Alcohols and Ethers

Alcohols, which contain a hydroxy group -OH, and ethers, which contain an ether linkage -O-, both have the molecular formula C_nH_2n+2O and are functional-group isomers.

Basic Knowledge

Classification of alcohols: By the number of hydroxyl groups: monohydric, dihydric, and trihydric. By the number of carbon groups attached to the carbon bearing -OH: primary(two H atoms attached to the carbon bearing -OH), secondary(one H), and tertiary(zero H).
Main alcohols: methanol CH₃OH(highly toxic, blindness and death), ethanol C₂H₅OH(alcoholic beverages), glycerol CH₂OH-CHOH-CH₂OH(trihydric, cosmetics).
Main reactions:
① Reaction with Na: 2C2H5OH + 2Na → 2C2H5ONa + H2(common to all alcohols)
② Oxidation: primary→aldehyde→carboxylic acid, secondary→ketone, tertiary→difficult to oxidize. Oxidizing agents include K₂Cr₂O₇+H₂SO₄ and KMnO₄.
③ Dehydration: concentrated sulfuric acid+heating. At low temperature(130–140℃)→intermolecular dehydration produces ether 2C2H5OH → C2H5OC2H5 + H2O; at high temperature(160–170℃)→intramolecular dehydration produces alkene C2H5OH → CH2=CH2 + H2O.
④ Esterification(reaction with a carboxylic acid, reversible): R-OH + R'-COOH ⇄ R'-COOR + H2O.
Iodoform reaction: Compounds with the structure CH₃-CH(OH)-R(ethanol, 2-propanol, lactic acid, etc.) produce a yellow CHI₃ precipitate with I₂+NaOH.

Further Exploration (Principles and Applications)

Industrial preparation of ethanol: ① Alcoholic fermentation of sugar C6H12O6 → 2C2H5OH + 2CO2(enzymes), ② ethylene+water(phosphoric acid catalyst, synthetic method). Bioethanol(from sugarcane and corn) is attracting attention as a renewable fuel.
Methanol is an important industrial chemical raw material(formalin production, methyl tert-butyl ether MTBE, etc.), but it is metabolized in the human body to formaldehyde→formic acid, causing central nervous system toxicity(blindness and death). It is the causative substance in accidents involving illegally made alcohol and ingestion of windshield-washer fluid.
Ethers are represented by diethyl ether(C₂H₅)₂O, historically used as an anesthetic and still used as an organic solvent. Its boiling point is low at 35℃, it is highly volatile, and it can form an explosive mixture with air, so caution is required.
Hydrogen bonding in alcohols: Because -OH can form hydrogen bonds, alcohols have much higher boiling points than ethers even when their molecular masses are the same, such as diethyl ether(boiling point 35℃)<1-butanol(boiling point 117℃).

Points to Note(Common Confusions and Frequent Mistakes)

① Ethanol and dimethyl ether have the same C₂H₆O formula but are completely different substances(functional-group isomers). ② Positive examples of the iodoform reaction: ethanol, 2-propanol, acetone, acetaldehyde, lactic acid, and others with a CH₃-CO- or CH₃-CH(OH)- structure. Methanol is negative(the carbon attached to -CH₃ has neither -OH nor >C=O). ③ Use different temperatures for intermolecular and intramolecular dehydration. ④ Tertiary alcohols are not oxidized by ordinary oxidizing agents such as K₂Cr₂O₇(C-C bond cleavage is required).

Practice

1. State the products formed when ethanol is heated with concentrated sulfuric acid at low and high temperatures.
2. State the product formed by oxidation of 2-propanol and whether its iodoform reaction is positive or negative.
3. Compare the oxidation products of primary, secondary, and tertiary alcohols.