Phenols: methods of preparation, chemical reaction

Phenols: methods of preparation, chemical reaction

1. POC-II BP 301P A.Y. 2020-21 UNIT:II PHENOLS Mrs. Ambreen Kauser RCPER, Malegaon

2. PHEN O LS  Aromatic compounds containing one or more OH groups directly attached with carbon of benzene ring are called Phenols.  Simplest phenol is Carbolic Acid C 6 H 5 OH  Term Phenol is derived from an old name of benzene – Phene  Phenyl : C 6 H 5

3. NOMENCLATURE OF PHENOLS  In IUPAC –OH group is represented as hydroxyl. It is used as a prefix, while benzene part of the molecule is used as suffix . 1,2- dihydorxybenzene O-hydroxyl phenol (Catechol) Phenol 1,3- dihydorxybenzene m-hydroxyl phenol (Resorcinol) 1,4- dihydorxybenzene p-hydroxyl phenol (Hydroquinone)

4. NOMENCLATURE 2-nitrophenol O-nitrophenol 3-nitrophenol m-nitrophenol 4-nitrophenol p-nitrophenol 2,4,6-trinitrophenol Picric Acid

5. STRUCTURE OF PHENOLS The alcohol functional group consists of O atom bonded to sp 2 hybridized aromatic C atom and H atom via σ bond Both C-O and O-H bonds are polar  Conjugation exist between an unshared electron pair of the O and Benzene Ring This results in, as compared to alcohols: A shorter C-O bond A more basic OH group A more acidic OH proton

6. PHYSICAL PROPERTIES OF PHENOLS  Colorless, crystalline, poisonous solid with phenolic odor  Melting point 41 o C and Boiling Point 182 o C  Sparingly soluble in water forming pink solution at room temperature  Completely soluble above 68.5 o C  Causes blisters on skin  Used as disinfectants .

7. ACIDITY OF PHENOLS Phenols are more acidic (pKa ≈ 10) than alcohol (pKa ≈ 16-20) Phenols are less acidic than Carboxylic acids (pKa ≈ 5) COMPARISON OF ACIDITY OF PHENOLS AND ALCOHOLS  Phenol exists as resonance hybrid of following structures

8.  Due to resonance O atom acquires a positive charge and hence attracts electron pair of O-H bond leading to the release of H +  Carbon atom of C-OH group of phenol (sp 2 hybridized) is more electrophilic than Carbon atom in Alcohols (sp 3 hybridized)  In phenols, a greater inductive effect facilitated release of proton Thus phenols are more acidic than alcohols because resonance is impossible in alcohols Phenoxide is more resonance stabilized than phenol but in case of alcohol, alkoxide is not stable because there is no possibility for the delocalization of negative charge.

9. COMPARISON OF ACIDITY OF PHENOLS AND CARBOXYLIC ACIDS Resonating structures of carboxylic acids: Carboxylic acids ionize as: R-COOH  R-COO - + H + The carboxylate anion exhibits following resonating structures: The resonating structures of RCOOH (I and II) are not equivalent and hence less stable The resonating structures of RCOO - ion (III and IV) are equivalent and hence more stable Thus RCOOH have tendency to undergo ionization and form more stable carboxylate ion and proton.

10.  The resonating structures of phenoxide ion are not equivalent as shown below:  The resonating structures of RCOO - ion are equivalent. Hence RCOO - ion is relatively more resonance stabilized that Phenoxide ion. Thus a carboxylic acid is more acidic than a phenol. RELATIVE ACIDITY ORDER OF SOME COMMON COMPOUNDS: RCOOH > H 2 CO 3 > C 6 H 5 OH > H 2 O > ROH

11. EFFECT OF SUBSTITUENTS ON THE ACIDITY OF PHENOLS Electron attracting substituents tend to disperse negative charge of the phenoxide ion thus stabilize the ion and increase the acidity of phenols.  Electron releasing substituents tend to intensify the charge, destabilize the ion, diminish the resonance and decrease the acidity.

12. PREPARATION OF PHENOL REACTION OF SODIUM SALT OF BENZENE SULFONIC ACID WITH NaOH:  Sodium benzene sulfonate on fusion with strong alkali like NaOH at 300 o C give sodium phenoxide which on treatment with HCl gives phenol

13. PREPARATION OF PHENOL 2. BASE HYDROLYSIS OF CHLOROBENZEN (DOW’S METHOD)  Chlorobenzene is hydrolysed by heating with 10% NaOH at 360 o C under high pressure to form sodium phenoxide which on treating with HCl gives phenol

14. PREPARATION OF PHENOL 3. ACIDIC OXIDATION OF CUMENE  It is recently developed commercial method for preparation of phenol. Cumene is oxidized by atmospheric oxygen is presence of metal catalyst into Cumene Hydroperoxide.  The hydroperoxide is converted into phenol through acid catalyzed arrangement

15. PREPARATION OF PHENOL 4. Preparation of phenol from Aryl Diazonium salts  Aryl diazonium salts are prepared by reaction of aryl amines with nitrous acid  Aryl diazonium salts can be converted into phenols using H 2 O/H 2 SO 4 / heat

16. PREPARATION OF PHENOL 5. From phenolic acids:

17. REACTIVITY AND REACTIONS OF PHENOLS Phenols are very reactive towards electrophilic aromatic substitution. OH group is strongly activating ortho-/para- directing group. ELECTROPHILIC AROMATIC SUBSTITUTION

18.  Strong activation means milder reaction conditions than those used for benzene. Phenols are so activated that poly-substitution can be a problem. OXIDATION OF PHENOLS Phenols are very reactive towards oxidizing agent. The oxidation takes place through several steps eventually destroying the ring. Reaction Benzene Phenol Nitration Conc. HNO 3 / H 2 SO 4 Dil. HNO 3 in H 2 O or CH 3 COOH Sulfonation H 2 SO 4 or SO 3 /H 2 SO 4 Conc. H 2 SO 4 Halogenation X 2 /Fe or FeX 3 X 2 Alkylation RCl/AlCl 3 ROH/H + or RCl/AlCl 3 Acylation RCOCl/AlCl 3 RCOCl/AlCl 3 Nitrosation - Aq. NaNO 2 /H +

19.  REACTION WITH SODIUM METAL / CARBOXYLATION OF PHENOLS (KOLBE- SCHMITT REACTION) “The reaction of sodium salt of phenol with CO 2 is called Kolbe Reaction. It is carbonation of phenol.” At low temperature Sodium salicylate (sodium-o-hydroxyl benzoate) is formed, at higher temperature o-product is isomerizes to p-isomer CO 2 act as electrophilic center in this reaction. Acidification of the salt gives corresponding hydroxyl acid.

20. PHENOL - REACTIONS OF THE OH GROUP Water phenol is a weak acid it dissolves very slightly in water to form a weak acidic solution it is a stronger acid than aliphatic alcohols the ring helps weaken the O-H bond and stabilises the resulting anion C 6 H 5 OH( aq ) C 6 H 5 O¯( aq ) + H + ( aq ) NaOH phenol reacts with sodium hydroxide to form a salt - sodium phenoxide it is ionic and water soluble C 6 H 5 OH( aq ) + NaOH( aq ) ——> C 6 H 5 O¯ Na + ( aq ) + H 2 O(l) Sodium phenol reacts with sodium to form an ionic salt - sodium phenoxide hydrogen is also produced this reaction is similar to that with aliphatic alcohols such as ethanol 2C 6 H 5 OH(s) + 2Na(s) ——> 2C 6 H 5 O¯ Na + (s) + H 2 (g)

21. ELECTROPHILIC SUBSTITUTION / Halogenation reaction: Bromine the OH group is electron releasing it increases the electron density of the delocalised system it makes substitution much easier compared to benzene the electron density is greatest at the 2,4 and 6 positions substitution takes place at the 2,4 and 6 positions phenol reacts readily with bromine water WITHOUT A CATALYST it is so easy that multiple substitution takes place other electrophiles such as NO 2 + react in a similar way PHENOL - REACTIONS OF THE AROMATIC RING

22. OXIDATION OF PHENOL 1 . 2 .

23. KOLBE REACTION REIMER TIEMANN REACTION

24. QUALITATIVE TESTS OF PHENOLS:

25. STRUCTURE AND USES OF COMPOUNDS: Phenol: (C6H5OH)(Carbolic Acid) Aromatic compound Slightly acidic nature Soluble in water have high May be toxic and caustic Uses: Powerful antiseptic in soap, lotion. Synthesis of Bakelite plastic , resin etc. Partial Hydrogenation of phenol gives cyclohexanone which is a precursor of nylon. Extraction of nucleic acid. Manufacturing of sunscreens, skin lightening cream, hair coloring lotions. Manufacturing of Drugs like salol , aspirin, salicylic acid. Manufacturing of picric acid, preservative for ink.

26. 2. Cresols: (C7H8O) Hydroxy Toluene: It is yellowish to brown red tint in colour. Liquid or solid depends on temperature. Exposure to cresol in high concentration can cause irritation and burning of skin, eye, mouth and abdominal pain etc. Uses: Chlorination and etherification of o- cresol gives 2-methyl-4-chlorophenoxy acetic acid (MCPA) which is used as herbicides. Synthesis of vitamin E. Antiseptic Solvent for polymers. Preservative in some insulin. P-cresols is used in the production of butylated hydroxytoulene (BHT) which is an antioxidant.

27. 3. Resorcinol (C6H6O2): It is colorless, readily soluble in water, alcohol and ethers but insoluble in chloroform. It becomes pink with exposure to light. It has sweet taste. Uses: Antiseptic, disinfectant, Analgesics, bactericidal, preservative. Its ointment up to 05 to 10 % used for treatment of psoriases, eczema. Manufacturing of antidandruff shampoo and sunscreens cosmetics. Production of diazodyes, plasticizers and UV absorber in resins. Manufacturing of glue, adhesive.

28. 4. Naphthols ( C10 H7OH) It is highly polar in nature. α -Naphthol is white solid, fluorescent compound and soluble in alcohols, ethers, and chloroform. ß-Naphthol is colorless or yellow crystalline solid fluorescent compound and soluble in alcohols, ethers, and chloroform. Uses: Used for detection of carbohydrates(content of molisch’s reagent) Precursor of Insecticides like carbanyl . Detection of presence of arginine in proteins by Sakaguchi test. Preparlation of sudan dyes. Synthesis of BINOL which is used as ligand for transition of metal catalysed synthesis. Antiseptic, fungicide, antioxidant , in perfumes.

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