Class 12 Biology - Alcohols, Phenols, & Ethers Study Notes

Alcohols, Phenols & Ethers

Introduction to Alcohols, Phenols and Ethers

• Definition of Alcohols: Compounds where one or more hydrogen atoms from aliphatic carbons are replaced by -OH (hydroxyl) groups. The general formula is represented as R-OH where R represents an alkyl group.

• Definition of Phenols: Compounds where hydrogen atoms attached to a benzene ring are replaced by -OH groups.

• Definition of Ethers: Compounds characterized by the presence of an ether group, -O-, between two alkyl or aryl groups, represented as R-O-R'.

Classification of Alcohols

1. Based on the number of -OH groups

• Monohydric Alcohols: Contain only one -OH group.

  • Example: Methanol (CH₃OH)

• Dihydric Alcohols: Contain two -OH groups.

  • Example: Ethylene glycol (HOCH₂-CHOH)

• Polyhydric Alcohols: Contain more than two -OH groups.

  • Example: Glycerol (C₃H₈O₃)

2. Monohydric Alcohol Classification by Carbon Bonding

• Based on the hybridization of carbon:

  1. Primary Alcohols (1°): -OH group is attached to a primary carbon (sp³ hybridized).

  • Example: Ethyl alcohol (CH₃-CH₂-OH)

  1. Secondary Alcohols (2°): -OH group is attached to a secondary carbon.

  • Example: Isopropyl alcohol (CH₃-CHOH-CH₃)

  1. Tertiary Alcohols (3°): -OH group is attached to a tertiary carbon.

  • Example: Tert-butyl alcohol (C(CH₃)₃OH)

  1. Allylic Alcohols: Alcohols where -OH group is attached to a carbon atom adjacent to a double bond.

  • Example: 2-Penylethanol (CH₂=CH-CHOH-CH₃)

  1. Benzylic Alcohols: Alcohols with the hydroxyl group attached to a benzylic position (a carbon adjacent to an aromatic ring).

3. Alcohols Containing sp² Hybridized Carbon

• Vinylic Alcohols: Where the -OH is directly attached to an sp² hybridized carbon.

  • Example: Vinyl alcohol (CH₂=CHOH)

Classification of Ethers

1. On the Basis of Structure

• Symmetrical Ethers: Both alkyl groups are the same.

  • Example: Dimethyl ether (CH₃-O-CH₃)

• Unsymmetrical Ethers: Different alkyl groups attached to the ether oxygen.

  • Example: Ethyl methyl ether (CH₃-O-CH₂CH₃)

Nomenclature

1. Alcohols

• IUPAC Naming Structure: IUPAC Prefix + Root Word + ‘-ol’ suffix for alcohols.

• Common Names: Often named based on alkyl groups plus the term alcohol.

Examples

• Methanol: IUPAC: Methanol, Common: Methyl alcohol.

• Ethanol: IUPAC: Ethanol, Common: Ethyl alcohol.

2. Phenols

• Common Name: Carbolic acid (when substituent groups are present).

• IUPAC Examples: Carbolic acid (phenol), 2-Methylphenol (o-Cresol).

Physical Properties of Alcohols and Phenols

1. Boiling Points

• Alcohols and phenols have higher boiling points compared to ethers and hydrocarbons of comparable molar mass due to hydrogen bonding.

• Higher atomic numbers lead to increased boiling points, yet branching in the chain decreases it due to reduced surface area.

2. Solubility

• Lower alcohols are more soluble in water due to their ability to form hydrogen bonds. As the carbon chain length increases, solubility decreases.

Chemical Reactions of Alcohols

1. Oxidation

• Alcohols can undergo oxidation reactions to form aldehydes, ketones, or carboxylic acids depending on primary, secondary, or tertiary alcohol types.

• Oxidation Agents: KMnO₄, CrO₃, etc. Weak oxidizing agents convert primary alcohols to aldehydes, while strong oxidizing agents can further oxidize aldehydes to carboxylic acids.

2. Dehydration

• Alcohols can undergo dehydration to form alkenes. The reaction is favored by strong acids (e.g., H₂SO₄).

• Example: CH₃-CH₂-OH → CH₂=CH₂ + H₂O under heat with H₂SO₄.

3. Reactions with Hydrogen Halides

• Alcohols react with hydrogen halides (HX) to form alkyl halides, following either SN1 or SN2 mechanisms depending on the structure of the alcohol.

Chemical Reactions of Phenols

1. Electrophilic Substitution

• Phenols readily undergo electrophilic aromatic substitution due to the activating effect of the hydroxyl group on the benzene ring.

  • Examples: Nitration forms nitrophenol; bromination forms tribromophenol.

2. Reactions with Metals

• Phenols react with strong bases to form phenoxide salts.

3. Acidity of Phenols

• Phenols are weak acids due to the resonance stabilization of phenoxide ions, which allows them to donate protons more easily than alcohols.

• Comparison: Phenols have a greater tendency to lose a proton compared to regular alcohols.

Summary of Key Points

• Alcohols, phenols, and ethers are important classes of organic compounds with unique properties and reactions.

• Understanding their classifications, nomenclatures, and chemical behaviors is pivotal in organic chemistry.

1. Basic Definitions (Basic Paribhashayein)

• Alcohols: Jab kisi aliphatic hydrocarbon (jaise methane ya ethane) se ek hydrogen hata kar $-OH$ group lagaya jaye.

  • General Formula: $R-OH$

  • Example: $CH_{3}OH$ (Methanol).

• Phenols: Jab benzene ring ke carbon se seedhe $-OH$ group juda ho.

  • Example: $C<em>{6}H</em>{5}OH$ (Phenol).

• Ethers: Jab do alkyl ya aryl groups ke beech mein ek oxygen atom ($-O-$) aa jaye.

  • General Formula: $R-O-R'$

  • Example: $CH<em>{3}-O-CH</em>{3}$ (Dimethyl ether).

2. Classification of Alcohols (Alcohols ka Vargikaran)

A. Number of -OH Groups ke Aadhar par

• Monohydric: Sirf ek $-OH$ group.

  • Example: $CH<em>{3}CH</em>{2}OH$ (Ethanol).

• Dihydric: Do $-OH$ groups.

  • Example: Ethylene glycol ($HO-CH<em>{2}-CH</em>{2}-OH$).

• Polyhydric: Do se zyada $-OH$ groups.

  • Example: Glycerol ($CH<em>{2}OH-CHOH-CH</em>{2}OH$).

B. Carbon ki Hybridization ($sp^{3}$) ke Aadhar par

• Primary (1°): $-OH$ us carbon se juda hai jo sirf ek dusre carbon se juda ho (ya zero).

  • Example: $CH<em>{3}CH</em>{2}OH$.

• Secondary (2°): $-OH$ us carbon se juda hai jo do aur carbons se juda ho.

  • Example: Isopropyl alcohol ($CH<em>{3}-CH(OH)-CH</em>{3}$).

• Tertiary (3°): $-OH$ us carbon se juda hai jo teen aur carbons se juda ho.

  • Example: Tert-butyl alcohol ($C(CH<em>{3})</em>{3}OH$).

C. Special Cases (Exam Important)

• Allylic Alcohol: $-OH$ group double bond ke thik agle carbon ($sp^{3}$) par hota hai.

  • Example: $CH<em>{2}=CH-CH</em>{2}OH$.

• Benzylic Alcohol: Benyl ring ke side chain wale carbon par $-OH$ laga ho.

• Vinylic Alcohol: $-OH$ seedhe double bonded carbon ($sp^{2}$) par laga ho.

  • Example: $CH_{2}=CH-OH$.

3. Nomenclature (Naamkaran)

• IUPAC Rule: 'e' hatao aur 'ol' lagao. (Alkane $→$ Alkanol).

• Ethers: Chote alkyl group ko 'alkoxy' aur bade ko 'alkane' kehte hain.

  • Example: $CH<em>{3}-O-C</em>{2}H_{5}$ (Methoxyethane).

4. Physical Properties (Bhautik Guna)

1. Boiling Point:

   • Alcohols aur Phenols ka boiling point bahut high hota hai kyunki inme Intermolecular Hydrogen Bonding hoti hai.

   • Branching Rule: Jaise-jaise chain mein branching badhti hai, surface area kam hota hai aur boiling point bhi kam ho jata hai.

2. Solubility:

   • Shuruat ke alcohols paani mein ghul jaate hain (due to H-bonding), lekin jaise-jaise hydrocarbon part ($R$) bada hota hai, solubility ghatne lagti hai.

5. Chemical Reactions (Chemical Abhikriyayein)

A. Oxidation (Ek bahut important topic)

• Primary Alcohol (1°) $→$ Aldehyde $→$ Carboxylic Acid.

• Secondary Alcohol (2°) $→$ Ketone.

• Tertiary Alcohol (3°): Inka oxidation mushkil hota hai, ye dehydrate hokar alkene bana dete hain.

• Oxidizing Agents:

  • $KMnO_{4}$ (Strong - for acids).

  • $CrO_{3}$ aur PCC (Mild - for aldehydes).

B. Dehydration

• Jab alcohol ko concentrated $H<em>{2}SO</em>{4}$ ke saath heat kiya jaye, toh paani ($H_{2}O$) nikal jata hai aur Alkene banta hai.

• Example: Ethanol $→$ Ethene ($CH<em>{2}=CH</em>{2}$).

C. Acidity of Phenols

• Phenols alcohols se zyada acidic hote hain. Iska kaaran hai Phenoxide Ion ka resonance ke dwara stabilize hona.

• Phenols neutral ferric chloride ($FeCl_{3}$) ke saath violet colour dete hain.

D. Electrophilic Substitution in Phenols

• Nitration: Dilute $HNO_{3}$ ke saath o-nitrophenol aur p-nitrophenol ka mixture milta hai.

• Bromination: Bromine water ke saath ye white precipitate (2,4,6-Tribromophenol) deta hai.