Phenols

Phenol

The antiseptic property of phenol was discovered in 1865 by Joseph Lister, a physician in Scotland.

Listerine, named after him, contains phenol as the active ingredient.

Today, over two million tons of phenol are made each year in the US alone.

Phenol is used for resins, glue to make plywood, plastics, and pharmaceuticals.

OH

Examples of Phenols

OH

CH3

CH(CH3)2

HO

CH3ON

C

O

H

OH

OCH3

eugenol (cloves; dental analgesic/antiseptic)

OH

OCH3

COH

vanillin

capsaicin (hot peppers; analgesic, anti-inflammatory)

OH

COCH3

O

methyl salicylate(oil of wintergreen)

thymol in thyme)

CH3 CH3

OHHO

bisphenol-A

More phenols…

OH

OH

C15H29

urushiol(s)(poison ivy, oak)

OH

C(CH3)3(CH3)3C

CH3

OH

C(CH3)3(CH3)3C

OCH3BHT (food preservative)

BHA (food preservative)

OH

Cl

Cl

Cl

Cl

Cl

pentachlorophenol(wood preservative)

OH

OH

CH2CH2CH2CH2CH2CH3

n-hexyl resorcinol (antiseptic)

HO

OH

OH

resveratrol(antioxidant in red grapes)

Slight Acidity of Phenol

Phenol has a Ka = 10-10 (pKa =10) Alcohols have a Ka = 10-16 (pKa =16)

The reason phenol is more acidic than an alcohol is resonance stabilization of the phenolate anion (next slide)

OH O

+ H

ROH + HRO

Resonance of Phenolate ion

O O

O O O

Substituents that stabilize an anion enhance the acidity of phenol.Phenol has a pKa = 10; p-nitrophenol has a pKa = 7.1

Picric acid (2,4,6-trinitrophenol)

2,4,6-trinitrophenol is so acidic that it is called picric acid; it has a Ka = 10-1 (pKa =1)

The enhanced acidity compared to phenol itself (Ka = 10-10) is due to the increased resonance stabilization of the conjugate base (phenolate anion) by the nitro groups:

O

N

O

O

O

N

O

O

O

N

O

O

IR spectrum of phenol

O-H

C=CC-O

IR spectrum of cyclohexanol

O-H C-O

O-H C-OO-HC-O

CH3CH2CH2CH2OH

IR spectrum of 1-butanol

OH

OH

H

H

H

H

H

H

H

H

CDCl3 (solvent)

CH2 OH

CH3

CH3CH2CH2CH2OH

CH2

CH2

CH3CH2CH2CH2OH

CH3

CH3

CH2

CH2

CH2

CH2

CH2 O

CH2 O

OH

OH

Summary of Spectral Features

IR: O-H stretch, 3600-3000 cm-1, broad

(alcohols and phenols)

C-O stretch, 1250 to 1050 cm-1, (higher

for phenols than for alcohols)

13C-NMR: C-O, 60-65 alcohols phenols

1H-NMR:-CH2-O, 3.5