Phenols Ar-OH

Phenols are compounds with an –OH group attached to an aromatic carbon. Although they share the same functional group with alcohols, where the –OH group is attached to an aliphatic carbon, the chemistry of phenols is very different from that of alcohols.

Nomenclature.

Phenols are usually named as substituted phenols. The methylphenols are given the special name, cresols. Some other phenols are named as hydroxy compounds. OH OH CH 3 OH OH COOH phenol OH OH Br

m

-bromophenol OH

o

-cresol OH salicylic acid COOH OH catechol resorcinol OH hydroquinone OH

p

-hydroxybenzoic acid

physical properties

phenols are polar and can hydrogen bond phenols are water insoluble phenols are stronger acids than water and will dissolve in 5% NaOH phenols are weaker acids than carbonic acid and do not dissolve in 5% NaHCO 3

Intramolecular hydrogen bonding is possible in some ortho-substituted phenols. This intramolecular hydrogen bonding reduces water solubility and increases volatility. Thus,

o

-nitrophenol is steam distillable while the isomeric

p

-nitrophenol is not.

OH O H N O O NO 2

o

-nitrophenol bp 100 o C at 100 mm 0.2 g / 100 mL water volatile with steam

p

-nitrophenol bp

decomposes

1.69 g / 100 mL water non-volatile with steam

phenols, syntheses: 1. From diazonium salts

N 2 H 2 O,H + OH

2. Alkali fusion of sulfonates

SO 3 Na NaOH,H 2 O 300 o ONa H + OH

Reactions: alcohols 1. HX 2. PX 3 3. dehydration 4. as acids 5. ester formation 6. oxidation phenols NR NR NR phenols are more acidic similar NR

Phenols, reactions: 1. as acids 2. ester formation 3. ether formation 4. EAS a) nitration b) sulfonation c) halogenation d) Friedel-Crafts alkylation e) Friedel-Crafts acylation f) nitrosation g) coupling with diaz. salts h) Kolbe i) Reimer-Tiemann

as acids: with active metals :

OH Na ONa + H 2 (g) sodium phenoxide

with bases:

CH 4 < NH 3 < HC  CH < ROH < H 2 O < phenols < H 2 CO 3 < RCOOH < HF OH ONa SA + NaOH SB WB + H 2 O WA

CH 4 < NH 3 < HC  CH < ROH < H 2 O < phenols < H 2 CO 3 < RCOOH < HF OH + NaOH SA water insoluble SB OH + NaHCO 3 ONa WB water soluble + H 2 O WA NR phenol < H 2 CO 3

CH 4 < NH 3 < HC  CH < ROH < H 2 O < phenols < H 2 CO 3 < RCOOH < HF water 5% NaOH 5% NaHCO 3 phenols

insoluble soluble insoluble

carboxylic acids

insoluble soluble soluble

We use the ionization of acids in water to measure acid strength (Ka): HBase + H 2 O H 3 O + + Base Ka = [H 3 O + ][ Base ] / [ HBase] ROH Ka ~ 10 -16 - 10 -18 ArOH Ka ~ 10 -10 Why are phenols more acidic than alcohols?

ROH + H 2 O H 3 O + + RO OH ArOH + H 2 O H 3 O + + ArO OH O O O O O Resonance stabilization of the phenoxide ion, lowers the PE of the products of the ionization, decreases the ΔH, shifts the equil farther to the right, makes phenol more acidic than an alcohol

effect of substituent groups on acid strength?

OH O G + H 2 O G + H 3 O

Electron withdrawing groups

will decrease the negative charge in the phenoxide, lowering the PE, decreasing the ΔH, shifting the equil farther to the right,

stronger acid

.

Electron donating

groups will increase the negative charge in the phenoxide, increasing the PE, increasing the ΔH, shifting the equilibrium to the left,

weaker acid .

Number the following acids in decreasing order of acid strength (let # 1 = most acidic, etc.) OH OH OH OH OH 3 NO 2 CH 3 Br 5 1 4 2

SO 3 H COOH OH CH 2 OH 1 2 3 4

2. ester formation

(similar to alcohols) OH CH 3 + O CH 3 CH 2 C OH H + OH COOH + (CH 3 CO) 2 O O CH 3 CH 2 C O H 3 C H 3 C O C O COOH + H 2 O salicyclic acid aspirin

H 3 C O C O COOH analgesic anti-inflamatory antipyrretic anticoagulant aspirin Reye's syndrome not to be used by children with high fevers!

OH H 3 C C O NH acetaminophen aspirin substitute Tylenol Kidney damage!

3. ether formation (Williamson Synthesis)

Ar-O Na + + R-X  Ar-O-R + NaX

note: R-X must be 1 o or CH 3

Because phenols are more acidic than water, it is possible to generate the phenoxide

in situ

using NaOH.

OH OCH 2 CH 3 + CH 3 CH 2 Br, NaOH CH 3 CH 3

4. Electrophilic Aromatic Substitution

The –OH group is a powerful activating group in EAS and an

ortho/para

director.

a) nitration

OH OH HNO 3 dilute HNO 3 O 2 N OH NO 2

polynitration!

NO 2 OH NO 2 + OH NO 2

b) halogenation

OH OH Br 2 (aq.) Br 2 , CCl 4 non-polar solvent Br OH Br Br OH Br + no catalyst required use polar solvent

polyhalogenation!

OH Br

c) sulfonation

OH H 2 SO 4 , 15-20 o C OH SO 3 H OH H 2 SO 4 , 100 o C SO 3 H At low temperature the reaction is non-reversible and the lower Eact

ortho

product is formed (rate control).

At high temperature the reaction is reversible and the more stable

para

product is formed (kinetic control).

d) Friedel-Crafts alkylation

.

OH + CH 3 H 3 C C CH 3 Cl AlCl 3 OH H 3 C C CH 3 CH 3

e) Friedel-Crafts acylation

OH + O CH 3 CH 2 CH 2 C Cl AlCl 3 OH O Do not confuse FC acylation with esterification: OH + O CH 3 CH 2 CH 2 C Cl O O

Fries rearrangement of phenolic esters.

OH + O CH 3 CH 2 CH 2 C Cl O O OH AlCl 3 O

f) nitrosation

OH OH HONO

p

-nitrosophenol NO EAS with very weak electrophile NO+ OH CH 3 NaNO 2 , HCl OH CH 3 NO

g) coupling with diazonium salts

(EAS with the weak electrophile diazonium) OH CH 3 + N 2 Cl benzenediazonium chloride OH CH 3 N N an azo dye

h) Kolbe reaction (carbonation)

ONa + CO 2 125 o C, 4-7 atm.

OH COONa EAS by the weakly electrophilic CO 2  O C O sodium salicylate H + OH COOH salicylic acid

i) Reimer-Tiemann reaction

OH CHCl 3 , aq. NaOH 70 o C H + OH CHO salicylaldehyde The salicylaldehyde can be easily oxidized to salicylic acid

Spectroscopy of phenols: Infrared:

O—H stretching, strong, broad 3200-3600 cm -1 C—O stretch, strong, broad ~1230 cm -1 (alcohols ~ 1050 – 1200)

nmr:

O—H 4-7 ppm (6-12 ppm if intramolecular hydrogen bonding)

o

-cresol O--H C--O

c

o

-cresol OH b CH 3 a c b a

ethyl salicylate (intramolecular hydrogen bonding) d OH O C O CH 2 CH 3 b a c d c b a