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Benzene and aromaticity

- in the early days of org chem, the word aromatic was used to describe such fragrant

substances as

Benzenefrom coal distillate

Benzaldehydefrom cherries, peaches and almonds

Toluenefrom Tolu balsam

Today, aromatic refers to benzene and its structural relatives.

- chemists of the early 19th

century were correct abt there being a difference b/w aromatic cpds

and others, but the assoctn of aromaticity w/ fragrance has long been lost.

- many cpds isolated from natural sources are aromatic in part

Steroidal hormone analgesic Synthetic drug tranquilizer

Benzenecauses bone marrow depression and leukopenia (lowered WBC count) on prolonged

exposure

- should be handled cautiously when used as a laboratory solvent

Two sources of aromatic hydrocarbons: coal and petroleum

coala complex mixture primarily made up of benzene-like rings

- thermal breakdown at 1000oC in the absence of air produces

volatile pdts called coal tar, whose fractional distn pdts include

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Petroleumcont few aromatic cpds and consists largely of alkanes

- during refining, aromatic molecules are formed when alkanes

are passed over a catalyst at abt 500oC at high P.

- for ex, heptane toluene

Naming aromatic cpdsalthough the use of nonsystematic names is

discouraged, IUPAC rules allow for some of the more widely used names to be retained

Monosubstituted benzenes are systematically named in the same manner as other hydrocarbons, with

benzene as the parent name.

dehydrogenation

cyclization

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Arenesalkyl-substituted benzenes

- named in different ways depending on the size of the alkyl grp

- if the alkyl substituent is smaller than the ring ( 6 or fewer C), the arene is named as an alkyl-

substituted benzene

- if the alkyl substituent is larger than the ring (more than 6 C), the cpd is named as a phenyl-

substituted alkane

- phenyl, sometimes abbreviated as Ph or (phi), is used for

theC6H5unit when the benzene is considered as substituent

- phenyl comes from Greekpheno (I bear light) to commemorate that benzene was

discovered by Michael Faraday in 1825 from the oily residue in illuminating gas used in London street

lamps.

Disubstituted benzenes are named using one of the prefixes

ortho- (o)

meta- (m), or

para (p).

This system of nomenclature is also useful when discussing reactions.

For ex, Reaction of bromine with toluene occurs at the para position.

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Benzenes with more than two substituents are named by numbering the position of each substituent so

that the lowest possible numbers are used.

- the substituents are listed alphabetically when writing the name

Note in the third example that toluene is used as the parent name rather than benzene.

Any of the monosubstituted aromatic cps previously retaining their nonsystematic names can serve as a

parent name, with the principal substituent (-CH3in toluene) assumed to be on C1.

Structure and stability of benzene

- Benzene is more stable than typical alkenes

- reacts slowly with Br2

- cyclic conjugation of the benzene ring is retained

A comparison of the heats of hydrogenation

- benzene is 150 kJ/mol or 36 kcal/mol more stable that might be expected for a cyclotriene.

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Further evidence for the unusual nature of benzene

- all C-C bonds have the same length of 139 pm (value is intermediate b/w typical single bond of

154 pm, double bond of 134 pm

- electrostatic potential map shows that

e- density in all C-C bonds is identical.

Thus, benzene is

- planar molecular with the shape of a regular hexagon

- all C-C-C bond angles are 120o

- all C atoms are sp2-hybridized

- each C atom has a p orbital perpendicular to the plane of the

6-membered ring

In benzene,- each p orbital overlaps equally well with both neighboring p orbitals

- thus, all 6 pi e-s are completely delocalized around the ring

- structure is the hybrid of two equivalent forms

cyclohexene

cyclohexane

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Alternative representations of benzene

circle representation must be used carefully since it does not indicate the no. of pi e-s in the ring.

Aromaticity and the Hckel 4n + 2 rule

Erich HckelGerman physicist (1931)

- a molecule is aromatic only if

a. it has a planar, monocyclic system of conjugation

b. contains a total of 4n + 2 e-s

where n is an integer (n = 0, 1, 2, 3, )

thus, only molecules with 2, 6, 10, 14, 18, e-s can be aromatic

Some examples:

- Antiaromatic

- actually highly reactive and shows none of the properties assocd w/

aromaticity

- dimerizes by a self-Diels-Alder rxn

(one molecules behaves as a diene while the other as a dienophile)

Other examples of aromatic cpds following Hckel 4n + 2 rule

4n + 2 = 6 when n = 1

Thus, aromatic

- Not aromatic

- pi e-s are localized into four double bonds rather than delocalized

around the ring

- molecule is tub-shaped rather than planar

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Aromatic ions

follows the Hckel 4n + 2 rule

The forms that are not ionic are not aromatic.

Only one wh

is aromatic

Only one which

is aromatic

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Aromatic heterocycles: pyridine and pyrrole

heterocyclea cyclic cpd that contains an atom or atoms other than carbon in its ring

--- Could be N, O, S, or P

Pyridine Pyrrole

Polycyclic aromatic cpds: Naphthalene

Hckel 4n + 2 rule is strictly applicable only to monocyclic cpds.

However, the concept of aromaticity can include polycyclic aromatic cpds;

One of cancer-causing substances isolated from tobacco smoke

All polycyclic aromatic HCs can be represented by resonance forms. Napthalene has three resonance

forms.

Naphthalene also shows many aromatic properties.

- heat of hydrogenation show aromatic stabilization E of 250 kJ/mol (60 kcal/mol)

- reacts slowly with electrophiles like Br2 to give substn rather than addn pdts

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- full delocalization of the 10 pi e-s

throughout both rings

Practice problem

Thiophene, a sulfur-contg heterocycle, undergoes typical aromatic substitution reactions rather than

addition reactions. Explain why thiophene is aromatic.

Strategy

Aromatic cpds are planar, cyclic, conjugated, with 4n + 2 e-s.

See how these facts apply to thiophene.

Solution

Thiopene is the sulfur analog of pyrrole.

The sulfur atom is sp2-hybridized and has a lone pair of e-s in a p orbital perpendicular to the plane of

the ring.

Sulfur also has a second lone pair of e-s in the ring plane.