Alcohols and phenols are organic compounds formed when one or more hydrogen atoms in a hydrocarbon are replaced by a hydroxyl (-OH) functional group.
Structural Differences
- Alcohols: The hydroxyl group is attached to an sp3 hybridized carbon atom of an aliphatic chain or alkyl group (e.g., Ethanol, CH3CH2OH).
- Phenols: The hydroxyl group is attached directly to an sp2 hybridized carbon atom of an aromatic ring system (e.g., Phenol, C6H5OH).
Classification Based on Hydroxyl Groups
Alcohols and phenols are classified based on the number of hydroxyl groups present in their structure:
- Monohydric: Contain one -OH group (e.g., Methanol, Phenol).
- Dihydric: Contain two -OH groups (e.g., Ethane-1,2-diol/Glycol, Benzene-1,2-diol/Catechol).
- Trihydric: Contain three -OH groups (e.g., Propane-1,2,3-triol/Glycerol).
- Polyhydric: Contain multiple -OH groups (e.g., Sorbitol).
Sub-classification of Monohydric Alcohols
Monohydric alcohols are further categorized based on the hybridization and substitution of the carbon atom bearing the -OH group:
| Alcohol Type | Structural Definition | Structural Example |
| Primary (1°) | The -OH group is attached to a carbon bonded to only one other carbon atom. | Ethanol (CH3CH2OH) |
| Secondary (2°) | The -OH group is attached to a carbon bonded to two other carbon atoms. | Propan-2-ol (CH3CH(OH)CH3) |
| Tertiary (3°) | The -OH group is attached to a carbon bonded to three other carbon atoms. | 2-Methylpropan-2-ol (C(CH3)3OH) |
| Allylic Alcohol | The -OH group is attached to an sp3 hybridized carbon next to a carbon-carbon double bond. | Allyl alcohol (CH2 = CH-CH2OH) |
| Benzylic Alcohol | The -OH group is attached to an sp3 hybridized carbon next to an aromatic ring. | Benzyl alcohol (C6H5CH2OH) |
Physical Properties
The physical properties of alcohols and phenols are heavily governed by the presence of the highly polar hydroxyl group, which facilitates intermolecular interactions.
Boiling Points
Alcohols and phenols possess significantly higher boiling points compared to hydrocarbons, ethers, and haloalkanes of comparable molecular masses. This anomaly is due to the presence of intermolecular hydrogen bonding.
- Trend within Alcohols: Boiling points increase with an increase in the number of carbon atoms due to van der Waals forces. Conversely, boiling points decrease with increased branching because the surface area decreases, weakening the van der Waals forces.
- Boiling Point Order: Primary (1°) > Secondary (2°) > Tertiary (3°).
Solubility
The lower members of alcohols (Methanol, Ethanol, Propanol) are completely miscible in water in all proportions because they form hydrogen bonds with water molecules. Solubility decreases as the size of the alkyl or aryl group (the hydrophobic part) increases.
Chemical Properties and Reactivity
Alcohols function both as nucleophiles (when the O-H bond breaks) and as electrophiles (when the C-O bond breaks).
1. Acidity of Alcohols and Phenols
- Alcohols: Exhibit weakly acidic behavior, reacting with active metals (like Na, K) to liberate hydrogen gas and form alkoxides. Electron-donating alkyl groups (+I effect) increase electron density on oxygen, making the O-H bond harder to break. Therefore, the acidity order is: Primary (1°) > Secondary (2°) > Tertiary (3°).
- Phenols: Phenols are significantly more acidic than alcohols (with a lower pKa value). This enhanced acidity occurs because the conjugate base, the phenoxide ion, is stabilized by the resonance delocalization of the negative charge into the aromatic ring. Alcohols lack this resonance stabilization for their alkoxide ions.
2. Esterification
Alcohols and phenols react with carboxylic acids, acid chlorides, or acid anhydrides in the presence of an acid catalyst to form esters.
3. Oxidation Reactions
The oxidation of alcohols involves the formation of a carbon-oxygen double bond with the clearance of bonds on the oxygen and carbon atoms.
- Primary Alcohols (1°): Oxidize to form Aldehydes, which can oxidize further into Carboxylic acids. Using a mild oxidizing agent like Pyridinium Chlorochromate (PCC) stops the reaction at the aldehyde stage.
- Secondary Alcohols (2°): Oxidize to form Ketones when treated with Chromic anhydride (CrO3).
- Tertiary Alcohols (3°): Do not easily undergo oxidation under neutral or alkaline conditions because they lack an α-hydrogen. Under harsh acidic conditions and high temperatures, they undergo dehydration to form alkenes.
4. Electrophilic Aromatic Substitution of Phenols
Because the -OH group is highly activating and ortho/para-directing due to resonance, phenols undergo rapid substitution:
- Nitration: Phenol reacts with dilute HNO3 at low temperatures (298 K) to yield a mixture of ortho-nitrophenol and para-nitrophenol. Ortho-nitrophenol is steam-volatile due to intramolecular hydrogen bonding, whereas para-nitrophenol is less volatile due to intermolecular hydrogen bonding.
- Kolbe’s Reaction: Phenoxide ion reacts with carbon dioxide (CO2) followed by acidification to produce 2-hydroxybenzoic acid (Salicylic acid), a key precursor for aspirin.
- Reimer-Tiemann Reaction: Treating phenol with chloroform (CHCl3) in the presence of sodium hydroxide (NaOH) introduces a formyl (-CHO) group at the ortho position, producing Salicylaldehyde.
UPSC Prelims Fact File: Industrial Applications and Applied Chemistry
Distinguishing Tests for Alcohols
- Lucas Test: Used to differentiate primary, secondary, and tertiary alcohols based on their reactivity with Lucas reagent (a mixture of concentrated HCl and anhydrous ZnCl2):
- Tertiary alcohols: Produce immediate turbidity/cloudiness at room temperature due to the rapid formation of insoluble alkyl chlorides.
- Secondary alcohols: Produce turbidity within 5 minutes.
- Primary alcohols: Do not produce turbidity at room temperature; turbidity appears only upon heating.
Commercially Vital Alcohols and Phenols
1. Methanol (CH3OH)
- Trivia: Known historically as wood spirit because it was produced via the destructive distillation of wood. Today, it is synthesized industrially by catalytic hydrogenation of carbon monoxide (CO).
- Toxicity: Highly poisonous. Ingestion of even small quantities causes blindness (due to its oxidation to methanal and methanoic acid in the liver, which degrades the optic nerve), while larger doses are fatal.
- Denatured Alcohol: Industrial alcohol is made unfit for human consumption by adding toxic substances like methanol, copper sulfate (to impart color), and pyridine (to impart a foul odor).
2. Ethanol (CH3CH2OH)
- Production: Produced on a large industrial scale via the fermentation of sugars present in molasses, grapes, or starches using inverted enzymes found in yeast (Invertase and Zymase).
- Power Alcohol: A blend of absolute ethanol (approx. 20%) and petrol (80%) along with a co-solvent. It is used as an internal combustion engine fuel to lower carbon emissions and decrease reliance on fossil fuel imports (aligned with India’s National Policy on Biofuels).
3. Glycerol / Glycerin (Propane-1,2,3-triol)
- Origin: Obtained as a byproduct during the saponification (soap-making) industry.
- Applications: Used as a humectant in cosmetics, a moistening agent in tobacco, and in the manufacturing of nitroglycerin, an explosive substance used to make dynamite.
4. Phenol / Carbolic Acid (C6H5OH)
- Trivia: Joseph Lister first used phenol as a surgical antiseptic in 1867.
- Applications: It is an essential industrial raw material for manufacturing Bakelite (a thermosetting phenol-formaldehyde plastic), phenolphthalein (a common pH indicator), and pharmaceuticals like aspirin. A dilute aqueous solution (0.1%) acts as an antiseptic, while higher concentrations (1%) function as a disinfectant.