Alcohols, Carboxylic Acids, and Derivatives
Examines the structure, properties, and characteristic reactions of alcohols, phenols, and acids.
This topic covers the chemistry of organic compounds containing the hydroxyl (-OH), carboxyl (-COOH), and ester (-COO-) functional groups. Their properties are largely determined by the polarity of these groups and their ability to form hydrogen bonds.
### Alcohols
Alcohols are organic compounds containing the hydroxyl (-OH) functional group attached to a saturated carbon atom. Their general formula is R-OH. They are classified based on the number of carbon atoms attached to the carbon bearing the -OH group.
* Primary (1°) Alcohols: The carbon atom bonded to the -OH group is attached to one other alkyl group (e.g., ethanol, CH₃CH₂OH).
* Secondary (2°) Alcohols: The carbon atom bonded to the -OH group is attached to two other alkyl groups (e.g., propan-2-ol, CH₃CH(OH)CH₃).
* Tertiary (3°) Alcohols: The carbon atom bonded to the -OH group is attached to three other alkyl groups (e.g., 2-methylpropan-2-ol, (CH₃)₃COH).
Key Reactions of Alcohols:
* Primary alcohols are oxidised first to aldehydes and then further to carboxylic acids. To obtain the aldehyde, the product is immediately distilled from the reaction mixture. To obtain the carboxylic acid, the mixture is heated under reflux.
CH₃CH₂OH + [O] → CH₃CHO + H₂O (distillation)
CH₃CH₂OH + 2[O] → CH₃COOH + H₂O (reflux)
* Secondary alcohols are oxidised to ketones. Ketones are resistant to further oxidation under these conditions.
CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O (reflux)
* Tertiary alcohols are resistant to oxidation because they lack a hydrogen atom on the carbon bonded to the -OH group.
### Phenols
Phenols are compounds where the -OH group is directly attached to a benzene ring. The simplest phenol is C₆H₅OH. Unlike alcohols, phenols are weakly acidic. This increased acidity is due to the delocalisation of the lone pair of electrons on the oxygen atom into the benzene ring's pi system. This stabilises the resulting phenoxide ion (C₆H₅O⁻) more than the undissociated phenol, making proton loss more favourable.
Key Reactions of Phenols:
C₆H₅OH + NaOH → C₆H₅O⁻Na⁺ + H₂O
### Carboxylic Acids
Carboxylic acids contain the carboxyl (-COOH) functional group. They are weak acids because they partially dissociate in water to release a proton (H⁺) and form a carboxylate ion (RCOO⁻). The stability of the carboxylate ion is enhanced by resonance, where the negative charge is delocalised over both oxygen atoms.
Key Reactions of Carboxylic Acids:
2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂
CH₃COOH + NaOH → CH₃COONa + H₂O
RCOOH + R'OH ⇌ RCOOR' + H₂O (conc. H₂SO₄ catalyst)
### Esters
Esters have the general formula RCOOR' and are known for their pleasant, fruity smells. They are formed from the condensation reaction between a carboxylic acid and an alcohol.
Key Reaction of Esters: Hydrolysis
Hydrolysis is the breakdown of an ester by water. It is the reverse of esterification and can be catalysed by acid or alkali.
CH₃COOCH₂CH₃ + H₂O ⇌ CH₃COOH + CH₃CH₂OH
CH₃COOCH₂CH₃ + NaOH → CH₃COONa + CH₃CH₂OH
Key Points to Remember
- 1Alcohols are classified as primary (1°), secondary (2°), or tertiary (3°), which determines their oxidation products (aldehyde/acid, ketone, or no reaction).
- 2Phenols are more acidic than alcohols due to the delocalisation of charge in the phenoxide ion, allowing them to react with strong bases like NaOH.
- 3Carboxylic acids are weak acids that react with alcohols in the presence of an acid catalyst to form esters in a reversible process called esterification.
- 4The -OH group in phenol is activating and directs electrophilic substitution to the 2,4,6-positions of the benzene ring.
- 5Esters undergo hydrolysis, which is reversible under acidic conditions but irreversible under alkaline conditions (saponification).
- 6Saponification is the alkaline hydrolysis of an ester to produce the salt of a carboxylic acid and an alcohol.
- 7Hydrogen bonding is responsible for the relatively high boiling points and water solubility of small-chain alcohols and carboxylic acids.
- 8Powerful reducing agents like LiAlH₄ can reduce carboxylic acids to primary alcohols.
Pakistan Example
Ethanol Production from Molasses in Pakistan's Sugar Industry
Pakistan is a major producer of sugarcane, and its sugar industry generates vast quantities of molasses as a by-product. This molasses is a key feedstock for producing ethanol, a primary alcohol. The process involves the **fermentation** of the sucrose in molasses by yeast (Saccharomyces cerevisiae) in large vats. The yeast enzymes convert the sugars into ethanol and carbon dioxide: C₆H₁₂O₆ → 2CH₃CH₂OH + 2CO₂. The resulting mixture, with an ethanol concentration of about 10-15%, is then subjected to **fractional distillation** to produce highly concentrated ethanol (around 95%). This industrial-scale application demonstrates the synthesis of a primary alcohol, which is then used as a solvent in the pharmaceutical industry, as a biofuel, and in the production of other chemicals.
Quick Revision Infographic
Chemistry — Quick Revision
Alcohols, Carboxylic Acids, and Derivatives
Key Concepts
Ethanol Production from Molasses in Pakistan's Sugar Industry
Pakistan is a major producer of sugarcane, and its sugar industry generates vast quantities of molasses as a by-product. This molasses is a key feedstock for producing ethanol, a primary alcohol. The process involves the **fermentation** of the sucrose in molasses by yeast (Saccharomyces cerevisiae) in large vats. The yeast enzymes convert the sugars into ethanol and carbon dioxide: C₆H₁₂O₆ → 2CH₃CH₂OH + 2CO₂. The resulting mixture, with an ethanol concentration of about 10-15%, is then subjected to **fractional distillation** to produce highly concentrated ethanol (around 95%). This industrial-scale application demonstrates the synthesis of a primary alcohol, which is then used as a solvent in the pharmaceutical industry, as a biofuel, and in the production of other chemicals.