chapter 12: aromatic compounds

benzene

six-membered ring with alternating single and double bonds

reactivity of benzene

benzene is very stable, therefore, unreactive

• requires a lewis acid catalyst to undergo substitution

structure of benzene

• planar, flat, all carbons are sp² hybridized

• all bond lengths are equal

• has resonance of double bonds

• electron rich so is a nucleophile

heat of hydrogenation of benzene

benzene is very low in energy, less than a typical cyclohexane, so it has a low heat of hydrogenation

how to name a monosubstituted benzene

name the substituent group and add the word benzene

• three exceptions:

  • toluene

  • phenol

  • aniline

how to name disubstituted benzenes

• use normal naming rules

• add meta, ortho, or para based on placement of groups

  • ortho = 1, 2

  • meta = 1, 3

  • para = 1,4 (opp sides)

how to name polysubstituted benzenes

• use normal naming rules

• if a substituent is part of a common root, name the molecule as derivative of that molecule, numbering that substituent as 1

benzylic position

carbon adjacent to benzene ring

• stable carbocations and radicals

• don’t see resonance in benzylic reactions, need to maintain aromaticity

criteria for aromaticity

1. molecule must be cyclic

2. molecule must be planar

3. molecule must be completely conjugated

4. must satisfy Hückel’s rule:

   1. 4n + 2 pi electrons (2,6,10,14,etc)

   2. 4n pi electrons are antiaromatic (unstable)

aromatic molecules

cyclic, completely conjugated, planar, 4n + 2 pi electrons

antiaromatic molecules

cyclic, completely conjugated, planar, 4n pi electrons

annulenes

single ring with alternating double bonds

• to name: indicate # of atoms in the ring in brackets and add annulene

  • eg: [14]-annulene

heterocycles

• contain a heteroatom

• must determine wither the lone pair is localized or delocalized on heteroatom to determine aromaticity

  • lone pair + double bond on same atom = not aromatic