[2] Phenols | Chemical Properties of Phenol | Nature of substituents on the benzene ring | BSc 2nd Y

Chemical properties of phenols

Electrophilic substitution reactions

The presence of OH group on benzene increases the electron density on the benzene ring making it more susceptible to attack by an electrophile. The reactions involving benzene ring are electrophilic substitution reaction. The presence of OH group makes the orthoand para carbon of benzene more electron rich than meta position. The OH group is called o ‒, p ‒ directing group.

Fries rearrangement:

Esters of phenol gives phenolic ketones on rearrangement in the presence of anhydrous AlCl3. This reaction is called fries rearrangement.

Substituent Effects

Activation and Deactivation

Because benzene acts as a nucleophile in electrophilic aromatic substitution, substituents that make the benzene more electron-rich can accelerate the reaction. Substituents that make the benzene moor electron-poor can retard the reaction. In the mid-twentieth century, physical organic chemists including Christopher Ingold conducted a number of kinetic studies on electrophilic aromatic substitution reactions. In table 1, you can see that some substituents confer a rate of reaction that is much higher than that of benzene (R = H). Phenol, C6H5OH, undergoes nitration a thousand times faster than benzene does.  Nitrobenzene, C6H5NO2, undergoes the reaction millions of times more slowly.

Directing Effects

In addition to exerting an effect on the speed of reaction, substituents on the benzene ring also influence the regiochemistry of the reaction. That is, they control where the new substituent appears in the product.

Remember, there are three different positions on the benzene ring where a new substituent can attach, relative to the original substituent.  Substitution could actually occur on five positions around the ring, but two pairs are related by symmetry. Isomerism in disubstituted benzenes can be described by numbering the substituents (1,2- etc) or by the relationships ortho-, meta– and para-.  There are two positions ortho– to the initial substituent and two positions meta– to it.

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Time stamp
00:00- Chemical Properties of Phenol
1:18- Acidic Nature of Phenol
10:55-Nature of substituents on the benzene ring
25:36- Williamson's Synthesis
27:40- Claisen Rearrangement with mechanism
34:00- Fries Rearrangement with mechanism

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