Reactions of Alcohols

Primary Alcohols: These undergo a two-stage oxidation process. Initial oxidation produces an aldehyde, which can be further oxidized to a carboxylic acid if heated under reflux with excess oxidizing agent.

Secondary Alcohols: These are oxidized to ketones. Unlike aldehydes, ketones do not undergo further oxidation under standard laboratory conditions because there is no hydrogen atom attached to the carbonyl carbon.

Tertiary Alcohols: These do not undergo oxidation with standard reagents like acidified potassium dichromate ($VI$) because they lack a hydrogen atom on the carbon bonded to the hydroxyl group.

Oxidizing Agents: Common reagents include acidified $K_2Cr_2O_7$ (which changes from orange to green) and acidified $KMnO_4$ (which changes from purple to colorless).

Halogenation: The hydroxyl group can be replaced by a halogen atom to form a halogenoalkane. This is a critical method for functional group interconversion.

Reagents for Chlorination: $PCl_5$ reacts at room temperature producing $HCl$ gas (steamy fumes), while $PCl_3$ requires heating. $SOCl_2$ is often preferred because the by-products ($SO_2$ and $HCl$) are gases, simplifying purification.

Bromination and Iodination: These typically involve reacting the alcohol with $HX$. For bromination, $HBr$ is often generated in situ by reacting $KBr$ with concentrated $H_2SO_4$ or $H_3PO_4$.

Mechanism: Dehydration involves the removal of a water molecule (an $-OH$ group and a neighboring $-H$ atom) to form a $C=C$ double bond, resulting in an alkene.

Conditions: This reaction requires a catalyst and heat. Common catalysts include concentrated sulfuric acid ($H_2SO_4$), concentrated phosphoric acid ($H_3PO_4$), or passing alcohol vapor over hot aluminum oxide ($Al_2O_3$).

Regioselectivity: In unsymmetrical alcohols, multiple alkene products may form. The major product is typically the more substituted alkene, following Zaitsev's Rule.

Alkoxide Formation: Alcohols react with reactive metals like sodium ($Na$) to form a sodium alkoxide and release hydrogen gas ($H_2$).

Reactivity: This reaction is similar to the reaction of sodium with water but is generally less vigorous. The rate of reaction decreases as the length of the hydrocarbon chain in the alcohol increases.

Acidity: This reaction demonstrates the weakly acidic nature of the $O-H$ bond in alcohols, as the metal displaces the hydrogen atom.

Distinguishing Alcohols: Use the oxidation test with $K_2Cr_2O_7$. If the solution stays orange, the alcohol is tertiary. If it turns green, it is primary or secondary.

Product Prediction: Always check the degree of the alcohol (1°, 2°, or 3°) before predicting oxidation products. Tertiary alcohols are a common 'trick' question.

Reagent Specificity: Remember that $PCl_5$ is a diagnostic test for the $-OH$ group because it produces visible steamy fumes of $HCl$.

Ester Naming: Be careful with the order. The 'alcohol' part is the prefix (e.g., methyl) and the 'acid' part is the suffix (e.g., ethanoate).