Physical and Chemical Properties

Phenols :

Physical properties:

1.Low molecular weight phenols are normally liqueids or low melting solids

2.Due to H- bonding ,most low molecular weight phenols are water soluble

3.Phenols tend to have higher boiling pointings than alchols of similar weight because they have stonger intermolecular H-bonding

Chemical properties:

I.]pHEonols are acidic because of the ease with which the oxygen atom will release the hydrogen bonded to it.This section describes typical reactions that occur as a result of the acidity of phenols.

1. Reactions with bases:Because phenol is acidic, it reacts with bases to form salts.

2. Esterification of phenol:Phenols form esters with acid anhydrides and acid chlorides.

3. Williamson ether synthesis:Ethers are produced from phenol by the Williamson method via an SN mechanism.

II.] Reactions of Phenolic Benzene Rings:

The hydroxy froup in a phenol molecule exhibts a strong activating effect on the bengene ring because it provides a ready source of electron density for the ring. This directing influence is so strong that you can often accomplish substitutions on phenols
without the use of a catalyst.

Properties

Phenol is appreciably soluble in water, with about 8.3 g dissolving in 100 mL (0.88 M). Homogeneous mixtures of phenol and water at phenol to water mass ratios of ~2.6 and higher are also possible. The sodium salt of phenol, sodium phenoxide, is far more water soluble.

[edit] Acidity

Phenol is weakly acidic but at high pH''s gives the phenolate anion C₆H₅O⁻ (also called phenoxide):^[5]

PhOH PhO^- + H⁺ K = 10^-10

Compared to aliphatic alcohols, phenol is about 1 million times more acidic, although it is still considered a weak acid. It reacts completely with aqueous NaOH to lose H⁺, whereas most alcohols react only partially. Phenols are less acidic than carboxylic acids, and even carbonic acid.

One explanation for the increased acidity over alcohols is resonance stabilization of the phenoxide anion by the aromatic ring. In this way, the negative charge on oxygen is delocalized on to the ortho and para carbon atoms.^[6] In another explanation, increased acidity is the result of orbital overlap between the oxygen''s lone pairs and the aromatic system.^[7] In a third, the dominant effect is the induction from the sp² hybridised carbons; the comparatively more powerful inductive withdrawal of electron density that is provided by the sp² system compared to an sp³ system allows for great stabilization of the oxyanion.

The pK_a of the enol of acetone is 10.9, comparable to that for phenol.^[8] The acidities of phenol and acetone enol diverge in the gas phase owing to the effects of solvation. About 1/3 of the increased acidity of phenol is attributable to inductive effects, with resonance accounting for the remaining difference.^[9]

[edit] Phenoxide anion

Resonance structures of the phenoxide anion

The phenoxide anion has a similar nucleophilicity to free amines, with the further advantage that its conjugate acid (neutral phenol) does not become entirely deactivated as a nucleophile even in moderately acidic conditions. Phenols are sometimes used in peptide synthesis to "activate" carboxylic acids or esters to form Activated ester (page does not exist). Phenolate esters are more stable toward hydrolysis than acid anhydrides and acyl halides but are sufficiently reactive under mild conditions to facilitate the formation of amide bonds.

[edit] Tautomerism

Phenol-cyclohexadienone tautomerism

Phenol exhibits keto-enol tautomerism with its unstable keto tautomer cyclohexadienone, but only a tiny fraction of phenol exists as the keto form. The equilibrium constant for enolisation is approximately 10⁻¹³, meaning that only one in every ten trillion molecules is in the keto form at any moment.^[10] The small amount of stabilisation gained by exchanging a C=C bond for a C=O bond is more than offset by the large destabilisation resulting from the loss of aromaticity. Phenol therefore exists entirely in the enol form.^[11]

Phenoxides are enolates stabilised by aromaticity. Under normal circumstances, phenoxide is more reactive at the oxygen position, but the oxygen position is a "hard" nucleophile whereas the alpha-carbon positions tend to be "soft".^[12]

[edit] Reactions

Neutral phenol substructure "shape". An image of a computed electrostatic surface of neutral phenol, showing neutral regions in green, electronegative areas in orange-red, and the electropositive phenolic proton in blue.

Phenol is highly reactive toward electrophilic aromatic substitution as the oxygen atom''s pi electrons donate electron density into the ring. By this general approach, many groups can be appended to the ring, via halogenation, acylation, sulfonation, and other processes. However, phenol''s ring is so strongly activated — second only to aniline - that bromination or chlorination of phenol leads to substitution on all carbons ortho and para to the hydroxy group, not only on one carbon.

Aqueous solution of phenol is weakly acidic and turns blue litmus slightly to red. Phenol is easily neutralized by sodium hydroxide forming sodium phenate or phenolate, but it being weaker than carbonic acid cannot be neutralized by sodium bicarbonate or sodium carbonate to liberate carbon dioxide

C₆H₅OH + NaOH → C₆H₅ONa + H₂O

When a mixture of phenol and benzoyl chloride when shaken in presence of dilute sodium hydroxide solution, Phenyl benzoate (page does not exist) is formed. This is an example of Schotten-Baumann reaction:

C₆H₅OH + C₆H₅COCl → C₆H₅OCOC₆H₅ + HCl

Phenol is reduced to benzene when it is distilled with zinc dust or its vapour is passed over granules of zinc at 400 °C:^[13]

C₆H₅OH + Zn → C₆H₆ + ZnO

When phenol is reacted with diazomethane in the presence of boron trifluoride (BF₃), anisole is obtained as the main product and nitrogen gas is released:

C₆H₅OH + CH₂N₂ → C₆H₅OCH₃ + N₂

[edit] Production

Because of phenol''s commercial importance, many methods have been developed for its production. The dominant current route, accounting for 95% of production (2003), involves the partial oxidation of cumene (isopropylbenzene) via the Hock rearrangement:^[4]

C₆H₅CH(CH₃)₂ + O₂ → C₆H₅OH + (CH₃)₂CO

Compared to most other processes, the cumene-hydroperoxide process uses relatively mild synthesis conditions, and relatively inexpensive raw materials. However, to operate economically, there must be demand for both phenol, and the acetone by-product.

An early commercial route, developed by Bayer and Monsanto in the early 1900s, begins with the reaction of a strong base with benzenesulfonate:^[14]

C₆H₅SO₃H + 2 NaOH → C₆H₅OH + Na₂SO₃ + H₂O

Other methods under consideration involve:

• hydrolysis of chlorobenzene, using base or steam (Raschig-Hooker process):^[15]

C₆H₅Cl + H₂O → C₆H₅OH + HCl

• direct oxidation of benzene with nitrous oxide, a potentially "green" process:

C₆H₆ + N₂O → C₆H₅OH + N₂

• oxidation of toluene, as developed by Dow Chemical:

C₆H₅CH₃ + 2 O₂ → C₆H₅OH + CO₂ + H₂O

In the Lummus Process, the oxidation of toluene to benzoic acid is conducted separately.

Phenol is also a recoverable byproduct of coal pyrolysis.^[15]

[edit] Uses

The major uses of phenol, consuming two thirds of its production, involve its conversion to precursors to plastics. Condensation with acetone gives bisphenol-A, a key precursor to polycarbonates and epoxide resins. Condensation of phenol, alkylphenols, or diphenols with formaldehyde gives phenolic resins, a famous example of which is Bakelite. Partial hydrogenation of phenol gives cyclohexanone, a precursor to nylon. Nonionic detergents are produced by alkylation of phenol to give the alkylphenols, e.g., nonylphenol, which are then subjected to ethoxylation.^[4]

Phenol is also a versatile precursor to a large collection of drugs, most notably aspirin but also many herbicides and pharmaceutical drugs. Phenol is also used as an oral anesthetic/analgesic in products such as Chloraseptic or other brand name and generic equivalents, commonly used to temporarily treat pharyngitis.