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Page 1
CHAPTER 17: AROMATICS
GENERAL INFO
EXAMPLES OF AROMATICS
UV spectrum octyl methoxycinnamate
UVA: 315-400 nmUVB: 280-315 nmUVC: 100-280 nm
Page 2
NOMENCLATURE
COMMON NAMES TO MEMORIZE
Toluene Phenol Anisole Aniline Benzaldehyde Benzoic acid
1-2 SUBSTITUENTS
1-bromo-4-ethylbenzene para-bromoethylbenzene
p-bromoethylbenzene
ortho, or o- meta, or m- para, or p-
If using a common name, the parent group (for example the OH of phenol) is understood to be carbon #1.
3+ SUBSTITUENTS
AROMATICS AS A SUBSTITUENT
If a more important functional group is present, or if the parent chain is longer than 6 carbon atoms, the aromatic is a substituent.
Benzene + CH2 = benzyl Benzene alone = phenyl
CH3 OH OCH3 NH2 H
O
OH
O
Br
CH3
CH3
Page 3
PROPERTIES OF BENZENE
BOND LENGTHS1
REACTION PATTERNS
BENZENE ENERGY Hydrogenation data2
1 Jones, M., Fleming, S.A., Organic Chemistry, 4th ed., Norton, 2010, pp. 621. 2 Jones, M., Fleming, S.A., Organic Chemistry, 4th ed., Norton, 2010, pp. 627.
1.53 Å 1.32 Å 1.48 Å
H˚(kcal/mol)
-28.6 kcal/mol
Page 4
NMR SPLITTING PATTERNS
ORTHO, META, PARA COUPLING CONSTANTS
Ortho coupling
Meta coupling
Para coupling
J range
Typical J
SPLITTING PATTERNS
Tree diagram of Ha:
Page 5
BENZYLIC REACTIONS
BENZYLIC POSITION
REACTIVITY IN SN1 REACTIONS
Relative Rate3 0.0001 1.7 14 30,000
REACTIVITY IN SN2 REACTIONS
Relative Rate 1 33
3 Jones, M., Fleming, S.A., Organic Chemistry, 4th ed., Norton, 2010, pp. 613.
Page 6
AROMATICITY
COMPOUNDS WITH SPECIAL STABILITY
Resonance Energy4
(kcal/mol)
CRITERIA FOR AROMATICITY
CRITERION 1
To be “aromatic” a compound must have:
CRITERION 2
To be “aromatic” a compound must also have:
4 Most resonance energies from: Smith, M.B., March, J., March’s Advanced Organic Chemistry, 5th ed., Wiley, 2001, pp.5.
O
O
Erich Huckel (German chemist), used a quantum mechanical explanation for aromaticity in 1931.
Page 7
Molecular Orbitals of Benzene:
Explanation of Huckel’s Rule, using the π molecular orbitals:
Compound
π M.O.’s
π electrons 4 6 8 10
If flat: Unstable (high E) Aromatic (low E) Unstable (high E) Aromatic (low E)
π1
π6
π2 π3
π4 π5
Page 8
Frost’s Circle for predicting the π M.O. pattern:
CATEGORIES
AROMATIC
Aromatic compounds are lower in energy than a similar system of unconjugated alkenes.
ANTIAROMATIC
Antiaromatic compounds are higher in energy than a similar system of unconjugated alkenes.
NONAROMATIC
Nonaromatic compounds are roughly equal in energy to a similar system of unconjugated alkenes.
Cyclooctatetraene bond lengths5
5 Smith, M.B., March, J., March’s Advanced Organic Chemistry, 5th ed., Wiley, 2001, pp.62.
Page 9
EXAMPLES
O
Page 10
ANNULENES (LARGE RINGS)
[10]-Annulene [14]-Annulene [16]-Annulene [18]-Annulene
MORE EXAMPLES
Serotonin (neurotransmitter)
Ref 6
Ref 7
6 This doubly charged ion is stable in solution at -50 ˚C, but unstable at temperatures higher than -30 ˚C. Smith, M.B., March, J., March’s Advanced Organic Chemistry, 5th ed., Wiley, 2001, pp.54. 7 This structure is stable. Smith, M.B., March, J., March’s Advanced Organic Chemistry, 5th ed., Wiley, 2001, pp.63.
NH
OHH2NN
N
N
N
OH
H2N
HN
O
HN
O
OH
O OHFolic acid (vitamin)
Page 11
APPLICATION OF AROMATIC CONCEPTS
REACTIVITY IN SN1 REACTIONS
Rank the following in order of increasing rate in SN1 reactions.
DIFFERENCES IN ACIDITY
Explain the differences in pKa’s.
A B C D
pKa 15 pKa 39
Page 12
NMR SIGNALS
Shifts for [18]-Annulene8
Shifts for this compound9
8 Solomons, G., Fryhle, C.B., Organic Chemistry, 8th ed., Wiley, 2004, pp. 636. 9 Smith, M.B., March, J., March’s Advanced Organic Chemistry, 5th ed., Wiley, 2001, pp.63.