chapter 17: aromatics - chem 22, o-chem ii€¦ · to be “aromatic” a compound must have:...

CHAPTER 17: AROMATICS

GENERAL INFO

EXAMPLES OF AROMATICS

UV spectrum octyl methoxycinnamate

UVA: 315-400 nmUVB: 280-315 nmUVC: 100-280 nm

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

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

NMR SPLITTING PATTERNS

ORTHO, META, PARA COUPLING CONSTANTS

Ortho coupling

Meta coupling

Para coupling

J range

Typical J

SPLITTING PATTERNS

Tree diagram of Ha:

BENZYLIC REACTIONS

BENZYLIC POSITION

REACTIVITY IN SN1 REACTIONS

Relative Rate3 0.0001 1.7 14 30,000


Relative Rate 1 33

3 Jones, M., Fleming, S.A., Organic Chemistry, 4th ed., Norton, 2010, pp. 613.

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.

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

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.

EXAMPLES

O

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)

APPLICATION OF AROMATIC CONCEPTS


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

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.

chapter 17: aromatics - chem 22, o-chem ii€¦ · to be “aromatic” a compound must have:...

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