Nitration of alkanes is a reaction in which a hydrogen atom of an alkane is replaced by a nitro group (–NO2) using nitric acid. This reaction occurs through a free radical substitution mechanism, where nitric acid forms reactive radicals that help in the substitution process. The reaction is not very selective and generally produces a mixture of nitroalkanes, especially in higher alkanes. Nitration of alkanes is important for the preparation of nitro compounds used in organic chemistry.
Reagents and Conditions
The nitration of alkanes is carried out using nitric acid (HNO3) as the main reagent. Nitric acid acts as a nitrating agent, which provides the nitro group (–NO2) required for the reaction.
- The reaction requires high temperature (about 400–500 K).
- At this temperature, nitric acid decomposes to form highly reactive free radicals such as nitrogen dioxide (NO2•) and hydroxyl radicals (OH•).
- These free radicals initiate the reaction by attacking the alkane molecule and help in the substitution of a hydrogen atom by the nitro group.
- The reaction generally occurs in the vapour phase and is not very selective, especially in higher alkanes.
Reaction of Nitration of Alkanes
In this reaction, an alkane reacts with nitric acid at high temperature (400–500 K) to form a nitroalkane and water. In this process, one hydrogen atom of the alkane is replaced by a nitro group (–NO₂), so it is a substitution reaction.
Example:
CH4 + HNO3 → CH3NO2 + H2O
Nitration Mechanism of Alkanes
The nitration of alkanes takes place through a free radical substitution mechanism, which involves three main steps:
1. Formation of free radicals
At high temperature, nitric acid decomposes to form free radicals. These radicals start the reaction.
2. Propagation (Chain reaction steps)
Step 1: Formation of alkyl radical
Step 2: Formation of nitroalkane
In these steps:
- Alkane forms an alkyl radical (R•)
- Then it reacts with NO₂ to form nitroalkane
3. Termination
Free radicals combine to stop the reaction: