from the acid-catalyzedhydration of (hydration of (E)E)-3-methyl-2-pentene is:-3-methyl-2-pentene is:

A)A) chiralchiral

B)B) achiralachiral

Addition of Alcohol to Alkenes(ethers)

RegioRegiochemistrychemistry withstanding, in order to withstanding, in order to understand the understand thestereochemistry of thestereochemistry of the product, you must consider two things:product, you must consider two things:

(1) Stereochemistry of the starting (1) Stereochemistry of the starting alkene alkene (cis or trans; (cis or trans;

Z or E)Z or E)

(2) Stereochemistry of the addition (syn or anti)(2) Stereochemistry of the addition (syn or anti)

Stereochemistry of Addition to AlkenesStereochemistry of Addition to Alkenes

CC CC ++ EE——YY CC CCEE YY

Optically inactive reactants produce optically inactive products.Optically inactive reactants produce optically inactive products.((RacemicRacemic mixtures are optically inactive) mixtures are optically inactive)

The The correlary correlary is that an optically active starting material is that an optically active starting material MAY produce an MAY produce an optically active product depending on the mechanism.optically active product depending on the mechanism.

Stereochemistry of Addition to AlkenesStereochemistry of Addition to Alkenes

CC CC ++ EE——YY CC CCEE YY

QuestionQuestion

The product isolated from the acid-catalyzedThe product isolated from the acid-catalyzedhydration of (hydration of (E)- or (Z)E)- or (Z)-3-methyl-2-pentene is:-3-methyl-2-pentene is:

A)A) optically activeoptically active

B)B) an optically inactivean optically inactive racemic racemic mixture mixture

C) an optically inactive C) an optically inactive enantiomer enantiomer

6.126.12Hydroboration-Oxidation of AlkenesHydroboration-Oxidation of Alkenes

Needed: a method for hydration of Needed: a method for hydration of alkenes with a regioselectivity opposite to alkenes with a regioselectivity opposite to Markovnikov's rule.Markovnikov's rule.

Suppose you wanted to prepare 1-decanol Suppose you wanted to prepare 1-decanol from 1-decene?from 1-decene?

SynthesisSynthesis

OHOH

Two-step reaction sequence called Two-step reaction sequence called hydroborationhydroboration--oxidation converts alkenes to alcohols with a oxidation converts alkenes to alcohols with a regiochemistry regiochemistry opposite to opposite to Markovnikov's Markovnikov's rule.rule.

1. 1. hydroborationhydroboration

2. oxidation2. oxidation

SynthesisSynthesis

OHOH

++ H H——BHBH22CC CC HH BHBH22CC CC

Hydroboration Hydroboration can be viewed as the addition ofcan be viewed as the addition ofborane borane (BH(BH33) to the double bond. But BH) to the double bond. But BH33 is is not the reagent actually used.not the reagent actually used.

Hydroboration StepHydroboration Step


Hydroboration reagents:Hydroboration reagents:

HH22BB

HH

HH

BHBH22

Diborane (BDiborane (B22HH66))normally used in an normally used in an ether-like solventether-like solventcalled "diglyme"called "diglyme"



Hydroboration Hydroboration reagents:reagents:

BoraneBorane--tetrahydrofurantetrahydrofurancomplex (Hcomplex (H33B-THF)B-THF)


+ OO

BHBH33–

••••

HH22OO22, HO, HO––

HH BHBH22CC CC HH OOHHCC CC

Organoborane formed in the hydroborationOrganoborane formed in the hydroborationstep is oxidized with hydrogen peroxide.step is oxidized with hydrogen peroxide.

Oxidation StepOxidation Step

1. B1. B22HH66, , diglymediglyme

2. H2. H22OO22, HO, HO––

(93%)(93%)

ExampleExample

OOHH (98%)(98%)

HH33CC

HH33CC

CHCH33

HH

CC CC1. 1. HH33B-THFB-THF

2. H2. H22OO22, HO, HO––

HH

CC CCCHCH33

CHCH33

CHCH33

HH OOHH

ExampleExample

hydration of alkeneshydration of alkenes

regioselectivity opposite to Markovnikov's rule regioselectivity opposite to Markovnikov's rule

no rearrangementno rearrangement

stereospecific stereospecific synsyn addition addition

Features of Hydroboration-OxidationFeatures of Hydroboration-Oxidation

1. B1. B22HH66, , diglymediglyme

2. H2. H22OO22, HO, HO––

(82%)(82%)

ExampleExample

OOHH

6.136.13Stereochemistry of Hydroboration-OxidationStereochemistry of Hydroboration-Oxidation

hydration of alkeneshydration of alkenes

regioselectivity opposite to Markovnikov's rule regioselectivity opposite to Markovnikov's rule

no rearrangementno rearrangement

stereospecific stereospecific synsyn addition addition

Features of Hydroboration-OxidationFeatures of Hydroboration-Oxidation

H and OH become attached to same H and OH become attached to same face of double bondface of double bond

1. B1. B22HH66

2. H2. H22OO22, NaOH, NaOH

only product is only product is transtrans-2-methylcyclopentanol-2-methylcyclopentanol(86%) yield(86%) yield

syn Additionsyn Addition

CHCH33

HH

HH

CHCH33

HH

HHOO

QuestionQuestion

HydroborationHydroboration-oxidation of which one of the-oxidation of which one of thefollowing yields a primary alcohol as the majorfollowing yields a primary alcohol as the majorproduct?product?

A) A) B)B)

C)C) D)D)

HH33CC

HH33CC

CHCH33

HH

CC CC1. 1. HH33B-THFB-THF

2. H2. H22OO22, HO, HO––

HH

CC CCCHCH33

CHCH33

CHCH33

HH OOHH

QuestionQuestion

A)A) The product isThe product is achiral achiralB)B) The product is optically activeThe product is optically activeC)C) The product is aThe product is a racemic racemic mixture mixtureD)D) The product is a The product is a single enantiomersingle enantiomer

6.156.15

Addition of HalogensAddition of Halogens

to Alkenesto Alkenes

++ XX22 XX XX

electrophilic electrophilic addition to double bondaddition to double bondforms a vicinalforms a vicinal dihalide dihalide

CC CCCC CC

General featuresGeneral features

CHCH33CHCHCHCHCH(CHCH(CH33))22

(100%)(100%)

CHClCHCl3300°°CC

CHCHCH(CHCH(CH33))22CHCH33CHCHBrBr22

ExampleExample

BrBr BrBr

FF22 addition proceeds with explosive violence addition proceeds with explosive violence

II22 addition is endothermic: vicinal addition is endothermic: vicinal diiodides diiodides

dissociate to andissociate to an alkene alkene and Iand I22

limited to Cllimited to Cl22 and Br and Br22

ScopeScope HalogenationHalogenation

anti additionanti additionanti addition

6.166.16Stereochemistry of Halogen AdditionStereochemistry of Halogen Addition

BrBr22

transtrans-1,2--1,2-DibromocyclopentaneDibromocyclopentane80% yield; only product80% yield; only product

ExampleExample

HH

HH

BrBr

BrBr

HH

HH

ClCl22

transtrans-1,2--1,2-DichlorocyclooctaneDichlorocyclooctane73% yield; only product73% yield; only product

ExampleExample

HH

HH

HH

HH

ClCl

ClCl

6.176.17Mechanism of Halogen Addition toMechanism of Halogen Addition to

Alkenes:Alkenes: Halonium Halonium IonsIons

BrBr22 is not polar, but it is is not polar, but it is polarizable polarizable

two stepstwo steps(1) (1) formation offormation of bromonium bromonium ionion(2)(2) nucleophilic nucleophilic attack onattack on bromonium bromonium

ion by bromideion by bromide

Mechanism isMechanism is electrophilic electrophilic additionadditionAddition of Halogens to Alkenes

ethyleneethylene HH22C=CHC=CH22 11

propenepropene CHCH33CH=CHCH=CH22 6161

2-2-methylpropene methylpropene (CH(CH33))22C=CHC=CH22 54005400

2,3-2,3-dimethyldimethyl-2--2-butene butene (CH(CH33))22C=C(CHC=C(CH33))22 920,000920,000

More highly substituted double bonds react faster.More highly substituted double bonds react faster.

Alkyl groups on the double bond make itAlkyl groups on the double bond make it

more more ““electron rich.electron rich.””

Relative Rates ofRelative Rates of Bromination Bromination QuestionQuestion

Arrange the alkenes in order of decreasing rateArrange the alkenes in order of decreasing rateof reaction toward bromine addition:of reaction toward bromine addition:2-methyl-1-butene, 2-methyl-2-butene, and 3-2-methyl-1-butene, 2-methyl-2-butene, and 3-methyl-1-butenemethyl-1-buteneA)A) 2-methyl-1-butene > 3-methyl-1-butene >2-methyl-1-butene > 3-methyl-1-butene >

2-methyl-2-butene2-methyl-2-buteneB)B) 3-methyl-1-butene > 2-methyl-1-butene >3-methyl-1-butene > 2-methyl-1-butene >

2-methyl-2-butene2-methyl-2-buteneC)C) 2-methyl-2-butene > 2-methyl-1-butene >2-methyl-2-butene > 2-methyl-1-butene >

3-methyl-1-butene3-methyl-1-buteneD)D) 2-methyl-2-butene > 3-methyl-1-butene >2-methyl-2-butene > 3-methyl-1-butene >

2-methyl-1-butene2-methyl-1-butene

BrBr22

transtrans-1,2--1,2-dibromocyclopentanedibromocyclopentane80% yield; 2 asymmetric carbon atoms,80% yield; 2 asymmetric carbon atoms,

Chiral Chiral product,Optically inactive,product,Optically inactive,

RacemicRacemic mixture, only products formed mixture, only products formed

ExampleExample

HH

HH

BrBr

BrBr

HH

HH

DiastereomersDiastereomers

In general, In general, diastereomers diastereomers fall into two categories:fall into two categories:

geometric isomersgeometric isomers

cis-transcis-trans

stereoisomers stereoisomers containing two or morecontaining two or moreasymmetric atoms; (that are not asymmetric atoms; (that are not enantiomersenantiomers))

C

C

CH

CH3

CH3

H

CH

CH3

H

CH3

C

C

CH

CH3

CH3

H

CH

CH3

H

CH3

CH3HH

BrCl

CH3

CH3BrH Cl

H

CH3

CH3HH

BrCl

CH3

CH3BrH Cl

H

CH3

diastereomers

(S)

(R)

(R)

(R)


Diastereomers Diastereomers have different physicalhave different physicalproperties:properties:

BP, MP, density, refractive index, solubilityBP, MP, density, refractive index, solubility

Can be separated through conventionalCan be separated through conventionalmeans (distillation, means (distillation, recrystallizationrecrystallization,,chromatography)chromatography)

H BrCO2H

CO2HBr H

CO2H

Co2H

HH

BrBr

H BrCO2H

CO2HBr H

CO2H

CO2H

HH

BrBr

MP = 158oC MP = 256oC


A compound with A compound with ““nn”” asymmetric carbon atoms asymmetric carbon atomscan have a maximum of 2can have a maximum of 2nn stereoisomersstereoisomers.. CH3

CH3

BrBrH

HBr

BrH

H

CH3

CH3CH3

CH3

HH

BrBr

CH3

BrBr

HH

CH3

CC

CC

CC

CC

CH3

CH3

BrBrH

HBr

BrH

H

CH3

CH3CH3

CH3

HH

BrBr

CH3

BrBr

HH

CH3

CC

CC

CC

CC

CH3

CH3

BrBrH

HBr

BrH

H

CH3

CH3CH3

CH3

HH

BrBr

CH3BrBr

HH

CH3

CC

CC

CC

CC

CH3

CH3

BrBrH

HBr

BrH

H

CH3

CH3CH3

CH3

HH

BrBr

CH3BrBr

HH

CH3

CC

CC

CC

CC

2,3-dibromobutane

Enantiomers Same compound(meso)diastereomers

σ

(S)

(S) (R)

(R)

(R)

(S)

(S)

(R)

Only 3 stereoisomers for 2,3-dibromobutane.

ThreeThree stereoisomers stereoisomers of 2,3-of 2,3-dibromobutanedibromobutane

22RR,3,3RR 22SS,3,3SS 22RR,3,3SS

chiralchiral chiralchiral achiralachiral

CHCH33

CHCH33

BrBrHH

HHBrBrHH BrBr

CHCH33

CHCH33

HHBrBr HH

CHCH33

CHCH33

BrBr

BrBrHH

The addition of bromine to The addition of bromine to ciscis-2-butene-2-buteneproduces:produces:

A)A) A singleA single enantiomer enantiomerB)B) A product with one asymmetric carbon atomA product with one asymmetric carbon atomC)C) An optically inactiveAn optically inactive meso meso product productD)D) A A racemicracemic mixture mixtureE)E) 4 different4 different stereoisomers stereoisomers each with 2 each with 2 chiral chiral

carbon atomscarbon atoms

QuestionQuestion

The addition of bromine to The addition of bromine to transtrans-2-butene-2-buteneproduces:produces:

A)A) A singleA single enantiomer enantiomerB)B) A product with one asymmetric carbon atomA product with one asymmetric carbon atomC)C) An optically inactiveAn optically inactive meso meso product productD)D) A A racemicracemic mixture mixtureE)E) 4 different4 different stereoisomers stereoisomers each with 2 each with 2 chiral chiral

carbon atomscarbon atoms

QuestionQuestionDiastereomersDiastereomers

When naming compounds containing multipleWhen naming compounds containing multiplechiral chiral atoms, you must give the configurationatoms, you must give the configurationaround each around each chiral chiral atom:atom:

position number and configuration of eachposition number and configuration of eachchiral chiral atom, separated by commas, all in ( )atom, separated by commas, all in ( )at the start of the compound nameat the start of the compound name

C

CH3

CH3

Cl

ClBr HH

H

C

CH3

CH3

Br

HCl

H(S)

(S)(2S, 3S)-2-bromo-3-chlorobutane


Which ones areWhich ones are chiral chiral? Name each one. Give? Name each one. Givethethe stereochemical stereochemical relationship between them. relationship between them.

CH3

CH2CH3

ClCl

HH

CH3

CH2CH3

CH3

CH2CH3

CH3

CH2CH3

ClCl

HH

H ClCl H

Cl HH Cl

CH3

CH2CH3

ClCl

HH

CH3

CH2CH3

CH3

CH2CH3

CH3

CH2CH3

ClCl

HH

H ClCl H

Cl HH Cl

CH3

CH2CH3

ClCl

HH

CH3

CH2CH3

CH3

CH2CH3

CH3

CH2CH3

ClCl

HH

H ClCl H

Cl HH Cl

CH3

CH2CH3

ClCl

HH

CH3

CH2CH3

CH3

CH2CH3

CH3

CH2CH3

ClCl

HH

H ClCl H

Cl HH Cl

All of them are chiral.


CH3

CH2CH3

ClCl

HH

CH3

CH2CH3

CH3

CH2CH3

CH3

CH2CH3

ClCl

HH

H ClCl H

Cl HH Cl

CH3

CH2CH3

ClCl

HH

CH3

CH2CH3

CH3

CH2CH3

CH3

CH2CH3

ClCl

HH

H ClCl H

Cl HH Cl

CH3

CH2CH3

ClCl

HH

CH3

CH2CH3

CH3

CH2CH3

CH3

CH2CH3

ClCl

HH

H ClCl H

Cl HH Cl

CH3

CH2CH3

ClCl

HH

CH3

CH2CH3

CH3

CH2CH3

CH3

CH2CH3

ClCl

HH

H ClCl H

Cl HH Cl

(2S, 3R)-2,3-dichloropentane (2R, 3S)-2,3-dichloropentane

(2S, 3S)-2,3-dichloropentane (2R, 3R)-2,3-dichloropentane


CH3

CH2CH3

ClCl

HH

CH3

CH2CH3

CH3

CH2CH3

CH3

CH2CH3

ClCl

HH

H ClCl H

Cl HH Cl

CH3

CH2CH3

ClCl

HH

CH3

CH2CH3

CH3

CH2CH3

CH3

CH2CH3

ClCl

HH

H ClCl H

Cl HH Cl

CH3

CH2CH3

ClCl

HH

CH3

CH2CH3

CH3

CH2CH3

CH3

CH2CH3

ClCl

HH

H ClCl H

Cl HH Cl

CH3

CH2CH3

ClCl

HH

CH3

CH2CH3

CH3

CH2CH3

CH3

CH2CH3

ClCl

HH

H ClCl H

Cl HH Cl

A B C D

A and B: enantiomersC and D: enantiomersA and C: diastereomersA and D: diastereomersB and C: diastereomersB and D: diastereomers

6.186.18Conversion of Alkenes to VicinalConversion of Alkenes to Vicinal

HalohydrinsHalohydrins

Addition of Halogens in the Presence of Water

(halohydrins) ++ X X22 XX XXCC CCCC CC

alkenes react with Xalkenes react with X22 to form vicinal to form vicinal dihalides dihalides

++ X X22 XX XXCC CCCC CC

++ H H22OO OHOH

+ + HH——XX

++ X X22 XX CC CCCC CC

alkenes react with Xalkenes react with X22 to form vicinal to form vicinal dihalides dihalides

alkenes react with Xalkenes react with X22 in water to give vicinal in water to give vicinal halohydrinshalohydrins

ClCl22

anti anti addition: only productaddition: only product

HH22OO

HH22CC CHCH22 BrBrCHCH22CHCH22OOHH++ BrBr22

HH22OO

(70%)(70%)

ExamplesExamples

HH

HH

OOHH

ClCl

HH

HH

ClCl22

anti anti addition: only productaddition: only product

HH22OO

HH22CC CHCH22 BrBrCHCH22CHCH22OOHH++ BrBr22

HH22OO

(70%)(70%)

ExamplesExamples

HH

HH

OOHH

ClCl

HH

HH

Perspective formula

Fischer projection

(77%)(77%)

HH33CC

CC CHCH22

HH33CC

CHCH33

OOHH

CC CHCH22BrBrCHCH33

Markovnikov's Markovnikov's rule applied torule applied to halohydrin halohydrinformation: formation: the halogen adds to the carbonthe halogen adds to the carbonhaving the greater number ofhaving the greater number of hydrogens hydrogens..

BrBr22

HH22OO

RegioselectivityRegioselectivity

CHCH22

HH33CC

CCHH33CC

transition state for attack of water ontransition state for attack of water on bromonium bromonium ion hasion has carbocation carbocationcharacter; more stable transition state (left) has positive charge oncharacter; more stable transition state (left) has positive charge on

more highly substituted carbonmore highly substituted carbon

HH33CC CHCH22

HH33CC

BrBr:: ::δ+δ+

δ+δ+

HH

OOHH δ+δ+....

BrBr:: ::

HH

OO HHδ+δ+....

CC

δ+δ+

δ+δ+

ExplanationExplanation

acid-catalyzed hydration of alkenes -...

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