Reactions of Alcohols

Complete Combustion: When ignited in the presence of excess oxygen, alcohols undergo complete combustion to produce carbon dioxide and water. This reaction is highly exothermic, making alcohols effective fuels.

General Equation: The process can be represented by the general formula: $C_nH_{2n+1}OH + (1.5n)O_2 \rightarrow nCO_2 + (n+1)H_2O$. As the carbon chain length increases, the amount of oxygen required for complete combustion also increases.

Observation: A clean blue flame typically indicates complete combustion, whereas a yellow, sooty flame suggests incomplete combustion where carbon (soot) or carbon monoxide is produced due to limited oxygen.

Chlorination: Primary and secondary alcohols react vigorously with phosphorus(V) chloride ($PCl_5$) at room temperature to produce chloroalkanes, $POCl_3$, and $HCl$ gas. The evolution of steamy white fumes of $HCl$ serves as a qualitative chemical test for the presence of the hydroxyl group.

Tertiary Chlorination: Tertiary alcohols can be converted to chloroalkanes more simply by shaking the alcohol with concentrated hydrochloric acid ($HCl$) at room temperature, a method that does not work efficiently for primary or secondary alcohols.

Bromination and Iodination: These reactions typically require the 'in situ' generation of the halogenating agent. For bromination, a mixture of potassium bromide and $50\%$ concentrated sulfuric acid is warmed with the alcohol to produce $HBr$, which then reacts. For iodination, red phosphorus and iodine are heated under reflux with the alcohol to form $PI_3$ as the active intermediate.

Oxidising Agent: The standard reagent is acidified potassium dichromate(VI) ($K_2Cr_2O_7/H_2SO_4$). During the reaction, the orange dichromate(VI) ions ($Cr_2O_7^{2-}$) are reduced to green chromium(III) ions ($Cr^{3+}$), providing a clear color change.

Primary Alcohols: These can be oxidized in two stages. Partial oxidation using distillation produces an aldehyde (R-CHO). Complete oxidation under reflux with excess oxidant produces a carboxylic acid (R-COOH).

Secondary and Tertiary Alcohols: Secondary alcohols are oxidized to ketones (R-CO-R) and cannot be oxidized further because there is no C-H bond remaining on the carbonyl carbon. Tertiary alcohols do not undergo oxidation under these conditions because the carbon bonded to the -OH group lacks a hydrogen atom.

Distillation with Addition: This technique is used to isolate aldehydes from primary alcohols. Because aldehydes have lower boiling points than the parent alcohols (due to the lack of hydrogen bonding), they vaporize immediately upon formation and are condensed into a separate receiver, preventing further oxidation.

Heating Under Reflux: This setup involves a vertical condenser that returns vapors back into the reaction flask. It is used when complete oxidation is required (e.g., forming a carboxylic acid or a ketone), as it allows for prolonged heating with excess oxidising agent without loss of volatile reactants or products.

Anti-bumping Granules: In both setups, small granules are added to the liquid to promote smooth boiling and prevent 'bumping,' which can cause the mixture to splash into the condenser.

Tollens' Reagent: An alkaline solution of silver nitrate in ammonia. When warmed with an aldehyde, the $Ag^+$ ions are reduced to metallic silver, forming a 'silver mirror' on the test tube. Ketones give a negative result.

Fehling's Solution: Contains copper(II) ions in an alkaline solution. Aldehydes reduce the blue $Cu^{2+}$ ions to a brick-red precipitate of copper(I) oxide ($Cu_2O$). Ketones do not react.

Significance: These tests are essential for distinguishing between the products of alcohol oxidation, specifically identifying if a primary alcohol was partially oxidized to an aldehyde or if the compound is a ketone.

Classification First: Always identify if the alcohol is primary, secondary, or tertiary before predicting oxidation products. This is the most common source of errors in multi-step synthesis questions.

Reagent Precision: When describing oxidation, always specify 'acidified' potassium dichromate(VI). Omitting the acid ($H^+$) will often result in lost marks as the dichromate ion cannot act as an oxidant in neutral or alkaline conditions.

Observation Details: Be prepared to describe the specific color changes (orange to green for dichromate) and physical observations (steamy fumes for $PCl_5$, silver mirror for Tollens').

Equation Balancing: In dehydration reactions, remember that the catalyst ($H_3PO_4$) is not consumed and should not appear as a reactant in the stoichiometric equation, though it may be written over the arrow.