Aromatic Compounds

Early in the history of organic chemistry (late 18th, early 19th century) chemists discovered a class of compounds which were unusually stable

A number of these compounds had a distinct odor

Hence these compounds were called “aromatic”

Today the term aromatic is used regardless of the odor of the compound Some “aromatic” compounds have little to no odor

Benzene

The parent aromatic compound was discovered to have a molecular formula of C6H6

This 1:1 ratio of carbon to hydrogen is extremely low compared to other known compounds

It was also quickly discovered that these aromatic compounds did not react like other alkene compounds

Structure

Before NMR and other spectroscopic tools it was hard to determine the structure of organic compounds

Ultimately the symmetry of the molecule revealed its structure

All carbon atoms, and all carbon-carbon bonds, are symmetrically equivalent

To account for these observations the proposed structure consisted of a cyclic compound stabilized by resonance

Each resonance structure is equal in energy and thus each contributes equally to the overall structure

Stability

The resonance structures imply an extra stability, but the amount of stability in benzene is much more than a typical resonance structure

Consider reactivity:

ORCO3H

RCO3H O

RCO3H No reaction

The same reactivity behavior is observed for almost any alkene reaction

Aromatic Compounds Have More Stability than Conjugated Alkenes

Can measure stability by hydrogenation

H2catalyst

The energy required for this hydrogenation indicates the stability of the alkene

28.6 Kcal/mol

57.4 Kcal/mol 55.4 Kcal/mol

Almost double in energy

2 Kcal/mol Conjugation stability

How much energy should be in the hydrogenation of Benzene? Have three double bonds in conjugation, so therefore should expect ~79 Kcal/mol

(~24 Kcal/mol more than 55 Kcal/mol for 1,3-cyclohexadiene)

49.8 Kcal/mol

Benzene is ~ 30 Kcal/mol more stable than predicted!!

Aromatic Stabilization

This ~30 Kcal/mol stabilization is called “aromatic stabilization”

It is the cause of the difference in reactivity between normal alkenes

It would cost ~30 Kcal/mol to break the aromaticity and thus the normal alkene reactions do not occur with benzene

Somehow having these three double bonds in resonance in a cyclic system offers a tremendous amount of energy

Cyclic system alone, however, is not sufficient for aromatic stabilization

Consider a four membered ring

Cyclobutadiene also has a ring structure with conjugated double bonds

This compound however is highly reactive and does not exist with equivalent single and double bonds

In solution it reacts with itself in a Diels-Alder reaction

Why the Difference in Stability?

Can already see in electron density maps that cyclobutadiene is not symmetric

Benzene 6-fold symmetry

Cyclobutadiene Not symmetric

Consider the Molecular Orbitals for Benzene

For benzene there are 6 atomic p orbitals in conjugation therefore there will be 6 MO’s As the number of nodes increase, the energy increases

For lowest energy MO there are zero nodes, therefore bonding interactions between each carbon-carbon bond

Benzene model Top view with orbitals Side view

Entire MO Picture for Benzene

E

Zero nodes

2 nodes 2 nodes

4 nodes 4 nodes

6 nodes

Notice all electrons are in bonding MO’s

All the antibonding MO’s are unfilled With a cyclic system we obtain degenerate orbitals

(orbitals of the same energy)

Overall this electronic configuration is much more stable than the open chain analog

This is now the definition of an aromatic compound (not aroma), Flat conjugated cyclic system is MORE stable than the open chain analog

Consider Cyclobutadiene

E

Unlike benzene, cyclobutadiene has two electrons at the nonbonding energy level (these electrons do not stabilize the electronic structure)

Antiaromatic

Cyclobutadiene is less stable than butadiene

If a cyclic conjugated system is less stable than the open chain analog it is called antiaromatic

Part of the reason for cycobutadiene to be antiaromatic is the presence of two MO’s at the nonbonding level

In butadiene all electrons are in bonding MO’s therefore the electrons are more stable in butadiene relative to cyclobutadiene

<

Hückel’s Rule

In order to determine if a system is aromatic or antiaromatic, without needing to determine the overall electronic energy of the closed form versus the open form, Hückel’s rule was developed

First the cyclic system must have a p orbital on all atoms in a continuous cyclic chain (if there is an atom without a p orbital in the cycle then the system is nonaromatic)

In practice this means the cyclic system must be flat (to allow overlap of p orbitals)

If these criteria are met then:

If the system has 4n+2 π electrons, it is aromatic

If the system has 4n π electrons, it is antiaromatic

Examples

If there is an atom without a p orbital in the ring, the compound is nonaromatic

6 π electrons, 4n+2 where n=1 Therefore aromatic

4 π electrons, 4n where n=1 Therefore antiaromatic

nonaromatic

Remember that the cyclic ring must have overlap of p orbitals to be considered aromatic or antiaromatic

Molecule adopts a non-flat low energy conformation

top view side view

Aromatic Ions

Benzene is a neutral aromatic compound

Any compound with 4n+2 electrons in a continuous loop is considered aromatic regardless of the number of carbons in the loop

There are many aromatic compounds with a different number of electrons than atoms in the loop

Due to this difference usually these compounds are ions, hence aromatic ions

Cyclopentadienyl Anion

Cyclopentadiene is nonaromatic since there is not a p orbital on one of the carbons in the ring

Upon removal of a proton, however, there is now a p orbital on each carbon

6 electrons in system, therefore according to Hückel this is aromatic

base

nonaromatic pKa ~16

Due to this aromaticity cyclopentadienyl anion has unique properties

First, the compound is very acidic, pKa of ~16 compared to other allyl positions of ~45 Due to stability of anion once formed

Since it is aromatic it is more stable than the open chain anion analog (pentadienyl anion)

It will still react with electrophiles in an SN2 reaction

According to Hückel’s rule, however, the carbocation should be antiaromatic

Cyclopentadienyl cation has only 4 electrons in the continuous loop of p orbitals, therefore it is an antiaromatic compound

Since it is antiaromatic it will not form, too high in energy

Other Common Aromatic Ions

Any compound that will have 4n+2 electrons in a continuous loop for planar conjugated compound will be favored due to aromatic nature

Nomenclature of Benzene Derivatives

The IUPAC name of 1,3,5-cyclohexatriene is never used The common name of benzene dominates naming of these structures

In addition, another common naming tool for benzene derivates is for disubstituted compounds (ortho, meta, para)

ortho- dimethylbenzene

meta- dimethylbenzene

para- dimethylbenzene

Other naming follows rules already learned

Number along ring to give lowest number First priority substituent is at the 1-position

Other common names

CH3 OHO

OH

toluene phenol benzoic acid

If the benzene group is being considered as a substituent instead of as a root name, then it is given a phenyl prefix (reason for the word phenol)

Another common name is used for the substituted toluene (called benzyl)

Heterocyclic Aromatic Compounds

Compounds that contain atoms besides carbon can also be aromatic

Need to have a continuous loop of orbital overlap and follow Hückel’s rule for the number of electrons in conjugation

Common noncarbon atoms to see in aromatic compounds include oxygen, nitrogen, and sulfur

Pyridine

One common aromatic compound with nitrogen is pyridine

N

One carbon atom of benzene has been replaced with nitrogen

Consider the placement of electrons

N

Lone pair is orthogonal to conjugated electrons in ring

The number of electrons in conjugation is 6 (don’t include lone pair that is orthogonal to ring)

therefore pyridine follows Hückel’s rule and is aromatic

Pyridine can be protonated in acidic conditions and it will still be aromatic, protonation occurs at lone pair

Pyrrole

A similar aromatic compound is pyrrole

N H

With pyrrole the lone pair is included in the conjugated ring Have 6 electrons in loop and therefore this compound is aromatic

If protonated, however, pyrrole will become nonaromatic since the nitrogen would thus be sp3 hybridized without a p orbital for conjugation

HN

Difference in electron placement affects properties

Excess electron density of lone pair is localized orthogonal to ring in pyridine while the electron density is conjugated in ring with pyrrole

pyridine pyrrole

Some other common heterocyclic aromatic compounds

All of these compounds have 6 electrons conjugated in ring Consider where the lone pair(s) are located for each heteroatom

O S N NNHN

furan thiophene pyrimidine imidazole

Fused Rings

Compounds with more than one fused ring can also be aromatic

The simplest two ring fused system is called naphthalene

Like benzene, naphthalene is an aromatic compound with 10 electrons in a continuous ring around the cyclic system

(one p orbital on each carbon is conjugated)

The reactivity of naphthalene is similar to benzene

It is unreactive toward normal alkene reactions because any addition would lower the aromatic stabilization

If it did react, however, there would still be one benzene ring intact

HBrBr

Hypothetical reaction – does not occur

With larger fused ring systems normal alkene reactions start to occur

Anthracene

Br2

Br

Br

Reactions occur at central ring due to large aromatic stabilization remaining

Two intact benzene rings

Diels-Alder reactions can also occur about this central ring

NO2 NO2

Fused Heterocyclics

Fused ring systems with heterocyclics can also be aromatic

Extremely important compounds biologically and medicinally

Two of the four constituents of base pairs in DNA consist of fused aromatic rings, the other two bases, cytosine (C) and thymine (T), are one ring aromatic base pairs

Spectroscopy of Aromatic Compounds

We have already seen how aromatic benzene compounds have a relatively large downfield NMR shift due to aromatic ring current

Therefore any of these aromatic systems, which by definition have a ring current, have a large downfield shift

Can use as a characteristic of aromaticity

Mass Spectrometry

A characteristic peak in a MS for a benzenoid compound is the presence of a peak at m/z 91 (if formation is possible)

Due to resonance stabilized benzyl cation