Benzene and Aromaticity

Goals

w After this chapter you should be able to:

n Identify and describe aromatic compounds

n Name simple aromatic compounds

n Explain Aromatic Substitution Reactions

n Use Hückel 4n+2 rule and MOT to predict aromaticity and reactivity

n Identify Prominent IR peaks

n Identify prominent NMR peaks

Aromaticity

w Compounds were once classified as aromatic because they stink!

w However today aromatic compound are considered benzene ring containing compounds

w Or any compound that obeys the Hückel 4n+2 rule.

Sources of Aromatic Compounds

w Fractional distillation of coal tar yields

n Benzene

n Toluene

n Xylenes

n Naphthalene

n Anthracene

n Phenanthrene

n Etc.

Aromatic Nomenclature

w Monosubstituted (Best name underlined)

n X= -Br Bromobenzene

n X= -NO₂ Nitrobenzene

n X= -CH₃ Methylbenzene or Toluene

n X= -OH Hydroxybenzene or Phenol

n X= -CH₂CH₂CH₃ Propylbenzene

n X= -NH₂ Aminobenzene or Aniline

n X= -CHO Benzaldehyde

n X= -CO₂H Benzoic acid

Nomenclature

w Phenyl group

w Benzyl group

Nomenclature

w Ortho indicates the #2 position relative to another substituent group or functional group.

w Meta indicates the #3 position relative to another substituent group or functional group.

w Para indicates the #4 position relative to another substituent group or functional group.

w When giving this designation o-, p- and m- are used in front of the substituent group’s name.

Disubstituted Aromatics

w Orthodi____ benzene

n O-di_____ benzene

n 1,2-di______ benzene

w Metadi____ benzene

n M-di_____ benzene

n 1,3-di_____ benzene

w Paradi____ benzene

n P-di_____ benzene

n 1,4-di_____ benzene

Disubstituted Aromatics

w When naming disubstituted benzenes, the lowest number is given to the substituent group(s), the names of the substituent groups is given in alphabetic order and the name benzene or the particular benzene derivative is given last.

w For benzenes with a special name ortho, meta, and para designations are also used.

Common Names

Nomenclature Examples

w Give two names for each of the following.

More Nomenclature

w Insert your CD and click on the appropriate link for more practice.

w D:\OCOL_HTM\OCOL\CH15\CH15.HTM

w E:\OCOL_HTM\OCOL\CH15\CH15.HTM

Bromination of Benzene

Bromination of Benzene

w

Substitution reactions with benzene rings require an electrophile.

w

The acidic FeBr₃ polarized the Br-Br bond making a good electrophile.

Molecular Orbitals of Benzene

Method to Determine Relative MO Energy

w

Inscribed polygon method for benzene

The Hückel 4n+2 rule

w
To be aromatic, a p system must have 4n+2 electrons, where n = 0,1,2,3,4…

Ch15 Files\aromatics are flat.C3D

Not 4n+2

w

Ch15 Files\aromatics are flat.C3D

Conversion to 4n+2

w

Ch14 Files\cyclooctatetraene.C3D

Why 2n+2?

w The electrons in an aromatic system must occupy all bonding molecular orbitals.

Problem

w Use inscribed polygon method to determine why cycloheptatriene does not have resonance, but cycloheptatrienyl cation does.

Reactivities to Produce an Aromatic

Problem

w Use inscribed polygon method to determine why cyclooctatetraene does not have resonance, but cyclooctatetraene²⁺ anion does.

Reactivities to Produce an Aromatic

w The most acidic hydrocarbon.

(Or, it really wants to become aromatic!)

IR Spectroscopy of Aromaics

w

C-H bond stretch occurs as 3030 cm^-1

Prominent IR Peaks

^w Monosubstituted 690-710 cm^-1 and

^w
730-770 cm^-1

Prominent IR Peaks

w Ortho-disubstituted 690-700 cm^-

^w Meta-disubstituted690-700 cm^-1

^w 810-850 cm^-1

w Para-disubstituted 810-840

w http://spectra.galactic.com/spconline/default.asp

¹H NMR Spectra for Aromatics

w Absorption peaks d = 6.5 to 8.0

w A Para-Disubstuted Benzene