15- Benzene

How is an aromatic compound unique?

Distinguished by electronic configuration & special conjugated system

What are the sources of aromatic hydrocarbons?

• high temp distillation of coal tar

• heating petroleum at high temp & pressure over catalyst

Phenyl

• benzene ring is a substituent

• phenyl = benzene plus something added

Benzyl

C6H5CH2

How should benzenes be named?

based on relative positions on ring, choose lowest numbering, list substituent alphabetically

What is unique about the structure of benzene?

• all C-C bonds same length (139 pm)

• planar hexagonal structure

• C-C-C bond angles 120

• each C is sp2 w/ perpendicular p orbital

What are stability indicators?

substitution reactions instead of addition

heats of hydrogentation show 150 kj more

What is the Huckel 4n+2 Rule?

4n+2 pi electrons, n=0,1,2,3,4, minimum 6

What is the criteria for aromatic compounds?

• cyclic w/ conjugated pi bond

• each ring atom has free p orbital

• planar structure

• more stable than expected

Anti-Aromatic

• cyclic w/ conjugated pi bond

• each ring atom has free p orbital

• planar structure

Non-Aromatic

no continuous overlapping p orbitals

Pyridine

Nitrogen lone pair not part of aromatic system, aromatic heterocycle

Pyrrole

Lone pair electrons in aromatic ring, aromatic heterocycle

Aromatic → heteroatom contributes additional pi electrons from lone pair

Polycyclic Aromatic Compounds

• fused aromatic rings

• resonance energy decreases w/ more rings (lose 2 electrons)

• more willing to undergo addition reactions

IR of Aromatic Compounds

C-H stretching at 3030 cm-1

UV of Aromatic Compounds

Peaks near 205 nm & 255-275 nm

1H NMR of Aromatic Compounds

aromatic hydrogens between 6.5-8.0

13C NMR of Aromatic Compounds

Carbons absorb at 110-140

What is the bond angle in benzene’s structure?

120

What type of reaction do aromatic compounds undergo instead of addition reactions?

Substitution

What makes cyclobutadiene anti-aromatic

has 4 pi electrons

What is the heat of hydrogenation for benzene?

206 kJ/mol instead of 356 expected

What determines if a ring is names as a substituent/phenyl or main chain?

length of alkyl group attached (>6 = phenyl)

Why does the 4n+2 rule exist?

takes 2 electrons to fill lowest orbital & 4 electrons for each succeeding level

What happens when polycyclic aromatic compounds have more fused rings in terms of their reactivity?

Become more willing to undergo addition reactions

Why is pyrrole a weak base compared to normal amines?

protonation destroys aromaticity

Why is pyrrole a weak base compared to normal amines?

protonation doesn’t affect aromaticity

Ortho

next to substituent

Meta

1 away from substituent

Para

across from substituent

Aromatic Ions

• 4n+2 rule applies to ions & neutral species

• have 6 pi electrons in a ring of continuous p orbitals