Alcohols, Phenols, and Ethers

1. Alcohols

Alcohols are organic compounds characterized by the presence of a hydroxyl (-OH) functional group attached to a saturated carbon atom. The general formula for saturated monohydric alcohols is C_nH_2n+1OH. They are a crucial class of organic compounds with diverse applications.

1.1. Classification of Alcohols

Alcohols can be classified based on the number of hydroxyl groups and the nature of the carbon atom to which the hydroxyl group is attached.

1.1.1. Based on the Number of Hydroxyl Groups

• Monohydric Alcohols: Contain one hydroxyl group (e.g., Ethanol, CH₃CH₂OH).
• Dihydric Alcohols (Glycols): Contain two hydroxyl groups (e.g., Ethane-1,2-diol, HOCH₂CH₂OH).
• Trihydric Alcohols (Glycerols): Contain three hydroxyl groups (e.g., Propane-1,2,3-triol, HOCH₂CH(OH)CH₂OH).
• Polyhydric Alcohols: Contain more than three hydroxyl groups.

1.1.2. Based on the Nature of Carbon Atom Attached to -OH Group

This classification is particularly important for understanding the reactivity of alcohols. The carbon atom directly bonded to the hydroxyl group is considered.

• Primary (1°) Alcohols: The carbon atom bearing the -OH group is attached to at most one other carbon atom. The general structure is R-CH₂-OH. Examples include Methanol (CH₃OH) and Ethanol (CH₃CH₂OH).
• Secondary (2°) Alcohols: The carbon atom bearing the -OH group is attached to two other carbon atoms. The general structure is R₂CH-OH. An example is Propan-2-ol ((CH₃)₂CHOH).
• Tertiary (3°) Alcohols: The carbon atom bearing the -OH group is attached to three other carbon atoms. The general structure is R₃C-OH. An example is 2-Methylpropan-2-ol ((CH₃)₃COH).

1.2. Nomenclature of Alcohols

Alcohols are named according to IUPAC rules. The suffix '-e' of the parent alkane is replaced by '-ol'. The position of the hydroxyl group is indicated by a number. For dihydric alcohols, the suffix '-diol' is used, and for trihydric alcohols, '-triol'.

• Methanol (CH₃OH)
• Ethanol (CH₃CH₂OH)
• Propan-1-ol (CH₃CH₂CH₂OH)
• Propan-2-ol ((CH₃)₂CHOH)
• Butan-1-ol (CH₃CH₂CH₂CH₂OH)
• Butan-2-ol (CH₃CH₂CH(OH)CH₃)
• 2-Methylpropan-1-ol ((CH₃)₂CHCH₂OH)
• 2-Methylpropan-2-ol ((CH₃)₃COH)

1.3. Preparation of Alcohols

Alcohols can be prepared by various methods:

1.3.1. From Alkenes

• Acid-Catalysed Hydration: Alkenes react with water in the presence of an acid catalyst (like H₂SO₄) to form alcohols. This reaction follows Markovnikov's rule.

  CH₃-CH=CH₂ + H₂O $\xrightarrow{H^+}$ CH₃-CH(OH)-CH₃ (Propan-2-ol)

• Hydroboration-Oxidation: This method yields anti-Markovnikov addition of water across the double bond, producing primary alcohols from terminal alkenes.

  CH₃-CH=CH₂ $\xrightarrow{1. B_2H_6, THF}$ $\xrightarrow{2. H_2O_2, OH^-}$ CH₃-CH₂-CH₂OH (Propan-1-ol)

1.3.2. From Carbonyl Compounds

• Reduction of Aldehydes and Ketones: Aldehydes and ketones can be reduced to primary and secondary alcohols, respectively, using reducing agents like LiAlH₄ or NaBH₄, or by catalytic hydrogenation (H₂/Ni, Pt, Pd).

  R-CHO (Aldehyde) $\xrightarrow{[H]}$ R-CH₂OH (Primary Alcohol)

  R₂CO (Ketone) $\xrightarrow{[H]}$ R₂CHOH (Secondary Alcohol)

• Reduction of Carboxylic Acids and Esters: Carboxylic acids and esters are reduced to primary alcohols using strong reducing agents like LiAlH₄.

  R-COOH (Carboxylic Acid) $\xrightarrow{LiAlH_4}$ R-CH₂OH (Primary Alcohol)

  R-COOR' (Ester) $\xrightarrow{LiAlH_4}$ R-CH₂OH + R'-OH

1.3.3. From Grignard Reagents

Grignard reagents react with aldehydes and ketones to form alcohols. The product depends on the type of carbonyl compound and the Grignard reagent used.

• With Formaldehyde (HCHO): Produces primary alcohols.

  HCHO + R-MgX $\rightarrow$ R-CH₂-OMgX $\xrightarrow{H_2O}$ R-CH₂OH

• With Other Aldehydes (R-CHO): Produces secondary alcohols.

  R'-CHO + R-MgX $\rightarrow$ R'-CH(R)-OMgX $\xrightarrow{H_2O}$ R'-CH(R)OH

• With Ketones (R₂CO): Produces tertiary alcohols.

  R'₂CO + R-MgX $\rightarrow$ R'₂C(R)-OMgX $\xrightarrow{H_2O}$ R'₂C(R)OH

1.4. Physical Properties of Alcohols

• Boiling Points: Alcohols have higher boiling points than corresponding alkanes or ethers of similar molecular mass. This is due to the presence of intermolecular hydrogen bonding between the hydroxyl groups of alcohol molecules. Primary alcohols have higher boiling points than secondary, and secondary higher than tertiary, due to differences in surface area and hydrogen bonding efficiency.
• Solubility: Lower alcohols (methanol, ethanol, propanol) are soluble in water due to their ability to form hydrogen bonds with water molecules. As the hydrocarbon chain length increases, the hydrophobic character increases, and solubility decreases.

1.5. Chemical Reactions of Alcohols

Alcohols undergo reactions involving the cleavage of the O-H bond and the C-O bond, as well as reactions involving the alkyl group.

1.5.1. Reactions involving O-H Bond Cleavage

• Acidity: Alcohols are weakly acidic and react with active metals like sodium to liberate hydrogen gas.

  2R-OH + 2Na $\rightarrow$ 2R-O^-Na⁺ + H₂

  The acidity order is: Methanol > Primary > Secondary > Tertiary alcohols. This is because the alkoxide ion (R-O^-) formed is more stable when the alkyl group is smaller, due to its electron-donating inductive effect destabilizing the negative charge.

• Esterification: Alcohols react with carboxylic acids in the presence of an acid catalyst (like concentrated H₂SO₄ or HCl) to form esters. This is a reversible reaction.

  R-COOH + R'-OH $\rightleftharpoons$ R-COOR' + H₂O (Acid Catalyst)

1.5.2. Reactions involving C-O Bond Cleavage

• Reaction with Hydrogen Halides (HX): Alcohols react with hydrogen halides to form alkyl halides. The reactivity order of hydrogen halides is HI > HBr > HCl. The reactivity of alcohols is Tertiary > Secondary > Primary. This reaction proceeds via an S_N1 mechanism for 3° and 2° alcohols (in acidic conditions) and S_N2 for 1° alcohols.

  R-OH + HX $\rightarrow$ R-X + H₂O

  Lucas Test: This test is used to distinguish between primary, secondary, and tertiary alcohols. The reagent is anhydrous ZnCl₂ dissolved in concentrated HCl.

  • Tertiary alcohols react immediately to give a cloudy solution (alkyl halide formation).
  • Secondary alcohols react in 5-10 minutes.
  • Primary alcohols do not react at room temperature but react upon heating.

• Dehydration: Alcohols undergo dehydration to form alkenes (intramolecular dehydration, usually at high temperatures with acid catalysts like H₂SO₄ or Al₂O₃) or ethers (intermolecular dehydration, at lower temperatures).
  • Intramolecular Dehydration (Formation of Alkenes):

    CH₃CH₂OH $\xrightarrow{H_2SO_4, 443K}$ CH₂=CH₂ + H₂O

    Reactivity order: Tertiary > Secondary > Primary alcohols.

  • Intermolecular Dehydration (Formation of Ethers):

    2CH₃CH₂OH $\xrightarrow{H_2SO_4, 413K}$ CH₃CH₂-O-CH₂CH₃ + H₂O

    This method is suitable for preparing symmetrical ethers from primary alcohols.

• Oxidation: The oxidation of alcohols depends on the type of alcohol (primary, secondary, tertiary) and the oxidizing agent used.
  • Primary Alcohols: Can be oxidized to aldehydes (using mild oxidizing agents like PCC - Pyridinium Chlorochromate, or with Cu at 573K) or further to carboxylic acids (using strong oxidizing agents like acidified KMnO₄ or K₂Cr₂O₇).

    R-CH₂OH $\xrightarrow{PCC}$ R-CHO

    R-CH₂OH $\xrightarrow{KMnO_4/H^+}$ R-COOH

  • Secondary Alcohols: Are oxidized to ketones.

    R₂CHOH $\xrightarrow{[O]}$ R₂CO

  • Tertiary Alcohols: Are generally resistant to oxidation under mild conditions. Strong oxidizing agents under harsh conditions can lead to the cleavage of C-C bonds.

1.6. Important Alcohols

• Methanol (CH₃OH): Also known as wood alcohol. It is produced by the destructive distillation of wood. It is a toxic substance and can cause blindness or death if ingested. It is used as a solvent, in the production of formaldehyde, and as a fuel additive.
• Ethanol (C₂H₅OH): Also known as grain alcohol. It is produced by the fermentation of sugars by yeast. It is the alcohol present in alcoholic beverages. It is also used as a solvent, in fuels (bioethanol), and as a disinfectant.
• Ethane-1,2-diol (Ethylene Glycol): Used as an antifreeze in car radiators.
• Propane-1,2,3-triol (Glycerol): A viscous liquid, used in cosmetics, pharmaceuticals, and as a sweetening agent.

2. Phenols

Phenols are organic compounds in which a hydroxyl group (-OH) is directly attached to a benzene ring (or other aromatic system). They are aromatic analogues of alcohols.

2.1. Nomenclature of Phenols

The parent compound is phenol (C₆H₅OH). When other substituents are present on the benzene ring, they are given locant numbers. Common derivatives include cresols (methylphenols) and xylenols (dimethylphenols).

• Phenol (C₆H₅OH)
• o-Cresol (2-methylphenol)
• m-Cresol (3-methylphenol)
• p-Cresol (4-methylphenol)
• Catechol (Benzene-1,2-diol)
• Resorcinol (Benzene-1,3-diol)
• Hydroquinone (Benzene-1,4-diol)

2.2. Preparation of Phenols

• From Halobenzenes: Phenol can be prepared by the hydrolysis of chlorobenzene under severe conditions (high temperature and pressure) with aqueous sodium hydroxide.

  C₆H₅Cl + 2NaOH $\xrightarrow{623K, 320 atm}$ C₆H₅ONa + NaCl + H₂O

  C₆H₅ONa + H₂O + CO₂ $\rightarrow$ C₆H₅OH + NaHCO₃

• From Benzene Sulfonic Acid: Benzene can be sulfonated, and the resulting benzene sulfonic acid is fused with sodium hydroxide.

  C₆H₆ + conc. H₂SO₄ $\rightarrow$ C₆H₅SO₃H

  C₆H₅SO₃H + 2NaOH $\xrightarrow{fusion}$ C₆H₅ONa + Na₂SO₃ + H₂O

  C₆H₅ONa $\xrightarrow{H^+}$ C₆H₅OH

• From Diazonium Salts: Diazonium salts, obtained from the reaction of primary aromatic amines with nitrous acid at low temperatures, are readily hydrolyzed to phenols.

  C₆H₅NH₂ + NaNO₂ + 2HCl $\xrightarrow{273-278K}$ [C₆H₅N₂]⁺Cl^- + NaCl + 2H₂O

  [C₆H₅N₂]⁺Cl^- + H₂O $\xrightarrow{boiling}$ C₆H₅OH + N₂ + HCl

• Industrial Method (Cumene Process): This is the most important commercial method for producing phenol. Cumene (isopropylbenzene) is oxidized by air to cumene hydroperoxide, which is then decomposed by acid to phenol and acetone.

  C₆H₅CH(CH₃)₂ (Cumene) $\xrightarrow{O_2}$ C₆H₅C(OOH)(CH₃)₂ (Cumene hydroperoxide)

  C₆H₅C(OOH)(CH₃)₂ $\xrightarrow{H^+}$ C₆H₅OH + (CH₃)₂CO (Acetone)

2.3. Physical Properties of Phenols

• Phenol is a colorless crystalline solid. It melts at about 41°C.
• It is sparingly soluble in water but soluble in organic solvents.
• Phenols have higher boiling points than benzene or corresponding aromatic hydrocarbons due to hydrogen bonding.

2.4. Chemical Reactions of Phenols

Phenols are more acidic than alcohols. This is because the phenoxide ion (C₆H₅O^-) formed after the loss of a proton is resonance-stabilized, with the negative charge delocalized over the benzene ring.

2.4.1. Acidity of Phenols

Phenol reacts with strong bases like NaOH to form sodium phenoxide. It does not react with weak bases like NaHCO₃, unlike carboxylic acids.

C₆H₅OH + NaOH $\rightarrow$ C₆H₅ONa + H₂O

C₆H₅OH + NaHCO₃ $\rightarrow$ No reaction

2.4.2. Electrophilic Substitution Reactions

The hydroxyl group is an activating and ortho-, para- directing group in electrophilic aromatic substitution reactions. This means that incoming electrophiles tend to attack the ortho and para positions of the benzene ring.

• Halogenation: Phenol reacts with bromine water to give 2,4,6-tribromophenol.

  C₆H₅OH + 3Br₂ (aq) $\rightarrow$ 2,4,6-C₆H₂Br₃OH + 3HBr

  With bromine in a non-polar solvent (like CS₂), it gives a mixture of ortho- and para-bromophenols.

• Nitration:
  • With dilute HNO₃, phenol gives a mixture of o- and p-nitrophenols.
  • With concentrated HNO₃, phenol undergoes vigorous reaction to form 2,4,6-trinitrophenol (picric acid).
• Sulfonation: Phenol reacts with concentrated sulfuric acid to give a mixture of o- and p-phenolsulfonic acids depending on temperature. At lower temperatures (293K), o-phenolsulfonic acid is the major product. At higher temperatures (373K), p-phenolsulfonic acid is favored.
• Friedel-Crafts Alkylation and Acylation: Phenols undergo Friedel-Crafts reactions, but the Lewis acid catalyst (like AlCl₃) often complexes with the hydroxyl group, making the reaction difficult or leading to rearrangement.

2.4.3. Kolbe's Reaction

Phenol reacts with sodium hydroxide to form sodium phenoxide, which then reacts with carbon dioxide under pressure and heat to form sodium salicylate. Acidification yields salicylic acid.

C₆H₅ONa + CO₂ $\xrightarrow{400K, 125 atm}$ o-HOC₆H₄COONa (Sodium salicylate)

o-HOC₆H₄COONa + H⁺ $\rightarrow$ o-HOC₆H₄COOH (Salicylic acid)

2.4.4. Reimer-Tiemann Reaction

When phenol is heated with chloroform (CHCl₃) in the presence of a strong base (like NaOH or KOH), an aldehyde group (-CHO) is introduced ortho to the hydroxyl group, forming salicylaldehyde.

C₆H₅OH + CHCl₃ + NaOH $\rightarrow$ o-HOC₆H₄CHO (Salicylaldehyde)

This reaction also introduces some para isomer, but the ortho isomer is the major product.

2.4.5. Reaction with Acid Chlorides/Anhydrides

Phenols react with acid chlorides or acid anhydrides in the presence of a base like pyridine or an acid catalyst to form esters. This is similar to the esterification of alcohols.

C₆H₅OH + CH₃COCl $\xrightarrow{Pyridine}$ C₆H₅OCOCH₃ + HCl (Phenyl acetate)

2.5. Important Phenols

• Phenol: Used as an antiseptic (carbolic acid), in the production of plastics (Bakelite), and in the synthesis of dyes and pharmaceuticals.
• Cresols: Used as disinfectants and preservatives.
• Picric Acid (2,4,6-trinitrophenol): Used as an antiseptic and in the manufacture of explosives.
• Salicylic Acid: Used in the treatment of skin diseases and as a precursor to aspirin.

3. Ethers

Ethers are organic compounds in which an oxygen atom is bonded to two alkyl or aryl groups. Their general formula is R-O-R', where R and R' can be the same or different alkyl or aryl groups.

3.1. Classification of Ethers

• Simple Ethers: Both alkyl/aryl groups are the same (e.g., Diethyl ether, CH₃CH₂-O-CH₂CH₃).
• Mixed Ethers: The two alkyl/aryl groups are different (e.g., Ethyl methyl ether, CH₃-O-CH₂CH₃).

3.2. Nomenclature of Ethers

According to IUPAC nomenclature, ethers are considered as alkoxy derivatives of alkanes. The smaller alkyl group along with the oxygen atom is named as an alkoxy group (e.g., methoxy, ethoxy).

• Methoxyethane (CH₃OCH₂CH₃)
• Ethoxyethane (CH₃CH₂OCH₂CH₃)
• Methoxybenzene (C₆H₅OCH₃) - Anisole

Common names are derived by naming the two alkyl/aryl groups alphabetically followed by the word 'ether' (e.g., Diethyl ether, Ethyl methyl ether).

3.3. Preparation of Ethers

• Williamson Synthesis: This is a widely used method for preparing ethers, especially simple ethers and unsymmetrical ethers. It involves the reaction of a sodium alkoxide with a primary alkyl halide or alkyl sulfate.

  R-ONa + R'-X $\rightarrow$ R-O-R' + NaX

  For preparing unsymmetrical ethers, it is best to use a sodium salt of a phenol and an alkyl halide, or a sodium alkoxide and an aryl halide (though the latter is difficult due to the low reactivity of aryl halides towards nucleophilic substitution).

  Williamson Synthesis Tip: Always use a primary alkyl halide (R'-X) to avoid elimination reactions (E2) which compete with substitution (S_N2), especially when using a bulkier alkoxide.
• From Alcohols (Intermolecular Dehydration): As mentioned earlier, heating primary alcohols with concentrated sulfuric acid at 413K yields ethers. This method is suitable for preparing symmetrical ethers.

  2R-OH $\xrightarrow{H_2SO_4, 413K}$ R-O-R + H₂O

• From Alcohols using Diazomethane: Methanol can be converted to methoxy derivatives using diazomethane (CH₂N₂) in the presence of a catalyst like BF₃.

  ROH + CH₂N₂ $\xrightarrow{BF_3}$ R-O-CH₃ + N₂

• From Phenols: Phenols react with alkyl halides in the presence of a base (like K₂CO₃ or NaOH) to form aryl alkyl ethers.

  C₆H₅OH + RX $\xrightarrow{Base}$ C₆H₅OR + HX

3.4. Physical Properties of Ethers

• Ethers are generally colorless liquids with characteristic sweet odors.
• They have lower boiling points than alcohols of comparable molecular mass because they cannot form intermolecular hydrogen bonds. However, their boiling points are higher than corresponding alkanes due to dipole-dipole interactions.
• Lower ethers are sparingly soluble in water, but solubility decreases with increasing molecular size.
• Ethers are relatively inert and are good solvents for many organic compounds, fats, oils, and alkaloids.

3.5. Chemical Reactions of Ethers

Ethers are less reactive than alcohols. The C-O bond is strong, and they do not undergo reactions like oxidation or reduction easily.

• Cleavage of C-O Bond: Ethers can be cleaved by strong acids like HI or HBr. The reaction proceeds via protonation of the oxygen atom, followed by nucleophilic attack by the halide ion.

  R-O-R' + HI $\rightarrow$ R-I + R'-OH (if R' is primary or secondary)

  If excess HI is used, the alcohol formed can further react to form another alkyl halide.

  R'-OH + HI $\rightarrow$ R'-I + H₂O

  Overall: R-O-R' + 2HI $\rightarrow$ R-I + R'-I + H₂O

  If one of the groups is phenyl, the phenyl-oxygen bond is harder to break. For example, anisole (C₆H₅OCH₃) reacts with HI to give phenol and iodomethane, or iodobenzene and methanol, depending on conditions. Typically, the alkyl group is cleaved first.

  C₆H₅-O-CH₃ + HI $\rightarrow$ C₆H₅OH + CH₃I

• Reaction with Carbon Monoxide (CO): Under high pressure and in the presence of certain catalysts (like BF₃), ethers can react with CO to form alkyl formates.
• Electrophilic Substitution Reactions (Aryl Ethers): Aryl ethers undergo electrophilic substitution reactions similar to benzene, with the alkoxy group being an activating and ortho-, para- directing group.

3.6. Important Ethers

• Diethyl Ether (CH₃CH₂OCH₂CH₃): Commonly known as ether. It is a volatile liquid used as a general anesthetic (though its use has declined due to safety concerns) and as a solvent in laboratories and industry. It is highly flammable.
• Anisole (Methoxybenzene, C₆H₅OCH₃): Used in perfumery and as an intermediate in organic synthesis.