## 17.3 Preparations of Alkyl Halides This section outlines the primary methods for synthesizing alkyl halides (also known as halogenoalkanes) from various starting materials, including alcohols, alkanes, and alkenes. ### 1. From Alcohols Alkyl halides can be prepared from alcohols by reacting them with specific halogenating agents. The hydroxyl group (-OH) of the alcohol is substituted by a halogen atom (-X). Common reagents used for this conversion are: * Hydrogen chloride ($HCl$) in the presence of a catalyst like anhydrous zinc chloride ($ZnCl_2$). * Thionyl chloride ($SOCl_2$) in the presence of a base like pyridine. * Phosphorus trihalides ($PX_3$, e.g., $PCl_3$, $PBr_3$). * Phosphorus pentahalides ($PX_5$, e.g., $PCl_5$). The general reactions are summarized below: $$ \mathrm{R-OH} + \mathrm{HCl} \xrightarrow{\text{anhydrous } ZnCl_2} \mathrm{R-Cl} + \mathrm{H_2O} $$ $$ \mathrm{R-OH} + \mathrm{SOCl_2} \xrightarrow{\text{Pyridine}} \mathrm{R-Cl} + \mathrm{SO_2} + \mathrm{HCl} $$ $$ 3\mathrm{R-OH} + \mathrm{PX_3} \rightarrow 3\mathrm{R-X} + \mathrm{H_3PO_3} $$ $$ \mathrm{R-OH} + \mathrm{PX_5} \rightarrow \mathrm{R-X} + \mathrm{POX_3} + \mathrm{HX} $$ The reaction with $HCl$ and $ZnCl_2$ is specifically used to distinguish between primary, secondary, and tertiary alcohols. ### 2. From Alkanes Alkanes undergo **free-radical halogenation** when treated with chlorine ($Cl_2$) or bromine ($Br_2$) in the presence of diffused sunlight or ultraviolet (UV) light. In this reaction, a hydrogen atom of the alkane is substituted by a halogen atom. The reaction proceeds via a **free radical mechanism**. **Example:** Chlorination of ethane. $$ \mathrm{CH}_{3}-\mathrm{CH}_{3} + \mathrm{Cl}_{2} \xrightarrow{h\nu} \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{Cl} + \mathrm{HCl} $$ *Ethane* $\rightarrow$ *Chloroethane* *Note:* This method is generally not preferred for preparing pure alkyl halides because the reaction is difficult to control. It often results in a mixture of products, including polyhalogenated alkanes (e.g., dichloroethane, trichloroethane). ### 3. From Alkenes Alkenes, with their reactive carbon-carbon double bonds, are excellent precursors for alkyl halides through addition reactions. #### a) Hydrohalogenation (Addition of Halogen Acids) Alkenes react with aqueous solutions of halogen acids ($HX$, where X = Cl, Br, I) at room temperature to form alkyl halides. This reaction is an **electrophilic addition** where the $\pi$-bond of the alkene breaks. * **Mechanism:** The reaction proceeds via the formation of a carbocation intermediate. * **Regioselectivity:** The addition of an unsymmetrical reagent (like $HBr$) to an unsymmetrical alkene (like propene) follows **Markonikov's Rule**. This rule states that the hydrogen atom adds to the carbon atom of the double bond that already has more hydrogen atoms. **Order of Reactivity of Halogen Acids:** $$ \mathrm{HI} > \mathrm{HBr} > \mathrm{HCl} $$ **Example:** Addition of $HBr$ to propene. #### b) Halogenation (Addition of Halogens) Alkenes react with halogens ($Br_2$ or $Cl_2$) in an inert solvent, such as carbon tetrachloride ($CCl_4$), at room temperature. The reaction results in the formation of **vicinal dihalides** (dihaloalkanes where the two halogen atoms are on adjacent carbon atoms). **Example:** Addition of $Br_2$ to ethene. *Ethene* $\rightarrow$ *1,2-Dibromoethane* * **Reactivity of Halogens:** * **Fluorine ($F_2$):** Too reactive, the reaction is violent and difficult to control. * **Chlorine ($Cl_2$) and Bromine ($Br_2$):** Effective electrophilic reagents for this addition. * **Iodine ($I_2$):** Generally does not react under these conditions. ### 4. Halide Exchange Reaction Alkyl iodides are often difficult to prepare by direct iodination or addition of $HI$. They are conveniently prepared by reacting alkyl chlorides or bromides with sodium iodide in acetone.