Alcohols, Phenols, and Ethers

Alcohols, phenols, and ethers are fundamental organic compounds that play an important role in NEET Examinations. Understanding their structure, properties, preparation methods, classification, and chemical reactions is crucial for mastering these topics. Let’s get into a detailed overview of each compound.

Alcohols- Introduction, Classification, Structure, Properties, and Reactions

Introduction to Alcohols

Alcohols are organic compounds in which a hydroxyl group (-OH) is attached to a saturated carbon atom. Alcohols are classified based on the number of hydroxyl groups present in the molecule and the type of carbon to which the -OH group is attached. Alcohols are widely used in industry, medicine, and as solvents.

Classification of Alcohols

1. Based on the Number of Hydroxyl Groups

• Monohydric Alcohols-Contain one hydroxyl group (e.g., methanol, ethanol).

• Dihydric Alcohols- Contain two hydroxyl groups (e.g., ethylene glycol).

• Trihydric Alcohols- Contain three hydroxyl groups (e.g., glycerol).

• Polyhydric Alcohols- Contain more than three hydroxyl groups (e.g., sorbitol).

2. Based on the Type of Carbon Bonded to Hydroxyl Group-

• Primary Alcohols (1°)- The hydroxyl group is attached to a primary carbon (e.g., ethanol, methanol).

• Secondary Alcohols (2°)- The hydroxyl group is attached to a secondary carbon (e.g., isopropyl alcohol).

• Tertiary Alcohols (3°)- The hydroxyl group is attached to a tertiary carbon (e.g., tert-butyl alcohol).

3. Allylic Alcohols- The hydroxyl group is attached to a carbon adjacent to a double bond (e.g., 3-hydroxypropene).

4. Benzylic Alcohols- The hydroxyl group is attached to a carbon adjacent to an aromatic ring (e.g., benzyl alcohol).

Structure of Alcohols

• C-O Bond- The carbon and oxygen in the hydroxyl group are bonded through sp3 hybridized orbitals. The bond angle between the C-O-H is 108.9°, which is slightly less than the typical tetrahedral angle (109.5°) due to electron pair repulsion.

• Bond Length- In methanol, the bond length of the C-O bond is 142 pm.

Properties of Alcohols

1. Boiling Point- Alcohols have higher boiling points than hydrocarbons due to hydrogen bonding between hydroxyl groups. The boiling point increases with the size of the molecule.

2. Solubility- Alcohols are soluble in water due to hydrogen bonding. Solubility decreases as the carbon chain length increases.

3. Acidity- Alcohols are weak Bronsted acids. The acidity decreases with increasing electron-donating alkyl groups. The order of acidity is- 1° > 2° > 3°.

Reactions of Alcohols

1. Nucleophilic Substitution-

• Alcohols can react with HX (hydrogen halides) to form alkyl halides.

• Reaction with metal to form alkoxides.

2. Oxidation-

• Primary alcohols are oxidized to aldehydes, which can further be oxidized to carboxylic acids.

• Secondary alcohols are oxidized to ketones.

• Tertiary alcohols are resistant to oxidation.

3. Dehydrogenation- Alcohols undergo dehydrogenation when heated with copper at 573K, resulting in the formation of aldehydes or ketones.

4. Reaction with Carboxylic Acids- Alcohols react with carboxylic acids to form esters in the presence of concentrated sulfuric acid.

Phenols- Introduction, Classification, Structure, Properties, and Reactions

Introduction to Phenols

Phenols are organic compounds where the hydroxyl group (-OH) is attached to an aromatic ring. Phenols are weaker acids than carboxylic acids, but they are stronger acids than alcohols. They are commonly used in antiseptics, industrial chemicals, and resins.

Classification of Phenols

1. Based on the Number of Hydroxyl Groups

• Monohydric Phenols- Contain one hydroxyl group (e.g., phenol).

• Dihydric Phenols- Contain two hydroxyl groups (e.g., catechol).

• Polyhydric Phenols- Contain more than two hydroxyl groups (e.g., resorcinol).

Structure of Phenols

Phenols consist of a C-O bond where the carbon is part of an aromatic ring (sp2 hybridized). The bond angle between C-O-H is 109°, and the bond length of C-O is 136 pm.

• The hydroxyl group in phenols can participate in resonance, stabilising the structure and making the molecule slightly acidic.

Properties of Phenols

1. Boiling Point- Phenols have higher boiling points than alcohols and ethers due to hydrogen bonding.

2. Solubility- Phenol is soluble in water, but solubility decreases as the hydrophobic part of the molecule increases.

3. Acidity- Phenols are more acidic than alcohols because the negative charge on the oxygen after deprotonation is stabilized by the delocalization of electrons in the aromatic ring. The acidity order is Nitrophenol > Phenol > Cresol > Ethanol.

Reactions of Phenols

1. Electrophilic Substitution-

• Phenols undergo electrophilic substitution reactions such as nitration, halogenation, and sulfonation.

2. Reaction with NaOH- Phenols react with sodium hydroxide to form sodium phenoxide.

3. Oxidation- Phenols can undergo oxidation to form quinones.

4. Reaction with Carboxylic Acids- Phenols react with carboxylic acids to form esters.

Ethers- Introduction, Classification, Structure, Properties, and Reactions

Introduction to Ethers

Ethers are organic compounds where an oxygen atom is bonded to two alkyl or aryl groups. They are known for their low reactivity and are widely used as solvents. Ethers are generally less reactive than alcohols and phenols.

Classification of Ethers

1. Symmetrical Ethers- Both alkyl groups attached to oxygen are the same (e.g., dimethyl ether).

2. Unsymmetrical Ethers- The alkyl or aryl groups attached to oxygen are different (e.g., ethyl methyl ether).

Structure of Ethers

Ethers consist of an oxygen atom bonded to two alkyl or aryl groups. The C-O-C bond angle is approximately 111.7°, which is slightly larger than the tetrahedral angle due to repulsion between bulky alkyl groups. The C-O bond length is around 141 pm, similar to alcohols.

Properties of Ethers

1. Boiling Point- Ethers have lower boiling points than alcohols because they do not form hydrogen bonds.

2. Solubility- Ethers are soluble in water and many organic solvents.

3. Acidity- Ethers are less acidic than alcohols and phenols. They are weak acids but are more acidic than hydrocarbons.

Reactions of Ethers

1. Electrophilic Substitution-

• Ethers undergo electrophilic substitution reactions.

2. Friedel-Crafts Reaction- Ethers can participate in Friedel-Crafts alkylation reactions.

3. Cleavage of C-O Bond- Ethers undergo cleavage of the C-O bond under acidic conditions, forming alcohols or alkyl halides.

Classification of Alcohols, Phenols and Ethers

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