Ch. 15 Benzene

Benzene does not readily do addition due to

high degrees of unsaturation

**needs catalyst to activate (Lewis acid) + will not react w/Br

results in substitution, not addition

true structure of benzene

resonance hybrid

true bond lengths are intermediates b/w single and double bonds

each carbon in benzene

trigonal planar (sp²)

has p-orbital with 1 e^- that extends above and below plane of molecule

overlap of 6 p-orbitals creates…

2 rings of e^- density (1 above, 1 below ring)

makes benzene e^- rich, reacts with strong e^- phile

Naming 1 substituent benzene

name substituent + add benzene

toluene

monosub. w/common name

methylbenzene

phenol

monosub. w/common name

hydroxybenzene

aniline

monosub. w/common names

aminobenzene

naming 2 diff substituent names

alphabetize 2 diff substituent names

Criteria for aromaticity

1. must be cyclic

2. molecule must be planar

3. completely conjugated

4. # of electrons in pi system = [4n+2]

Aromaticity - must be cyclic

each p-orbital must overlap with p-orbitals on adjacent atoms so they can share

Aromaticity - planar

ensures p-orbitals are aligned, delocalizes electron density

aromaticity - completely conjugated

p-orbital on every atom

aromaticity - 4n+2 pi electrons

Huckel’s rule = 4n + 2pi electrons, where n=0, 1, 2, 3, etc.

antiaromatic

1. fully conjugated

2. cyclic

3. planar

4. 4n pi electrons

nonaromatic compound

lacks 1+ req. for aromaticity (planar, fully conjugated, cyclic)

heterocycles can be aromatic

• lone pair must be part of delocalized system (not localized to heteroatom)

• ex: pyridine, pyrrole

Cyclopentadienyl anion vs cation vs radical

anion has 6 pi electrons, fully conj, planar, cyclic —> aromatic

cation has 4 pi electrons, fully conj, planar, cyclic —> anti-aromatic

radical has 5 pi electrons —> non aromatic

Electrophilic Aromatic Substitution (EAS)

H atom replaced by electrophile (lewis acid)

Aromatic Sulfonation

1. sulfur trioxide is activated by fuming H2SO4

2. rate determining step: attack of electrophile by aromatic ring

3. deprotonate to restore aromaticity

Nitration of benzene

1. Generate highly reactive NO₂⁺ (nitronium ion) by reacting HNO₃ with H2SO4

2. attack of electrophile (aromatic ring) — RDS

3. deprotonation with HSO₄^-

desulfonation of benzene

reverse sulfonation at high temps

Aromatic halogenation

FeBr₃ or FeCl₃ (AlCl₃) catalyst required to activate halogen to strong electrophile

attack of electrophile by benzene ring

deprotonation to restore aromaticity

Friedel crafts akylation

alkyl group sub. on to aromatic ring using AlCl₃

not good for forming primary connection because hydride shifts

carbocation is made and serves as electrophile, can rearrange

1. activation

2. attack of electrophile ring, rate determining

3. deprotonate

Friedel-Crafts Acylation

places acyl group on ring (RC=O)

must use acid chloride and catalyst

better way to make a primary connection, can’t make tertiary connection b/c starts w/carbonyl

NO rearranging

1. activation of electrophile (acylium ion is the active electrophile)

2. attack of electrophilic ring - RDS

3. deprotonation

Clemmensen reduction of ketones

done under strongly acidic conditions

Wolff-Kishner Reduction of Ketones

• uses hydrazine (NH2NH2) as reducing agent in presence of strong base (KOH) —- done under strongly basic conditions

  • in a high-boiling protic solvent (ethylene glycol)

Reduction of Nitro to amine — Hydrogenation of Benzene

2 general methods

1. reduction with a metal in the presence of acid

2. Reduction with a noble metal in presence of H2

Diazonium Salts From Aromatic Amines

1. treatment of aromatic amine with nitrous acid (HNO2) in presence of strong acid (HCl)

2. leads to loss of H2O and forms new N-N triple bond

Sandmeyer reactions

transform diazonium salt by treating with copper

Three key examples are:

1. CuCl transforms aryl diazonium salts into aryl chlorides

2. CuBr transforms aryl diazonium salts into aryl bromides

3. CuCN transforms aryl diazonium salts into aryl cyanides (nitriles).

Heteroaromatic compounds

lower resonance energies than benzene

activating groups activate ___ sites

ortho and para

deactivating groups

deactivate ortho/para sites, direct to meta sites

substitution sites is dictated by

resonance stability

inductive effects are produced by

ewg and edg through sigma bond

ewg diminish ring’s nucleophilic strength

substituents direct position of added group

stronger activating group dictates substitution

methyl directs

ortho para

nitro directs

meta

S_NAR (Nucleophilic Aromatic Subsitution)

substituted aromatic ring = electrophile

• only benzene with strong ewg can undergo snar

• LG must be ortho/para to ewg