Organic Chemistry chp. 14 - Aromatic Compounds

Benzene nomenclature

All are attached to benzene

• “Halogen” benzene - halogen

• phenol - alcohol

• Toluene - CH3

• Aniline - NH2

• Benzenesulfonic acid - SO3H

• Benzoic acid - carboxylic acid

• Acetophenone - ketone

• Anisole - ether

Disubstituted benzene names

• Ortho - 1,2

• Meta - 1,3

• para - 1,4

Xylene

disubstituted benzene with two methyls

phenyl group naming trend

• If the attached chain is straight than use “prefix” benzene

• If attached chain is branched use phenyl + chain name

Benzyl vs phenyl reactivity

Benzyl groups are much more reactive and susceptible to substitution rxns

Phenyls are more inert

Non reactive benzene rxns

• Br2 - no halogenation

• KMnO4 - no oxidation

• H3O+ - no hydration

• H2/Ni - slow addition at high temp

Halogenation of benzene outcome

Leads to a substitution rather than an addition.

1 Br is added not 2

Best way to depict a benzene ring

circle in the center indicating delocalization of electrons around entire ring

Kekule Benzene depiction

cyclohexane with three double bonds. This depiction has problems such as the fact that isomers cannot be created and it would have a higher heat of hydrogenation compared to the actual benzene structure.

Stability of benzene explained

Benzene has a much lower heat of hydrogenation than the Kekule depiction due to resonance stability that gives each DB less heat of hydrogenation compared to DBs in cyclohexene

Why is benzene an example of perfection in nature

All bonds lengths and angles are equal

• Bond lengths: 1.09 b/w C+H 1.39 b/w C+C

• Angle: 120

Frost circle rules

• cyclic structure is surrounded by circle

• Must be a C touching the bottom

• A line must be drawn across the middle

• Bonding pie orbitals are below the line and antibonding pie orbitals are on the top

• Amount of Bonding pie orbitals is determined by number of C below or above center line

• if center line crosses C’s than nonbonding orbitals are available

Orbital type ranked by strength

• Antibonding pie: Least stable

• Nonbonding pie: Middle stability

• Bonding pie: Most stable

Annulene nomenclature

Cyclic structures with CnHn formula (ex. benzene C6H6)

[#C]annulene (ex. Benzene - [6]annulene)

Huckels rule for aromaticity

Aromatic:

• Cyclic

• Planar

• Fully conjugated

• 4n+2 resonating electrons

Anti aromatic:

• cyclic

• planar

• Fully conjugated

• 4n resonating electrons

Non aromatic:

• exception to rules above

Exception of [10]annulene and [8]annulene

nonaromatic because they are not planar due to steric hinderance or angle strain

Trick for antiaromatic annulenes

The ones with nonbonding pie orbitals will be antiaromatic

Numbering polyaromatic compounds

Carbons connecting rings get #a

ex. Naphthalene has 4a and 8a carbons

Can anions and cations be aromatic

yes

ex. cyclopentadiene (-) and cycloheptatriene (+)

Common Aromatic heterocycles

• pyridine (6 w/ N)

• pyrrole (5 w/ N)

• Furan (5 w/ O)

• Thiophene (5 w/ S)

two types of electrons in conjugated pie systems

• coplanar

• ortaganol

Are lone pairs included in aromaticity

Depends

If it is aromatic w/o than no

If it is aromatic w/ than yes

Imidazole Basicity

Accepts hydrogen at dehydrated nitrogen in order to maintain aromaticity

Furan and Thiophene lone pair electrons

One pair is coplanar and used in pie orbital to be delocalized around ring

One pair is ortaganol can be used as electron donor (Base)

Two benzene based amino acids

• Phenylalanine

• Tyrosine

where heterocycles are seen in bio

• Tryptophan (indole)

• Purines and Pyridines

• NAD+

NADH and NAD+ used in bio

NAD+ is aromatic but gets hydrogenated to become NADH which not aromatic then NADH gets oxidized in the ETC and becomes NAD+ restoring aromaticity

Polycyclic benzenoid aromatic compounds to remember

Naphthalene

anthracene

pyrene

Know Summary