Carey - Organic Chemistry - sgchapt06

Carey - Organic Chemistry - sgchapt06

(Parte 1 de 3)

CHAPTER 6

6.1 Catalytic hydrogenation converts an alkene to an alkane having the same carbon skeleton. Since 2-methylbutane is the product of hydrogenation, all three alkenes must have a four-carbon chain with a one-carbon branch. The three alkenes are therefore:

6.2 Themosthighlysubstituteddoublebondisthemoststableandhasthesmallestheatofhydrogenation.

2-Methyl-2-butene: most stable (trisubstituted)

112 kJ/mol (26.7 kcal/mol)

2-Methyl-1-butene (disubstituted)

118 kJ/mol (28.2 kcal/mol)

3-Methyl-1-butene (monosubstituted)

126 kJ/mol (30.2 kcal/mol) Heat of hydrogenation:

2-Methyl-1-butene

2-Methyl-2-butene 2-Methylbutane 3-Methyl-1-butene metal catalyst

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6.3(b)Begin by writing out the structure of the starting alkene. Identify the doubly bonded carbon that has the greater number of attached hydrogens; this is the one to which the proton of hydrogen chloride adds. Chlorine adds to the carbon atom of the double bond that has the fewer attached hydrogens.

By applying Markovnikov’s rule, we see that the major product is 2-chloro-2-methylbutane.

(c)Regioselectivity of addition is not an issue here, because the two carbons of the double bond are equivalent in cis-2-butene. Hydrogen chloride adds to cis-2-butene to give 2-chlorobutane.

(d)One end of the double bond has no attached hydrogens, but the other end has one. In accordance with Markovnikov’s rule, the proton of hydrogen chloride adds to the carbon that already has one hydrogen. The product is 1-chloro-1-ethylcyclohexane.

This is the carbocation that leads to the observed product, 2-chloro-2-methylbutane.

(c)Asecondary carbocation is an intermediate in the reaction of cis-2-butene with hydrogen chloride.

Capture of this carbocation by chloride gives 2-chlorobutane.

HC lCC H3CC H3 cis-2-Butene Hydrogenchloride Secondary carbocation Chloride

C H3C

HC l H

2-Methyl-1-butene Hydrogenchloride Tertiary carbocation Chloride

CH3

Hydrogenchloride Ethylidenecyclohexane 1-Chloro-1-ethylcyclohexane

HClCH3CH CH3CH2 cis-2-Butene Hydrogenchloride 2-Chlorobutane

HCl CH3CH2CHCH3 Cl

2-Methyl-1-butene 2-Chloro-2-methylbutane

Chlorine adds tothis carbon.Hydrogen adds to this carbon.

HCl CH3CH2CCH3 Cl

CH3

REACTIONS OF ALKENES: ADDITION REACTIONS125

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This carbocation is captured by chloride to give the observed product, 1-chloro-1- ethylcyclohexane.

6.5The carbocation formed by protonation of the double bond of 3,3-dimethyl-1-butene is secondary. Methyl migration can occur to give a more stable tertiary carbocation.

The two chlorides are 3-chloro-2,2-dimethylbutane and 2-chloro-2,3-dimethylbutane.

6.6The structure of allyl bromide (3-bromo-1-propene) is CH2?CHCH2Br. Its reaction with hydrogen bromide in accordance with Markovnikov’s rule proceeds by addition of a proton to the doubly bonded carbon that has the greater number of attached hydrogens.

Addition according to Markovnikov’s rule:

Addition of hydrogen bromide opposite to Markovnikov’s rule leads to 1,3-dibromopropane. Addition contrary to Markovnikov’s rule:

Allyl bromide

Hydrogen bromide HBr 1,3-Dibromopropane

BrCH2CH2CH2Br

Allyl bromide

Hydrogen bromide HBr 1,2-Dibromopropane

CH3CHCH2Br Br

3,3-Dimethyl-1-butene Secondary carbocation Tertiary carbocation

CH2CH3CCH CH3

CH3 HCl methyl migration

CH3

CH3

3-Chloro-2,2-dimethylbutane

CHCH3CH3C CH3 Cl

CH3

CH3

2-Chloro-2,3-dimethylbutane

CH3Cl

CH3CH H

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The opposite regioselectivity is observed when peroxides are present. The product is 1-bromo-2-methylbutane.

(c)Both ends of the double bond in cis-2-butene are equivalently substituted, so that the same product (2-bromobutane) is formed by hydrogen bromide addition regardless of whether the reaction is carried out in the presence of peroxides or in their absence.

The regioselectivity of addition is reversed in the presence of peroxides, and the product is (1-bromoethyl)cyclohexane.

6.8The first step is the addition of sulfuric acid to give cyclohexyl hydrogen sulfate.

CyclohexeneCyclohexyl hydrogen sulfate

Hydrogenbromide Ethylidenecyclohexane (1-Bromoethyl)cyclohexane

HBrCH3CH CH3CH Br peroxides

Hydrogenbromide Ethylidenecyclohexane 1-Bromo-1-ethylcyclohexane

HBrCH3CH CH3CH2

2-BromobutaneHydrogenbromide cis-2-Butene

CH3CH2CHCH3BrCC

H CH3 H

CH3 HBr

1-Bromo-2-methylbutane Hydrogenbromide 2-Methyl-1-butene

CH3CH2CCH2Br H

CH3 C

HBr peroxides

2-Bromo-2-methylbutane Hydrogenbromide 2-Methyl-1-butene

CH3CH2CCH3 Br

CH3 C

H HBr

REACTIONS OF ALKENES: ADDITION REACTIONS127

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6.9The presence of hydroxide ion in the second step is incompatible with the medium in which the reaction is carried out. The reaction as shown in step 1 is performed in acidic solution. There are, for all practical purposes, no hydroxide ions in aqueous acid, the strongest base present being water itself. It is quite important to pay attention to the species that are actually present in the reaction medium whenever you formulate a reaction mechanism.

6.10The more stable the carbocation, the faster it is formed. The more reactive alkene gives a tertiary carbocation in the rate-determining step.

6.11The mechanism of electrophilic addition of hydrogen chloride to 2-methylpropene as outlined in text Section 6.6 proceeds through a carbocation intermediate. This mechanism is the reverse of the E1 elimination. The E2 mechanism is concerted—it does not involve an intermediate.

6.12(b)The carbon–carbon double bond is symmetrically substituted in cis-2-butene, and so the regioselectivity of hydroboration–oxidation is not an issue. Hydration of the double bond gives 2-butanol.

1. hydroboration 2. oxidation

3-Ethyl-2-pentanol C(CH2CH3)2CH3CH3-Ethyl-2-pentene

2. oxidation 1. hydroboration

Cyclopentene Cyclopentanol OH

CH2 CH2OHH2. oxidation

1. hydroboration

CyclobutylmethanolMethylenecyclobutane

2-Butanol cis-2-Butene

CH3CHCH2CH3OHCC

H H3C H

CH3 1. hydroboration

2. oxidation

CHCH3Protonation ofgives a secondary carbocation.CH

CH2

CH3

CH3 Tertiary carbocation

CH21(CH3)2CH3O(CH3)3CH2O

128REACTIONS OF ALKENES: ADDITION REACTIONS

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6.13The bottom face of the double bond of -pinene is less hindered than the top face.

Syn addition of H and OH takes place and with a regioselectivity opposite to that of Markovnikov’s rule.

6.14Bromine adds anti to the double bond of 1-bromocyclohexene to give 1,1,2-tribromocyclohexane. The radioactive bromines (82Br) are vicinal and trans to each other.

6.15Alkyl substituents on the double bond increase the reactivity of the alkene toward addition of bromine.

1-Bromo-3-methyl-2-butanol3-Methyl-1-butene

3-Bromo-2-methyl-2-butanol2-Methyl-2-butene

2-Methyl-2-butene (trisubstituted double bond; most reactive)

3-Methyl-1-butene (monosubstituted double bond; least reactive)

2-Methyl-1-butene (disubstituted double bond)

82Br

82Br

Br H

1,1,2-Tribromocyclohexane

H 1-Bromocyclohexene Bromine

82Br 82Br

CH3

Hydroboration occurs from this direction.

Methyl group shields top face.

REACTIONS OF ALKENES: ADDITION REACTIONS129

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6.17The structure of disparlure is as shown.

Its longest continuous chain contains 18 carbon atoms, and so it is named as an epoxy derivative of octadecane. Number the chain in the direction that gives the lowest number to the carbons that bear oxygen. Thus, disparlure is cis-2-methyl-7,8-epoxyoctadecane.

6.18Disparlure can be prepared by epoxidation of the corresponding alkene. Cis alkenes yield cis epoxides upon epoxidation. cis-2-Methyl-7-octadecene is therefore the alkene chosen to prepare disparlure by epoxidation.

6.19The products of ozonolysis are formaldehyde and 4,4-dimethyl-2-pentanone.

The two carbons that were doubly bonded to each other in the alkene become the carbons that are doubly bonded to oxygen in the products of ozonolysis. Therefore, mentally remove the oxygensand connect these two carbons by a double bond to reveal the structure of the starting alkene.

6.20From the structural formula of the desired product, we see that it is a vicinal bromohydrin.Vicinal bromohydrins are made from alkenes by reaction with bromine in water.

CH2C(CH3)3 2,4,4-Trimethyl-1-pentene

CO CH3

CH2C(CH3)3 4,4-Dimethyl-2-pentanoneFormaldehyde

H cis-2-Methyl-7-octadecene

OHH Disparlure peroxy acid

H Br

CH3 trans-2-Bromo-1-methylcyclopentanol 1-Methylcyclopentene

130REACTIONS OF ALKENES: ADDITION REACTIONS

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Since the starting material given is tert-butyl bromide, a practical synthesis is:

6.21Catalytic hydrogenation of the double bond converts 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl- 2-pentene to 2,2,4-trimethylpentane.

6.22This problem illustrates the reactions of alkenes with various reagents and requires application of Markovnikov’s rule to the addition of unsymmetrical electrophiles.

2-Iodopentane I peroxides

Br 2-Bromopentane

Cl 2-Chloropentane1-Pentene orC H

2,4,4-Trimethyl-1-pentene 2,2,4-Trimethylpentane

H3C C

2,4,4-Trimethyl-2-pentene

CH3CH2OH heat

1-Bromo-2-methyl-2-propanol2-Methylpropenetert-Butyl bromide

REACTIONS OF ALKENES: ADDITION REACTIONS131

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(k)When the ozonide in part (j) is hydrolyzed in the presence of zinc, formaldehyde and butanal are formed.

6.23When we compare the reactions of 2-methyl-2-butene with the analogous reactions of 1-pentene, we find that the reactions proceed in a similar manner.

2-Bromo-3-methylbutane Br peroxides

2-Bromo-2-methylbutane Br

2-Chloro-2-methylbutane2-Methyl-2-butene Cl

Formaldehyde HCCH2CH2CH3

Butanal H2O

ZnH2COO

1,2-Epoxypentane Acetic acid

H2C 1-Bromo-2-pentanol

Br2 BrCH2CHCH2CH2CH3 OH

CHCH2CH2CH3

Br2 BrCH2CHCH2CH2CH3 Br

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HBr

1-Bromo-1-methylcyclohexane

CH3 Br

CH3 HCl

CH3 Cl

1-Methylcyclohexene 1-Chloro-1-methylcyclohexane

Acetone Acetaldehyde O

O HH3C CH3H3C

O HH3C CH3H3C

Ozonide

2-Methyl-2,3-epoxybutane

3-Bromo-2-methyl-2-butanol OH

Br H2O

2,3-Dibromo-2-methylbutane Br

Br CCl4

3-Methyl-2-butanol OH

2-Methyl-2-butanol OH

2-Iodo-2-methylbutane I

REACTIONS OF ALKENES: ADDITION REACTIONS133

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CH3O O O

CH3

6-Oxoheptanal

CH3 CH3

Ozonide

1,2-Epoxy-1-methylcyclohexane trans-2-Bromo-1- methylcyclohexanol trans-1,2-Dibromo-1- methylcyclohexane

CH3Br CCl4

CH3 trans-2-Methylcyclohexanol

CH3

1-Methylcyclohexanol

CH3 HI

1-Iodo-1-methylcyclohexane

CH3

HBr CH3

1-Bromo-2-methylcyclohexane (mixture of cis and trans) peroxides 134REACTIONS OF ALKENES: ADDITION REACTIONS

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6.25We need first to write out the structures in more detail to evaluate the substitution patterns at the double bonds.

(d)( Z)-2,2,5,5-Tetramethyl-3-hexene Two tert-butyl groups cis

Compound d, having two cis tert-butyl groups, should have the least stable (highest energy) double bond. The remaining alkenes are arranged in order of increasing stability (decreasing heats of hydrogenation) according to the degree of substitution of the double bond: monosubstituted, cisdisubstituted, trans-disubstituted, trisubstituted. The heats of hydrogenation are therefore:

6.26In all parts of this exercise we deduce the carbon skeleton on the basis of the alkane formed on hydrogenation of an alkene and then determine what carbon atoms may be connected by a double bond in that skeleton. Problems of this type are best done by using carbon skeleton formulas.

(b)May be formed by hydrogenation ofProduct is 2,3-dimethylbutane. or

(c)May be formed by hydrogenation ofProduct is methylcyclobutane. oror

REACTIONS OF ALKENES: ADDITION REACTIONS135

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6.27Hydrogenation of the alkenes shown will give a mixture of cis- and trans-1,4-dimethylcyclohexane.

Only when the methyl groups are cis in the starting alkene will the cis stereoisomer be the sole product following hydrogenation. Hydrogenation of cis-3,6-dimethylcyclohexane will yield exclusively cis-1,4-dimethylcyclohexane.

2. H2O2, HO CH3CH3-Ethyl-2-pentanol

CH3CH 3-Ethyl-3-pentanol

C(CH2CH3)2 HBrperoxidesCH3CH 2-Bromo-3-ethylpentane

C(CH2CH3)2 HClCH3CH 3-Chloro-3-ethylpentane

CCl4CH3CH

2,3-Dibromo-3-ethylpentane3-Ethyl-2-pentene

CH3CHC(CH2CH3)2 Br Br

H2catalystH3CC H3 cis-3,6-Dimethylcyclohexene

H3CC H3 cis-1,4-Dimethylcyclohexane cis-1,4-Dimethylcyclohexane trans-1,4-Dimethylcyclohexane

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2,2-Dimethyl-1-propanol cannot be prepared by hydration of an alkene, because no alkene can have this carbon skeleton. (b)Hydroboration–oxidation of alkenes is the method of choice for converting terminal alkenes to primary alcohols.

2-Methyl-1-butene 2-Methyl-2-butanol

2-Methyl-2-butene 2-Methyl-2-butanol

CH3CH2CH2CH CH3CH2CH2CH2CH2OHCH2 1. B2H6

1-Pentanol1-Pentene

3-Methyl-1-butanol3-Methyl-1-butene

CH3CH2CC H3CH2CHCH2OHCH2 CH3 CH3

2-Methyl-1-butene 2-Methyl-1-butanol

1-Pentanol 2-Methyl-1-butanol 3-Methyl-1-butanol 2,2-Dimethyl-1-propanol CH3CH2CH2CH2CH2OH CH3CH2CHCH2OH (CH3)2CHCH2CH2OH (CH3)3CCH2OH

CH3

CH3CH CH3 CH2CH3

3-Ethyl-2,3-epoxypentane

REACTIONS OF ALKENES: ADDITION REACTIONS137

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(b)In the presence of peroxides, hydrogen bromide adds with a regioselectivity opposite to that predicted by Markovnikov’s rule. The product is the corresponding primary bromide.

(f)In aqueous solution bromine reacts with alkenes to give bromohydrins. Bromine is the electrophile in this reaction and adds to the carbon that has the greater number of attached hydrogens.

2-Methyl-2-butene Bromine

3-Bromo-2-methyl-2-butanol (observed yield 7%)

CH3 2-Methyl-1-pentene

BrCH2CCH2CH2CH3 CH3

1,2-Dibromo-2-methylpentane (observed yield 60%)

CHCl3

OH CH3

CH3

CH3

1,2-Dimethylcyclohexene cis-1,2-Dimethylcyclohexanol (observed yield 82%)

2-tert-Butyl-3,3-dimethyl-1-butanol(observed yield 65%) 2-tert-Butyl-3,3-dimethyl-1-butene

(CH3)2CHCH2CH2CH2CH2CH2Br(CH3)2CHCH2CH2CH2CH CH2 HBr peroxides

1-Bromo-6-methylheptane(observed yield 92%) 6-Methyl-1-heptene

CH3CH2CH CHCH2CH3 HBr no peroxides

3-Bromohexane (observed yield 76%)Hydrogenbromide 3-Hexene

CH3CH2CH2CHCH2CH3 Br

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6.31The product is epoxide B.

Epoxidation is an electrophilic addition; oxygen is transferred to the more electron-rich, more highly substituted double bond. Atetrasubstituted double bond reacts faster than a disubstituted one.

6.32(a)There is no direct, one-step transformation that moves a hydroxyl group from one carbon to another, and so it is not possible to convert 2-propanol to 1-propanol in a single reaction. Analyze the problem by reasoning backward. 1-Propanol is a primary alcohol. What reactions dowehaveavailableforthepreparationofprimaryalcohols?Onewayisbythehydroboration– oxidation of terminal alkenes.

(Parte 1 de 3)

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