Oxidation of Alcohols

Acidified Potassium Dichromate(VI): This is the most common reagent, represented as $K_2Cr_2O_7/H^+$. The orange dichromate ion ($Cr_2O_7^{2-}$) is reduced to the green chromium(III) ion ($Cr^{3+}$) during the reaction, providing a clear visual confirmation of oxidation.

Acidified Potassium Manganate(VII): A stronger alternative, $KMnO_4/H^+$, changes from a deep purple ($MnO_4^-$) to a colorless solution ($Mn^{2+}$). This reagent is less selective and often drives reactions to completion more aggressively than dichromate.

The Role of Acid: The presence of $H^+$ ions (usually from dilute sulfuric acid) is essential for the reduction of the metal ions. Without an acidic environment, the oxidizing agent cannot effectively accept electrons from the alcohol.

Ketone Formation: Secondary alcohols are oxidized to ketones regardless of whether distillation or reflux is used. Because ketones lack a hydrogen atom on the carbonyl carbon, they cannot be further oxidized into carboxylic acids without breaking carbon-carbon bonds.

Tertiary Resistance: Tertiary alcohols do not react with acidified dichromate or manganate. In a laboratory test, the orange dichromate solution would remain orange, serving as a diagnostic tool to distinguish tertiary alcohols from primary and secondary ones.

Fehling's Solution: This alkaline solution contains $Cu^{2+}$ ions (blue). When warmed with an aldehyde, the aldehyde is oxidized to a carboxylate ion, and the blue $Cu^{2+}$ is reduced to a brick-red precipitate of copper(I) oxide ($Cu_2O$). Ketones give no reaction.

Tollens' Reagent: Also known as ammoniacal silver nitrate, it contains the $[Ag(NH_3)_2]^+$ complex. Aldehydes reduce the silver ions to metallic silver, which deposits on the glass as a 'silver mirror,' while the aldehyde is oxidized to a carboxylic acid. Ketones do not react.

Distinguishing Conditions: Always specify 'distillation' for aldehydes and 'reflux' for carboxylic acids when discussing primary alcohols. Marks are frequently lost for omitting the specific experimental setup.

Color Changes: Memorize the specific color transitions ($K_2Cr_2O_7$: Orange to Green; $KMnO_4$: Purple to Colorless). These are often required in 'observation' style questions.

Equation Notation: Use $[O]$ to represent the oxygen provided by the oxidizing agent in balanced organic equations. Remember that forming an aldehyde produces one $H_2O$, while forming a carboxylic acid from an alcohol requires $2[O]$ and also produces one $H_2O$.