Chemical Properties of Phenols Preparation and Properties
The chemical properties of phenol are grouped under three catagories.
- Reactions of phenolic group (—OH group)
- Reactions of benzene ring
- Condensation Reactions .
Reactions of Phenolic (—OH) Group : Phenol behaves as a weak acid
Acidic Character :
→ Phenol reacts with active metals like sodium, potassium and aluminium to form phenõxide and hydrogen gas.
Phenol reacts with aqueous sodium hydroxide solution to form sodium phenoxide.
Phenol reacts with Grignard reagent to form alkane in which H atom of —OH group is substituted.
→ Reaction of phenol wìth metals, sodium hydroxide and Grignard reagent represents its acidic nature.
Acidic nature of phenol is due to following reasons:
→ In phenol, lone pair of electrons present on oxygen of —OH group is in conjugation with it electrons of ring.
→ Hence, due to resonance effect lone pair present on oxygen (O) is displaced towards benzene ring. In this way oxygen of —OH group gets positively charged. By which electrons of —OH bond are attracted towards oxygen and therefore, facilitate the release of proton.
→ The —OH group, in phenol ìs directly attached to the sp2 hybridised carbon. The sp2 hybridised carbon is electron withdrawing. Due to its high electron negativity electron density on oxygen atom decreases by which polarity of O—H bond increases and proton is easily released.
→ By ionisation of phenol, phenoxide ion forms. This ion shows resonance and it is more stable than resonating structures of phenol. Because of resonating structures of phenoxide ion only negative charge is delocalised, and there is no charge separation, whereas there is charge separation in phenol.
→ As phenoxide ion is more stable than phenol, it facilitate the ionisation of —OH group of phenol and hence phenol shows acidic character.
→ Effect of Substituents on the Acidic Character of Phenols
→ Presence of electron withdrawing group : When electron withdrawing group which produce —I effect like nitro group is present on benzene ring, acidic strength increases. When these groups are present at ortho and para positions, then their effect is more because delocalisation of negative charge is more effective in phenoxide ion.
→ Presence of electron releasing group : When electron releasing groups like alkyl group is present on benzene ring, acidic strength decreases because these groups do not favour the formation of phenoxide ion.
→ Greater the value of Ka and lesser the value of pKa, stronger is the acid. Comparison of acidity of phenol and substituted phenol with the pka values can be done in following way:
→ Comparison of Acidity of Phenol and Alcohol Phenols show acidic nature but alcohols are almost neutral. —OH group in alcohol is attached to sp3 hybridised carbon. Electronegativity of sp3 hybridised carbon is less hence it cannot increase the polarity of —O—H bond and H cannot be released easily.
→ Beside this in alcohol alkyl group is also attached with —OH group. it has electron releasing effect (+I). As a result, polarity of —O—H group does not increase so much high that it can release W .An opposite to it we see that alcohol form alkoxide ion on releasing H. This ion is not stable because there is no resonance effect present in it which provide stability to it.
→ Due to above reasons acidity of alcohol is negligible. In comparison to alcohol water is a strong proton donor.
→ Hence wateris a strong acid than phenol.
→ Comparison of Acidity of Phenol and Carboxylic Acid Acidity of phenol is very less in comparison to carboxylic acid.
→ Carboxylic acid decomposes from sodium bicarbonate and CO2 gas evolve with effervescence. But phenol does not decompose from NaHCO3 because it is a weak acid.
→ This reaction is used to distinguish between alcohol and carboxylic acid.
Phenol reacts with strong base like NaOH and forms salt.
→ ReactIon with Zinc : When heated with zinc dust phenol is reduced to benzene.
→ ReactIon with Ammonia : Phenol reacts with ammonia in the presence of anhydrous zinc chloride to give aniline.
→ Reaction with Phosphorus P.ntachioeid. Phenol reacts with PCl5 to give chiorobenzene and phosphorus oxychioride. If phenol is taken in excess then it reacts with POCl3 and form triphenyl phosphate as end product.
→ Etheratlon Phenol reacts with sodium hydroxide in acetone to give sodium phenoxide which reacts with aikyl halide to give alkyl phenyl ether.
→ Methylation of phenol can also be done by diazomethane.
→ Esterification Phenol form ester on reaction with carboxylic acid, acid chloride and acid anhydride. Reaction of carboxylic acid and acid anhydride is carried out in the presence of concentrated suiphurid acid.
→ This reaction is reversible, therefore water formed as a product is removed instantly. Reaction of phenol with acid chloride is carried out in the presence of base (like pyridine).
→ Base neutralises the HCl formed during reaction and shift the equilibrium towards right.
→ Introduction of acetyl group (CH3CO—) into phenol is known as acetylation.
→ Benzoylation : Phenol react with henzoyl chloride to form phenyl benzoate.
→ The reaction is called scnotten Baumann reaction.
→ Oxidation Phenol is oxidised by chromyl chloride (CrO2Cl2) to form benzoquinone.
→ In presence of air phenol slowly gets oxidised and form quinones.
→ Reactions of Benzene Ring : Phenol undergoes electrophilic substitution reactions due to presence of benzene ring.
→ The —OH group attached to benzene ring is ortho-para directing and therefore eleftrophilic substitution occurs at ortho or para positions.
→ Nitration Phenol reacts with dilute nitric acid at low temperature (298 K) to give ortho-nitrophenol and para nitrophenol.
→ The electrophile NO2 which attacks on the ring during reaction can be formed as follows:
→ On nitration with a mixture of cone. nitric acid and conc. sulphuric acid 2, 4. 6-trinitro phenol forms which is also known as picric acid. Due to the presence of three electron withdrawing group on ring it behaves as strong acid.
→ Sulphonation : Phenol reacts with conc. sulphuric acid to form a mixture of o- and p-phenol suiphonic acid. The electrophile which attacks on ring is formed during the reaction as follows:
Friedel Craft Reaction :
→ Alkylation : When Phenol is heated with Alkyl halides in the presence of anhydrous aluminium chloride it forms o- and p-phenyl derivative
→ Reimer-Tiemann Reaction : Reaction of phenol with carbon tetrachioride in alkaline solution (NaOH/KOH) forms salicylic acid.
→ Kolbe’s Reaction or Kolbe Schmidt Reaction When phenol reacts with NaOH it forms sodium phenoxide. When sodium phenoxide is heated with CO2 at about 398 K and high pressure, sodium salicylate is formed. This on acidification gives salicyclic acid as major product.
→ Hydrogenation : Phenol gets hydrogenated in the presence of nickel or palladium to give cyclohexanol.
→ Reaction with Acetone : Phenol reacts with acetone in the presence of cone. HCl to form bis-phenol-A.
→ Reaction with Benzene Diazonium Salt: Phenol couples with henzene diazonium salt at para position to form -hydroxy azobenzene.
→ Reaction with PhthaUc Anhydride : Phenol reacts with phthalic anhydride in the presence of conc. H2SO4 to give phenolphthalein.
→ Reaction with formaldehyde (Ledrer Mannase) Reaction : Phenol condenses with formaldehyde in the presence of dil acid or alkali to give para hydroxy benzyl alcohol in the formof major product which condenses to form ‘Bakelite’ polymer with high molecular weight. Ortho-hydroxy benzyl alcohol forms in less amount.