## 18.2 Preparations of Alcohols This section outlines the primary methods for synthesizing alcohols, covering reactions involving alkenes, haloalkanes, carbonyl compounds, carboxylic acids, and esters. ### (a) By Electrophilic Addition of Steam to Alkenes Alcohols can be prepared by the direct hydration of alkenes. This industrial process involves reacting an alkene with steam under high temperature and pressure, using an acid catalyst. - **Reactants**: Alkene (e.g., ethene) and steam ($H_2O$). - **Conditions**: - **Catalyst**: Phosphoric(V) acid ($H_3PO_4$) adsorbed on silica. - **Temperature**: $300^{\circ}C$. - **Pressure**: $60-70$ atmospheres. - **Reaction**: $$CH_2=CH_2(g) + H_2O(g) \xrightarrow{H_3PO_4, \, 300^{\circ}C, \, 60-70 \text{ atm}} CH_3CH_2OH(g)$$ *Ethene reacts with steam to produce ethanol.* ### (b) By the Oxidation of Alkenes with Potassium Manganate(VII) Alkenes can be oxidized using a cold, dilute, and acidified solution of potassium manganate(VII) ($KMnO_4$) to form a diol (a compound with two hydroxyl groups). This reaction is also a test for unsaturation (the Baeyer's test), where the purple $KMnO_4$ solution is decolourised. - **Reactants**: Alkene, cold dilute acidified $KMnO_4$. - **Product**: Diol (e.g., ethane-1,2-diol). - **Reaction**: The oxidizing agent, $KMnO_4$, provides an oxygen atom, represented as [O]. $$CH_2=CH_2 + H_2O + [O] \xrightarrow{\text{Cold, dilute } KMnO_4} HO-CH_2-CH_2-OH$$ *Ethene is oxidized to ethane-1,2-diol (ethylene glycol).* ### (c) By the Nucleophilic Substitution of Haloalkanes Haloalkanes undergo nucleophilic substitution when heated with an aqueous alkali solution, such as sodium hydroxide ($NaOH$), to produce an alcohol. The hydroxide ion ($OH^-$) acts as the nucleophile, replacing the halogen atom. For more on the nature of halogens, see . - **Reactants**: Haloalkane (e.g., R-Br) and aqueous alkali ($NaOH_{(aq)}$ or $KOH_{(aq)}$). - **Conditions**: Heating under reflux. - **Reaction**: $$R-Br + OH^- \xrightarrow{\Delta} R-OH + Br^-$$ *A bromoalkane reacts with a hydroxide ion to form an alcohol and a bromide ion.* ### (d) By the Reduction of Carbonyl Compounds Carbonyl compounds (aldehydes and ketones) can be reduced to alcohols using suitable reducing agents. - **Reducing Agents**: - Sodium borohydride ($NaBH_4$). - Lithium aluminium hydride ($LiAlH_4$). - **Products**: - **Aldehydes** are reduced to **primary alcohols**. - **Ketones** are reduced to **secondary alcohols**. - **General Reactions**: - *Aldehyde Reduction*: $$\text{Aldehyde} + 2[\text{H}] \xrightarrow{NaBH_4 \text{ or } LiAlH_4} \text{Primary Alcohol}$$ - *Ketone Reduction*: $$\text{Ketone} + 2[\text{H}] \xrightarrow{NaBH_4 \text{ or } LiAlH_4} \text{Secondary Alcohol}$$ ### (e) By the Reduction of Carboxylic Acids and Esters Carboxylic acids and their derivatives, like esters, can be reduced to primary alcohols. This reaction requires a strong reducing agent. - **Reducing Agent**: Lithium aluminium hydride ($LiAlH_4$) is typically used as it is strong enough to reduce carboxylic acids. $NaBH_4$ is generally not effective for this reduction. - **Reaction (Carboxylic Acid)**: $$R-COOH + 4[\text{H}] \xrightarrow{LiAlH_4} R-CH_2-OH + H_2O$$ *The carboxyl group (-COOH) is reduced to a primary alcohol group (-CH$_2$OH).* ### (f) By the Hydrolysis of Esters Esters can be broken down (hydrolyzed) into an alcohol and a carboxylic acid (or its salt) by heating with either a dilute acid or a dilute alkali. #### Acid Hydrolysis This is a reversible reaction carried out by heating the ester with a dilute acid catalyst (e.g., $H_2SO_4$). - **Products**: Carboxylic acid and alcohol. - **Reaction**: $$RCOOR' + H_2O \underset{\Delta}{\stackrel{H^+}{\rightleftharpoons}} RCOOH + R'OH$$ #### Basic Hydrolysis (Saponification) This is an irreversible reaction where the ester is heated with a strong base (e.g., $NaOH$). - **Products**: Carboxylate salt and alcohol. - **Reaction**: $$RCOOR' + NaOH \xrightarrow{\Delta} RCOO^-Na^+ + R'OH$$ ### (g) Preparation using Grignard Reagents Grignard reagents ($RMgX$) are versatile tools in organic chemistry for synthesizing various classes of alcohols. - **Formaldehyde ($HCHO$)** reacts with Grignard reagents to give **Primary Alcohols**. - **Other Aldehydes ($RCHO$)** react to give **Secondary Alcohols**. - **Ketones ($RCOR$)** react to give **Tertiary Alcohols**.