⚗️ Visual Chemistry
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Alcohols, Aldehydes, and Ketones

25min Part 4 / Ch7 / Lesson 3
Prerequisites: 4-7-2

Objectives

  • Classify alcohols by degree and understand their oxidation reactions
  • Explain the differences between aldehydes and ketones
  • Know detection methods using the silver mirror and Fehling's tests

Classification of Alcohols

Alcohols R-OH are classified by the number of carbon groups on the carbon bearing -OH:

  • Primary (1°): R-CH₂OH → oxidizes to aldehyde → then to carboxylic acid
  • Secondary (2°): R₂CHOH → oxidizes to ketone
  • Tertiary (3°): R₃COH → resistant to oxidation
NameFormulaClassNotes
MethanolCH₃OH1°Toxic (causes blindness/death)
EthanolC₂H₅OH1°Drinkable, disinfectant
2-Propanol(CH₃)₂CHOH2°Disinfectant
Ethylene glycolHOCH₂CH₂OH—Antifreeze
GlycerolC₃H₅(OH)₃—Moisturizer, nitroglycerin precursor

Representative alcohols

Reactions of Alcohols

Dehydration:

  • Intramolecular (170°C, conc. H₂SO₄): ethanol → ethylene + H₂O
  • Intermolecular (130°C, conc. H₂SO₄): 2 ethanol → diethyl ether + H₂O

Esterification:

R-OH+R′-COOH⇌R′-COOR+H2O\mathrm{R\text{-}OH + R'\text{-}COOH \rightleftharpoons R'\text{-}COOR + H_2O}

Oxidation Stages

R-CH₂OHAlcohol[O]→R-CHOAldehyde[O]→R-COOHCarboxylic acid
Oxidation stages of a primary alcohol

Secondary alcohol: R2CHOH→[O]R2C=O\mathrm{R_2CHOH \xrightarrow{[O]} R_2C=O} (ketone) — resists further oxidation

Detecting Aldehydes

Aldehydes (-CHO) have reducing properties:

  1. Silver mirror test (Tollens’ reagent): Ag⁺ → Ag (silver mirror deposited)
  2. Fehling’s test: Cu²⁺ → Cu₂O (red precipitate)

Ketones (-CO-) do not give these reactions.

PropertyAldehyde R-CHOKetone R-CO-R’
Functional group-CHO-CO-
Reducing abilityYesNo
Silver mirror testPositiveNegative
Fehling’s testPositiveNegative
ExampleFormaldehyde HCHOAcetone (CH₃)₂CO

Aldehydes vs. ketones

Formaldehyde HCHO:

  • 37% aqueous solution = formalin (preservative)
  • Raw material for phenol resin

Acetaldehyde CH₃CHO:

  • Formed by oxidizing ethanol
  • Further oxidation → acetic acid

Check Your Understanding

Q1 What forms when a primary alcohol is mildly oxidized?

Q2 Which compound gives a positive silver mirror test?

Q3 Formalin is primarily a solution of...

Exercises

Q1. Write the chemical equations for ethanol → acetaldehyde → acetic acid (represent the oxidizer as [O]).

Solution
  1. Ethanol → Acetaldehyde:

C2H5OH→[O]CH3CHO+H2O\mathrm{C_2H_5OH \xrightarrow{[O]} CH_3CHO + H_2O}

  1. Acetaldehyde → Acetic acid:

CH3CHO→[O]CH3COOH\mathrm{CH_3CHO \xrightarrow{[O]} CH_3COOH}

This is the stepwise oxidation of a primary alcohol: alcohol → aldehyde → carboxylic acid.