Nomenclature, Methods of Preparation — Revision Notes

⚡ 30-Second Revision

Phenols: — OH directly on aromatic ring.
Nomenclature: — Common (phenol, cresol, catechol, resorcinol, hydroquinone), IUPAC (OH at C-1, lowest locants).
Dow's Process: — Haloarene + $NaOH_{(aq)}$, $623K$, $300atm$ $\rightarrow$ Phenol.
Diazonium Salts: — Aniline $\xrightarrow{NaNO_2/HCl, 0-5^circ C}$ Diazonium salt $\xrightarrow{H_2O, warm}$ Phenol + $N_2$.
Cumene Process: — Cumene $\xrightarrow{O_2}$ Cumene hydroperoxide $\xrightarrow{H^+}$ Phenol + Acetone.
Benzenesulphonic Acid: — $C_6H_5SO_3H + NaOH_{(s)}$, $573K$ $\rightarrow$ Phenoxide $\xrightarrow{H^+}$ Phenol.

2-Minute Revision

Phenols are aromatic compounds with a hydroxyl group directly attached to the benzene ring. Their naming involves both common names like 'cresol' (methylphenol) and systematic IUPAC names, where the carbon bearing the -OH group is always assigned position 1, and other substituents are numbered to achieve the lowest possible locants.

Key preparation methods include the Dow's process, which converts haloarenes (like chlorobenzene) to phenol using harsh conditions of aqueous NaOH at high temperature ($623K$) and pressure ($300atm$).

Another method involves benzenediazonium salts, formed by diazotization of aniline with $NaNO_2/HCl$ at $0-5^circ C$, which then yield phenol upon warming with water, releasing nitrogen gas. The most important industrial method is the Cumene process, where cumene (isopropylbenzene) is air-oxidized to cumene hydroperoxide, which then undergoes acid-catalyzed decomposition to produce both phenol and acetone.

Finally, phenol can also be prepared from benzenesulphonic acid by fusion with solid NaOH at $573K$, followed by acidification. Remember the specific reagents, conditions, and key intermediates for each method.

5-Minute Revision

Let's quickly review the essentials of phenol nomenclature and preparation. Phenols are distinct from alcohols because their -OH group is directly attached to an aromatic ring. This structural feature makes them more acidic and influences their reactivity.

Nomenclature:

Parent Name: — 'Phenol' is the base. The carbon with -OH is C-1.
Substituents: — Number for lowest locants. List alphabetically. E.g., 2-chlorophenol.
Common Names: — Crucial to know: Catechol (1,2-dihydroxybenzene), Resorcinol (1,3-dihydroxybenzene), Hydroquinone (1,4-dihydroxybenzene), o-, m-, p-Cresol (methylphenols).

Methods of Preparation:

1
From Haloarenes (Dow's Process):

* Reaction: Chlorobenzene $\xrightarrow{NaOH_{(aq)}, 623K, 300atm}$ Sodium Phenoxide $\xrightarrow{H^+}$ Phenol. * Key: Harsh conditions due to unreactive C-Cl bond.

1
From Benzenediazonium Salts:

* Step 1 (Diazotization): Aniline $\xrightarrow{NaNO_2/HCl, 0-5^circ C}$ Benzenediazonium Chloride ($C_6H_5N_2^+Cl^-$). * Step 2 (Hydrolysis): $C_6H_5N_2^+Cl^- \xrightarrow{H_2O, warm}$ Phenol + $N_2 \uparrow + HCl$. * Key: Low temperature for diazonium salt stability; $N_2$ gas evolution.

1
From Cumene (Industrial Method):

* Step 1 (Oxidation): Cumene $\xrightarrow{O_2, Catalyst}$ Cumene Hydroperoxide ($C_6H_5C(CH_3)_2OOH$). * Step 2 (Decomposition): Cumene Hydroperoxide $\xrightarrow{H^+}$ Phenol + Acetone ($CH_3COCH_3$). * Key: Cumene hydroperoxide intermediate, valuable acetone co-product.

1
From Benzenesulphonic Acid:

* Reaction: Benzenesulphonic Acid $\xrightarrow{NaOH_{(s)}, 573K}$ Sodium Phenoxide $\xrightarrow{H^+}$ Phenol. * Key: High-temperature fusion with solid NaOH.

Mini-Example: What is the IUPAC name of o-cresol? It's 2-methylphenol. What are the products of Cumene hydroperoxide decomposition? Phenol and acetone. Focus on these specific details for NEET success.

Prelims Revision Notes

Phenol Nomenclature & Preparation: NEET Quick Recall

I. Nomenclature of Phenols

Definition: — OH group directly attached to an aromatic ring.
Parent Name: — 'Phenol' (benzene with -OH). Carbon bearing -OH is C-1.
Numbering: — Assign lowest possible locants to other substituents. Alphabetical order for listing.

* *Example:* 2-Bromophenol, 4-Nitrophenol.

Common Names (Crucial for NEET):

* Cresols: Methylphenols. * *o*-cresol (2-methylphenol) * *m*-cresol (3-methylphenol) * *p*-cresol (4-methylphenol) * Dihydroxybenzenes: * Catechol: Benzene-1,2-diol (ortho) * Resorcinol: Benzene-1,3-diol (meta) * Hydroquinone (or Quinol): Benzene-1,4-diol (para) * Trihydroxybenzenes: Pyrogallol (Benzene-1,2,3-triol), Phloroglucinol (Benzene-1,3,5-triol).

II. Methods of Preparation

1
From Haloarenes (Dow's Process):

* Starting Material: Chlorobenzene ($C_6H_5Cl$) or other haloarenes. * Reagents/Conditions: $NaOH_{(aq)}$, $623K$ ($350^circ C$), $300atm$ (harsh conditions). * Intermediate: Sodium Phenoxide ($C_6H_5ONa$). * Final Step: Acidification ($H^+$) to yield Phenol ($C_6H_5OH$).

1
From Benzenediazonium Salts:

* Step 1: Diazotization (Formation of Diazonium Salt): * Starting Material: Aniline ($C_6H_5NH_2$). * Reagents/Conditions: $NaNO_2 + HCl$ (or $HNO_2$), $0-5^circ C$ (critical low temperature). * Product: Benzenediazonium Chloride ($C_6H_5N_2^+Cl^-$). * Step 2: Hydrolysis of Diazonium Salt: * Reagents/Conditions: Warm with water ($H_2O$). * Products: Phenol ($C_6H_5OH$) + $N_2 \uparrow$ + $HCl$.

1
From Cumene (Isopropylbenzene) - Industrial Method:

* Starting Material: Cumene ($C_6H_5-CH(CH_3)_2$). * Step 1: Air Oxidation: * Reagents/Conditions: Air ($O_2$), Catalyst. * Intermediate: Cumene Hydroperoxide ($C_6H_5-C(CH_3)_2-OOH$). * Step 2: Acid-catalyzed Decomposition: * Reagents/Conditions: Dilute acid ($H_2SO_4$). * Products: Phenol ($C_6H_5OH$) + Acetone ($CH_3COCH_3$) (valuable co-product).

1
From Benzenesulphonic Acid:

* Starting Material: Benzenesulphonic acid ($C_6H_5SO_3H$). * Step 1: Fusion: * Reagents/Conditions: Solid $NaOH_{(s)}$, $573K$. * Intermediate: Sodium Phenoxide ($C_6H_5ONa$). * Step 2: Acidification: * Reagents/Conditions: $H^+$. * Product: Phenol ($C_6H_5OH$).

Key Points for NEET: Memorize specific temperatures, pressures, catalysts, and intermediate products for each method. Understand why certain conditions are necessary (e.g., low temperature for diazonium salts, high for Dow's).

Vyyuha Quick Recall

To remember the main preparation methods for Phenol, think of 'D.A.C.S.':

Dow's Process: From Dichloro- (Haloarenes)
Aniline: From Aniline (via Diazonium salts)
Cumene: From Cumene (Industrial method)
Sulphonic Acid: From BenzeneSulphonic Acid

This helps recall the starting materials for the four major methods.