Alcohols, Phenols and Ethers Class 12 Notes – Reactions, Properties, NCERT Solutions & CBSE Guide 2025-26

Unit 11 – Alcohols, Phenols and Ethers

11.1 Classification
11.2 Nomenclature
11.3 Structures of Functional Groups
11.4 Alcohols and Phenols
11.5 Some Commercially Important Alcohols
11.6 Ethers

I. Introduction to Alcohols, Phenols, and Ethers

Alcohols and phenols are formed when one or more hydrogen atoms in a hydrocarbon (aliphatic and aromatic, respectively) are replaced by a hydroxyl (-OH) group. These compounds are fundamental for various applications in industry and daily life, such as detergents, antiseptics, and fragrances. For instance, ethanol, a hydroxyl-containing compound, is the main component of spirit used for polishing wooden furniture. Sugar, cotton, and paper also consist of compounds with -OH groups. This unit specifically discusses the chemistry of alcohols, phenols, and ethers.

II. Classification

Alcohols, phenols, and ethers are classified to facilitate systematic study.

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

1. Based on Number of Hydroxyl Groups:
   • Monohydric compounds: Contain one hydroxyl group.
   • Dihydric compounds: Contain two hydroxyl groups.
   • Trihydric or Polyhydric compounds: Contain three or many hydroxyl groups, respectively.
2. Based on Hybridisation of Carbon Atom Attached to -OH Group:
   • Compounds containing C(sp³)-OH bond: The -OH group is attached to an sp³ hybridised carbon atom of an alkyl group. These are further classified as:
     • Primary alcohols: The -OH group is attached to a primary carbon atom.
     • Secondary alcohols: The -OH group is attached to a secondary carbon atom.
     • Tertiary alcohols: The -OH group is attached to a tertiary carbon atom.
     • Allylic alcohols: The -OH group is attached to an sp³ hybridised carbon adjacent to a carbon-carbon double bond (an allylic carbon). Allylic alcohols can be primary, secondary, or tertiary.
     • Benzylic alcohols: The -OH group is attached to an sp³ hybridised carbon atom next to an aromatic ring. Benzylic alcohols can also be primary, secondary, or tertiary.
   • Compounds containing C(sp²)-OH bond: These alcohols have the -OH group bonded to a carbon-carbon double bond (vinylic carbon) or an aryl carbon. These are also called vinylic alcohols, e.g., CH₂=CH-OH.

B. Phenols Phenols are classified as mono-, di-, or trihydric compounds depending on whether they contain one, two, or three hydroxyl groups, respectively, on the aromatic ring. The simplest is phenol itself.

C. Ethers Ethers are classified based on the alkyl or aryl groups attached to the oxygen atom:

• Simple or Symmetrical ethers: The alkyl or aryl groups attached to the oxygen atom are the same.
  • Example: Diethyl ether, C₂H₅OC₂H₅.
• Mixed or Unsymmetrical ethers: The two groups attached to the oxygen atom are different.
  • Example: C₂H₅OCH₃ (ethylmethyl ether) and C₂H₅OC₆H₅ (ethyl phenyl ether).

III. Nomenclature

Both common names and IUPAC (International Union of Pure and Applied Chemistry) system names are used for these compounds.

A. Alcohols

• Common name: Derived from the common name of the alkyl group with the word "alcohol" added to it.
  • Example: CH₃OH is methyl alcohol.
• IUPAC name: Derived from the name of the alkane from which the alcohol is derived, by substituting the "e" of alkane with the suffix "ol".
  • The longest carbon chain (parent chain) is numbered starting from the end nearest to the hydroxyl group.
  • The positions of the -OH group and other substituents are indicated by numbers.
  • For polyhydric alcohols, the "e" of alkane is retained, and "ol" is added, preceded by multiplicative prefixes (di, tri, etc.).
    • Example: HO-CH₂-CH₂-OH is named ethane-1,2-diol.
• Cyclic alcohols: Named using the prefix "cyclo" and the -OH group is considered attached to C-1.
  • Example: Cyclohexanol, 2-Methylcyclopentanol.

B. Phenols

• Common and IUPAC name: The simplest hydroxy derivative of benzene is phenol, which is both its common and an accepted IUPAC name.
• Substituted phenols: Terms like ortho (1,2-disubstituted), meta (1,3-disubstituted), and para (1,4-disubstituted) are often used in common names.
  • Example: o-Cresol (2-Methylphenol), m-Cresol (3-Methylphenol), p-Cresol (4-Methylphenol).
• Dihydroxy derivatives of benzene: Named as benzenediols.
  • Example: Catechol (Benzene-1,2-diol), Resorcinol (Benzene-1,3-diol), Hydroquinone or Quinol (Benzene-1,4-diol).

C. Ethers

• Common names: Derived by naming the alkyl/aryl groups attached to the oxygen atom as separate words in alphabetical order, followed by the word "ether".
  • Example: CH₃OC₂H₅ is ethylmethyl ether. If both alkyl groups are the same, the prefix 'di' is added, e.g., C₂H₅OC₂H₅ is diethyl ether.
• IUPAC names: Ethers are considered hydrocarbon derivatives where a hydrogen atom is replaced by an -OR or -OAr group (alkoxy or aryloxy). The larger (R) group is chosen as the parent hydrocarbon.
  • Example: CH₃OCH₃ is methoxymethane; C₂H₅OC₂H₅ is ethoxyethane.
  • Anisole (methoxybenzene) and Phenetole (ethoxybenzene) are also commonly accepted names for specific aryl alkyl ethers.

IV. Structures of Functional Groups

In alcohols, the oxygen of the -OH group is attached to carbon by a sigma (σ) bond. This bond is formed by the overlap of an sp³ hybridised orbital of carbon with an sp³ hybridised orbital of oxygen. The bond angle in alcohols is slightly less than the tetrahedral angle (109°28') due to repulsion from unshared electron pairs on oxygen.

In phenols, the -OH group is attached to an sp² hybridised carbon of an aromatic ring. The carbon-oxygen bond length (136 pm) in phenol is slightly shorter than in methanol. This is attributed to:

1. Partial double bond character due to the conjugation of the unshared electron pair of oxygen with the aromatic ring.
2. The sp² hybridization of the carbon in the benzene ring.

V. Preparation Methods

A. Preparation of Alcohols Alcohols can be prepared through various reactions:

• From Alkenes:
  • Hydration in presence of acid: Alkenes react with water in the presence of an acid catalyst to form alcohols.
  • Hydroboration-oxidation reaction.
• From Carbonyl Compounds:
  • Reduction of Aldehydes and Ketones:
    • Aldehydes and ketones are reduced to corresponding alcohols by catalytic hydrogenation (addition of hydrogen in presence of catalysts like platinum, palladium, or nickel).
    • They can also be prepared by treating aldehydes and ketones with sodium borohydride (NaBH₄) or lithium aluminium hydride (LiAlH₄).
    • Aldehydes yield primary alcohols, while ketones yield secondary alcohols.
    • Specifically, methanal produces a primary alcohol, other aldehydes yield secondary alcohols, and ketones give tertiary alcohols.
  • Reduction of Carboxylic Acids and Esters: Carboxylic acids are reduced to primary alcohols in excellent yields using lithium aluminium hydride (LiAlH₄), a strong reducing agent.

B. Preparation of Phenols Phenols can be prepared from:

• Haloarenes: Chlorobenzene, for example, is fused with NaOH at high temperature (623K) and pressure (320 atm), and the resulting sodium phenoxide is acidified to yield phenol.
• Benzenesulphonic acid: Benzene is sulphonated with oleum to form benzenesulphonic acid, which is then converted to sodium phenoxide by heating with molten sodium hydroxide. Acidification gives phenol.
• Diazonium Salts: An aromatic primary amine reacts with nitrous acid (NaNO₂ + HCl) at low temperatures (273-278 K) to form a diazonium salt. This salt is then hydrolyzed to phenol by warming with water or treating with dilute acids.
• Cumene (Isopropylbenzene): This is the most common industrial method for phenol production globally. Cumene is oxidized in the presence of air to form cumene hydroperoxide, which is then treated with dilute acid to produce phenol and acetone (a valuable by-product).

C. Preparation of Ethers Ethers can be prepared by:

• Dehydration of Alcohols: Alcohols undergo dehydration in the presence of protic acids (like concentrated H₂SO₄ or H₃PO₄).
  • The reaction conditions are crucial; at 413 K with H₂SO₄, ethanol primarily yields ethoxyethane (an ether), whereas at 443 K, it yields ethene (an alkene).
  • This method is most suitable for primary alkyl groups and requires unhindered alkyl groups and low temperatures to favor ether formation over alkene formation.
  • Dehydration of secondary and tertiary alcohols to ethers is generally unsuccessful because elimination (alkene formation) competes strongly with substitution.
  • The formation of ether from alcohol involves a nucleophilic bimolecular reaction (SN2) where an alcohol molecule attacks a protonated alcohol.
• Williamson Synthesis: This method involves the reaction of alkyl halides with sodium alkoxides or aryloxides. Phenols can also be converted to ethers via this method by using a phenoxide moiety.

VI. Physical Properties

The properties of alcohols and phenols are primarily due to their hydroxyl group, with the alkyl and aryl groups modifying these properties.

A. Boiling Points

• The boiling points of alcohols and phenols increase with an increase in the number of carbon atoms due to increased van der Waals forces.
• In alcohols, boiling points decrease with increasing branching in the carbon chain because branching reduces surface area, leading to weaker van der Waals forces.
• Alcohols and phenols have significantly higher boiling points compared to hydrocarbons, ethers, haloalkanes, and haloarenes of comparable molecular masses.
  • Example: Ethanol (b.p. 351 K) has a higher boiling point than propane (comparable molecular mass).
  • The boiling point of methoxymethane is intermediate between ethanol and propane.
• This high boiling point is mainly due to the presence of intermolecular hydrogen bonding in alcohols and phenols, which is absent in ethers and hydrocarbons.

B. Solubility

• Alcohols and phenols are soluble in water because their -OH groups can form hydrogen bonds with water molecules.
• The solubility decreases as the size of the alkyl/aryl (hydrophobic) groups increases.
• Lower molecular mass alcohols are miscible with water in all proportions.
• Ethers also exhibit miscibility with water, comparable to alcohols of similar molecular mass, because the oxygen atom in ethers can form hydrogen bonds with water molecules. For example, ethoxyethane and butan-1-ol are similarly miscible in water, while pentane is immiscible.

VII. Chemical Reactions

A. Reactions of Alcohols Alcohols are versatile, acting as both nucleophiles and electrophiles.

1. Reactions involving cleavage of O-H bond (Alcohol as Nucleophile):
   • Acidity of Alcohols and Phenols:
     • Alcohols and phenols react with active metals (e.g., sodium, potassium, aluminium) to produce corresponding alkoxides/phenoxides and hydrogen gas, indicating their acidic nature.
     • Phenols are stronger acids than alcohols and water. For instance, phenol is millions of times more acidic than ethanol. This is because the hydroxyl group in phenol is attached to an sp² hybridised carbon of the benzene ring, which acts as an electron-withdrawing group, causing the oxygen of the -OH group to become positive. This effect and resonance stabilize the phenoxide ion.
     • Electron-withdrawing groups (e.g., nitro group) enhance the acidic strength of phenol, especially at ortho and para positions, due to effective delocalization of the negative charge in the phenoxide ion.
     • Electron-releasing groups (e.g., alkyl groups) decrease the acid strength as they do not favor phenoxide ion formation. Cresols, for example, are less acidic than phenol.
   • Esterification: Alcohols and phenols react with carboxylic acids, acid chlorides, and acid anhydrides to form esters.
2. Reactions involving cleavage of C-O bond in alcohols (Protonated Alcohol as Electrophile):
   • Reaction with Hydrogen Halides: Alcohols react with hydrogen halides (HX) to form alkyl halides (R-X) and water.
     • The Lucas test uses the difference in reactivity with HCl to distinguish primary, secondary, and tertiary alcohols. Tertiary alcohols produce immediate turbidity (form halides easily), while primary alcohols do not produce turbidity at room temperature.
   • Reaction with Phosphorus Trihalides: Alcohols are converted to alkyl bromides by reacting with phosphorus tribromide (PBr₃).
   • Dehydration: Alcohols undergo dehydration (removal of water) to form alkenes when treated with a protic acid (e.g., concentrated H₂SO₄, H₃PO₄) or catalysts like anhydrous zinc chloride or alumina.
     • Example: Ethanol dehydrates with concentrated H₂SO₄ at 443 K to yield ethene.
   • Oxidation:
     • Primary alcohols yield aldehydes with mild oxidizing agents (e.g., CrO₃) and carboxylic acids with strong oxidizing agents (e.g., KMnO₄).
     • Secondary alcohols are oxidized to ketones by chromic anhydride (CrO₃).
     • Tertiary alcohols do not undergo oxidation readily. Under strong conditions, C-C bonds may cleave, forming a mixture of carboxylic acids with fewer carbon atoms.
     • When vapors of primary or secondary alcohols are passed over heated copper at 573 K, dehydrogenation occurs, forming an aldehyde or ketone, respectively. Tertiary alcohols undergo dehydration under these conditions.
   • Methanol Poisoning: Methanol is highly poisonous. In the body, it is oxidized to methanal and then methanoic acid, which can cause blindness and death. Diluted ethanol can be used as a treatment by overwhelming the enzyme responsible for methanal oxidation, allowing methanol to be excreted.

B. Reactions of Phenols The -OH group in phenols activates the aromatic ring toward electrophilic substitution and directs incoming groups to ortho and para positions due to resonance effects.

1. Electrophilic Aromatic Substitution Reactions:
   • Nitration:
     • With dilute nitric acid at low temperature (298 K), phenol yields a mixture of ortho and para nitrophenols. These isomers can be separated by steam distillation; o-nitrophenol is steam volatile due to intramolecular hydrogen bonding, while p-nitrophenol is less volatile due to intermolecular hydrogen bonding.
     • With concentrated nitric acid, phenol is converted to 2,4,6-trinitrophenol (picric acid), which is a strong acid due to three electron-withdrawing -NO₂ groups. Picric acid is typically prepared by first treating phenol with concentrated sulfuric acid (to form phenol-2,4-disulphonic acid) and then with concentrated nitric acid.
   • Halogenation: Phenol reacts with bromine differently under varying conditions.
   • Kolbe’s Reaction: Phenoxide ion (formed by treating phenol with NaOH) is even more reactive than phenol towards electrophilic aromatic substitution. It undergoes reaction with a weak electrophile like carbon dioxide to form ortho hydroxybenzoic acid as the main product.
   • Reimer-Tiemann Reaction: When phenol is treated with chloroform in the presence of sodium hydroxide, a -CHO group is introduced at the ortho position of the benzene ring, leading to the formation of salicylaldehyde.
2. Reaction with Zinc Dust: Phenol is converted to benzene upon heating with zinc dust.
3. Oxidation: Oxidation of phenol with chromic acid produces benzoquinone, a conjugated diketone. Phenols can also slowly oxidize in air to dark-colored mixtures containing quinones.

C. Reactions of Ethers Ethers are generally less reactive among functional groups.

1. Cleavage of C-O bond in Ethers: This reaction occurs under drastic conditions with an excess of hydrogen halides (HI > HBr > HCl is the order of reactivity) at high temperatures.
   • Dialkyl ethers yield two alkyl halide molecules.
   • Alkyl aryl ethers are cleaved at the alkyl-oxygen bond (due to the more stable aryl-oxygen bond), yielding phenol and alkyl halide.
   • For mixed ethers with two different alkyl groups (primary or secondary), the lower alkyl group forms the alkyl iodide via an SN2 mechanism.
   • The reaction mechanism involves protonation of the ether molecule, followed by nucleophilic attack by the halide ion on the least substituted carbon of the oxonium ion.
2. Electrophilic Substitution: The alkoxy group (-OR) is ortho, para directing and activates the aromatic ring towards electrophilic substitution, similar to phenol.
   • Halogenation: Phenylalkyl ethers (e.g., anisole) undergo bromination with bromine in ethanoic acid even without an iron (III) bromide catalyst due to the activating effect of the methoxy group. The para isomer is typically obtained in high yield (90%).
   • Friedel-Crafts Reaction: Anisole undergoes Friedel-Crafts alkylation and acylation, where alkyl and acyl groups are introduced at the ortho and para positions, using alkyl halide and acyl halide respectively, in the presence of anhydrous aluminium chloride (a Lewis acid).
   • Nitration: Anisole reacts with a mixture of concentrated sulfuric and nitric acids to yield a mixture of ortho and para nitroanisole.

VIII. Commercially Important Alcohols

A. Methanol (CH₃OH)

• Also known as "wood spirit".
• Historically produced by destructive distillation of wood.
• Today, primarily produced by catalytic hydrogenation of carbon monoxide at high pressure and temperature, using a ZnO-Cr₂O₃ catalyst.
• Properties: Colourless liquid, boils at 337 K, highly poisonous.
• Ingestion of even small quantities can cause blindness, and large quantities can be fatal. A methanol-poisoned patient is often treated with intravenous infusions of diluted ethanol, which helps the body excrete methanol.
• Uses: Solvent in paints and varnishes, chiefly for making formaldehyde.

B. Ethanol (C₂H₅OH)

• Commercially obtained through fermentation, an ancient method using sugars.
  • Sugar from molasses, sugarcane, or fruits (like grapes) is converted to glucose and fructose (C₆H₁₂O₆) by the enzyme invertase.
  • Glucose and fructose then undergo fermentation, catalyzed by the enzyme zymase (found in yeast), to produce ethanol and carbon dioxide.
• Properties: Colourless liquid, boiling point 351 K.
• Uses: Solvent in the paint industry and for preparing various carbon compounds.
• Denaturation of alcohol: Commercial alcohol is made unfit for drinking by adding substances like copper sulfate (for color) and pyridine (for foul smell). This is known as denaturation.
• Physiological Effects: Ingesting ethanol acts on the central nervous system. Moderate amounts impair judgment and lower inhibitions. Higher concentrations can cause nausea, loss of consciousness, and even interfere with spontaneous respiration, which can be fatal.
• Large quantities of ethanol are also obtained by the hydration of ethene.

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Analogy: Think of alcohols, phenols, and ethers as different types of "molecular buildings" all using oxygen as a key structural element, but with different foundations and connections.

• Alcohols are like houses built on a flexible, sp³-hybridized alkyl "land". They are versatile and can share their "front door" (O-H bond) or their "back door" (C-O bond) depending on who's visiting. Their strong "sticky notes" (hydrogen bonds) make them cling together and dissolve well in "water".
• Phenols are similar, but their oxygen is attached to a rigid, sp²-hybridized aromatic "city block". This "city block" influences the oxygen, making it more acidic and directing new "buildings" (substituents) to specific locations. They also have strong "sticky notes".
• Ethers are like houses where the oxygen is more like a "bridge" between two alkyl or aryl "lands". They're less reactive and don't have the same "sticky notes" as alcohols, so they don't cling as tightly to each other, resulting in lower boiling points. They can still form "sticky notes" with water, allowing some solubility.

This analogy highlights the structural differences and their impact on physical and chemical properties.

NCERT Solutions Highlights

Nomenclature:

• IUPAC names examples:

  • 2,2,4-Trimethylpentan-3-ol

  • 5-Ethylheptane-2,4-diol

  • Ethoxybenzene

  • 1-Phenoxyheptane

Structures to Know:

• 2-Methylbutan-2-ol

• 1-Phenylpropan-2-ol

Properties and Tests:

• Phenol is more acidic than ethanol because phenoxide ion is resonance stabilized; ethoxide ion is not.

• Lucas test distinguishes alcohol types based on reaction times for turbidity.

• Phenol reacts with FeCl₃ to give a violet color; ethanol does not.

Reactions:

• Phenol from cumene: cumene → cumene hydroperoxide → phenol + acetone

• Acid-catalyzed dehydration of ethanol involves protonation, water loss forming carbocation, and deprotonation.

• Williamson ether synthesis: alkoxide ion reacts with alkyl halide to form ether.

CBSE Past Exam Questions

1 Mark Questions:

• Phenol + bromine in CS₂ → 2,4,6-tribromophenol

• IUPAC name of CH₃CH₂OCH₂CH₃ = Ethoxyethane

• Phenol + dilute nitric acid → ortho- and para-nitrophenol

• Lucas test positive in tertiary alcohols at room temperature

2 Mark Questions:

• Explanation of phenol’s greater acidity than ethanol (resonance stabilization)

• Alcohols have higher boiling points than ethers due to hydrogen bonding

• FeCl₃ test distinguishes phenol from ethanol (violet color vs no reaction)

3 Mark Questions:

• Mechanism of ethanol dehydration (steps: protonation, carbocation, elimination)

• Mechanism of reaction with HBr (protonation, carbocation formation, nucleophilic attack)

• Arranging acidity: ethanol < water < phenol

• Boiling points order: diethyl ether < methanol < ethanol < ethylene glycol

5 Mark / Long Answer:

• Phenol synthesis from benzene sulphonic acid by fusion with NaOH, then acidification

• Williamson Ether Synthesis mechanism

• Distinguishing primary, secondary, tertiary alcohols using the Lucas test timings

Additional Tips

• Master IUPAC nomenclature and structure drawings

• Understand reaction mechanisms stepwise with intermediates

• Practice chemical tests differentiating alcohols, phenols, and ethers

• Focus on acidity and boiling point trends explained by hydrogen bonding and resonance

• Solve exemplar and higher-order thinking questions for conceptual clarity

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