Alcohols, Phenols and Carboxylic Acid : (Class 12 Chemistry)
Phenol
Structure and Bonding:
- Phenol is an aromatic compound with the chemical formula C₆H₅OH.
- It consists of a benzene ring directly attached to a hydroxyl (-OH) group.
- The carbon atom in the benzene ring attached to the -OH group is sp² hybridized.
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Physical Properties:
- Phenol is a white crystalline solid at room temperature.
- It has a distinct odor.
- Phenol is slightly soluble in water due to hydrogen bonding with water molecules. Solubility decreases with increasing hydrocarbon chain length.
- Phenol has a higher boiling point compared to similar molecular weight hydrocarbons due to hydrogen bonding.
Acidity:
- Phenol is a weak acid due to the ease of releasing a proton (H⁺) from the -OH group.
- The presence of the benzene ring withdraws electron density from the -OH group, making the O-H bond more polar and facilitating the release of the proton.
Chemical Reactions:
- Acid-Base Reactions: Phenol reacts with bases like sodium hydroxide (NaOH) to form phenoxide salts.
- Electrophilic Substitution Reactions: Due to the activating effect of the hydroxyl group, phenol undergoes electrophilic substitution reactions more readily than benzene. Examples include halogenation (with Br₂) and nitration (with HNO₃/H₂SO₄). The electrophile preferentially attacks the ortho and para positions of the ring due to increased electron density in these positions.
- Acylation Reactions: Phenol reacts with acid chlorides or anhydrides in the presence of a catalyst to form esters.
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Important Uses of Phenol:
- Production of Bakelite, a polymer used in various applications like electrical insulators and adhesives.
- Manufacturing of aspirin and other pharmaceutical products.
- Disinfection due to its mild antiseptic properties.
Additional Points:
- Phenol is a corrosive substance and can cause skin irritation.
- Distinguish between phenol and ethanol (alcohol) based on their reactivity with sodium metal. Phenol reacts with sodium to form sodium phenoxide and hydrogen gas, while ethanol reacts to form sodium ethoxide and hydrogen gas.
Carboxylic Acids
Structure and Functional Group:
- Carboxylic acids are organic compounds containing a carboxyl group (COOH) attached to a hydrocarbon chain (R-COOH).
- The carboxyl group consists of a carbonyl group (C=O) bonded to a hydroxyl group (OH).
Acidity:
- Carboxylic acids are weak acids in aqueous solutions due to the ability to release a proton (H⁺) from the carboxyl group.
- The acidity arises from the resonance stabilization of the resulting carboxylate anion (R-COO⁻).
- Factors affecting acidity include:
- Electron withdrawing groups (like Cl, NO₂) on the R group increase acidity by stabilizing the anion.
- Electron donating groups (like CH₃) decrease acidity by destabilizing the anion.
Physical Properties:
- Lower carboxylic acids (with shorter carbon chains) are colorless liquids with pungent odors.
- Solubility in water decreases with increasing chain length due to the hydrophobic nature of the hydrocarbon chain.
- Carboxylic acids exhibit hydrogen bonding between molecules, leading to higher boiling points compared to similar-sized alkanes.
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Chemical Reactions:
- Nucleophilic Substitution: Carboxylic acids can undergo nucleophilic substitution reactions at the carbonyl carbon due to its electrophilic nature. Examples include:
- Esterification: Reaction with alcohols to form esters (RCOOR').
- Amide formation: Reaction with ammonia or amines to form amides (RCONH₂ or RCONHR').
- Reduction: Carboxylic acids can be reduced to primary alcohols using reducing agents like LiAlH₄.
- Decarboxylation: Removal of CO₂ from the molecule under specific conditions.
Important Uses of Carboxylic Acids:
- Organic synthesis of various compounds like esters, amides, and polymers.
- Production of many everyday products such as plastics, fibers, and paints.
- As food additives (e.g., citric acid, acetic acid) and preservatives.
- In pharmaceuticals and personal care products.
Additional Points:
- Carboxylic acids are generally corrosive and can irritate skin.
- Distinguish between carboxylic acids, alcohols, and aldehydes based on functional group tests.
- Understand the difference between carboxylic acids and strong inorganic acids like sulfuric acid or nitric acid in terms of their relative strengths.
For more details, please read your textbook or books.
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