Alcohols, Phenols and Ethers
15.0 Chemical properties of phenols
15.0 Chemical properties of phenols
Chemical properties are as follows:
1. Acidity of phenols
2. Ester formation
3. Phenol in Williamson’s Synthesis
4. Reaction with Ammonia
5. Reaction with Zn dust
6. Fries Rearrangement
7. Alkylation
8. Bromination
9. Nitration
10. Sulphonation
11. Kolbe Reaction
12. Reimer-Tiemann reaction
1.Acidity of phenols: The most characteristic property of phenols is their acidity. Phenols are more acidic that alcohols which are even more weakly acidic than water, but phenols are less acidic than carboxylic acids.
$R - COOH{\text{ }} > {\text{ }}Ar - OH{\text{ }} > {\text{ }}H - OH{\text{ }} > {\text{ }}R - OH$
A summary of acidic character of phenols is given here.
(i) Greater acidity of a phenol than an alcohol is due to possibility of resonance in phenol which leads to electron-deficient oxygen atom. Presence of electron-deficient oxygen atom (see structures II, III and IV) in turn weakens the$ - \mathop O\limits^ + - \leftarrow H$ bond, and thus facilitates release of proton.
Such structures are not possible in alcohols.
(ii) Once hydrogen atom is removed from phenol, the ion (phenoxide) is very much stabilized due to delocalization of its negative charge.
Remember that phenoxide ion is very much more stable than the parent compound phenol because phenoxide ion does not involve charge separation, while in phenol three equivalent resonanting structures (II to IV) involve charge separation.
(iii) electron-withdrawing substituents increase the acidity of phenols; while electron releasing substituents decrease acidity. Thus substituents affect acidity of phenols in the same way as they affect acidity of carboxylic acids; it is of course, opposite to the way these groups affect basicity of amines.
G withdraws electrons, thus disperses the –ve charge of the ion, stabilizes it and hence increases ionization of the parent phenol.
$(where{\text{ }}G{\text{ }} = {\text{ }}-N{O_2},{\text{ }}-CN,{\text{ }}-CHO,{\text{ }}-COOH, - \mathop N\limits^ + {R_{3'}} - X)$
G releases electrons, thus intensifies the –ve charge of the ion, destabilizes it and hence decreases ionization of the parent phenol.
$\left( {where{\text{ }}G{\text{ }} = - R, - OR, - N{R_2}} \right)$
2.Ester formation: Phenols reacts with carboxylic acid unhydride and acid chloride to form esters. These reactions are quite similar to these of alcohols.
3. Phenol in Williamson’s Synthesis: Phenols canbe converted to ethers through the williamson’s synthesis Because phenols are more acidic than alcohols. they can be converted to sodium phenoxide through the use of sodium hydroxide.
4.Reaction with Ammonia: Phenol is converted to aniline when heated with ammonia under pressure or in presence of ${Z_n}C{l_2}.$
5.Reaction with Zn dust: Phenol when treated with Zn dust it gives benzene.
6. Fries Rearrangement: Phenyl acetate undergoes the Fries rearrangement with $AlC{l_3}$ to form ortho and para hydroxyl acetophenone. The ortho isomer is separated from the mixture by the steam-distillation.
Intramolecular Mechanism: If may be assumed to be Friedel-Craft’s acylation type in which the acylium ion is supplied by the substrate, i.e.,. it is self-acylation.
First of all, $AlC{l_3}$ complexes with the oxygen of the phenoxy group from which the acylium ion is generated, the acylium ion then attacks the benzene ring as in the case of Friedel-Crafts acylation.
Similar attack at p-position gives the para isomer.
7.Alkylation: When phenol is treated with allyl halide in the presence of ${N_a}OH$ formation of aryl allyl ether takes place.
Allyphenyl ethers rearrange to o– allyl phenol when heated at 200°C.
This rearrangement is known as Claisen rearrangement
If ortho position is free, only ortho rearrangement occurs. However, if both ortho positions are substituted then rearrangement takes place at para position.
An interesting feature of the rearrangement is that when migration takes place to the ortho position, the g-carbon of allyl group attaches itself to the benzene ring. In other words, there is an inversion of allylic chain. However, no such inversion of allylic chain takes place with p-migration.
Reactions Due To Benzene Ring
8.Bromination: The hydroxyl group is a powerful activating group and an o-p director in electrophilic substitution. Phenol reacts with bromine in aqueous solution to yield
2,4, 6-tribromophenol.
Monobromination of phenol can be achived by carrying out the reaction in carbon disulphide at a low temperature condition that reduce electrophilic reactivity of bromine. The major product is para-isomer.
9. Nitration: Phenol reacts with dilute nitric acid to yield a mixture of o and p-nitrophenol.
But when phenol is treated with nitrating mixture it gives 2,4,6-trinitrophenol (picric acid)
10.Sulphonation: Phenol reacts with concentrated sulphuric acid to yield mainly the orthosulphonated product of the reaction is carried out at 25°C and mainly the parasulphonated product at 100°C.
11. Kolbe Reaction: Phenol in the presence of base NaOH, reacts with carbon dioxide at high temperature and pressure to yield sodium salicylate which on acidification yields salicylic acid. This reaction is electrophlic substitution reactionin which carbon dioxide behaves as an electrophile. The reaction is known as Kolbe reaction.
The Kolbe reaction is usually carried out by allowing sodium phenoxide to absorb carbon dioxide and then heating the product to 125°C under a pressure of several atmospheres of carbon dioxide. The unstable intermediate undergoes a proton shift that leads to sodium salicylate subsequent acidification of the mixture produces salicylic acid.
In this reaction substitution takes place only at ortho position.
12.Reimer-Tiemann reaction: This reaction is carried out by refluxing an alkaline solution of phenol with chloroform at 60C, distilling off the excess of chloroform, acidifying the residual liquid with sulphuric acid, and then steam-distilling it. Unchanged phenol ando-hydroxy-benzaldehyde distill over, leaving behind p-hydroxybenzaldehyde.
The mechanism of the Reimer-Tiemann reaction is believed to involve the formation of dichlormethylene.
Phenols with blocked p-positions give cyclohexadienones containing the dichromethyl group.
In the Reimer-Tiemann reaction, the o-isomer predominates, but if one of the o-positions is occupied, the aldehyde group tends to go the p-positions; e.g., guaiacol forms vanillin
Test for Phenol: Phenol gives a violet colour with ferric chloride; this reaction is characteristic of all compounds containing the grouping $--{\text{ }}C\left( {OH} \right){\text{ }} = {\text{ }}C.$
