Problem 31 - Basic Principles of Organic Chemistry

Question

Suggest a route for the synthesis of each of the following compounds from the indicated starting material: a. 2 -methylpropanenitrile from 2 -methylpropanal b. \(\left(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{CH}_{2}\right)_{3} \mathrm{C}-\mathrm{NO}_{2}\) from nitromethane c. N-tert-butylbenzenecarboxamide from benzenecarbonitrile (benzeonitrile)

Step-by-Step Solution

Answer

1. Convert aldehyde to nitrile using NaCN. 2. Alkylate nitromethane with methyl chloroacetate. 3. Add tert-butylamine to benzenecarbonitrile with a reducing agent.

Step 1: Oxidation to Nitrile

For synthesizing 2-methylpropanenitrile from 2-methylpropanal, the aldehyde group in 2-methylpropanal needs to be converted to a nitrile. This can be achieved through the use of an oxidizing agent like PCC (Pyridinium chlorochromate) followed by PCl5 to convert the aldehyde to a corresponding alkyl chloride, and subsequently using a cyanide source like NaCN to perform a nucleophilic substitution to form the nitrile group.

Step 2: Michael Addition

To synthesize \((\text{CH}_3\text{CO}_2\text{CH}_2)_3\text{C}-\text{NO}_2\) from nitromethane, perform a Michael addition reaction. Initially, deprotonate nitromethane to form its anionic form, then perform an alkylation with methyl chloroacetate to introduce one ester group. Repeat the alkylation process two more times to form the desired product, making use of base-catalyzed deprotonation and substitution reactions to attach all three ester groups.

Step 3: Amide Formation

Starting with benzenenitrile, add tert-butylamine to the nitrile in the presence of a reducing agent like lithium aluminum hydride (LiAlH4) or a metal catalyst with hydrogen gas to form N-tert-butylbenzenecarboxamide. This reaction involves the conversion of the nitrile group to an amide through nucleophilic addition of the amine and subsequent reduction.

Key Concepts

Nitrile Synthesis

Nitrile synthesis involves transforming an aldehyde into a nitrile group, which introduces a carbon-nitrogen triple bond. This process is particularly useful in organic synthesis for creating molecules with applications in pharmaceuticals and agricultural chemicals. Let's take a deeper look into this technique.
To synthesize 2-methylpropanenitrile from 2-methylpropanal, the first step involves using Pyridinium chlorochromate (PCC) as an oxidizing agent. PCC transforms 2-methylpropanal into an acyl chloride, aided by PCl5. Adding PCl5 is crucial as it helps in further facilitating the conversion of the acyl chloride.
An essential part of this synthesis involves performing a nucleophilic substitution reaction with a cyanide source, such as sodium cyanide (NaCN). During this step, the chloride ion is replaced by the nucleophilic cyanide ion, successfully forming the nitrile group.

• PCC as an oxidizing agent can selectively oxidize alcohols and aldehydes without affecting other functional groups.
• PCl5 is used to activate the aldehyde carbon, converting it to a more reactive intermediate.
• Using NaCN allows for the introduction of the nitrile group via nucleophilic substitution.

This synthesis route is useful for creating carbon-nitrogen bonds, greatly expanding the diversity of organic compounds.

Michael Addition

The Michael Addition is a pivotal reaction in organic chemistry, employed to form carbon-carbon bonds. It is especially popular within the synthesis of complex molecules because it allows for the construction of intricate carbon skeletons.
In the task of synthesizing \((\text{CH}_3\text{CO}_2\text{CH}_2)_3\text{C}-\text{NO}_2\) from nitromethane, the first objective is the deprotonation of nitromethane. This generates an anion that is more nucleophilic, setting the stage for subsequent reactions.
Next is an alkylation step, attaching a methyl chloroacetate molecule to the nitromethane ion. This process is repeated a total of three times to attach all needed ester groups, eventually achieving the target molecule. The use of base-catalyzed deprotonation aids each step of alkylation by generating a more reactive nucleophile.

• Deprotonation uses bases to generate anions from acidic hydrogens.
• Alkylation introduces ester groups, increasing the molecular complexity.
• Utilizing a base ensures offensive power of anions for efficient alkylation.

This strategy showcases a powerful means of functional group substitution and linkage, vital for organic synthesis development.

Amide Formation

Amide formation is an important transformation in organic chemistry, converting a nitrile group into an amide. This conversion is achieved by adding an amine to a nitrile in the presence of a reducing agent, which facilitates the reduction.
To synthesize N-tert-butylbenzenecarboxamide from benzenecarbonitrile, the conversion starts by reacting the nitrile with tert-butylamine. Common reducing agents like lithium aluminum hydride (LiAlH4) or using a metal catalyst with hydrogen gas are implemented in this reaction.
In this specific reaction, the nitrile acts as an electrophile, while the tert-butylamine serves as a nucleophile. The nucleophilic addition of tert-butylamine to the nitrile group results in an intermediate imine, which rapidly reduces to the desired amide under the influence of a reducing agent.

• Nitriles are versatile electrophiles capable of forming strong carbon-nitrogen bonds.
• Tert-butylamine's nucleophilic nature drives the electrophilic attack on nitriles.
• Reduction stabilizes and converts the intermediate to yield a stable amide product.

Through this conversion, diverse amides are synthesized, pivotal in developing bioactive molecules and materials.