Chapter 15 Benzene and Aromaticity - HCC Learning Web

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Chapter 15 Benzene and Aromaticity

Aromatic Compounds

• Aromatic – Originally used to describe fragrant substances

– Refers to a class of compounds that meets Hückel criteria for aromaticity

Aromatic Compounds

• Aromatic – Originally used to describe fragrant substances

– Refers to a class of compounds that meets Hückel criteria for aromaticity

Aromatic Compounds

• The Hückel 4n + 2 Rule

– Developed by Erich Hückel in 1931

– States that a molecule can be aromatic only if:

• It has a planar, monocyclic system of conjugation

• It contains a total of 4n + 2 molecules – n = 0,1,2,3…

• 4n electrons are considered antiaromatic

Aromatic Compounds: Source

• Coal and petroleum are the major sources of simple aromatic compounds

• Coal primarily comprises of large arrays of conjoined benzene-like rings

• When heated to 1000°C, coal thermally breaks down to yield coal tar

https://grist.files.wordpress.com/2013/09/lump-o-coal.jpg

A representative structure of bituminous coal Proc. Natl. Acad. Sci. USA 79, 3365 (1982)

Fractional distillation of coal tar yields many aromatic compounds

• Petroleum primarily comprises alkenes and few aromatic compounds

• Formation of more aromatic molecules occur when alkanes are passed over a catalyst at high pressure and temperature

Petroleum

http://www.investigroup.com/wp-content/uploads/2014/07/5.jpg

Aromatic Compounds: Nomenclature

• Aromatic compounds naming system uses:

– Nonsystematic names

• Aromatic compounds naming system uses:

– Nonsystematic names

– International Union of Pure and Applied Chemistry (IUPAC) Rules

• Allows use of widely used names

• International Union of Pure and Applied Chemistry (IUPAC) Rules

– Monosubstituted benzenes have systematic names with –benzene being the parent name

• International Union of Pure and Applied Chemistry (IUPAC) Rules – Arenes are alkyl-substituted benzenes

• Alkyl-substituent benzenes are smaller than the ring (<6 carbons)

– Phenyl-substituted benzenes • Phenyl-substituted benzenes are larger than the ring (>7

carbons) – The term phenyl (Ph or Φ) is used in substituent benzene ring –

– The term benzyl is used for the C6H5CH2– group

– Disubstituted Benzenes

• Names based on the placement of substituents – Ortho- is 1,2 disubstituted

– Meta- is 1,3 disubstituted

– Para- is 1,4 disubstituted

– Provides clarity in the discussion of reactions

– Disubstituted Benzenes

• Names based on the placement of substituents – Ortho (o), meta (m) , and para (p)

» Provide clarity in the discussion of reactions

– Benzenes +2 or more substituents

• Numbers with the lowest possible values are chosen

• List substituents alphabetically with hyphenated numbers

• Common names, such as toluene can serve as root name(as in TNT)

Worked Example

• Provide the IUPAC name for the following compound

• Solution: – The compound is 1-Ethyl-2,4-dinitrobenzene

• Substituents on trisubstituted rings receive the lowest possible numbers

STRUCTURE AND STABILITY OF BENZENE

Aromatic Compounds: Stability of Benzene

• The reactivity of benzene is much lesser than that of alkenes despite having six fewer hydrogens

– Benzene - C6H6

– Cycloalkane - C6H12

Comparison of the heats of hydrogenation proves the stability of benzene

Remember

• Heat of Hydrogenation is the heat produced when alkene is reduced to an alkane – Alkene with lower (less negative) value is more stable – Reduction is exothermic (converting weaker pi bond

to stronger sigma bond) – Depends on degree of substitution of double bond

(greater substitution, lower heat of hydrogenation) – Trans alkene is lower than cis alkene

Comparison of the heats of hydrogenation proves the stability of benzene

Aromatic Compounds: Structure of Benzene

• All its C-C bonds are the same length: 139 pm — between single (154 pm) and double (134 pm) bonds

• Electron density in all six C-C bonds is identical

• Structure is planar, hexagonal

1.39 Å

• Carbon atoms and p orbitals in benzene are equivalent – Impossible to define three localized bonds in which

a given p orbital overlaps only one neighboring p orbital

• All electrons move freely in the entire ring due to equal overlap of all p orbitals

1.39 Å

• Structure is in resonance

– Resonance influences its rate of reactivity

1.39 Å

• Benzene resonance forms can be represented by a single structure with a circle in the center to indicate the equivalence of the carbon–carbon bonds – The ring does not indicate the number of electrons

in the ring but is a reminder of the delocalized structure

• Molecular orbital description of benzene – The 6 p-orbitals

combine to give: • Three bonding

orbitals with 6 electrons

• Three antibonding with no electrons

• Orbitals with the same energy are degenerate

Aromatic Compounds

• Observations about benzene and benzene like aromatic compounds – Unusually stable - Heat of hydrogenation 150

kJ/mol less negative than a hypothetical cyclic triene

– Planar hexagon - Bond angles are 120°, carbon-carbon bond length is 139 pm

– Undergoes substitution rather than electrophilic addition

– Resonance hybrid with structure between two line-bond structures

AROMATICITY AND THE HÜCKEL 4N+2 RULE

Aromatic Compounds: Hückel Rule

• States that a molecule can be aromatic only if:

– It has a planar, monocyclic system of conjugation

– It contains a total of 4n + 2 electrons

• n = 0,1,2,3…

• Antiaromatic if 4n electrons are considered

• Does molecule contain (4n+2) or 4n pi electrons

– Cyclobutadiene

• Four pi electrons

• Antiaromatic

– It reacts readily and exhibits none of the properties

corresponding to aromaticity

– It dimerizes by a Diels-Alder reaction at –78 °C

– Benzene possesses six electrons (4n + 2 = 6 when n = 1)

– Aromatic

– Cyclooctatetraene possesses eight electrons

– Not aromatic

– Comprises four double bonds

Aromatic Compounds: Stability and Molecular Orbital Theory

• Molecular orbitals for cyclic conjugated molecules

– Always contain a single lowest-lying MO

– Above lowest MO, MOs come in degenerate pairs

Energy Levels of the Six Benzene Molecular Orbitals

Worked Example

• To be aromatic, a molecule must have 4n + 2 electrons and must have a planar, monocyclic system of conjugation

– Explain why cyclodecapentaene has resisted all attempts at synthesis though it has fulfilled only one of the above criteria

Worked Example

• Solution:

– Cyclodecapentaene possesses 4n + 2 (n = 2) but is not flat

– If cyclodecapentaene were flat, the starred hydrogen atoms would crowd each other across the ring • To avoid this interaction, the ring system is distorted from

planarity

Aromatic Ions

• The 4n + 2 rule applies to ions as well as neutral substances

– Both the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic

Aromatic Ions

• How are ions aromatic?

– Starting with a neutral saturated hydrocarbon

– Remove one hydrogen from the saturated CH2

– Rehybridize the carbon from sp3 to sp2

– Result is a fully conjugated product with a p orbital on every product

Aromatic Ions

• Methods to remove hydrogen from saturated CH2 – Removing the hydrogen with both electrons (H:–) from

the C–H bond results in a carbocation

– Removing the hydrogen with one electron (H·) from the C–H bond results in a carbon radical

– Removing the hydrogen without any electrons (H+) from the C–H bond results in a carbanion

Aromatic Ions: Cyclopentadienyl Anion

• Disadvantages of the four--electron cyclopentadienyl cation and the five--cyclopentadienyl radical

– Highly reactive

– Difficult to prepare

– Not stable enough for aromatic systems

• Advantages of using the six--electron cyclopentadienyl cation

– Easily prepared

– Extremely stable

– pKa =16

• Acidicty of a hydrogen atom 37

Worked Example

• Cyclooctatetraene readily reacts with potassium metal to form the stable cyclooctatetraene dianion, C8H8

– Explain why this reaction occurs so easily

– Determine the geometry for the cyclooctatetraene dianion

Worked Example

• Solution:

– When cyclooctatetrene accepts two electrons, it becomes a (4n + 2) electron aromatic ion

– Cyclooctatetraenyl dianion is planar with a carbon–carbon bond angle of 135°, that of a regular octagon

AROMATIC HETEROCYCLES: PYRIDINE AND PYRROLE

Aromatic Heterocycles

• Heterocycle: Cyclic compound that comprises atoms of two or more elements in its ring

– Carbon along with nitrogen, oxygen, or sulfur

Aromatic compounds can have elements other than carbon in the ring

Aromatic Heterocycles: Pyridine

• Six-membered heterocycle with a nitrogen atom in its ring

• Pyridine is a relatively weak base compared to normal amines but protonation does not affect aromaticity

Aromatic Heterocycles: Pyridine

• The nitrogen lone pair electrons are not part of the aromatic system (perpendicular orbital)

• The structure of pyridine is quite similar to that of benzene – All five sp2-hybridized ions possess a p orbital

perpendicular with one to the plane of the ring – Each p orbital comprises one electron – The nitrogen atom is also sp2-hybridized and possesses

one electron in a p orbital 43

Aromatic Heterocycles: Pyrimidine

• Pyrimidine comprises two nitrogen atoms in a six-membered, unsaturated ring

– The sp2-hybridized nitrogen atoms share an electron each to the aromatic system

Aromatic Heterocycles: Pyrrole and Imidazole

Aromatic Heterocycles: Pyrimidine and Imidazole

• Significant in biological chemistry

• Pyrimidine is the parent ring system present in cytosine, thymine, and uracil

• Histidine contains an aromatic imidazole ring

Worked Example

• Draw an orbital picture of Furan to show how the molecule is aromatic

Worked Example • Solution:

– Oxygen contributes two lone-pair electron from a p

orbital perpendicular to the plane of the ring

– It possesses 6 electrons on a cyclic, conjugated system; it is aromatic

– Furan is an oxygen analog of pyrrole

POLYCYCLIC AROMATIC COMPOUNDS

Polycyclic Aromatic Compounds

• Hückel rule is relevant only to monocyclic compounds

• Aromaticity can also be applied to polycyclic aromatic compounds

Polycyclic Aromatic Compounds: Naphthalene Orbitals

• Three resonance forms and delocalized electrons

• Naphthalene and other polycyclic aromatic hydrocarbons possess certain chemical properties that correspond to aromaticity

– Heat of hydrogenation in naphthalene is approximately 250 kJ/mol

Polycyclic Aromatic Compounds: Naphthalene Orbitals

Comparison of the heats of hydrogenation

Naphthalene is approximately 250 kJ/mol

Polycyclic Aromatic Compounds: Naphthalene

• Naphthalene possesses a cyclic, conjugated electron system – p orbital overlap is present along the ten-carbon

periphery of the molecule and across the central bond • Aromaticity is due to the electron delocalization

caused by the presence of ten electrons (Hückel number)

Polycyclic Aromatic Compounds: Analogs of Naphthelene

• Quinolone, isoquinolone, and purine have pyridine-like nitrogens that share one electron

• Indole and purine have pyrrole-like nitrogens that share two electrons

Worked Example

• Azulene, a beautiful blue hydrocarbon, is an isomer of naphthalene

– Determine whether it is an aromatic

– Draw a second resonance form of azulene in addition to the form shown below

Worked Example

• Solution:

– Azulene is an aromatic because it has a conjugated cyclic electron system containing ten electrons (a Hückel number)

SPECTROSCOPY OF AROMATIC COMPOUNDS

Spectroscopy of Aromatic Compounds: IR

• Infrared Spectroscopy – C–H stretching absorption is seen at 3030 cm–1

• Usually of low intensity • Left of typical saturated C-H stretch

– A series of peaks are present between 1450 and 1600 cm–1 • Caused by the complex molecular motions of the ring

Spectroscopy of Aromatic Compounds: Ultraviolet Spectroscopy

• Presence of a conjugated system makes ultraviolet spectroscopy possible

– Intense absorption occurs near 205 nm

– Less intense absorption occurs between 255 nm and 275 nm

Spectroscopy of Aromatic Compounds: NMR Spectroscopy

• The aromatic ring shields hydrogens – Absorption occurs between 6.5 and 8.5 δ

• The ring current is responsible for the difference in chemical shift between aromatic and vinylic protons – Ring current is the magnetic field caused by the

circulation of delocalized electrons when the aromatic ring is perpendicular to a strong magnetic field • The effective magnetic field is greater than the applied field

• Aromatic protons appear as two doublets at 7.04 and 7.37 δ

• Benzylic methyl protons appear as a sharp singlet at 2.26 δ

Spectroscopy of Aromatic Compounds: 13C NMR Spectroscopy

• Carbons in aromatic ring absorb between 110 and 140 δ

• Shift is distinct from alkane carbons but in same range as alkene carbons

• Mode of substitution influences the formation of two, three, or four resonances in the proton-decoupled 13C NMR spectrum

The Proton-Decoupled 13C NMR Spectra of the Three Isomers of

Dichlorobenzene

Summary

• The term aromatic refers to the class of compounds that are structurally similar to benzene

• The Hückel rule states that in order to be aromatic, a molecule must possess 4n + 2 electrons, where n = 0,1,2,3, and so on

• Apart from IUPAC terms, disubstituted benzenes are also called ortho, meta, or para derivatives – The C6H5 unit is called a phenyl group

– The C6H5CH2 unit is called a benzyl group

Summary

• Planar, cyclic, conjugated molecules with other numbers of electrons are antiaromatic

• Pyridine and pyrimidine are six-membered, nitrogen containing, aromatic heterocycles

Give the shape of the benzene molecule.

a)Tetrahedral

b)Bent

c) Trigonal pyramidal

d)Planar

Give the shape of the benzene molecule.

a) Tetrahedral

b) Bent

c) Trigonal pyramidal

d) Planar

Explanation:

All six carbons and six hydrogens are in the same plane.

Give the hybridization of each carbon in benzene.

b) sp2

c) sp3

d) sp4

Give the hybridization of each carbon in benzene.

b) sp2

c) sp3

d) sp4

Explanation:

Each carbon in benzene is sp2 hybridized.

Give the bond angle of the atoms in benzene.

a)45°

b)60°

c) 90°

d)109.5°

e)120°

Give the bond angle of the atoms in benzene.

a) 45°

b) 60°

c) 90°

d) 109.5°

e) 120°

Explanation:

The carbons are trigonal planar with angles of 120°.

Classify

a)Aromatic

b)Antiaromatic

c) Nonaromatic

d)Acyclic

Classify

a) Aromatic

b) Antiaromatic

c) Nonaromatic

d) Acyclic

Explanation:

The compound gives a whole number for N in Hückel’s rule

(4N + 2 = 6, N = 1).

Classify

a)Aromatic

b)Antiaromatic

c) Nonaromatic

d)Acyclic

Classify

a) Aromatic

b) Antiaromatic

c) Nonaromatic

d) Acyclic

Explanation:

The compound is cyclic and has continuous delocalized electrons, but

does not give a whole number for Hückel’s rule (4N + 2 = 8, N = 3/2).

Classify

a)Aromatic

b)Antiaromatic

c) Nonaromatic

d)Acyclic

Classify

a) Aromatic

b) Antiaromatic

c) Nonaromatic

d) Acyclic

Explanation:

This cyclic compound does not have a continuous, overlapping ring of

p orbitals and is nonaromatic.

a)Pyridine

b)Pyrrole

c) Pyrimidine

d)Imidazole

a) Pyridine

b) Pyrrole

c) Pyrimidine

d) Imidazole

Explanation:

Pyrrole is a heterocyclic aromatic compound.

a)Imidazole

b)Pyrimidine

c) Pyridine

d)Purine

e)Furan

a) Imidazole

b) Pyrimidine

c) Pyridine

d) Purine

e) Furan

Explanation:

Pyrimidine is an aromatic compound with nitrogens in the 1 and 3

positions.

Classify

a)Aromatic

b)Antiaromatic

c) Nonaromatic

d)Acyclic

Classify

a) Aromatic

b) Antiaromatic

c) Nonaromatic

d) Acyclic

Explanation:

Furan is a heterocyclic aromatic compound.

a)Pyrimidine

b)Imidazole

c) Purine

d)Furan

e)Thiophene

a) Pyrimidine

b) Imidazole

c) Purine

d) Furan

e) Thiophene

Explanation:

Thiophene is a heterocyclic aromatic compound.

a)Anthracene

b)Naphthalene

c) Phenanthrene

d)Benzene

a) Anthracene

b) Naphthalene

c) Phenanthrene

d) Benzene

Explanation:

Naphthalene contains two benzene rings fused together.

a)Anthracene

b)Naphthalene

c) Phenanthrene

d)Benzene

a) Anthracene

b) Naphthalene

c) Phenanthrene

d) Benzene

Explanation:

Anthracene contains three benzene rings fused together.

a)Purine

b)Indole

c) Benzimidazole

d)Quinoline

a) Purine

b) Indole

c) Benzimidazole

d) Quinoline

Explanation:

Indole contains a benzene ring with a five-membered ring fused to it.

a)Purine

b)Indole

c) Benzimidazole

d)Quinoline

a) Purine

b) Indole

c) Benzimidazole

d) Quinoline

Explanation:

Benzimidazole contains a benzene ring with an imidazole fused to it.

a)4-Bromo-3-chloroaniline

b)4-Bromo-3-chlorophenol

c) 4-Bromo-3-chloroanisole

d)1-Bromo-2-chloro-4-aniline

e)1-Bromo-2-chloro-4-phenol

a) 4-Bromo-3-chloroaniline

b) 4-Bromo-3-chlorophenol

c) 4-Bromo-3-chloroanisole

d) 1-Bromo-2-chloro-4-aniline

e) 1-Bromo-2-chloro-4-phenol

Explanation:

Aniline is the parent compound. The NH2 is at position one.

a) p-Methylphenol

b) m-Methylphenol

c) o-Methylphenol

d) 4-Methylphenol

e) 3-Methylphenol

a) p-Methylphenol

b) m-Methylphenol

c) o-Methylphenol

d) 4-Methylphenol

e) 3-Methylphenol

Explanation:

The groups are on adjacent carbons, which is ortho.

a)3-Amino-5-benzaldehyde

b)5-Amino-3-benzaldehyde

c) 3-Aminobenzaldehyde

d)5-Nitro-3-benzaldehyde

e)3-Nitrobenzaldehyde

a) 3-Amino-5-benzaldehyde

b) 5-Amino-3-benzaldehyde

c) 3-Aminobenzaldehyde

d) 5-Nitro-3-benzaldehyde

e) 3-Nitrobenzaldehyde

Explanation:

Benzaldehyde is the parent compound.

a)1,3-Dinitrophenol

b)1-Hydroxy-2,4-dinitrobenzene

c) 2,4-Dinitrobenzen-1-ol

d)2,4-Dinitrophenol

e)4,6-Dinitrophenol

a) 1,3-Dinitrophenol

b) 1-Hydroxy-2,4-dinitrobenzene

c) 2,4-Dinitrobenzen-1-ol

d) 2,4-Dinitrophenol

e) 4,6-Dinitrophenol

Explanation:

Phenol is the parent compound.

Name C6H5CH2CH2C≡CCH3

a)1-Phenylpent-3-yne

b)5-Phenylpent-2-yne

c) 4-Phenylpent-2-yne

d)1-Phenylbut-2-yne

e)1-Phenylbut-3-yne

Name C6H5CH2CH2C≡CCH3

a)1-Phenylpent-3-yne

b)5-Phenylpent-2-yne

c) 4-Phenylpent-2-yne

d)1-Phenylbut-2-yne

e)1-Phenylbut-3-yne

Chapter 15 Benzene and Aromaticity - HCC Learning Web

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