Alcohols, Phenols, and Ethers Comprehensive Study Notes

Fundamental Definitions and Classification of Alcohols, Phenols, and Ethers

• Alcohols: Compounds where a hydroxyl group ($-OH$) is attached to an aliphatic carbon system. The general formula can be represented as $R-OH$.

• Phenols: Compounds where a hydroxyl group ($-OH$) is attached to an aromatic carbon system (a benzene ring). The general formula is $Ar-OH$.

• Ethers: Compounds formed when a hydrogen atom in a hydrocarbon is replaced by an alkoxy ($-OR$) or aryloxy ($-OAr$) group. The general formula is $R-O-R$ or $R-O-Ar$.

Classification Based on Hydroxyl Groups

Compounds are categorized by the number of hydroxyl groups they contain:

• Monohydric: Contains one $-OH$ group (e.g., $C_2H_5OH$).

• Dihydric: Contains two $-OH$ groups (e.g., Ethane-1,2-diol: $CH_2(OH)-CH_2(OH)$).

• Trihydric: Contains three $-OH$ groups (e.g., Propane-1,2,3-triol: $CH_2(OH)-CH(OH)-CH_2(OH)$).

• Polyhydric: Contains many $-OH$ groups.

Classification based on $sp^3$ hybridized Carbon-Oxygen Bonds

• Primary ($1^∘$) Alcohols: The $-OH$ group is attached to a carbon atom that is bonded to only one other carbon (e.g., $R-CH_2-OH$).

• Secondary ($2^∘$) Alcohols: The $-OH$ group is attached to a carbon atom bonded to two other carbons (e.g., $R_2CH-OH$).

• Tertiary ($3^∘$) Alcohols: The $-OH$ group is attached to a carbon atom bonded to three other carbons (e.g., $R_3C-OH$).

Specialized Alcohol Classifications

• Allylic Alcohols: The $-OH$ group is attached to an $sp^3$-hybridized carbon atom adjacent to a carbon-carbon double bond (the allylic carbon).

  • Primary ($1^∘$): $CH_2=CH-CH_2-OH$

  • Secondary ($2^∘$): $CH_2=CH-CH(CH_3)-OH$

  • Tertiary ($3^∘$): $CH_2=CH-C(CH_3)_2-OH$

• Benzylic Alcohols: The $-OH$ group is attached to an $sp^3$-hybridized carbon atom next to an aromatic ring.

  • Primary ($1^∘$): $C_6H_5-CH_2-OH$

  • Secondary ($2^∘$): $C_6H_5-CH(CH_3)-OH$

  • Tertiary ($3^∘$): $C_6H_5-C(CH_3)_2-OH$

• Vinylic Alcohols: The $-OH$ group is bonded to an $sp^2$-hybridized carbon atom (part of a double bond), such as in Vinyl alcohol ($CH_2=CH-OH$).

Classification of Ethers

• Simple or Symmetrical Ethers: Alkyl or aryl groups attached to the oxygen atom are identical ($R = R'$). Examples: Dimethyl ether ($CH_3OCH_3$), Diethyl ether ($C_2H_5OC_2H_5$).

• Mixed or Unsymmetrical Ethers: Alkyl or aryl groups attached to the oxygen atom are different ($R \neq R'$). Examples: Ethyl methyl ether ($CH_3OC_2H_5$), Methyl phenyl ether ($C_6H_5OCH_3$).

Real-World Applications and Industrial Importance

Many common materials containing hydroxyl groups are integral to daily life:

• Ethanol: The primary component in ordinary spirit, widely used for polishing wooden furniture.

• Sugars: Essential dietary components.

• Cotton: The basis for manufacturing various fabrics.

• Paper: Used for writing, notebooks, currency, and magazines.

IUPAC Nomenclature and Structure

Alcohols Nomenclature

• The suffix "-e" of the parent alkane is replaced with "-ol".

• For multiple $-OH$ groups, the "-e" is retained, and suffixes like "-diol" or "-triol" are used.

  • Methanol: $CH_3OH$

  • Propan-1-ol: $CH_3CH_2CH_2OH$

  • Propan-2-ol: $CH_3CH(OH)CH_3$

  • Butan-2-ol: $CH_3CH(OH)CH_2CH_3$

  • 2-Methylpropan-2-ol: $(CH_3)_3COH$

  • Propane-1,2,3-triol (Glycerol): $CH_2(OH)CH(OH)CH_2(OH)$

  • Ethane-1,2-diol (Glycol): $CH_2(OH)CH_2(OH)$

Phenols Nomenclature

• Phenol: Benzene ring with one $-OH$ group ($C_6H_5OH$).

• Cresols: Methylphenols.

  • o-Cresol (2-methylphenol)

  • m-Cresol (3-methylphenol)

  • p-Cresol (4-methylphenol)

• Benzenediols:

  • Catechol: Benzene-1,2-diol

  • Resorcinol: Benzene-1,3-diol

  • Quinol (Hydroquinone): Benzene-1,4-diol

Ethers Nomenclature

• Named as Alkoxyalkanes. The smaller alkyl group is considered part of the alkoxy group, while the larger alkyl group is the parent hydrocarbon.

  • Methoxymethane: $CH_3OCH_3$

  • Methoxyethane: $CH_3OC_2H_5$

  • 2-Methoxypropane: $CH_3CH(OCH_3)CH_3$

  • Methoxybenzene (Anisole): $C_6H_5OCH_3$

  • 1-Phenoxyheptane: $C_6H_5O(CH_2)_6CH_3$

  • 1,2-Dimethoxyethane: $CH_3OCH_2CH_2OCH_3$

Structural Features of Functional Groups

• Methanol ($CH_3OH$):

  • Bond Angle ($C-O-H$): $108.9^∘$. This is slightly less than the tetrahedral angle of $109^∘ 28'$ due to the repulsion between the lone pairs of electrons on the oxygen atom.

  • Bond Length ($C-O$): $142\,pm$.

• Phenol ($C_6H_5OH$):

  • Bond Length ($C-O$): $136\,pm$. This is shorter than in methanol because the $C-O$ bond acquires partial double bond character due to resonance and the attachment of oxygen to $sp^2$ hybridized carbon.

Preparation of Alcohols

1. From Alkenes

• Acid-Catalyzed Hydration: Alkenes react with water in the presence of an acid catalyst ($H^+$) to form alcohols. It follows Markovnikov's rule.

  • Reaction: $CH_3CH=CH_2 + H_2O \xrightarrow{H^+} CH_3CH(OH)CH_3$

  • Mechanism:

    1. Protonation: Formation of a carbocation via the electrophilic attack of $H_3O^+$ on the alkene double bond.

    2. Nucleophilic Attack: Water acts as a nucleophile and attacks the carbocation.

    3. Deprotonation: Loss of a proton to form the final alcohol.

• Hydroboration-Oxidation: Reaction with diborane ($B_2H_6$) followed by oxidation with hydrogen peroxide in an alkaline medium. This process yields primary alcohols in excellent quantities and follows anti-Markovnikov orientation.

  • Example: Propene to Propan-1-ol.

2. From Carbonyl Compounds

• Reduction of Aldehydes and Ketones: Catalytic hydrogenation using $Pd$, or chemical reduction using Sodium borohydride ($NaBH_4$) or Lithium aluminium hydride ($LiAlH_4$).

  • Aldehydes reduce to Primary Alcohols ($1^∘$).

  • Ketones reduce to Secondary Alcohols ($2^∘$).

• Reduction of Carboxylic Acids and Esters: Carboxylic acids are reduced to primary alcohols by $LiAlH_4$ (a powerful and expensive reagent). Industrially, acids are converted to esters, which are then reduced via catalytic hydrogenation ($H_2, Ni$).

3. From Grignard Reagents ($R-Mg-X$)

Nucleophilic addition of Grignard reagents to the carbonyl group followed by hydrolysis.

• Methanal (Formaldehyde) + $R-Mg-X \rightarrow 1^∘$ alcohol.

• Other Aldehydes + $R-Mg-X \rightarrow 2^∘$ alcohol.

• Ketones + $R-Mg-X \rightarrow 3^∘$ alcohol.

Preparation of Phenols

• From Haloarenes (Dow's Process): Chlorobenzene is treated with $NaOH$ at $623\,K$ and $300\,atm$ pressure to form sodium phenoxide, which is acidified to phenol.

• From Benzene Sulphonic Acid: Benzene is sulfonated using oleum to benzene sulphonic acid, then fused with molten sodium hydroxide at high temperature, and finally acidified.

• From Diazonium Salts: Aniline is treated with $NaNO_2 + HCl$ at $0-5^∘ C$ to form benzene diazonium chloride. This is then warmed with water or treated with dilute acid to yield phenol.

• From Cumene (2-Phenylpropane): Cumene is oxidized by air into cumene hydroperoxide. Acidic hydrolysis produces phenol and a valuable byproduct, Propan-2-one (Acetone).

Physical and Chemical Properties

Boiling Points

• Boiling points increase with higher molar mass.

• Boiling points decrease with increased branching due to a reduction in surface area.

• Comparison: Alcohols and phenols have significantly higher boiling points than hydrocarbons, ethers, and haloalkanes of comparable mass because they form intermolecular hydrogen bonds.

Solubility

• Alcohols and phenols are soluble in water because they can form intermolecular hydrogen bonds with water molecules.

• Solubility decreases as the size of the hydrophobic alkyl/aryl group increases.

Acidity

• Reaction with Metals: Alcohols and phenols react with active metals ($Na, K, Al$) to release hydrogen gas and form alkoxides/phenoxides.

• Acidic Strength Comparison:

  • Alcohols are less acidic than water ($Alkoxide + Water \rightarrow Alcohol + OH^-$).

  • Phenols are much more acidic than alcohols. This is because the phenoxide ion formed is resonance-stabilized. Polarization of the $-OH$ bond is greater in phenol due to the $sp^2$ hybridized carbon of the ring.

• Substituent Effects on Phenol Acidity:

  • Electron Withdrawing Groups (EWG) (e.g., $-NO_2$): Increase acidity by stabilizing the phenoxide ion through dispersal of the negative charge. The effect is strongest at ortho and para positions.

  • Electron Donating Groups (EDG) (e.g., $-CH_3$): Decrease acidity by destabilizing the phenoxide ion.

  • pKa Relationship: Lower $pK_a$ values indicate greater acidity (e.g., Nitrophenols have lower $pK_a$ than phenol).

Oxidation of Alcohols

• Primary ($1^∘$): Oxidize to Aldehydes (using $PCC$ - Pyridinium chlorochromate) or directly to Carboxylic acids (using acidified $KMnO_4$ or $K_2Cr_2O_7$).

• Secondary ($2^∘$): Oxidize to Ketones (using $CrO_3$ or heating with $Cu$ at $573\,K$).

• Tertiary ($3^∘$): Do not undergo oxidation under normal conditions; instead, they undergo dehydration to form alkenes when heated with $Cu$ at $573\,K$.

Named Reactions of Phenols

• Nitration:

  • With dilute $HNO_3$ at $298\,K$: Yields a mixture of o-nitrophenol and p-nitrophenol.

  • Volatility: o-Nitrophenol is steam volatile due to intramolecular H-bonding; p-nitrophenol is less volatile due to intermolecular H-bonding.

  • With concentrated $HNO_3$: Yields 2,4,6-Trinitrophenol (Picric acid).

• Halogenation:

  • $Br_2$ in $CS_2$ (non-polar solvent) at $273\,K$: Yields mono-brominated phenols (p-bromophenol is major).

  • $Br_2$ in water (Bromine water): Yields a white precipitate of 2,4,6-Tribromophenol.

• Kolbe’s Reaction: Sodium phenoxide is treated with $CO_2$ followed by acidification to produce 2-Hydroxybenzoic acid (Salicylic acid).

• Reimer-Tiemann Reaction: Phenol is treated with Chloroform ($CHCl_3$) in the presence of aqueous sodium hydroxide to introduce an aldehyde group at the ortho position, resulting in Salicylaldehyde.

• Reaction with Zinc Dust: Phenol is reduced to Benzene when heated with zinc dust.

• Oxidation: Phenol oxidizes with chromic acid ($Na_2Cr_2O_7 + H_2SO_4$) to produce Benzoquinone.

Commercial Alcohols

• Methanol (Wood Spirit): Produced by catalytic hydrogenation of carbon monoxide ($CO$) over a $ZnO-Cr_2O_3$ catalyst at $573-673\,K$ and $200-300\,atm$. It is highly poisonous, causing blindness or death.

• Ethanol: Obtained by the fermentation of sugars (cane sugar). Invertase converts sucrose to glucose/fructose, and Zymase converts glucose to ethanol and $CO_2$.

• Denaturation: Commercial ethanol is made unfit for drinking by adding Copper Sulphate (for color) and Pyridine (foul-smelling). This is called denatured alcohol.

Preparation and Reactions of Ethers

Preparation Methods

• Dehydration of Alcohols: Ethanol heated with concentrated $H_2SO_4$ at $413\,K$ yields ethoxyethane. At higher temperatures ($443\,K$), it yields ethene (dehydration).

• Williamson Synthesis: An alkyl halide reacts with a sodium alkoxide. For unsymmetrical ethers, a primary alkyl halide and a tertiary alkoxide are preferred. If a tertiary alkyl halide is used, the major product is an alkene (elimination) rather than an ether.

Chemical Properties of Ethers

• Cleavage by $HX$: The $C-O$ bond is cleaved by hydrogen halides ($HI > HBr > HCl$). In mixed ethers, the halide ion attacks the smaller alkyl group ($S_N2$ mechanism). If one group is tertiary, it forms a tertiary halide ($S_N1$ mechanism).

• Electrophilic Substitution in Anisole: The alkoxy group is ortho-para directing and activating.

  • Halogenation: Bromination with $Br_2$ in ethanoic acid gives p-bromoanisole (major).

  • Friedel-Crafts Alkylation: Reaction with $CH_3Cl$ and anhydrous $AlCl_3$ gives 4-methoxytoluene (major).

  • Friedel-Crafts Acylation: Reaction with Ethanoyl chloride gives 4-methoxyacetophenone (major).

  • Nitration: Reaction with concentrated $H_2SO_4/HNO_3$ gives 4-nitroanisole (major).

Questions & Discussion

• Why is the bond angle in alcohols slightly less than the tetrahedral angle? It is due to the repulsion between the lone pairs of electrons on the oxygen atom.

• Why is the C-O bond length in phenol ($136\,pm$) shorter than in methanol ($142\,pm$)? This is due to partial double bond character between carbon and oxygen resulting from resonance.

• Steam Distillation of Nitrophenols: o-Nitrophenol is steam volatile because of intramolecular hydrogen bonding. p-Nitrophenol has strong intermolecular hydrogen bonding, raising its boiling point and making it less volatile.

• Solubility comparison: Alcohols are more soluble in water than hydrocarbons of comparable mass because alcohols can form hydrogen bonds with water molecules.

• Reagents for Specific Conversions:

  • Primary alcohol to Carboxylic acid: Acidified $KMnO_4$ or $K_2Cr_2O_7$.

  • Primary alcohol to Aldehyde: $PCC$ (Pyridinium chlorochromate).

  • Phenol to 2,4,6-tribromophenol: Bromine water ($Br_2, H_2O$).

  • Benzyl alcohol to Benzoic acid: Acidified $KMnO_4$.

  • Propan-2-ol to Propene: $85\% H_3PO_4$ at $440\,K$.

  • Butan-2-one to Butan-2-ol: $LiAlH_4$ or $NaBH_4$.