heteroaromatic : aromatic molecules containing 1 atom other than C in the aromatic ring

e.g.

6 p electrons, lone pair electrons not part of the p system

6 p electrons, aromatic, lone pair of electrons are part of the p system

If the lone pair of electrons were donated to a H⁺ or a substrate in a nucleophilic substitution, would the compound still be aromatic?

a. yes b. no

2 Guidelines for Lone Pair Electrons and Aromaticity

If the structure as written has a double bond to the heteroatom with the lone pair electrons, then those electrons are probably not part of the p system.

The p system will use as many electrons as it needs to make it aromatic.

Frost Circles

Hückel’s rule is based on the fact that the filled MO’s of aromatic rings are well below the energy of a nonbonding MO which makes them very stable. Antiaromatic rings have partially filled or completely filled nonbonding and antibonding MO’s. They may also have bonding MO’s that are unfilled or half filled.

The relative energies of the MO’s in aromatic rings can be determined by drawing Frost circles.

1. Draw a circle.

2. Inscribe the polygon representing the ring in the circle with the vertex pointing down.

3. Each vertex represents the relative energy of a MO.

4. Draw a line through the center of the circle. This represents the position of the relative energy of nonbonding MO’s (arbitrarily set to 0).

5. Fill in the electrons.

6. Electrons fill all bonding MO’s : aromatic

Electrons fill nonbonding MO’s or antibonding MO’s : antiaromatic

e. g. benzene cyclobutadiene

Frost Circles are of limited usefulness. They only apply to monocyclic fully conjugated systems. They may or may not help with heteroaromatic compounds.

Aromaticity: The special stability of planar cyclic fully conjugated molecules with 4n + 2 p electrons. Such molecules will have molecular orbital systems with all bonding molecular orbitals filled and all antibonding molecular orbitals empty. (also nonbonding)

Annulene: A cyclic polyene that is at least formally fully conjugated.

Hückel’s rule and Frost circle point to an exception to resonance stabilization.

7 resonance forms can be written indicating greater stability. Is it aromatic?

a. aromatic b. antiaromatic

As shown with cyclooctatetraene, antiaromaticity can be avoided by adopting a nonplanar structure.

In some cases, angle strain, torsional strain, and van der Waals strain destabilize a planar structure to a greater extent than aromaticity stabilizes it. The molecule then adopts a nonplanar structure and does not exhibit aromatic characteristics.

e.g.

This compound has 10 p electrons, but it is not aromatic because it cannot be planar.

This compound has 10 p electrons and is planar. Experimental evidence shows that it is aromatic.

Aromaticity and antiaromaticity are most pronounced for small rings (7 or less C’s).

Polynuclear Aromatic Hydrocarbons

naphthalene	quinoline	anthracene

fused

Aromaticity and Forms (Allotropes) of C

Diamonds

Graphite

structure of graphite

MoS₂ has a layered structure similar to graphite.

Buckminsterfullerene C₆₀

structure of buckyballs

Nomenclature of Aromatic Compounds

All aromatic rings with one 6 membered ring are given the base name benzene.

1. If there is only 1 substituent on the ring,

substituent name + benzene

There is no need to indicate the substituent position

iodobenzene	propylbenzene	a. neopropylbenzene b. isopropylbenzene c. sec-propylbenzene d. t-propylbeznene (common : cumene)

2. If there are two substituents on the benzene ring:

a. one can # the substituents on the ring using the same priority rules as in alkanes

b. one can identify the substituents as being in the o- (ortho), m- (meta), or p- (para) positions relative to each other.

1-bromo-2-fluorobenzene 1-bromo-2-fluorobenzene

o-bromofluorobenzene m-bromofluorobenzene

1-bromo-4-fluorobenzene

o-bromofluorobenzene

a. 1-ethyl-4-pentylbenzene

b. 4-ethyl-1-pentylbenzene

c. 1-pentyl-4-ethylbenzene

d. 1-hexyl-4-propylbenzene

e. 1-propyl-4-hexylbenzene

3. If there are 3 or more substituents on the aromatic ring:

a. only #’s are used to locate the substituents using the same priority rules as with alkanes.

b. the benzene ring is numbered to give the lowest possible numbers to the substituents

c. substituents are listed in alphabetical order

2-bromo-1,4-diethylbenzene

4. The benzene ring as a substituent.

a. when benzene is attached to a chain of > 6 C atoms, treated as a phenyl substituent (Ph)

4-phenylheptane

b. when benzene is attached to an alkene

4-phenyl-1-butene

c. when benzene is in a molecule with a high priority group (will be explained later)

Common Names Accepted as IUPAC

What is 2,4,6-trinitrotoluene more commonly known as?

a. The explosive TNT

b. Seasoning for Doritos

c. Heart medication known as “Nitro”

d. An antioxidant for plastics

Reactions of Aromatic Compounds

Chapter 13 and 14

(Chapter 14 Substitution Reactions of Aromatic Compounds)

Aromatic compounds tend to react in a manner that retains aromaticity.

a. p system of aromatic ring only temporarily disrupted during course of reaction

b. only substituents undergo reaction, not aromatic ring itself

c. only a few examples for reactions that successfully occur that result in loss of aromaticity

Electrophilic Aromatic Substitution

(EAS)

General Reaction

The reaction will work if other substituents are attached to the ring. However they will have an effect on the rate of the reaction and where E attaches.

Mechanism

In most cases, the first step will be the rate determining step (RDS).

Electrophilic Addition of an Alkene

EAS Reactions and the Nature of the Electrophile E⁺

A. Halogenation

Mechanism

Formation of E⁺

1.

2.

B. Sulfonation

If just concentrated H₂SO₄ or D₂SO₄ is used, then just H⁺ or D⁺ transfer occurs on the aromatic ring. The mechanism is once again exactly electrophilic aromatic substitution except that E⁺ is H⁺ or D⁺.

If however a mixture of H₂SO₄ and SO₃ known as oleum which contains very little water, then a different reaction occurs because SO₃ is also a strong electrophile. This reaction is called a sulfonation.

benzenesulfonic acid

p-toluenesulfonic acid p-toluenesulfonate anion, tosylate anion

Detergent Dye: amaranth red (sodium salt)

R = long chain alkyl group

C. Nitration

Formation of the Electrophile

nitronium ion

very good E⁺

Nitronium ion adds to aromatic ring using the same mechanism as the other electrophiles.

Aromatic nitro compounds are used in the synthesis of dyes, and a series of nitrations are used to make TNT.

D. Friedel-Crafts Alkylation

alkyl halide

e.g.

isopropyl replaces H

Formation of Electrophile

Where have you just seen a species with an analogous structure?

a. nitration

b. halogenation

c. sulfonation

d. H exchange

complex is the electrophile because the 1°carbocation is too unstable too form

Drawback to Friedel-Crafts Alkylation:

F. Friedel-Crafts Acylation

an acid chloride

acetyl chloride acetophenone

ethanoyl chloride

Mechanism

Formation of Electrophile

acylium ion

1.

2.

Conversion to alkylbenzenes

Clemmensen Reduction

Wolff-Kishner Reduction

only 1 isomer

In most cases, it is preferable to synthesize a compound in as few steps as possible. Occasionally, 2 steps will be more efficient than one. Use of the Friedel-Crafts acylation followed by reduction to make an alkylbenzene is an example. This 2 step method is most useful when Friedel-Crafts alkylation leads to rearrangements.

G. Reaction of aryl diazonium ions with “activated” aromatic rings

Other reagents for converting nitro groups to amines:

H₂, Pd/C, EtOH or 1. LiAlH₄ 2. H₃O⁺, H₂O

trifluoroperacetic acid

NaONO + HCl ® HONO + NaCl

sodium nitrite nitrous acid

sodium salt N,N-dimethylaniline methyl orange

of sulfanilic acid