NEET Exam > NEET Notes > Short Notes > Short Notes: Alcohols, Phenols and Ethers
Short Notes: Alcohols, Phenols and Ethers
2.1 Classification and Nomenclature
| Type | Structure | Example |
|---|---|---|
| Primary Alcohol (1°) | R-CH₂-OH | CH₃CH₂OH (Ethanol) |
| Secondary Alcohol (2°) | R₂CH-OH | (CH₃)₂CHOH (Propan-2-ol) |
| Tertiary Alcohol (3°) | R₃C-OH | (CH₃)₃COH (2-Methylpropan-2-ol) |
| Phenols | Ar-OH | C₆H₅OH (Phenol) |
| Ethers | R-O-R' | CH₃-O-CH₃ (Dimethyl ether) |
2.2 Preparation of Alcohols
| Method | Reaction | Type of Alcohol |
|---|---|---|
| From Alkenes | R-CH=CH₂ + H₂O → R-CH(OH)-CH₃ (H⁺) | Follows Markovnikov's rule |
| Hydroboration-Oxidation | R-CH=CH₂ + B₂H₆, then H₂O₂/OH⁻ → R-CH₂-CH₂OH | Anti-Markovnikov (1° alcohol) |
| From Haloalkanes | R-X + KOH (aq) → R-OH + KX | Hydrolysis |
| From Carbonyl (Reduction) | RCHO + LiAlH₄ → RCH₂OH R₂CO + LiAlH₄ → R₂CHOH | 1° from aldehyde 2° from ketone |
| Grignard Reagent | R-MgX + HCHO → RCH₂OH (1°) R-MgX + R'CHO → RR'CHOH (2°) R-MgX + R'₂CO → RR'₂COH (3°) | Versatile method |
2.3 Physical Properties of Alcohols
- Hydrogen bonding: Higher boiling points than corresponding alkanes
- Solubility: Lower alcohols soluble in water (H-bonding)
- Boiling point order: 1° > 2° > 3° (more H-bonding in 1°)
2.4 Chemical Reactions of Alcohols
| Reaction Type | Reaction | Product |
|---|---|---|
| With Metals | 2R-OH + 2Na → 2R-ONa + H₂ | Sodium alkoxide |
| With HX (Lucas Test) | R-OH + HCl (ZnCl₂) → R-Cl + H₂O | Alkyl halide 3° immediate, 2° ~5 min, 1° no reaction at RT |
| Esterification | R-OH + R'COOH → R'COOR + H₂O (H⁺) | Ester |
| Dehydration | R-CH₂-CH₂-OH → R-CH=CH₂ + H₂O (conc. H₂SO₄, 443K) | Alkene (follows Saytzeff rule) |
| Oxidation (1° alcohol) | RCH₂OH → RCHO → RCOOH (K₂Cr₂O₇/H⁺) | Aldehyde → Carboxylic acid |
| Oxidation (2° alcohol) | R₂CHOH → R₂CO (K₂Cr₂O₇/H⁺) | Ketone |
| Oxidation (3° alcohol) | No oxidation under mild conditions | Resistant to oxidation |
2.5 Phenols - Preparation
| Method | Reaction |
|---|---|
| From Chlorobenzene | C₆H₅Cl + NaOH → C₆H₅OH (623K, 300 atm) |
| From Benzene Sulphonic Acid | C₆H₅SO₃H + NaOH → C₆H₅ONa → C₆H₅OH |
| From Diazonium Salt | C₆H₅N₂⁺Cl⁻ + H₂O → C₆H₅OH + N₂ + HCl (warm) |
| From Cumene | C₆H₅CH(CH₃)₂ + O₂ → C₆H₅OH + CH₃COCH₃ |
2.6 Phenols - Properties and Reactions
Acidity of Phenol: Phenol is more acidic than alcohol due to resonance stabilization of phenoxide ion. Order: Carboxylic acid > Phenol > Water > Alcohol
| Reaction | Equation | Notes |
|---|---|---|
| With NaOH | C₆H₅OH + NaOH → C₆H₅ONa + H₂O | Forms sodium phenoxide |
| With Na | 2C₆H₅OH + 2Na → 2C₆H₅ONa + H₂ | Acidic nature |
| Bromination | C₆H₅OH + 3Br₂ (aq) → C₆H₂Br₃OH + 3HBr | 2,4,6-Tribromophenol (white ppt) |
| Nitration | C₆H₅OH + 3HNO₃ → C₆H₂(NO₂)₃OH | 2,4,6-Trinitrophenol (Picric acid) |
| Kolbe's Reaction | C₆H₅ONa + CO₂ → C₆H₄(OH)COOH (o-position) | Salicylic acid (under pressure) |
| Reimer-Tiemann | C₆H₅OH + CHCl₃ + NaOH → C₆H₄(OH)CHO | Salicylaldehyde (o-position) |
| Coupling (Diazo) | C₆H₅OH + C₆H₅N₂⁺Cl⁻ → C₆H₅-N=N-C₆H₄OH | Orange dye (p-position) |
| With FeCl₃ | C₆H₅OH + FeCl₃ → Violet color | Test for phenol |
2.7 Ethers - Preparation and Reactions
| Method/Reaction | Details |
|---|---|
| Williamson Synthesis | R-ONa + R'-X → R-O-R' + NaX (Best method for preparation) |
| Dehydration of Alcohols | 2R-OH → R-O-R + H₂O (413K, conc. H₂SO₄) |
| Cleavage with HI | R-O-R' + HI → R-I + R'-OH Excess HI: R-I + R'-I + H₂O |
| With PCl₅ | R-O-R + PCl₅ → R-Cl + R-Cl + POCl₃ |
2.8 Important Distinctions
| Test | Alcohol | Phenol |
|---|---|---|
| FeCl₃ | No color | Violet color |
| NaHCO₃ | No reaction | No reaction (phenol not strong enough) |
| NaOH | No reaction | Forms sodium phenoxide |
| Bromine water | No reaction | White ppt (2,4,6-tribromophenol) |
The document Short Notes: Alcohols, Phenols and Ethers is a part of the NEET Course Short Notes for NEET.
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FAQs on Short Notes: Alcohols, Phenols and Ethers
| 1. What are the general properties of alcohols? |  |
Ans. Alcohols are organic compounds characterised by the presence of one or more hydroxyl (-OH) groups. They are typically polar and can form hydrogen bonds, resulting in higher boiling points compared to hydrocarbons of similar molecular weight. Alcohols are soluble in water due to their polar nature, and they exhibit varying degrees of acidity based on the structure of the alkyl group attached to the hydroxyl group.
| 2. How do phenols differ from alcohols? | |
Ans. Phenols are a specific class of compounds that contain a hydroxyl group attached to an aromatic hydrocarbon ring. Unlike alcohols, which can be aliphatic or aromatic, phenols have distinct properties due to the resonance stabilisation of the aromatic ring. This resonance makes the -OH group in phenols more acidic compared to alcohols, leading to different reactivity and uses in various chemical processes.
| 3. What are ethers, and how are they structurally different from alcohols? | |
Ans. Ethers are organic compounds characterised by an oxygen atom bonded to two alkyl or aryl groups (R-O-R'). They differ from alcohols in that ethers do not have a hydroxyl group; instead, the oxygen atom is part of a C-O-C linkage. This structural difference imparts distinct physical and chemical properties to ethers, such as lower boiling points compared to alcohols and limited solubility in water.
| 4. What is the significance of the hydroxyl group in alcohols and phenols? | |
Ans. The hydroxyl group (-OH) is crucial in determining the chemical behaviour of alcohols and phenols. In alcohols, it contributes to their solubility in water and their ability to participate in hydrogen bonding. In phenols, the hydroxyl group not only enhances acidity but also plays a vital role in biological activity and the formation of complex organic compounds, making phenols important in pharmaceuticals and chemical synthesis.
| 5. How can alcohols be classified based on their structure? | |
Ans. Alcohols can be classified into three categories based on the number of carbon atoms bonded to the carbon bearing the hydroxyl group: primary (1°) alcohols, where the -OH group is attached to a carbon atom connected to one other carbon; secondary (2°) alcohols, where it is attached to a carbon connected to two other carbons; and tertiary (3°) alcohols, where it is attached to a carbon connected to three other carbons. This classification affects their reactivity and physical properties.
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