Organic Chemistry II:CHEM 2211

Chapter 14

Aromaticity

Chapter 14 Problem Set

14.1-14.15 (in chapter, help to understand materials);

14.16-29; 14.30-14.34; 14.38-14.42; Learning group problem #2,#3.

13. Retrosynthesis!!!

• Please review Org I reactions!

• Reagents and their structures!

• Elimination: KOtBu; NaOEt; Pyridine

• Reduction: H2, Pd/C; LiAlH4; NaBH4; DIBAL

• Oxidation: PCC; KMnO4; Jone’s; Swern oxidation12.4B

• Ozonalysis: 8.16B

• Epoxidation: mCPBA

14.1 Structure of benzene: snake bits its own tail!• Early 19th century chemists

derive benzene formula (C6H6)and molecular mass (78)

• Problems with Kekulé’sstructure

• If Kekulé’s structure were tohave two chloride substituentsreplacing two hydrogen atoms,there should be a pair of 1,2-dichlorobenzene isomers.

• benzene is fairly inert and failsto undergo reactions thatcharacterize normal alkenesKekulé’s structure

CCCCC

CH

H H

HH

H

CCCCC

CH

H H

ClCl

H

CCCCC

CH

H Cl

ClH

H

14.3 Reactivity: not like C=C!

14.5 Improved stability

+ H2

HH

ΔHo = -120 kJ/mol

+ H2

HH

ΔHo = -232 kJ/mol2H

H

+ H2

HH

ΔHo = -208 kJ/mol3H

H

H

H

14.3 Reactivity: not like C=C!

Three pi bonds lying within a ring.

• Each atom is sp2 • Planarity.

• 6 pi electrons (4n +2 = 6, n = 1).

• “All C-C, bond lengths are equal,

and halfway between C-C and C=C.

All C-H bonds are equal.

All C-C-C bond angles are equal,

and all H-C-C bond angles are equal at 120°

14.2 Nomenclature: functional group and position

F

fluorobenzene

NO2

nitrobenzene

CH3

toluene

OH

phenol

NH2

aniline

COOH

benzoic acid

CHO

benzaldehyde styrene

CN

benzonitrile

14.2 Nomenclature: di-substituted benzene

o-xylene m-xylene p-xylene

12

1

3

1

2

BrBr

1,2-dibromobenzene

o-dibromobenzene

Ortho- meta- para-

14.2 Nomenclature: multi-substituted benzene

O2N

Br

COOH

3-bromo-5-nitrobenzoic acid

Br1-(3-bromo-5-(tert-butyl)phenyl)ethan-1-one

O

13

5

Br

H

O

Br

t-Bu

2-bromo-5-(tert-butyl)-1,3-dimethylbenzene

NO2

ClCH3

14.6 Aromaticity

naphthalene

anthracene

Are they aromatic?

14.6 Aromaticity!!!!

• Cyclic

• Conjugate

• Planar

• 4n+2= # of pi electron (n=0 or positive integer)

14.7 Aromatic, anti-aromatic and non-aromatic

Cyclic?Yes

Conjugated?

No

Planar?

4n+2?

Yes

Yes

Yes

Aromatic!

Non-aromaticNo

No

NoAnti-aromatic

Examples:

N

NH N

HN

4n

14.7 Aromatic, anti-aromatic and non-aromatic

Exception:

Examples:

[10]-annulene

14.9 Hetero-Aromatic compounds

ONH

N

S

O

NH

indole furan thiophene

pyridine pyrrolepyrylium