||Organic Chemistry 4e Carey||
|Online Learning Center
Chapter 24: Phenols
Preparation of Phenols |
Reactions of Phenols |
Reactions of Aryl Ethers |
Spectroscopic Analysis |
Self Assessment |
Reactions of Phenol
Reaction type: Electrophilic
- Phenols are potentially
very reactive towards electrophilic aromatic substitution.
- This is because the
hydroxy group, -OH, is a strongly activating, ortho- /
para- directing substituent (review)
- Substitution typically
occurs para to the hydroxyl group unless the para position is
blocked, then ortho substitution occurs.
- The strong activation
often means that milder reaction conditions than those used for benzene itself
can be used (see table below for a comparison)
- Phenols are so activated
that polysubstitution can be a problem (similar
problems occur with anilines)
in H2O or CH3CO2H
or SO3 / H2SO4
Fe or FeX3
ROH / H+
or RCl / AlCl3
RCl / AlCl3
RCOCl / AlCl3
RCOCl / AlCl3
||C-acylation = electrophilic
||O-acylation = nucleophilic
- Phenols are examples
of bidentate nucleophiles, meaning that they can react at two positions:
- Reagents :
: acylating agent (acyl chloride or anhydride) and AlCl3
: acylating agent (acyl chloride or anhydride)
- The product of C-acylation
is more stable and predominates under conditions of thermodynamic control
(i.e. when AlCl3 is present).
- The product of O-acylation
forms faster and predominates under conditions of kinetic control
can be promoted by either:
- acid catalysis via
protonation of the acylating agent, increasing its' electrophilicity or
- base catalysis via
deprotonation of the phenol, increasing its' nucleophilicity.
- It is also known that
aryl esters readily rearrange to aryl ketones in the presence of AlCl3,
a reaction known as the Fries rearrangement:
of Phenols (Kolbe-Schmitt reaction)
Reaction type: Electrophilic
- Heating the nucleophilic
phenolate salt with carbon dioxide under high pressure / temperature results
in regioselective ortho-substitution.
- This process is also
known as the Kolbe-Schmitt synthesis.
- o-hydroxybenzoic acid
is more commonly known as salicyclic acid.
out the CHIME image for the intramolecular hydrogen bond by looking the
the position of the shared H atom with respect to the two oxygen atoms
Aspirin is the acetyl ester of salicylic acid (hence ASA), how would you
make aspirin from salicylic acid ?
Can you think of other reactions of nucleophilic species with carbon dioxide
Consider the phenolate to be an enolate, hence reactions at the a-C are typically
of Aryl Ethers
CARBOXYLATION OF PHENOLS
The nucleophilic phenolate (reacting
like an enolate) reacts with the electrophilic carbon of carbon
dioxide in the ortho position (compare this with an Aldol reaction)
The non-aromatic cyclohexadienonecarboxylate intermediate tautomerises
to the more stable aromatic enol which is further stabilized by an intramolecular
hydrogen bond. An acidic work-up will generate the carboxylic
Reaction type : Nucleophilic
- Reagents : Use a base
such as Na2CO3 to prepare the phenoxide, then add the
- Since the reaction is
SN2, the halide should be methyl or primary alkyl halides (or tosylates).
- This reaction is the
Williamson ether synthesis applied
to aromatic alcohols.
Is this method suitable for the preparation of diaryl ethers such as Ph-O-Ph ?
- In general, phenols
are more easily oxidized than simple alcohols.
- Oxidation can achieved
by reaction with silver oxide (Ag2O) or chromic acid (Na2Cr2O7),
or other oxidizing agents.
- Particularly important
are the oxidation of 1,2- and 1,4-benzenediol (pyrocatechol and hydroquinone,
respectively) and their derivatives (see examples below):
types of systems are important in biological redox-systems such as coenzyme
Q and vitamin K.
a closer look at the two one electron transfers that are believed to take
place when hydroquinone is oxidized to benzoquinone
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