Question

The Hinsberg test of a compound, \(\mathrm{C}_{5} \mathrm{H}_{14} \mathrm{~N}_{2}\) produces a solid that is insoluble in \(10 \%\) aq. \(\mathrm{NaOH}\). This solid derivative dissolves in \(10 \%\) aqueous sulphuric acid. Which of the following would best describe these facts? (A) \(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{~N}\left(\mathrm{CH}_{3}\right)_{2}\) (B) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NCH}_{2} \mathrm{CH}_{2} \mathrm{NHCH}_{3}\) (C) \(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{C}\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (D) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NCH}_{2} \mathrm{~N}\left(\mathrm{CH}_{3}\right)_{2}\)

Short answer

The compound is a secondary amine because it forms a solid insoluble in 10% NaOH during the Hinsberg test. Therefore, the best structure to describe these facts is (B) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NCH}_{2} \mathrm{CH}_{2} \mathrm{NHCH}_{3}\).

Step-by-step solution

Understanding the Hinsberg test reactions

The Hinsberg test has a series of reactions according to the amine type: - Primary amines react with Hinsberg reagent to form an N-alkyl benzenesulphonyl amide, which is soluble in aqueous NaOH due to the formation of the sodium salt of the amide. - Secondary amines react with Hinsberg reagent to form N,N-dialkyl benzenesulphonyl amines, which are neutral and insoluble in NaOH. - Tertiary amines don't react with the Hinsberg reagent and remain in the organic phase. Since the compound in question produces a solid that is insoluble in 10% NaOH, this indicates the molecule is a secondary amine. AssemblyFileVersion

Key concepts

Secondary Amines

Secondary amines play an important role in the Hinsberg test because of their unique reactions compared to primary and tertiary amines. A secondary amine is an organic compound where the nitrogen atom is bonded to two alkyl or aryl groups rather than to hydrogen. These amines bear the general formula \( R_2NH \).

When a secondary amine reacts during the Hinsberg test, it does not form an ionic product. Instead, it results in the formation of a neutral compound called N,N-dialkyl benzenesulphonyl amine. This means:

• Secondary amines react with Hinsberg reagent to produce a neutral, insoluble product in aqueous NaOH.
• They remain uncharged because the sulphonamide formed lacks any proton to release.
• This characteristic insolubility in NaOH is what makes identifying secondary amines simple yet effective.

The neutral nature of the product is the key to differentiating secondary from primary amines in this test.

Chemical Solubility

Chemical solubility is an important characteristic in chemistry that dictates how substances dissolve in solvents. In the Hinsberg test:

• Primary amines form products that are soluble in aqueous NaOH due to the formation of salts.
• Secondary amines form neutral products, which largely remain insoluble in NaOH.
• Further behaviors such as solubility in acids provide additional insights.

For the chemical compound discussed in the exercise, its insolubility in NaOH indicates a secondary amine. Additionally, the compound's derivative's ability to dissolve in aqueous sulphuric acid (a stronger acid) reinforces the amine's secondary nature.

This behavior is crucial because it helps chemists predict and understand the structure and behavior of specific compounds in reactions.

Amines Reactivity

Amines exhibit a variety of reactivity patterns due to the lone pair of electrons located on their nitrogen atom. In the context of the Hinsberg test, this reactivity is key in determining the type of amine:

• Primary amines react to form soluble benzenesulphonyl amides.
• Secondary amines react to form a neutral, insoluble N,N-dialkyl sulphonamide.
• Tertiary amines do not participate in the reaction owing to the steric hindrance that impedes bonding with the Hinsberg reagent.

The ability of amines to react or not in a specific manner helps chemists to rapidly identify and classify the amine under test.

The presence or absence of a proton exchange capability during the test highlights the fundamental differences in structure and reactivity between amine types. This proposed activity underscores the usefulness of the Hinsberg test in distinguishing the varied reactivities of primary, secondary, and tertiary amines.