Phenols & Aromatic Amines - Unit II PDF

**.-Unit- II: Phenols & Aromatic Amines** **2.1. Phenols** **Phenols:** are organic compounds with a hydroxyl (―OH) group attached to a carbon atom that is part of an aromatic ring or directly to an aromatic ring **Derivatives of Phenol** **Structure of Phenols** **1**. In phenol, all ring carb...

**.-Unit- II: Phenols & Aromatic Amines** **2.1. Phenols** **Phenols:** are organic compounds with a hydroxyl (―OH) group attached to a carbon atom that is part of an aromatic ring or directly to an aromatic ring **Derivatives of Phenol** **Structure of Phenols** **1**. In phenol, all ring carbon atoms are SP^2^ hybridized and the oxygen atom of the OH group is SP^3^ hybridized **2**. All rings of carbon atoms also have a p -- orbital & these are perpendicular to the plane of σ bonds of the ring. The lateral overlap of this p -- orbital produces a delocalized π molecular orbital. The two lone pair of electrons on the oxygen atom residue in the SP^3^ hybrid orbitals **3**. One of the oxygen atom in the OH group overlaps with the delocalized π molecular orbital **Nomenclature of Phenols** **2.1.2. Preparation** **Question:** Discuss any five methods of preparation of phenol **1. Hydrolysis of diazonium salts**: A diazonium salt is formed by treating an aromatic primary amine with nitrous acid then it undergoes hydrolysis to form phenol compound **2. From sodium benzesulfonamide:** This involves fusion of sodium benzene sulfonate with NaOH to form phenol **3. Alkali fusion of sulfonates:** Sodium benzene **sulfonate** is **fused** or reacts with sodium hydroxide (at around 350°C) to produce sodium phenoxide which on acidification yields phenol. **4. From Chlorobenzene (Dow process)** This involves the hydrolysis of chlorobenzene with aqueous NaOH to form phenol **5. From Cumene:** Benzene is reacted with propene in the presence of phosphoric acid (H~3~PO~4~) or sulphuric acid (H~2~SO~4~) to form cumene and oxidized by using oxygen to give cumene hyper peroxide then hydrolyzed to produce phenol compound **2.1.3. Reaction of phenols** **1. Acidity salt formation** **2. Ester formation** **3. Ether formation. Williamson synthesis:** It is the most reliable & versatile method for the preparation of unsymmetrical & symmetrical ethers. This reaction involves the treatment of sodium phenoxide with an alryl halide to give ethers **4. Ring substitution of phenols:** Electrophilic aromatic **substitution** reactions are the reactions where an electrophile replaces one or more hydrogen atoms attached to the aromatic **ring**. **Phenols** are highly prone to electrophilic **substitution** reactions due to rich electron density **i) Nitration** **ii) Sulfonation** **iii) Halogenation** **iv) Friedel -- crafts alkylation** **v) Fridel crafts acylation. Fries rearrangement:** It is a [rearrangement reaction](https://en.wikipedia.org/wiki/Rearrangement_reaction) of a phenolic ester to a [hydroxy](https://en.wikipedia.org/wiki/Hydroxyl) [aryl](https://en.wikipedia.org/wiki/Aryl) [ketone](https://en.wikipedia.org/wiki/Ketone) by [catalysis](https://en.wikipedia.org/wiki/Catalysis) of [Lewis acids](https://en.wikipedia.org/wiki/Lewis_acid). It involves the migration of an [acyl](https://en.wikipedia.org/wiki/Acyl) group of [phenol](https://en.wikipedia.org/wiki/Phenol) ester to the [aryl](https://en.wikipedia.org/wiki/Aryl) ring. The reaction is [para-selective](https://en.wikipedia.org/wiki/Arene_substitution_patterns) and one of the two products can be favored by changing reaction conditions, such as [temperature](https://en.wikipedia.org/wiki/Temperature) and [solvent](https://en.wikipedia.org/wiki/Solvent). The reaction of an aryl ester with a Lewis acid catalyst followed by an aqueous acid to give phenols is known as **Fries rearrangement** **vi) Nitrosation: Nitrosation** is a process of converting organic compounds into nitroso derivatives, i.e. compounds containing the R-NO functionality **vii) Carbonation: Kolbe reaction**: is an organic **reaction** used to convert a phenol to a hydroxy benzoic acid using carbon dioxide gas, and base. **viii) Aldehyde formation. Reimer -- Tiemann reaction:** The Reimer- Tiemann is a chemical reaction used to produce aldehyde compound. Treatment of phenol with chloroform in presence of hydroxide ion to form salicyaldehyde (aldehyde) **Reaction**: **2.1.4. Acidity of Phenols** The acidity of phenols is due to its ability to lose hydrogen ion to form phenoxide ions. Phenols are converted into their salts by aq. Hydroxides. The salts are converted into the free phenols by aqueous mineral acids or carbonic acids **Resonance form of phenoxide ion** 1\. Phenols are much more acidic than alcohols but less acidic than carboxylic acid 2\. The Pka value of phenol is 10.1 3\. In phenol molecule, the SP^2^ hybridized carbon atom of benzene ring attached directly to the hydroxyl group acts as an electron withdrawing group. This SP^2^ hybridized carbon atom of benzene ring attached directly to the hydroxyl group has higher electronegativity in comparison to hydroxyl group. Due to higher electronegativity of the carbon, electron density increases the polarity of O -- H bond and produces the acidity 4\. Less Pka value, more acidity **Effect of substituents on acidity** **1. Electron withdrawing substituents increase acidity of phenols** An electro withdrawing group likes NO~2~, Cl, CN, CHO, COOH on aromatic ring makes more acidic to phenol compound 4- Nitro phenol is more acidic than phenol, because it has electron withdrawing group like NO~2~ **2. Electron releasing groups** An electron releasing group likes - CH~3~, - OCH~3~, - NH~2~, on aromatic ring, make less acidic to phenol compound **Q.1. Phenols are more acidic than alcohols** Alcohols are less acidic than phenols because it is very tough to remove the H ion from alcohol. Phenol can lose ion easily because phenoxide ion formed which stabilized to some extent. This is as the negative charge on the oxygen atom is delocalized around the ring. **Q.2. Phenol/alcohol is a weaker acid than benzoic acid/Acetic acid** 1. The strength of weak acids, are measured on the Pka scale. The smaller the number of this scale, the stronger the acid is Pka values **Compound Pka** Benzoic acid 4.76 (lesser pka value greater) Phenol 10 Ethanol 16 2. The more electron withdrawing group (COOH) present in compound that would increases the acidity. The more donating (phenolic OH) present in compound would group decreases the activity - Phenol is more acidic than cresol. - In the cresol, there is one electron donating methyl group is attached to the ring which exerts +I effect and hyperconjugation resulting increase in nucleophilicity of negatively charged oxygen hence it accepts the proton quickly. - Hydrogen bonding and the vicinity of the −OH to the anion (for the stronger -I effect) make **catechol more acidic** than the phenol - Resorcinol has only -I effect and no counteracting electron-donating effects, which makes it the most acidic - Hydroquinone: strong +M effect and weak -I effect - Resorcinol: -I effect - **Hydroquinone:** strong +M effect and weak -I effect o **Phenol:** the normal resonance in all phenols **Resorcinol:** -I effect - **- Catechol:** +M effect, stronger -I effect and hydrogen bonding - Hydroquinone is **least acidic.** - Hydrogen bonding and the vicinity of the −OH to the anion (for the stronger -I effect) make catechol more acidic than the phenol despite its +M contribution. - Resorcinol has only -I effect and no counteracting electron-donating effects, which makes it the **most acidic.** - Hydroquinone \< Phenol \< Catechol \< Resorcinol (pKa 10.85) (pKa 10) (pKa 9.45) (pKa 9.26). - Less pKa value i**ncreases** the acidity & more pKa value **decreases** the acidity - −CH~3~​ shows + I effect, makes **more acidic** - NH~2~ show  −I effect, but weak inductive effect - −OCH~3~​ show −I effect, but the inductive effect is negligible in para position. **Q.11. Compare the acidity in between phenol and p -- hydroxy benzoic acid** - Less pka value increases the acidity - NO~2~ group is more electronegative than chloro, more electronegative group, its increases the acidity - Less Pka value increases the acidity - NO~2~ group is electron withdrawing group, its **increases** the acidity, while CH~3~ group is electron donating or releasing group, its **decreases** the acidity - Less Pka value increases the acidity - At meta position forms, weaker -- I effect because of NO~2~ group at m -- position withdraws of O --H bond - In o -- nitro phenol forms intermolecular H -- bonds & in p- nitro phenol do not forms intermolecular H -- bonds & decreases the acidity - Less ~pKa~ value is increases the acidity **2.1.5. Qualitative tests for phenols** **Question : Give the chemical identification test for Phenol functional group** **Ans** +-----------------------+-----------------------+-----------------------+ | **Test** | **Observation** | **Inference** | +=======================+=======================+=======================+ | **(i)** Take 0.2gm of | Dissolves completely, | Phenolic --OH group | | sub + | | | | | sometimes slight | | | 3 ml dil.NaOH | brown | | | | | | | | colored solution | | +-----------------------+-----------------------+-----------------------+ | **(ii)** 0.2 gm of | -Violet | Phenol, cresols, | | Comp + | | | | | -Violet blue | Resorcinol, | | 5 ml water(warm if | | B-napthol. | | | Greenish white | | | necessary) | opalescent | | | | | | | Take 1 ml of this | | | | solution. | | | | | | | | Add 1 drop of neutral | | | | | | | | ferric chloride | | | +-----------------------+-----------------------+-----------------------+ | **(iii)** Phthalein | Red colour | Phenol or O-cresols | | test | | | | | Blue colour | m- cresols | | 0.2 gm of phenol +0.2 | | | | gm | Yellowish Green | Catechol or | | | | pyrogallol | | Of phthalic anhydride | Fluorescence Green | | | + 3 | colour | Resorcinol | | | | | | drops of | Very faint green with | α-Napthol | | conc.Sulphuric | | | | | slight fluorescence | β- Napthol | | acid. | | | | | | | | The melt is cooled | | | | with 2 | | | | | | | | ml of water followed | | | | by | | | | | | | | 10% NaOH solution | | | | drop by | | | | | | | | Drop till the | | | | solution is | | | | | | | | just Alkaline. | | | +-----------------------+-----------------------+-----------------------+ **2.1.6. Structure and uses of Phenol, Cresols, Resorcinol, Naphthols** **i) Phenol** **Structure** **Uses:** 1\. **Phenols** are widely **used** as and disinfectants  2\. **Phenol compound is use as starting material for the synthesis of Aspirin drug (**reduce fever and relieve mild to moderate pain) 3\.  It is primarily **used** to synthesize plastics and related materials 4\. **Phenol** is also **used** in the cosmetic industry in the manufacturing of sunscreens, skin lightening creams and hair colouring solutions **ii) Cresols** **Structure** **Uses:** 1\. Cresols are precursors or synthetic intermediates to other compounds and materials, including plastics, pesticides, pharmaceuticals, and dyes. 2\. **Cresols** are used as disinfectants, perfumes, preserving agents or herbicides **iii) Resorcinol** **Structure** **Uses:** 1\. Resorcinol topical is used to treat **acne (skin related) eczema**, **psoriasis**, **seborrhea**, corns, callouses, **warts**, and other skin disorders 2\. **Resorcinol** is used as a disinfectant or an antiseptic 3\. **Resorcinol** is use as intermediate for drug synthesis 4\. Resorcinol is mainly used in the production of resins (Polymer) **iv) Naphthols** **Structure** **Uses:** 1\. 2-**Naphthol (β -- Napthol)** has several different **uses** including dyes, pigments, fats, oils, insecticides, pharmaceuticals, perfumes, antiseptics, synthesis of fungicides, and antioxidants for rubber. 2\. 2-**Naphthol (β -- Napthol) is intermediate for the synthesis of Naproxen drug, which act as antiinflammatory drug (NSAID)** 3\. 1-**Naphthol ( -- Napthol) is intermediate for the synthesis of** Nadolol **drug, which act as antihypertensive drug** **Question Bank:** **3-5 Marks** **Q.1.Draw the structures of resorcinol, α- napthol and β- napthol** **Q.2.Draw the structures of phenols & cresols** **Q.3. Explain the acidity of phenols** **Q.4. Comment on effect of substituents on acidity** **Q.5.Write Qualitative Analysis tests for phenol** **Q.6. Write any five general methods of preparation of phenols. Write uses of phenols** **Q.7. Write any five chemical reactions of phenols** **10 Marks** **Q.1. What are phenols? Explain acidity of phenols. Write any four methods of preparation and three reactions of phenols?**