Halogenoalkanes are organic compounds containing at least one halogen atom bonded to an sp3 hybridised carbon. Because halogens are more electronegative than carbon, the carbon-halogen bond is polar, making halogenoalkanes highly susceptible to attack by nucleophiles.
🔑 Key Principle
The polarity of the carbon-halogen bond arises from the difference in electronegativity. The carbon atom carries a partial positive charge (\(\delta+\)) and acts as an electrophilic centre. This carbon is attacked by nucleophiles, which donate an electron pair to form a new covalent bond while the halogen atom departs as a halide leaving group.
Polarity of the Carbon-Halogen Bond
Halogen atoms (Fluorine, Chlorine, Bromine, Iodine) have a higher electronegativity than carbon. This difference pulls the shared electron density in the C-X covalent bond towards the halogen atom:
- The halogen atom acquires a partial negative charge (\(\delta-\)).
- The carbon atom acquires a partial positive charge (\(\delta+\)).
This electron-deficient carbon atom makes halogenoalkanes reactive towards electron-pair donors, which are known as nucleophiles.
Key Definitions
An electron-pair donor. Nucleophiles are species that possess a lone pair of electrons and carry either a negative charge or a partial negative charge, allowing them to attack electron-deficient carbon atoms.
A reaction mechanism in which an electron-rich nucleophile attacks an electron-deficient carbon atom, leading to the replacement of a leaving atom or group of atoms (in this case, a halide ion) by the nucleophile.
An atom or group of atoms that departs from the organic molecule during a reaction, taking the shared pair of bonding electrons with it. In halogenoalkane substitution, the halide ion (\(\text{X}^-\)) is the leaving group.
Reactions of Halogenoalkanes
You need to know the specific reagents, reaction conditions, and products for three key nucleophilic substitution reactions of halogenoalkanes.
1. Reaction with Aqueous Hydroxide Ions (Formation of Alcohols)
When heated under reflux with aqueous sodium hydroxide (\(\text{NaOH}\)) or potassium hydroxide (\(\text{KOH}\)), halogenoalkanes undergo substitution to produce alcohols.
Reagent: Aqueous sodium hydroxide or potassium hydroxide
Conditions: Warm, aqueous solvent, reflux
General Equation:
\[ \text{R-X} + \text{OH}^- \rightarrow \text{R-OH} + \text{X}^- \]
It is vital to state the solvent conditions. If hydroxide ions are dissolved in aqueous solvent, they act as nucleophiles to undergo substitution. If they are dissolved in ethanolic solvent and heated strongly, they act as bases to undergo elimination. You must specify the solvent to gain full marks.
The Nucleophilic Substitution Mechanism
The mechanism below shows the pathway for the reaction between bromoethane and a hydroxide ion. The curly arrows show the movement of pairs of electrons:
2. Reaction with Potassium Cyanide (Formation of Nitriles)
Halogenoalkanes react with potassium cyanide (\(\text{KCN}\)) to produce nitriles. This reaction is extremely important in organic synthesis because it increases the length of the carbon chain.
Reagent: Ethanolic potassium cyanide (\(\text{KCN}\))
Conditions: Heated under reflux, ethanolic solvent
General Equation:
\[ \text{R-X} + \text{CN}^- \rightarrow \text{R-C}\equiv\text{N} + \text{X}^- \]
For example, reacting 1-bromobutane with potassium cyanide yields pentanenitrile:
\[ \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{Br} + \text{CN}^- \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CN} + \text{Br}^- \]
Cyanide ions are ambidentate, but they attack the carbon atom of the halogenoalkane through the carbon atom of the cyanide group, not the nitrogen. This is because the lone pair of electrons is located on the carbon atom of \(\text{:C}\equiv\text{N}^-\). Ensure your curly arrows start directly from the carbon lone pair.
3. Reaction with Excess Ammonia (Formation of Primary Amines)
Halogenoalkanes react with ammonia (\(\text{NH}_3\)) to form primary amines. The reaction requires excess ammonia to prevent the primary amine product from reacting further with the halogenoalkane.
Reagent: Excess ethanolic ammonia (\(\text{NH}_3\))
Conditions: Heated under pressure in a sealed tube
General Equation:
\[ \text{R-X} + 2\text{NH}_3 \rightarrow \text{R-NH}_2 + \text{NH}_4\text{X} \]
For example, reacting bromoethane with excess ammonia yields ethylamine:
\[ \text{CH}_3\text{CH}_2\text{Br} + 2\text{NH}_3 \rightarrow \text{CH}_3\text{CH}_2\text{NH}_2 + \text{NH}_4\text{Br} \]
Note the stoichiometry: you must use 2 molecules of ammonia in the overall equation. The first ammonia molecule acts as the nucleophile to attack the carbon. The second ammonia molecule acts as a base to remove a proton from the intermediate alkylammonium ion, forming the stable amine product and an ammonium halide salt.
Worked Examples
Step 1: Polarisation and nucleophilic attack.
The C-Br bond in 1-bromobutane is polar due to bromine's higher electronegativity. The carbon attached to the bromine is electron-deficient (\(\delta+\)), and the bromine is \(\delta-\). A curly arrow is drawn from a lone pair of electrons on the oxygen of the hydroxide ion (\(\text{OH}^-\)) to the \(\delta+\) carbon atom.
Step 2: Departure of the leaving group.
Simultaneously, the pair of electrons in the C-Br bond moves entirely to the bromine atom. A curly arrow is drawn starting from the middle of the C-Br bond and pointing to the bromine atom.
Step 3: Product formation.
The products formed are butan-1-ol and a bromide ion (\(\text{Br}^-\)):
\[ \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{Br} + \text{OH}^- \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH} + \text{Br}^- \]
Answer:
This reaction is highly valuable because it is a method of increasing the carbon chain length. Most organic starting materials obtained from crude oil are short-chain. The cyanide ion contains a carbon atom, so when it replaces the halogen, the carbon chain is extended by exactly one carbon atom.
In this example, 1-bromopropane contains three carbons. When it reacts with potassium cyanide, butanenitrile is formed, which contains four carbons. The nitrile group can then be converted into carboxylic acids or amines, facilitating the synthesis of longer-chain polymers, pharmaceuticals, and other complex materials.
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