Ch Chapter 14 Aromatic Compounds Modified from sides of William Tam & Phillis Chang

Nomenclature Naming monosubstituted benzenes Most: 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 benzenes With two substituents: Their relative positions are indicated by prefixes or numbers : ortho- (abbreviated o-, or 1,2-) meta-, (m-, 1,3-) para-, (p-, 1,4-)

examples

Dimethylbenzenes = xylenes

More than two groups 1. Positions must be indicated by numbers 2. Number the benzene ring to give Substituents 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 (C 6 H 5 ) 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 addition Reactions of Benzene [+ HBr] substitution

The Kekulé Structure for Benzene

These 1,2-dibromobenzenes are not isomers RESONANCE or an equilibrium X X

+ other Br 2 additons However aromatic character?

3-D structure π-electrons above and below ring Note: Planar structure All carbons sp 2 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, delocalized electrons should be aromatic

How To Diagram the Relative Energies of p Molecular Orbitals in Monocyclics Based on Hückel’s Rule circled polygon antibonding  orbitals nonbonding  orbitals bonding  orbitals type of  orbitals orbital energy 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 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 Electron Delocalization in Aromatic Compounds Delocalization in Aromatic Compounds 1 H NMR spectrum 1 H occurs at relatively high frequency Is compelling evidence for aromaticity

(  -3.0) (  9.3)

Aromatic Ions pk a = 16 pk a = 36 pKa unsaturated and saturated hydrocarbon 44-53

sp 3 sp 2 6 π electrons aromatic

strong base LA

Aromatic, Antiaromatic, and Nonaromatic Compounds 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 Compounds

Nonbenzenoid Aromatic Compounds

Fullerenes

Heterocyclic Aromatic Compounds heterocyclic 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

poor base: loss of aromaticity still aromaticity imidazole aromatic 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