Producing Halogenoalkanes

This method involves reacting an alkane with a halogen ($Cl_2$ or $Br_2$) in the presence of ultraviolet (UV) light or high temperatures.

The process follows a three-stage mechanism: Initiation (homolytic fission of the halogen bond), Propagation (chain reactions creating radicals), and Termination (radicals colliding to form stable molecules).

A significant drawback is the lack of control, often resulting in a mixture of multi-substituted products (e.g., $CH_3Cl$, $CH_2Cl_2$, $CHCl_3$, $CCl_4$) and structural isomers.

To favor the production of a mono-substituted halogenoalkane, a large excess of the alkane is typically used to ensure halogen radicals are more likely to collide with alkane molecules than with halogenoalkane products.

Alkenes react with hydrogen halides ($HX$) or pure halogens ($X_2$) at room temperature to produce halogenoalkanes or dihalogenoalkanes.

When adding $HX$ to an unsymmetrical alkene, the reaction follows Markovnikov's Rule, where the hydrogen atom attaches to the carbon with the most hydrogen atoms already present.

This preference occurs because the reaction proceeds via the most stable carbocation intermediate (tertiary > secondary > primary).

Addition of $X_2$ (like $Br_2$) is often used as a diagnostic test for unsaturation, as the orange-brown bromine water decolourises upon forming a colorless vicinal dihalogenoalkane.

Alcohols can be converted to halogenoalkanes using various halogenating agents that replace the $-OH$ group with a halogen atom.

Chlorination is commonly achieved using phosphorus(V) chloride ($PCl_5$) at room temperature, producing $POCl_3$ and $HCl$ gas as byproducts.

Sulfur dichloride oxide ($SOCl_2$) is often preferred in synthesis because its byproducts ($SO_2$ and $HCl$) are both gases, which escape the reaction mixture, leaving a pure liquid product.

Bromination and Iodination usually require in-situ generation of reagents, such as reacting red phosphorus with iodine to form $PI_3$, or using $NaBr$ and $H_2SO_4$ to generate $HBr$.

Identify the Mechanism: Always check if the question asks for 'Free Radical Substitution' (Alkanes) or 'Electrophilic Addition' (Alkenes); confusing these is a common way to lose marks.

Condition Specifics: Ensure you mention $UV$ light for alkane reactions; without it, the halogen bond will not undergo homolytic fission at standard temperatures.

Markovnikov Application: In alkene addition, always draw the intermediate carbocation to justify why the major product forms; examiners look for the 'stability of the carbocation' explanation.

Byproduct Awareness: For alcohol substitution, memorizing the byproducts for $PCl_5$ ($POCl_3 + HCl$) and $SOCl_2$ ($SO_2 + HCl$) is essential for completing balanced equations.