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 ($ZnC{l}_2$).
• Thionyl chloride ($SOC{l}_2$) in the presence of a base like pyridine.
• Phosphorus trihalides ($P{X}_3$, e.g., $PC{l}_3$, $PB{r}_3$).
• Phosphorus pentahalides ($P{X}_5$, e.g., $PC{l}_5$).

The general reactions are summarized below:

$\mathrm{R}-\mathrm{OH}+\mathrm{HCl}\stackrel{\text{anhydrous }ZnC{l}_2}{\to }\mathrm{R}-\mathrm{Cl}+{\mathrm{H}}_2\mathrm{O}$ $\mathrm{R}-\mathrm{OH}+\mathrm{SOC}{\mathrm{l}}_2\stackrel{\text{Pyridine}}{\to }\mathrm{R}-\mathrm{Cl}+\mathrm{S}{\mathrm{O}}_2+\mathrm{HCl}$ $3\mathrm{R}-\mathrm{OH}+\mathrm{P}{\mathrm{X}}_3\to 3\mathrm{R}-\mathrm{X}+{\mathrm{H}}_3\mathrm{P}{\mathrm{O}}_3$ $\mathrm{R}-\mathrm{OH}+\mathrm{P}{\mathrm{X}}_5\to \mathrm{R}-\mathrm{X}+\mathrm{PO}{\mathrm{X}}_3+\mathrm{HX}$

The reaction with $HCl$ and $ZnC{l}_2$ is specifically used to distinguish between primary, secondary, and tertiary alcohols.

2. From Alkanes

Alkanes undergo free-radical halogenation when treated with chlorine ($C{l}_2$) or bromine ($B{r}_2$) 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. ${\mathrm{CH}}_3-{\mathrm{CH}}_3+{\mathrm{Cl}}_2\stackrel{h\nu }{\to }{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{Cl}+\mathrm{HCl}$ Ethane $\to$ 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 ($B{r}_2$ or $C{l}_2$) in an inert solvent, such as carbon tetrachloride ($CC{l}_4$), 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 $B{r}_2$ to ethene.

Ethene $\to$ 1,2-Dibromoethane

• Reactivity of Halogens:
  • Fluorine ($F_2$): Too reactive, the reaction is violent and difficult to control.
  • Chlorine ($C{l}_2$) and Bromine ($B{r}_2$): Effective electrophilic reagents for this addition.
  • Iodine ($I_2$): Generally does not react under these conditions.

Halogens→

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.