Question

Which of the following will give a positive carbylamine test? 1\. \(\mathrm{H}_{3} \mathrm{C}-\mathrm{NH}_{2}\) 2\. \(\mathrm{H}_{3} \mathrm{C}-\mathrm{NH}-\mathrm{CH}_{3}\) 3\. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\) 4\. \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{NH}_{2}\) Select the correct answer using the codes given below. (a) 1 and 3 (b) 2 and 4 (c) 1 and 4 (d) 3 and 4

Short answer

Option (c) 1 and 4 will give a positive carbylamine test.

Step-by-step solution

Understanding the Carbylamine Reaction

The carbylamine test (or isocyanide test) is used to detect primary amines. Primary amines react with chloroform and alcoholic potassium hydroxide to form isocyanides (carbylamines), which have a characteristic foul smell. Only primary amines will give a positive result.

Analyzing Compound 1

Compound 1 is \(\mathrm{H}_{3} \mathrm{C}-\mathrm{NH}_{2}\), a primary amine. It possesses a single alkyl group attached to the nitrogen atom, fulfilling the condition needed to undergo a carbylamine reaction.

Analyzing Compound 2

Compound 2 is \(\mathrm{H}_{3} \mathrm{C}-\mathrm{NH}-\mathrm{CH}_{3}\), which is a secondary amine because it has two alkyl groups attached to the nitrogen. It will not react in the carbylamine test, as it is not a primary amine.

Analyzing Compound 3

Compound 3 is \((\mathrm{CH}_{3})_{3} \mathrm{~N}\), a tertiary amine with three methyl groups attached to the nitrogen atom. Tertiary amines do not give a positive carbylamine test.

Analyzing Compound 4

Compound 4 is \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{NH}_{2}\), aniline, which is a primary aromatic amine. Primary amines, including aromatic ones, will give a positive carbylamine reaction.

Identifying Positive Compounds

From the analysis, Compound 1 (a primary aliphatic amine) and Compound 4 (a primary aromatic amine) will give a positive carbylamine test.

Selecting the Correct Option

The correct answer is option (c) 1 and 4, as these are the primary amines among the given options that will test positive in the carbylamine test.

Key concepts

Primary Amines

Primary amines are a class of organic compounds that have a structure involving one nitrogen atom bonded to exactly one carbon-containing group, along with two hydrogen atoms. Essentially, they can be thought of as derivatives of ammonia, where one hydrogen atom is replaced by a carbon-containing group. This replacement can either be with an aliphatic or aromatic group. Some essential features of primary amines include their capacity to participate in key chemical reactions, their basic nature, and typically lower boiling points relative to others like alcohols. Common characteristics include:

• Basicity: They can accept a proton due to the lone pair of electrons on the nitrogen.
• Variable Physical States: Depending on the molecular weight, they can exist in gaseous, liquid, or solid forms.
• Reactivity: One of their notable reactions includes the formation of amides or participating in nucleophilic substitution reactions.

In general, understanding primary amines involves recognizing their fundamental role in organic chemistry as precursors to many compounds and functional groups.

Isocyanide Reaction

The isocyanide reaction, often called the carbylamine reaction, is a specialized reaction utilized primarily for the detection of primary amines. It involves reacting a primary amine with chloroform ( CHCl₃ ) and alcoholic potassium hydroxide ( KOH ). As a result, the reaction yields an isocyanide (carbylamine), emitting a characteristically unpleasant odor. This transformation showcases an intriguing mechanism where the base abstracts a hydrogen, leading to the formation of dichlorocarbene, which subsequently reacts with the amine.

Some key aspects of the isocyanide reaction include:

• Specificity: It only works with primary amines, making it a reliable detection test.
• Distinctive Odor: The produced isocyanides have a particularly foul smell, which aids in their detection.
• Chemical Transformation: This reaction involves significant structural changes, showcasing organic reaction mechanisms.

Overall, the isocyanide reaction holds historical and practical importance in laboratory settings for identifying primary amines from other types of amines.

Chemical Reactions Detection Method

The carbylamine test serves as a chemical reaction detection method specifically designed for primary amines. This detection approach is one of many used by chemists to qualitatively analyze chemical substances. A successful detection method like the carbylamine test needs to be both selective and sensitive.

Detection methods rely on unique properties of target compounds, such as specific reaction pathways or the generation of unique compounds (in this case, isocyanides). These methods are significant because:

• They aid in verifying the presence of specific functional groups within organic molecules.
• They are often simple and do not require advanced tools or complex procedures.
• They can often be performed quickly, offering rapid results for experimental needs.

Chemical reactions detection methods play an indispensable role in research and industrial applications, ensuring that compounds can be readily identified and assessed.