Carey - Organic Chemistry - sgchapt11

Carey - Organic Chemistry - sgchapt11

(Parte 1 de 2)

CHAPTER 1 ARENES AND AROMATICITY

Benzoic acid has a @CO2H substituent on the benzene ring.

and assuming that there is no resonance stabilization in 1,3,5-cycloheptatriene, we predict that its heat of hydrogenation will be three times that of cycloheptene or 330 kJ/mol (78.9 kcal/mol).

Cycloheptene Cycloheptane

Robinson symbolfor benzoic acid Kekulé forms of benzoic acid

Kekulé forms of toluene

CH3 CH3

Robinson symbol for toluene

CH3

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254ARENES AND AROMATICITY Themeasured heat of hydrogenation is

Therefore

Resonance energy 330 kJ/mol (predicted for no delocalization) 305 kJ/mol (observed) 25 kJ/mol (5.9 kcal/mol)

The value given in the text for the resonance energy of benzene (152 kJ/mol) is six times larger than this. 1,3,5-Cycloheptatriene is not aromatic.

Kekulé formulations of benzene. For chrysene, electrons are moved in pairs from the structure given to generate a more stable one:

1.5Birch reductions of monosubstituted arenes yield 1,4-cyclohexadiene derivatives in which the alkyl group is a substituent on the double bond. With p-xylene, both methyl groups are double-bond substituents in the product.

1,4-Dimethyl-1,4- cyclohexadiene

Na, NH3 CH3CH2OH

More stable: four rings havebenzene bonding pattern.Less stable: two rings have benzene bonding pattern.

NH2

NO2 p-Nitroaniline m-Chlorostyrene CH Cl

CH2

1,3,5-Cycloheptatriene Cycloheptane

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have two @CO2H groups. None of the hydrogens of a tert-butyl substituent on a benzene ring is benzylic, and so this group is inert to oxidation. Only the benzylic methyl groups of 4-tert-butyl-1,2- dimethylbenzene are susceptible to oxidation; therefore, the product is 4-tert-butylbenzene-1,2- dicarboxylic acid.

1.8Each of these reactions involves nucleophilic substitution of the SN2 type at the benzylic position of benzyl bromide.

CH2SH

Phenylmethanethiol CH2BrBenzyl bromide

Hydrogen sulfide ion

CH2NBenzyl azide CH2BrBenzyl bromide

Benzyl tert-butyl ether CH2BrBenzyl bromide tert-Butoxide ion

CH3 CH3

Susceptibleto oxidationNot benzylic hydrogens; not readily oxidized

4-tert-Butylbenzene- 1,2-dicarboxylic acid

CH2Br

OCH3

NO2 CH3

OCH3

NO2

Only these hydrogens are benzylic.

NBS 80 C, peroxides

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1.9The dihydronaphthalene in which the double bond is conjugated with the aromatic ring is more stable; thus 1,2-dihydronaphthalene has a lower heat of hydrogenation than 1,4-dihydronaphthalene.

(c)Bromine adds to alkenes in aqueous solution to give bromohydrins. Awater molecule acts as a nucleophile, attacking the bromonium ion at the carbon that can bear most of the positive charge, which in this case is the benzylic carbon.

1.1Styrene contains a benzene ring and will be appreciably stabilized by resonance, which makes it lower in energy than cyclooctatetraene.

1.12The dimerization of cyclobutadiene is a Diels–Alder reaction in which one molecule of cyclobutadiene acts as a diene and the other as a dienophile.

1.13(b)Since twelve 2porbitals contribute to the cyclic conjugated system of [12]-annulene, there will be 12 molecular orbitals. These MOs are arranged so that one is of highest energy, one is of lowest energy, and the remaining ten are found in pairs between the highest and lowest

Diene Dienophile Diels–Alder adduct

CH CH2

Structure contains an aromatic ring.

Styrene: heat of combustion 4393 kJ/mol (1050 kcal/mol)

Cyclooctatetraene (not aromatic): heat of combustion 4543 kJ/mol (1086 kcal/mol)

Styrene CH CH2

CH CH2 O

Peroxybenzoic acid

Benzoic acid

Styrene 2-Bromo-1-phenylethanol (82%)

CHCH2BrOH CH2

2-Phenylpropene

CH3 CH3 2-Phenyl-1-propanol (92%)

1,2-Dihydronaphthalene Heat of hydrogenation 101 kJ/mol (24.1 kcal/mol)

1,4-Dihydronaphthalene Heat of hydrogenation 113 kJ/mol (27.1 kcal/mol)

256ARENES AND AROMATICITY

BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website energy orbitals. There are 12 electrons, and so the lowest 5 orbitals are each doubly occupied, whereas each of the next 2 orbitals—orbitals of equal energy—is singly occupied.

1.14One way to evaluate the relationship between heats of combustion and structure for compounds that are not isomers is to divide the heat of combustion by the number of carbons so that heats of combustion are compared on a “per carbon” basis.

As the data indicate (within experimental error), the heats of combustion per carbonof the two aromatic hydrocarbons, benzene and [18]-annulene, are equal. Similarly, the heats of combustion per carbon of the two nonaromatic hydrocarbons, cyclooctatetraene and [16]-annulene, are equal. The two aromatic hydrocarbons have heats of combustion per carbon that are less than those of the nonaromatic hydrocarbons. On a per carbon basis, the aromatic hydrocarbons have lower potential energy (are more stable) than the nonaromatic hydrocarbons.

1.15The seven resonance forms for tropylium cation (cycloheptatrienyl cation) may be generated by moving electrons in pairs toward the positive charge. The resonance forms are simply a succession of allylic carbocations.

Benzene

Heats of combustion: 3265 kJ/mol (781 kcal/mol)

Heats of combustion per carbon:

544 kJ/mol (130 kcal/mol)

Cyclooctatetraene

4543 kJ/mol (1086 kcal/mol)

568 kJ/mol (136 kcal/mol)

[16]-Annulene

9121 kJ/mol (2182 kcal/mol)

570 kJ/mol (136 kcal/mol)

[18]-Annulene

9806 kJ/mol (2346 kcal/mol)

545 kJ/mol (130 kcal/mol)

Antibonding orbitals (5)

Nonbonding orbitals (2) Bonding orbitals (5)

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1.16Resonance structures are generated for cyclopentadienide anion by moving the unshared electron pair from the carbon to which it is attached to a position where it becomes a shared electron pair in a bond.

1.17The process is an acid–base reaction in which cyclopentadiene transfers a proton to amide ion (the base) to give the aromatic cyclopentadienide anion. The sodium ion (Na ) has been omitted from the equation.

1.18(b)Cyclononatetraenide anion has 10 electrons; it is aromatic. The 10 electrons are most easily seen by writing a Lewis structure for the anion: there are 2 electrons for each of four double bonds, and the negatively charged carbon contributes 2.

1.19Indole is more stable than isoindole. Although the bonding patterns in both five-membered rings are the same, the six-membered ring in indole has a pattern of bonds identical to benzene and so is highly stabilized. The six-membered ring in isoindole is not of the benzene type.

1.20The prefix benz- in benzimidazole (structure given in text) signifies that a benzene ring is fused to an imidazole ring. By analogy, benzoxazole has a benzene ring fused to oxazole.

Similarly, benzothiazole has a benzene ring fused to thiazole.

Benzothiazole

Benzimidazole

Benzoxazole

Indole more stable

Isoindole less stable

Six-membered ring corresponds to benzene.

Six-membered ring does not have same pattern of bonds as benzene.

1,3-Cyclopentadiene Amide ion Cyclopentadienideanion Ammonia

258ARENES AND AROMATICITY

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Protonation of N-1:

The species formed on protonation of N-1 is not aromatic. The electron pair of N-1 that contributes to the aromatic 6 -electron system of imidazole is no longer available for this purpose because it is used to form a covalent bond to the proton in the conjugate acid.

Protonation of N-3:

The species formed on protonation of N-3 is aromatic. Electron delocalization represented by the resonance forms shown allows the 6 -electron aromatic system of imidazole to be retained in its conjugate acid. The positive charge is shared equally by both nitrogens.

These are the four constitutional isomers. sec-Butylbenzene is chiral and so exists in enantiomeric Rand Sforms.

Allylbenzene CH2CH CH2

Butylbenzene (1-phenylbutane)

CH2CH2CH2CH3 sec-Butylbenzene (2-phenylbutane)

CHCH2CH3 CH3

Isobutylbenzene (2-methyl-1-phenylpropane) tert-Butylbenzene (2-methyl-2-phenylpropane)

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The two higher ranked substituents,phenyl and ethyl, are on opposite sides of the double bond. (c)The constitution of 2-phenyl-2-butene is . The Zstereoisomer is

Thetwohigherranked substituents,phenylandmethyl,areonthesamesideofthedoublebond. (d)1-Phenylethanol is chiral and has the constitution . Among the substituents

attached to the stereogenic center, the order of decreasing precedence is

In the Renantiomer the three highest ranked substituents must appear in a clockwise sense in proceeding from higher ranked to next lower ranked when the lowest ranked substituent is directed away from you.

(f)In p-chlorophenol the benzene ring bears a chlorine and a hydroxyl substituent in a 1,4- substitution pattern.

2-Nitrobenzenecarboxylic acid

Cl p-Chlorophenol

CH2OH Cl

H3C CO H

(R)-1-Phenylethanol

C CH3

CH2CH3

260ARENES AND AROMATICITY

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(j)Acetophenone (from text Table 1.1) is . Therefore

(k)Styrene is and numbering of the ring begins at the carbon that bears the side chain.

OCH3

CH2CH CH2

Estragole 4-Allylanisole

NO2

Diosphenol 2,6-Diiodo-4-nitrophenol

NH2 m-Xylidine 2,6-Dimethylaniline

CH3CH2

Br CH CH23 41

4-Bromo-3-ethylstyrene

C H3C

NO2 m-Nitroacetophenone

NH2

Br BrBr

2,4,6-Tribromoaniline p-Diisopropylbenzene

ARENES AND AROMATICITY261

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The prefixes o-,m-, and p- may not be used in trisubstituted arenes; numerical prefixes are used. Note also that benzenecarboxylicmay be used in place of benzoic. (c)In the various tribromophenols, we are dealing with tetrasubstitution on a benzene ring.

Again, o-,m-, and p- are not valid prefixes. The hydroxyl group is assigned position 1 because the base name is phenol.

2,4,5-Tribromophenol

OH Br

Br Br

2,4,6-Tribromophenol Br

BrOH Br

3,4,5-Tribromophenol

Br OH Br

2,3,4-Tribromophenol

OH Br

Br Br

2,3,5-Tribromophenol

Br OH

BrBr 2,3,6-Tribromophenol

Br Br OH

Cl Cl CO2H

2,6-Dichlorobenzoicacid 3,4-Dichlorobenzoic acid

Cl Cl

3,5-Dichlorobenzoic acid

CO2H Cl Cl

2,3-Dichlorobenzoic acid

CO2H Cl

2,4-Dichlorobenzoic acid

Cl Cl

2,5-Dichlorobenzoic acid

Cl Cl

CH3 NO2 o-Nitrotoluene (2-nitrotoluene)

CH3

NO2 p-Nitrotoluene(4-nitrotoluene) m-Nitrotoluene (3-nitrotoluene)

CH3 NO2

262ARENES AND AROMATICITY

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(d)There are only three tetrafluorobenzenes. The two hydrogens may be ortho, meta, or para to each other.

( f ) There are three isomeric bromoanthracenes. All other positions are equivalent to one of these.

Their relative stabilities are determined by steric effects. Mesitylene (the 1,3,5-trisubstituted isomer) is the most stable because none of its methyl groups are ortho to any other methyl group. Ortho substituents on a benzene ring, depending on their size, experience van der Waals strain in the same way that cis substituents on a carbon–carbon double bond do. Because the carbon–carbon bond length in benzene is somewhat longer than in an alkene, these effects are smaller in magnitude, however. The 1,2,4-substitution pattern has one methyl–methyl repulsion between ortho substituents. The least stable isomer is the 1,2,3-trimethyl derivative, because it is the most crowded. The energy differences between isomers are relatively small, heats of combustion being 5198, 5195, and 5193 kJ/mol (1242.4, 1241.6, and 1241.2 kcal/mol) for the 1,2,3, 1,2,4, and 1,3,5 isomers, respectively.

1,2,3-Trimethylbenzene

CH3 CH3

CH3 1,2,4-Trimethylbenzene

CH3

CH3 CH3

1,3,5-Trimethylbenzene

BrBr Br

1-Bromoanthracene 2-Bromoanthracene 9-Bromoanthracene

Naphthalene-1- carboxylic acid

Naphthalene-2- carboxylic acid

1,2,3,4-Tetrafluorobenzene

1,2,3,5-Tetrafluorobenzene

1,2,4,5-Tetrafluorobenzene H

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1.27 p-Dichlorobenzene has a center of symmetry. Each of its individual bond moments is balanced byan identical bond dipole oriented opposite to it. p-Dichlorobenzene has no dipole moment. o-Dichlorobenzene has the largest dipole moment.

1.28The shortest carbon–carbon bond in styrene is the double bond of the vinyl substituent; its length is much the same as the double-bond length of any other alkene. The carbon–carbon bond lengths of the ring are intermediate between single- and double-bond lengths. The longest carbon–carbon bond is the sp2to sp2single bond connecting the vinyl group to the benzene ring.

1.29Move electron pairs as shown so that both six-membered rings have an arrangement of bonds that corresponds to benzene.

By rewriting the benzenoid ring in its alternative Kekulé form, bothrings become benzenoid.

Both rings are benzenoid.

This six-membered ring is not benzenoid (does not correspond to Kekulé form of benzene).

This six-membered ring is benzenoid (corresponds to a Kekulé form of benzene).

Less stableMore stable

CH CH2 140 pm147 pm

134 pm p-Dichlorobenzene 0 D m-Dichlorobenzene 1.48 D o-Dichlorobenzene 2.27 D

Cl Cl

264ARENES AND AROMATICITY

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(c)The structure portrayed for phenanthrene contains two terminal benzenoid rings and a nonbenzenoid central ring. All three rings may be represented in benzenoid forms by converting one of the terminal six-membered rings to its alternative Kekulé form as shown:

(d)Neither of the six-membered rings is benzenoid in the structure shown. By writing the cyclooctatetraene portion of the molecule in its alternative representation, the two six-membered rings become benzenoid.

(b)Sodium and ethanol in liquid ammonia is the combination of reagents that brings about Birch reduction of benzene rings. The 1,4-cyclohexadiene that is formed has its isopropyl group as a substituent on one of the double bonds.

CH(CH3)2 Isopropylbenzene 2-Bromo-2-phenylpropane

N-Bromosuccinimide benzoyl peroxide,heat C

CH3

CH3 Br

CH(CH3)2 Isopropylbenzene Benzoic acid

CH(CH3)2 1-Isopropyl-1,4-cyclohexadiene CH(CH3)2 Isopropylbenzene

Na, ethanol NH3

CH(CH3)2 Isopropylbenzene

CH(CH3)2 Isopropylcyclohexane

H2 (3 mol) Pt

Six-membered ringsare not benzenoid.Six-membered rings are benzenoid.

Central ringnot benzenoidAll three rings benzenoid

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1.32All the specific reactions in this problem have been reported in the chemical literature with results as indicated.

(b)The compound contains a substituted benzene ring and an alkene-like double bond. When hydrogenation of this compound was carried out, the alkene-like double bond was hydrogenated cleanly.

(c)Free-radical chlorination will lead to substitution of benzylic hydrogens. The starting material contains four benzylic hydrogens, all of which may eventually be replaced.

(d)Epoxidation of alkenes is stereospecific.

H3C OH

cis-4-Methyl-1-phenylcyclohexanol H3C

4-Methyl-1- phenylcyclohexene (81%)

CH3COOH acetic acid excess Cl2

CCl4, lightCCH3H

Cl (65%)

1-Ethylindene 1-Ethylindan (80%) Pt

1-Phenylcyclobutene trans-2-Phenylcyclobutanol (82%)

2-Phenylpropene2-Bromo-2-phenylpropane

(Parte 1 de 2)

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