condition for aromaticity

Organic Chemistry II TA: Robert Spencer

Ch. 13: Conjugation and Aromaticity Stability of Aromatic Compounds As a result of cyclic delocalization and resonance, most aromatic compounds are more stable and less reactive than alkenes.

Heats of Hydrogenation

H2 Pd

- 118 kJ/mol

3H2 Pd

- 206 kJ/mol

Benzene Molecular Orbitals Frost Circle: a method f or determining the relative order of pi MOs in planar, fully conjugated monocyclic compounds

Note: When inscribing the polygon, ensure that one of the vertices is placed at the bottom. Each point where one of the vertices touches the circle represents the location of one of the molecular orbitals.

Antibonding orbitals

Bonding orbitals Energy Determining Aromaticity

Example: Cyclooctatetraene

Four Conditions: 1. Must be cyclic 2. Must be planar 3. Must be f ully-conjugated 4. Must have 4n+2 number of pi electrons A molecule is anti-aromatic if it meets the f irst three conditions but only has 4n pi electrons.

This compound is not actually planar:

A molecule is non-aromatic if it does not meet any of the f irst three conditions.

Aromatic Ions Cations:

Non-aromatic (sp3 carbon)

Anions:

4e-

Aromatic (now sp2)

5e-

Heterocyclic Compounds sp2-hybridized Lone pair in sp2 orbital (orthogonal) N

6 pi electrons N

Aromatic 6e-

Lone pair in p orbital

NH

6 pi electrons

N H

sp2-hybridized

Lone pair in sp2 orbital (orthogonal) O

6 pi electrons

O

In general, heteroatoms that are double bonded will not contribute their lone pairs because their p orbitals are already being used for the double bond. Heteroatoms single bonded to other atoms can contribute one lone pair to the pi system (the other pair is perpendicular). Acid and Base Properties Stronger acids have more stable conjugate bases. H N

Aromatic

Not Aromatic pK a = 15

Aromatic pK a = 23

N

Aromatic

Pyrrole (right) is considered a weak acid compared to cyclopentadiene (left). To determine the strength of a base, one must look at the stability of the conjugate acid. H N

H N

Pyrrole is a weak base because protonation destroys aromaticity H

Aromatic

N

NH

Imidazole

Cl

Not Aromatic pK a = - 4

In this compound, the lone pair on the left nitrogen is in an sp2 orbital that does not interact with the pi system. As a result, this nitrogen is more basic than the other.

Chemistry of Benzene The most common reaction of aromatic compounds is electrophilic aromatic substitution. H H

H

H +

E

H

E

Base

E

H H

Deuterated Acid: H

D3O+/D2O

H D

D

H D

D2O

H

D

D

Birch Reduction Example: Na/NH3 Na/NH3

EtOH

EtOH

OEt

OEt

COOH

COOH

1. Na/NH3, EtOH

1. Na/NH3, EtOH

2. H2O/H3O+

2. H2O/H3O+

Electron-releasing groups end up attached to a double bond

Electron-withdrawing groups end up on the methylene positions

Benzyl Compounds An orbital at the benzylic position is stabilized by overlap with adjacent orbitals. The benzyl position is similar to the allylic position.

CH2

benzyl position H2C

allylic position

CH2

Benzyl compounds are very active f or SN1 and SN2 reactions. Cl

(+)

(+)

(+)

(+)

Radical Reactions at the Benzyl Position

Br

NBS CCl4, hv

Br

Br2 hv

Need benzyl hydrogen!

Oxidation at Benzyl Position (CH)nCH3

COOH

KMnO4 or H2Cr2O7

KMnO4 or H2Cr2O7