Alcohols Phenols And Ethers Class 12 Chapter Notes

Key Definitions

Alcohols

Organic compounds characterized by the presence of a hydroxyl (-OH) functional group attached to a saturated carbon atom.

Phenols

Compounds in which the hydroxyl (-OH) group is directly attached to a benzene ring. They are more acidic than alcohols.

Ethers

Organic compounds having an oxygen atom bonded to two alkyl or aryl groups (R-O-R'). They can be simple (symmetric) or mixed (asymmetric).

Lucas Reagent

An equimolar mixture of concentrated HCl and anhydrous ZnCl₂ used to distinguish primary, secondary, and tertiary alcohols based on reactivity and turbidity formation.

Kolbe's Reaction (Kolbe-Schmitt Reaction)

A chemical reaction that involves the carboxylation of phenols to give salicylic acid derivatives. Phenol reacts with CO₂ in the presence of NaOH at high pressure and temperature.

Reimer-Tiemann Reaction

A reaction used for the ortho-formylation of phenols, typically using chloroform in the presence of a strong base to form o-hydroxybenzaldehyde (salicylaldehyde).

Williamson's Synthesis

A method for synthesizing ethers by reacting an alkyl halide with a sodium alkoxide or phenoxide. It is an SN2 reaction and works best with primary alkyl halides.

Alcohols: Classification and Nomenclature

Alcohols are classified based on the number of hydroxyl groups and the hybridization of the carbon atom to which the hydroxyl group is attached.

Classification by Number of -OH Groups:

• Monohydric Alcohols: Contain one -OH group (e.g., ethanol, CH₃CH₂OH).
• Dihydric Alcohols: Contain two -OH groups (e.g., ethane-1,2-diol or ethylene glycol).
• Polyhydric Alcohols: Contain three or more -OH groups (e.g., propane-1,2,3-triol or glycerol).

Classification by Hybridization of Carbon Bearing -OH Group:

1. Compounds containing C(sp³)-OH bond:

• Primary (1°): -OH group is attached to a primary carbon atom (R-CH₂OH).
• Secondary (2°): -OH group is attached to a secondary carbon atom (R₂CH-OH).
• Tertiary (3°): -OH group is attached to a tertiary carbon atom (R₃C-OH).
• Allylic Alcohols: -OH group is attached to a sp³ hybridized carbon atom next to a C=C double bond (e.g., CH₂=CH-CH₂OH).
• Benzylic Alcohols: -OH group is attached to a sp³ hybridized carbon atom next to an aromatic ring (e.g., C₆H₅-CH₂OH).

1. Compounds containing C(sp²)-OH bond:

• Vinylic Alcohols: -OH group is attached to a sp² hybridized carbon atom of a C=C double bond (e.g., CH₂=CH-OH). These are generally unstable and tautomerize to carbonyl compounds.
• Phenols: -OH group is directly attached to a sp² hybridized carbon atom of an aromatic ring.

Nomenclature (IUPAC):

• Replace the '-e' of the corresponding alkane with '-ol'.
• Number the carbon chain such that the -OH group gets the lowest possible number.
• For cyclic alcohols, the carbon bearing the -OH group is numbered 1.
• If other functional groups are present, priority is given to the -OH group for numbering.

Preparation of Alcohols

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Phenols: Preparation, Acidity and Reactions

Phenols are aromatic compounds with an -OH group directly attached to the benzene ring. Their distinct electronic environment leads to unique properties and reactions.

Preparation of Phenols:

1. From Haloarenes (Dow's Process): Chlorobenzene is fused with NaOH at high temperature (623 K) and pressure (300 atm) to give sodium phenoxide, followed by acidification. C₆H₅Cl + NaOH --(623K, 300atm)--> C₆H₅ONa --(H⁺)--> C₆H₅OH.
2. From Benzenesulphonic Acid: Benzene is sulphonated to benzenesulphonic acid, which is then fused with NaOH at high temperature, followed by acidification. C₆H₅SO₃H + 2NaOH --> C₆H₅ONa + Na₂SO₃ + H₂O --(H⁺)--> C₆H₅OH.
3. From Diazonium Salts: Aniline reacts with nitrous acid (NaNO₂ + HCl) at 0-5°C to form benzenediazonium chloride, which on warming with water hydrolyses to phenol. C₆H₅NH₂ --(NaNO₂/HCl, 0-5°C)--> C₆H₅N₂⁺Cl⁻ --(H₂O, warm)--> C₆H₅OH + N₂ + HCl.
4. From Cumene (Isopropylbenzene): Cumene is oxidized in air to cumene hydroperoxide, which is then treated with dilute acid to produce phenol and acetone. This is a major industrial method. C₆H₅CH(CH₃)₂ --(O₂/Air)--> C₆H₅C(OOH)(CH₃)₂ --(H⁺)--> C₆H₅OH + CH₃COCH₃.

Acidity of Phenols:

• Phenols are more acidic than alcohols but less acidic than carboxylic acids.
• This increased acidity is due to the resonance stabilization of the phenoxide ion (C₆H₅O⁻) formed after donating a proton. The negative charge is delocalized over the benzene ring.
• Electron-withdrawing groups (e.g., -NO₂, -X) at ortho and para positions increase acidity by stabilizing the phenoxide ion through resonance/inductive effect.
• Electron-donating groups (e.g., -CH₃, -OCH₃) decrease acidity by destabilizing the phenoxide ion.

Important Reactions of Phenols:

1. Electrophilic Aromatic Substitution: -OH group is strongly activating and ortho-para directing.

• Nitration: With dilute HNO₃, forms o-nitrophenol and p-nitrophenol. With conc. HNO₃, forms 2,4,6-trinitrophenol (picric acid).
• Halogenation: With Br₂ water, forms 2,4,6-tribromophenol (white precipitate). With Br₂ in CS₂, forms monobromo products (o- and p-).
• Sulphonation: Forms o- and p-phenolsulphonic acids depending on temperature.

1. Kolbe's Reaction: Reaction with CO₂ in the presence of NaOH at 400K, 4-7 atm, followed by acidification, yields salicylic acid (2-hydroxybenzoic acid).
2. Reimer-Tiemann Reaction: Reaction with chloroform (CHCl₃) and NaOH, followed by hydrolysis, yields salicylaldehyde (2-hydroxybenzaldehyde).
3. Reaction with Zinc Dust: Phenol is reduced to benzene. C₆H₅OH + Zn --> C₆H₆ + ZnO.
4. Oxidation: Phenol oxidizes to benzoquinone in the presence of chromic acid (Na₂Cr₂O₇/H₂SO₄).

Ethers: Preparation and Reactions

Ethers are characterized by an oxygen atom bonded to two alkyl or aryl groups. They are generally unreactive, making them excellent solvents.

Preparation of Ethers:

1. By Dehydration of Alcohols: Primary alcohols, when heated with concentrated H₂SO₄ at 413 K (140°C), undergo intermolecular dehydration to form ethers. This is an SN2 reaction. 2R-OH --(Conc. H₂SO₄, 413 K)--> R-O-R + H₂O. This method is suitable only for symmetrical ethers and primary alcohols to avoid alkene formation.
2. Williamson's Synthesis: This is a versatile method for preparing both symmetrical and unsymmetrical ethers. An alkyl halide reacts with a sodium alkoxide or phenoxide. R-X + R'-ONa → R-O-R' + NaX. For best yield of unsymmetrical ethers, a primary alkyl halide should be used with a tertiary or secondary alkoxide. If a tertiary alkyl halide is used, elimination (alkene formation) predominates.

Key Points to Remember

• Alcohols, phenols, and ethers are oxygen-containing organic compounds with distinct reactivities.
• Phenols are significantly more acidic than alcohols due to resonance stabilization of the phenoxide ion.
• Williamson's Synthesis is an SN2 reaction; use primary alkyl halide for good yields of ethers to prevent elimination.
• Lucas test differentiates 1°, 2°, 3° alcohols (turbidity: 3° immediate, 2° in 5-10 min, 1° no reaction at room temp).
• Kolbe's reaction yields salicylic acid; Reimer-Tiemann reaction yields salicylaldehyde from phenol.
• Ethers are generally unreactive but undergo cleavage with hot concentrated HI or HBr.
• Order of reactivity of alcohols towards hydrogen halides: 3° > 2° > 1° (SN1 mechanism).
• Dehydration of alcohols to alkenes follows Zaitsev's rule and uses concentrated H₂SO₄ at higher temperatures (443 K).
• Phenols give characteristic color with neutral FeCl₃ solution (violet, green, etc.).
• Grignard reagents are highly versatile for alcohol synthesis: formaldehyde (1°), other aldehydes (2°), ketones (3°).

Exam Tip: Distinguishing Reactions and Mechanisms

Pay close attention to reaction conditions (temperature, catalysts) as they often dictate the product. For instance, dehydration of alcohols at 413 K yields ethers, while at 443 K, it yields alkenes. In Williamson's synthesis, always choose a primary alkyl halide and the appropriate alkoxide/phenoxide to prevent unwanted elimination side reactions. For phenol reactions, remember the activating and ortho-para directing nature of the -OH group, which influences electrophilic substitution patterns. Clearly differentiate SN1 vs SN2 mechanisms in alcohol substitution reactions based on the substrate (1°, 2°, 3°).

Worked Mini-Examples

• Example 1: Williamson's Synthesis Synthesize methoxyethane (ethyl methyl ether) using Williamson's synthesis. Solution: To get CH₃OCH₂CH₃, we need a primary alkyl halide and a suitable alkoxide. We can use bromomethane (CH₃Br) and sodium ethoxide (CH₃CH₂ONa). CH₃Br + CH₃CH₂ONa → CH₃OCH₂CH₃ + NaBr
• Example 2: Conversion of Phenol to Salicylic Acid How is phenol converted to salicylic acid? Solution: This is Kolbe's Reaction. Phenol reacts with sodium hydroxide to form sodium phenoxide. Sodium phenoxide then reacts with carbon dioxide under pressure (400K, 4-7 atm) to form sodium salicylate, which upon acidification yields salicylic acid.

Practice Questions with Solutions

• Q: Why are phenols more acidic than alcohols? A: Phenols are more acidic because the phenoxide ion, formed after losing a proton, is resonance stabilized due to the delocalization of the negative charge over the benzene ring, making proton donation easier.
• Q: What is the main limitation of preparing ethers by dehydration of alcohols? A: The main limitation is that it is suitable primarily for preparing symmetrical ethers from primary alcohols. With secondary/tertiary alcohols or at higher temperatures, elimination (alkene formation) becomes a major side reaction.
• Q: Give the product of the reaction: Phenol + Br₂ water. A: When phenol reacts with bromine water, it gives a white precipitate of 2,4,6-tribromophenol. The -OH group is a strong activating group, causing substitution at all available ortho and para positions.
• Q: How would you distinguish between propan-1-ol and propan-2-ol using a chemical test? A: The Lucas test can distinguish them. Propan-2-ol (secondary alcohol) will react with Lucas reagent (conc. HCl + anhyd. ZnCl₂) to form turbidity within 5-10 minutes, while propan-1-ol (primary alcohol) will show no turbidity at room temperature.

Frequently Asked Questions

Frequently Asked Questions