Chemistry PHENOLS : ACIDIC NATURE, ELECTROPHIILIC SUBSTITUTION REACTION

Acidity of Phenols :

Phenols are weak acids (`pK_a = 10`). They form salts with aqueous `NaOH` but not with aqueous `NaHCO_3`.

Although Phenols are structurally similar to alcohols, they are much stronger acids. But phenol is a weak acid when compared to a carboxyllic acid, such as acetic acid (`pK_a = 4.7447`).

The greater acidity of phenol owes itself primarily to an electrical charge distribution in phenols that causes the `-OH` oxygen to be more positive; therefore the proton is held less strongly.

The factor influencing the electron distribution may be the contributions to the overall resonance hybrid of phenol made by the resonance structures shown below. The effect of these structures is to withdraw electrons from the hydroxyl group and to make the oxygen positive. See fig.1.

The considerably greater acid strength of `PhOH (pK_a = 10)` than that of `ROH (pK_a = 18)` can be accounted for as the negative charge on the alkoxide anion, `RO^(-)`, cannot be delocalized, but on `PhO^(-)` the negative charge is delocalized to the ortho and para ring positions as indicated by the starred sites in the resonance hybrid. See fig.2.

`PhO^(-)` is therefore a weaker base than `RO^(-)` and `PhOH` is a stronger acid the effect of

a) electron - attracting and

b) electron - releasing substituents on the acid strength of phenols

Electron-attracting substituents disperse negative charges and therefore stabilize `ArO^(-)` and increase acidity of `ArOH`. Electron- releasing substituents concentrate the negative charge on `O` destabilizes `ArO^(-)` and decreases acidity of `ArOH`.

In terms of resonance and inductive effects we can account for the following relative acidities.

a) `p-O_2NC_6H_4OH > m- O_2NC_6H_4OH > C_6H_5OH`

b) `m- ClC_6H_4OH > p-ClC_6H_4OH > C_6H_5OH`

a) The- `NO_2` is electron-withdrawing and acid strengthening. Its resonance effect, which occurs only from para and ortho positions,
predominates over its inductive effect, which occurs also from the meta position.

b) `Cl` is electron-withdrawing by inductive effect. This effect diminishes with increasing distance between `Cl` and `OH`. The meta is closer than the para positions and `m-Cl` is more acid - strengthening than the `p-Cl`. Other substituents in this category are `F`, `Br`, `I`, `text( )^(+)NR_3`.

Chemical Properties :

Chemical properties of phenols are as follows :

Acylation :

Phenols react with carboxylic acid anhydrides or acid chlorides to yield phenyl esters.

Fries Rearrangement :

The phenyl ester formed from the reaction of phenol and acid anhydride undergoes rearrangement when treated with anhydrous `AlCl_3` to yield ortho and para-hydroxyketones. The o-isomer is separated from the mixture by steam-distillation. See fig.1.

Generally low temperature (`60^oC` or less) favours the formation of the `p`-isomer whereas high temperature (above `160^oC`) favours the `o`-isomer.

`text(Mechanism :)`

The reaction proceeds as Friedel-Crafts acylation, acylium ion being supplied by the substrate itself. See fig.2.

Similarly, if the electrophile attacks at `p` position, `p`-isomer is formed.

Reaction with Benzoyl Chloride :

Alkaline solution of phenol when treated vigorously with benzoyl chloride yields phenylbenzoate. The reaction is known as Schotten- Baumann reaction.

Reaction with `Zn` Dust :

Phenol yields benzene when heated with `Zn`-dust.

`C_6H_5-OH + Zn overset(Delta)-> C_6H_6 + ZnO`

Reaction with Ammonia :

Phenol is converted into aniline when heated with ammonia under pressure or in presence of `ZnCl_2`.

`C_6H_5- OH + NH_3 overset(ZnCl_2)-> C_5H_5- NH_2 + H_2O`

Reaction with `PCl_5` :

Phenol reacts with `PCl_5` to give chlorobenzene along with `POCl_3` and `HCl`. Further with `POCl_3`, phenol reacts to produce triphenyl phosphate. With `PCl_3`, phenol reacts to give chlorobenzene and `H_3PO_3`.

`C_6H_5- OH + PCl_5 -> C_6H_5- Cl + + POCl_3 + HCl`

`3C_6H_5-OH + POCl_3 -> undersettext[(Triphenyl phosphate)](O=P(OC_6H_5)_3) + 3HCl`

`3C_6H_5-OH + PCl_3 -> 3C_6H_5-Cl + H_3PO_3`

Friedel-Crafts Reaction :

Phenol undergoes alkylation and acylation to form mainly `p`-derivative along with some `o`-derivative. See fig.1.

The yield is not good as the reactivity of phenol is diminished by the formation of a complex with Lewis acid `AlCl_3`. See fig.2.

Kolbe's Reaction :

Sodium phenoxide is heated with carbon dioxide at `120-140^oC` under pressure to yield sodium salicylate, which upon acidification produces salicylic acid. See fig.1.

A small amount of `p`-derivative is also formed. If the temperature rises above `140^oC`, the `p`-isomer is the main product.

`text(Mechanism :)` See fig.2.

Salicylic acid is used for the preparation of Aspirin, oil of winter green (methyl salicylate) and salol (phenyl salicylate). See fig.3.

Gattermann's Reaction :

When phenol is treated with liquid `HCN` and gaseous `HCl` in the presence of anhydrous `AlCl_3` and the product formed is hydrolysed, `p`-hydroxy benzaldehyde is formed.

`HCl + HCN overset(AlCl_3)-> HC = overset(oplus)NH Cl^(-)`

Libbermann Test :

When phenol is dissolved in concentrated sulphuric acid and a few drops of aqueous sodium nitrite is added, a red colour is obtained on dilution and turns blue when aqueous sodium hydroxide is added.