# 17.5 Reactions of Alkyl Halides Alkyl halides are a highly reactive class of organic compounds that serve as important intermediates in organic synthesis. They primarily undergo two types of reactions: - Nucleophilic substitution reactions ($S_N$) - Elimination reactions ## Nucleophilic Substitution ($S_N$) Reactions of Alkyl Halides Nucleophilic substitution reactions are those in which the halogen atom of an alkyl halide is substituted or replaced by an attacking nucleophile. The general reaction is represented as: $$ \text{Nu:}^- + \text{R}-\text{X} \rightarrow \text{R}-\text{Nu} + \text{X:}^- $$ Where: - **Nu:⁻** is the incoming *Nucleophile*. - **R-X** is the *Alkyl Halide (Substrate)*. - **R-Nu** is the *Product*. - **X⁻** is the *Leaving Group*. ### Key Components in $S_N$ Reactions * **Substrate:** The alkyl halide molecule on which the nucleophile attacks. The carbon atom bonded to the halogen is electrophilic due to the halogen's electronegativity. For more on the nature of halogens, see . * **Nucleophile:** A species with a lone pair of electrons that it can donate to the electrophilic carbon of the alkyl halide. A nucleophile can be neutral (e.g., $H_2O$, $NH_3$) or negatively charged (e.g., $OH^-$, $CN^-$). * **Leaving Group (LG):** The halogen atom that detaches from the alkyl halide, taking its bonding pair of electrons with it. A good leaving group is a weak base. The incoming nucleophile must be stronger than the departing one. - **Good Leaving Groups:** $Cl^-$, $Br^-$, $I^-$, $HSO_4^-$ - **Poor Leaving Groups:** $OH^-$, $OR^-$, $NH_2^-$ - *Note:* The iodide ion ($I^-$) is both a good nucleophile and a good leaving group. > ***Do You Know?*** > Alkyl halides are important intermediates in organic synthesis. They can be used to introduce functional groups or undergo various reactions leading to the formation of complex organic molecules. ### Examples of Nucleophiles | Formula | Name | Formula | Name | | :----------------------- | :------------------- | :--------------------- | :------------ | | **$HO^-$** | Hydroxide ion | **$NH_2^-$** | Amino group | | **$C_2H_5O^-$** | Ethoxide ion | **$Cl^-$** | Chloride ion | | **$HS^-$** | Hydrogen sulphide ion | **$Br^-$** | Bromide ion | | **$SCN^-$** | Thiocyanate ion | **$NH_3$** | Ammonia | | **$H_2O:$** | Water | | | ### Specific $S_N$ Reactions #### 1. Reaction with Aqueous Sodium Hydroxide When an alkyl halide is heated with an aqueous solution of sodium hydroxide ($NaOH$), an alcohol is produced. * **General Reaction:** $$ R-X + NaOH_{(aq)} \rightarrow R-OH + NaX $$ * **Example:** $$ CH_3-Br + NaOH_{(aq)} \rightarrow CH_3-OH + NaBr $$ *(Bromomethane)* *(Methanol)* #### 2. Reaction with Aqueous Potassium Cyanide When an alkyl halide is heated with potassium cyanide ($KCN$) in ethanol, the halogen is replaced by a cyano group ($-CN$) to form a nitrile. * **General Reaction:** $$ R-X + KCN \rightarrow R-CN + KX $$ * **Example:** $$ CH_3-Cl + KCN \rightarrow CH_3-CN + KCl $$ *(Chloromethane)* *(Acetonitrile / Ethanenitrile)* #### 3. Reaction with Ammonia Primary amines are formed when an alkyl halide is heated with excess ammonia ($NH_3$) in ethanol. * **General Reaction:** $$ R-X + NH_3 \rightarrow R-NH_2 + HX $$ * **Example:** $$ CH_3-Cl + NH_3 \rightarrow CH_3-NH_2 + HCl $$ *(Chloromethane)* *(Methanamine)* #### 4. Reaction with Silver Nitrate (Test for Halides) When an alkyl halide is heated with silver nitrate ($AgNO_3$) in ethanol, a nitroalkane is formed. This reaction is also used as a chemical test to identify the halogen present in the alkyl halide. The halide ion displaced combines with the silver ion ($Ag^+$) to form a silver halide precipitate. * **General Reaction:** $$ R-X + AgNO_{3(ethanol)} \rightarrow R-NO_3 + AgX_{(s)} $$ * **Identification of Halide Ions:** The color of the precipitate and its solubility in aqueous ammonia helps identify the halide: - **Chloride ($Cl^-$):** Forms a **white precipitate** ($AgCl$) which is **soluble** in aqueous ammonia. - **Bromide ($Br^-$):** Forms a **cream precipitate** ($AgBr$) which is **partially soluble** in aqueous ammonia. - **Iodide ($I^-$):** Forms a **yellow precipitate** ($AgI$) which is **insoluble** in aqueous ammonia. --- ## Elimination Reactions An elimination reaction is one in which two groups (a halogen and a hydrogen atom) are removed from adjacent carbon atoms of an alkyl halide to form a carbon-carbon double bond (an alkene). Since a hydrogen atom from the carbon adjacent to the halogen-bearing carbon (the $\beta$-carbon) is removed, this is often called **$\beta$-elimination**. ### Explanation The $\beta$-hydrogen atom in an alkyl halide is slightly acidic due to the electron-withdrawing inductive effect of the halogen atom. A strong base, such as sodium hydroxide in ethanol, can abstract this proton, initiating the elimination. * **Mechanism:** The base removes the $\beta$-hydrogen, the C-H bond electrons form a $\pi$-bond between the $\alpha$ and $\beta$ carbons, and the halogen departs as a leaving group. * **Example:** Elimination from chloroethane using ethanolic NaOH. $$ CH_3-CH_2-Cl + NaOH_{(ethanol)} \xrightarrow{\Delta} CH_2=CH_2 + NaCl + H_2O $$ *(Chloroethane)* *(Ethene)* --- ## Worked Examples / Concept Assessment **Concept Assessment Exercise 17.1:** Complete the following reactions. **(a) Markovnikov's Addition** $$ CH_3-CH=CH_2 + HBr \rightarrow CH_3-CH(Br)-CH_3 $$ **(b) Nucleophilic Substitution** $$ CH_3-CH_2-Br + NaOH_{(aq)} \rightarrow CH_3-CH_2-OH + NaBr $$ **(c) Nucleophilic Substitution** $$ C_2H_5-Cl + KCN \rightarrow C_2H_5-CN + KCl $$ **(d) Nucleophilic Substitution** $$ C_2H_5-Cl + NH_3 \rightarrow C_2H_5-NH_2 + HCl $$ --- ## Possible Questions/Answers - **Q:** What are the two main types of reactions that alkyl halides undergo? **A:** Alkyl halides primarily undergo nucleophilic substitution ($S_N$) reactions and elimination reactions. - **Q:** Which of the following is not a nucleophile: $H_2O$, $NH_3$, $BF_3$, $OH^-$? **A:** $BF_3$ (Boron trifluoride). It is an electron-deficient species and acts as a Lewis acid (an electrophile), not a nucleophile. - **Q:** What is a $\beta$-elimination reaction? **A:** It is an elimination reaction where a halogen atom and a hydrogen atom from the adjacent carbon ($\beta$-carbon) are removed from an alkyl halide to form an alkene.