Ch. 14 - 1

Chapter 14Chapter 14

Aromatic CompoundsAromatic Compounds

Modified from sides of William Tam & Phillis Chang

Nomenclature

Naming monosubstituted benzenesMost: benzene is the parent name and the substituent is a prefix

benzene

Other simple, common benzenes, have accepted parent name (for substituent and ring)

Disubstituted benzenesWith two substituents:

Their relative positions are indicated by prefixesor numbers : ortho- (abbreviated o-, or 1,2-) meta-, (m- , 1,3-) para-, (p- , 1,4-)

examples

Dimethylbenzenes = xylenes

More than two groups1. Positions must be indicated by numbers

2. Number the benzene ring to giveSubstituents the lowest possible numbers

More than two different substituents list in alphabetical order

A substituent gives “special” base name (aniline, anisole, etc.) that substituent is position 1

Benzene as a substituent = phenyl group (C6H5)

hydrocarbon with saturated chain and 1 benzene ring Base/parent is the larger structural unit.

butylbenzene t-butylbenzene (S)-2-phenylheptane

Unsaturated chains, the parent/base name is of that chain, (regardless of ring size)

trans-1-phenyl-2-butene

Benzyl (Bn) is a common name for the phenylmethyl group

recall

substitution not additionReactions of Benzene

[+ HBr] substitution

The Kekulé Structure for Benzene

These 1,2-dibromobenzenes are not isomers

RESONANCE

or an equilibrium

X

X

+ other Br2 additons

However

aromatic character?

3-D structure

π-electrons above and below ring

Note: Planar structure All carbons sp2 hybridized

Hückel’s Rule: The 4n + 2 π Electron Rule

(1) Planar monocyclic rings (2) containing 4n + 2 π electrons, where n = 0

or an integer (2, 6, 10, 14 . . .etc.) have substantial resonance energies, “aromatic”

i.e. a planar ring containing 6 π electrons is “aromatic”

Hückel’s rule states that planar monocyclic rings with 2, 6, 10, 14 . . . delocalized electrons should be aromatic

How To Diagram the Relative Energies of p How To Diagram the Relative Energies of p Molecular Orbitals in Monocyclics Molecular Orbitals in Monocyclics Based on HückelBased on Hückel’’s Rules Rule

circledpolygon

antibonding orbitals

nonbonding orbitals

bonding orbitals

type of orbitals

orbitalenergy levels

π molecular orbitals of cyclooctatetraene, if planar Predicted to have 2 nonbonding orbitals and

an unpaired electron in each nonbonding orbital

Not be expected to be aromatic

System not planar

The bonds alternately long and short; (1.48 and 1.34 Å)

The AnnulenesThe Annulenes

Hückel’s rule predicts that annulenes will be aromatic

if the molecule has 4n + 2 π electrons and have a planar carbon skeleton

All these (4n + 2)π, planar annulenes are aromatic

Non-planar (4n + 2)π annulenes are antiaromatic

(4n) non-planar annulenes are antiaromatic

NMR Spectroscopy: Evidence for ElectronNMR Spectroscopy: Evidence for Electron Delocalization in Aromatic CompoundsDelocalization in Aromatic Compounds

1H NMR spectrum 1H occurs at relatively high frequencyIs compelling evidence for aromaticity

(δ -3.0)

(δ 9.3)

Aromatic Ions

pka = 16 pka = 36

pKa unsaturated and saturated hydrocarbon 44-53

sp3 sp2

6 π electrons aromatic

strong base

LA

Aromatic, Antiaromatic, and Nonaromatic Aromatic, Antiaromatic, and Nonaromatic CompoundsCompounds

An aromatic compound has its π electrons delocalized over the entire ring and

It is stabilized by the π-electron delocalization

Evaluation: compare cyclic compound vs acyclic with same number of electrons.

Ring is aromatic if the ring has lower -electron energy then the acyclic chain

Based on sound calculations or experiments

Nonaromatic if the ring and the chain have the same -electron energy [non-planar]

Antiaromatic if the ring has greater π-electron energy than the open chain [4n e’s]

Cyclobutadiene

Benzene

Other Aromatic Compounds

Benzenoid polycyclic aromatic hydrocarbons having two or more fused benzene rings.

Benzenoid Aromatic CompoundsBenzenoid Aromatic Compounds

Nonbenzenoid Aromatic CompoundsNonbenzenoid Aromatic Compounds

FullerenesFullerenes

Heterocyclic Aromatic Compoundsheterocyclic compounds cyclic compounds with an

element(s) other than carbon, e.g. piperidine

aromatic heterocyclic:

Examples of useful heterocyclic aromatic compounds

Aromaticity

Basicity of nitrogen-containing heterocycles

Basicity of nitrogen-containing heterocycles

poor base:

loss of aromaticity

still aromaticityimidazolearomatic weak base

Aromatic Compounds in Biochemistry

Two amino acids necessary for protein synthesis contain the benzene ring

Derivatives of purine and pyrimidine are essential parts of DNA and RNA

Nicotinamide adenine dinucleotide important coenzymes in oxidations and reductions

-pyridine derivative (nicotinamide) -purine derivative (adenine)

O O