Problem 67 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

The reaction of propane with nitric acid in vapour phase gives (a) 1-nitropropane (b) 2-nitropropane (c) Nitromethane (d) Nitroethane

Step-by-Step Solution

Answer

The reaction gives nitroethane (d).

Step 1: Understanding the Reaction

The problem involves the reaction of propane with nitric acid in the vapor phase. This type of reaction generally involves nitrating agents reacting with hydrocarbons to yield nitro compounds.

Step 2: Identify the Hydrocarbon

Propane is a three-carbon alkane with the molecular formula \(C_3H_8\). When nitrating agents like nitric acid react with alkanes, it's typically on the more accessible and reactive carbon atoms.

Step 3: Predict the Products

In vapor phase nitration, the reaction typically results in the formation of nitroalkanes. Given propane's structure, possible products from nitration can be nitropropane isomers or breakdown products like nitromethane or nitroethane.

Step 4: Determine Typical Nitration Products for Propane

Vapor phase nitration of propane can produce both 1-nitropropane and 2-nitropropane because they are the simplest nitro derivatives possible when retaining all carbon atoms. However, reaction conditions can also fragment propane into smaller hydrocarbons.

Step 5: Identify Fragmentation Products

In vapor phase reactions, especially with nitrating agents, breaking of C-C bonds can yield smaller molecules. Nitroethane, a two-carbon nitro compound, is a likely product from such fragmentation.

Key Concepts

Nitroalkanes

Nitroalkanes are a group of organic compounds that include a nitro group \((-NO_2)\), bound to an alkyl group. These compounds are commonly used as building blocks in organic synthesis, due to their versatility. When discussing nitroalkanes in the context of vapor phase nitration, the focus is on how alkanes like propane react with nitrating agents, such as nitric acid. During the reaction, the nitro group attaches to the carbon chain of the alkane, transforming it into a nitroalkane.

Key aspects of their reactions include the:

• Specificity of the nitro group to attach to hydrogen in alkanes.
• Formation of isomers depending on the carbon position of attachment. For example, propane can form 1-nitropropane or 2-nitropropane as nitroalkanes.

Ultimately, the positions of the nitro groups determine the specific properties and applications of the resulting nitroalkane.

Propane Reactions

Propane is a simple straight-chain alkane with the formula \(C_3H_8\). When reacting in the vapor phase with a strong nitrating agent such as nitric acid, propane can undergo nitration or fragmentation, depending on reaction conditions.

The main reactions include:

• Direct Nitration: Attaching the nitro group to one of the carbon atoms in propane. This can result in either 1-nitropropane or 2-nitropropane depending on which hydrogen atom is replaced by the nitro group.
• Fragmentation: Breaking of the carbon-carbon bonds in propane, which results in smaller molecules often still carrying one or more nitro groups.

In practice, conditions like temperature, pressure, and nitric acid concentration define the predominance of direct nitration versus fragmentation reactions. Vapor phase conditions typically favor where alkanes undergo transformations more easily into their nitro derivatives.

Fragmentation Products

Fragmentation products result when the C-C bonds in propane break during a reaction with nitric acid, especially under elevated temperatures typical of vapor phase nitration. These smaller molecules maintain their nitro groups, but are now part of simpler hydrocarbons like nitromethane and nitroethane.

The main products from fragmentation can include:

• Nitromethane: A single carbon nitro compound formed when further breakdown of propane occurs.
• Nitroethane: A two-carbon fragment, often seen when one of the carbon bonds in propane cleaves.

Fragmentation not only affects the type of products obtained but influences the yield and selectivity of reactions. Understanding this aspect is crucial in optimizing the process to achieve desired outcomes in industrial or laboratory settings.