17.5 Reactions of Alkyl Halides | Halogenoalkanes | Chemistry 11

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: $Nu:^{-} + R - X \to R - Nu + X:^{-}$

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 12.1 Halogens→(/chemistry-11/12-halogens/12-1-halogens).
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., $H_{2} O$ , $N H_{3}$ ) or negatively charged (e.g., $O H^{-}$ , $C N^{-}$ ).
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: $C l^{-}$ , $B r^{-}$ , $I^{-}$ , $H S O_{4}^{-}$
- Poor Leaving Groups: $O H^{-}$ , $O R^{-}$ , $N H_{2}^{-}$
- 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
$H O^{-}$	Hydroxide ion	$N H_{2}^{-}$	Amino group
$C_{2} H_{5} O^{-}$	Ethoxide ion	$C l^{-}$	Chloride ion
$H S^{-}$	Hydrogen sulphide ion	$B r^{-}$	Bromide ion
$S C N^{-}$	Thiocyanate ion	$N H_{3}$	Ammonia
$H_{2} O :$	Water

Specific $S_{N}$ Reactions

1. Reaction with Aqueous Sodium Hydroxide

When an alkyl halide is heated with an aqueous solution of sodium hydroxide ( $N a O H$ ), an alcohol is produced.

General Reaction: $R - X + N a O H_{(a q)} \to R - O H + N a X$
Example: $C H_{3} - B r + N a O H_{(a q)} \to C H_{3} - O H + N a B r$ (Bromomethane) → (Methanol)

2. Reaction with Aqueous Potassium Cyanide

When an alkyl halide is heated with potassium cyanide ( $K C N$ ) in ethanol, the halogen is replaced by a cyano group ( $- C N$ ) to form a nitrile. This reaction is synthetically important because it increases the carbon chain length by one carbon atom.

General Reaction: $R - X + K C N \to R - C N + K X$
Example: $C H_{3} - C l + K C N \to C H_{3} - C N + K C l$ (Chloromethane) → (Acetonitrile / Ethanenitrile)

3. Reaction with Ammonia

Primary amines are formed when an alkyl halide is heated with excess ammonia ( $N H_{3}$ ) in ethanol.

General Reaction: $R - X + N H_{3} \to R - N H_{2} + H X$
Example: $C H_{3} - C l + N H_{3} \to C H_{3} - N H_{2} + H C l$ (Chloromethane) → (Methanamine)

4. Reaction with Silver Nitrate (Test for Halides)

When an alkyl halide is heated with silver nitrate ( $A g N O_{3}$ ) in ethanol, an alkyl nitrate ( $R$ - $O N O_{2}$ ) 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 ( $A g^{+}$ ) to form a silver halide precipitate.

General Reaction: $R - X + A g N O_{3 (e t han o l)} \to R - O N O_{2} + A g X_{(s)}$
Identification of Halide Ions: The color of the precipitate and its solubility in aqueous ammonia helps identify the halide:

Halide Precipitate Color Solubility in $N H_{3}$
$C l^{-}$ $A g C l$ White Soluble
$B r^{-}$ $A g B r$ Cream Partially soluble
$I^{-}$ $A g I$ Yellow Insoluble

Halide	Precipitate	Color	Solubility in $N H_{3}$
$C l^{-}$	$A g C l$	White	Soluble
$B r^{-}$	$A g B r$	Cream	Partially soluble
$I^{-}$	$A g I$	Yellow	Insoluble

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 $β$ -carbon) is removed, this is often called $β$ -elimination or dehydrohalogenation.

Explanation

The $β$ -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 $β$ -hydrogen, the C-H bond electrons form a $π$ -bond between the $α$ and $β$ carbons, and the halogen departs as a leaving group.
Example: Elimination from chloroethane using ethanolic NaOH. $C H_{3} - C H_{2} - C l + N a O H_{(e t han o l)} Δ C H_{2} = C H_{2} + N a C l + H_{2} O$ (Chloroethane) → (Ethene)

Saytzeff's Rule

When elimination can produce more than one alkene (due to the presence of different $β$ -hydrogens), Saytzeff's rule states that the more highly substituted (more stable) alkene is the major product.

Example: Elimination from 2-bromobutane gives 2-butene (major) rather than 1-butene (minor), because 2-butene has more alkyl substituents on the double bond and is therefore more stable.

Aqueous vs. Alcoholic NaOH — Substitution vs. Elimination

Condition	Reaction Type	Product
$N a O H_{(a q)}$ (aqueous)	Nucleophilic Substitution	Alcohol ( $R$ - $O H$ )
$N a O H_{(e t han o l)}$ (alcoholic)	Elimination	Alkene

Concept Assessment Exercise 17.1

Complete the following reactions:

(a) $C H_{3} C H_{2} B r + N a O H_{(a q)} Δ$ ?

Answer: $C H_{3} C H_{2} O H + N a B r$ (Ethanol formed by nucleophilic substitution)

(b) $C H_{3} C H_{2} C l + K C N e t han o l$ ?

Answer: $C H_{3} C H_{2} C N + K C l$ (Propanenitrile formed — chain extended by one carbon)

(c) $C H_{3} C H_{2} B r + N a O H_{(e t han o l)} Δ$ ?

Answer: $C H_{2} = C H_{2} + N a B r + H_{2} O$ (Ethene formed by elimination)

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: $Nu:^{-} + R - X \to R - Nu + X:^{-}$

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 12.1 Halogens→(/chemistry-11/12-halogens/12-1-halogens).
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., $H_{2} O$ , $N H_{3}$ ) or negatively charged (e.g., $O H^{-}$ , $C N^{-}$ ).
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: $C l^{-}$ , $B r^{-}$ , $I^{-}$ , $H S O_{4}^{-}$
- Poor Leaving Groups: $O H^{-}$ , $O R^{-}$ , $N H_{2}^{-}$
- Note: The iodide ion ( $I^{-}$ ) is both a good nucleophile and a good leaving group.

Examples of Nucleophiles

Formula	Name	Formula	Name
$H O^{-}$	Hydroxide ion	$N H_{2}^{-}$	Amino group
$C_{2} H_{5} O^{-}$	Ethoxide ion	$C l^{-}$	Chloride ion
$H S^{-}$	Hydrogen sulphide ion	$B r^{-}$	Bromide ion
$S C N^{-}$	Thiocyanate ion	$N H_{3}$	Ammonia
$H_{2} O :$	Water

Specific $S_{N}$ Reactions

1. Reaction with Aqueous Sodium Hydroxide

When an alkyl halide is heated with an aqueous solution of sodium hydroxide ( $N a O H$ ), an alcohol is produced.

General Reaction: $R - X + N a O H_{(a q)} \to R - O H + N a X$
Example: $C H_{3} - B r + N a O H_{(a q)} \to C H_{3} - O H + N a B r$ (Bromomethane) → (Methanol)

2. Reaction with Aqueous Potassium Cyanide

General Reaction: $R - X + K C N \to R - C N + K X$
Example: $C H_{3} - C l + K C N \to C H_{3} - C N + K C l$ (Chloromethane) → (Acetonitrile / Ethanenitrile)

3. Reaction with Ammonia

Primary amines are formed when an alkyl halide is heated with excess ammonia ( $N H_{3}$ ) in ethanol.

General Reaction: $R - X + N H_{3} \to R - N H_{2} + H X$
Example: $C H_{3} - C l + N H_{3} \to C H_{3} - N H_{2} + H C l$ (Chloromethane) → (Methanamine)

4. Reaction with Silver Nitrate (Test for Halides)

General Reaction: $R - X + A g N O_{3 (e t han o l)} \to R - O N O_{2} + A g X_{(s)}$
Identification of Halide Ions: The color of the precipitate and its solubility in aqueous ammonia helps identify the halide:

Halide Precipitate Color Solubility in $N H_{3}$
$C l^{-}$ $A g C l$ White Soluble
$B r^{-}$ $A g B r$ Cream Partially soluble
$I^{-}$ $A g I$ Yellow Insoluble

Halide	Precipitate	Color	Solubility in $N H_{3}$
$C l^{-}$	$A g C l$	White	Soluble
$B r^{-}$	$A g B r$	Cream	Partially soluble
$I^{-}$	$A g I$	Yellow	Insoluble

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 $β$ -carbon) is removed, this is often called $β$ -elimination or dehydrohalogenation.

Explanation

Mechanism: The base removes the $β$ -hydrogen, the C-H bond electrons form a $π$ -bond between the $α$ and $β$ carbons, and the halogen departs as a leaving group.
Example: Elimination from chloroethane using ethanolic NaOH. $C H_{3} - C H_{2} - C l + N a O H_{(e t han o l)} Δ C H_{2} = C H_{2} + N a C l + H_{2} O$ (Chloroethane) → (Ethene)

Saytzeff's Rule

Example: Elimination from 2-bromobutane gives 2-butene (major) rather than 1-butene (minor), because 2-butene has more alkyl substituents on the double bond and is therefore more stable.

Aqueous vs. Alcoholic NaOH — Substitution vs. Elimination

Condition	Reaction Type	Product
$N a O H_{(a q)}$ (aqueous)	Nucleophilic Substitution	Alcohol ( $R$ - $O H$ )
$N a O H_{(e t han o l)}$ (alcoholic)	Elimination	Alkene

Concept Assessment Exercise 17.1

Complete the following reactions:

(a) $C H_{3} C H_{2} B r + N a O H_{(a q)} Δ$ ?

Answer: $C H_{3} C H_{2} O H + N a B r$ (Ethanol formed by nucleophilic substitution)

(b) $C H_{3} C H_{2} C l + K C N e t han o l$ ?

Answer: $C H_{3} C H_{2} C N + K C l$ (Propanenitrile formed — chain extended by one carbon)

(c) $C H_{3} C H_{2} B r + N a O H_{(e t han o l)} Δ$ ?

Answer: $C H_{2} = C H_{2} + N a B r + H_{2} O$ (Ethene formed by elimination)