Question

An amine of unknown structure has the molecular formula \(\mathrm{C}_{8} \mathrm{H}_{11} \mathrm{~N}\). Benzenesulfony1 chloride was added to this amine. The result was a precipitate which was soluble in sodium hydroxide. If the amine had been treated with nitrous acid at \(0^{\circ} \mathrm{C}\), a reaction would have occurred. A gradual warming to room temperature in the presence of water would have caused a gas to be evolved. Write a possible structure of the amine.

Short answer

A possible structure for the amine is \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{N}(\mathrm{C}_{2} \mathrm{H}_{5})\mathrm{H}\), since it fits the molecular formula, C8H11N, and satisfies the conditions mentioned in the question with an aromatic ring and a secondary amine.

Step-by-step solution

Identify key properties of the amine

Based on the given information, we can deduce that this amine is possibly primary or secondary and aromatic.

Consider possible structures

We can consider various possible structures for the amine, with 8 carbons and one nitrogen atom.

Choose a structure that meets the criteria

A structure that satisfies all the conditions provided is an amine with the nitrogen attached to an aromatic ring (such as an aniline derivative). A possible structure could be: \[ \mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{N}(\mathrm{C}_{2} \mathrm{H}_{5})\mathrm{H} \] This structure has a total of 8 carbon atoms, 11 hydrogen atoms, and 1 nitrogen atom, fitting the molecular formula, C8H11N. Additionally, the structure contains an aromatic ring and a secondary amine, which satisfies the conditions mentioned in the problem.

Key concepts

Benzenesulfonyl Chloride Reaction

When identifying the structure of an amine, one common method is to react it with benzenesulfonyl chloride. This reaction is a useful tool because it forms a sulfonamide when it reacts with primary and secondary amines.

For primary amines, the product is insoluble in an aqueous solution but soluble in basic solutions like sodium hydroxide. Secondary amines also react to form sulfonamides but in a slightly less straightforward manner. The solubility of the resulting product can help differentiate between primary and secondary amines.

Example Application:

In the exercise, the amine reacts with benzenesulfonyl chloride to form a precipitate, indicating it is likely a primary or secondary amine. However, since the precipitate dissolves in sodium hydroxide, we can further deduce it's likely a primary amine, as secondary amine derivatives are usually not soluble in sodium hydroxide.

Solubility in Sodium Hydroxide

Sodium hydroxide, being a strong base, is known for its ability to dissolve various ionic and organic compounds. In the context of amine structures, the solubility in sodium hydroxide can help classify an amine.

Primary amines react with benzenesulfonyl chloride to form a sulfonamide that is not water-soluble but does dissolve in sodium hydroxide due to deprotonation, leading to a negative charge that increases its solubility. Example Application: The amine from the problem statement produces a water-insoluble precipitate with benzenesulfonyl chloride that dissolves in sodium hydroxide, which supports the identification of the amine as primary.

Reaction with Nitrous Acid

The reaction of amines with nitrous acid, particularly at low temperatures, is another distinguishing mechanism. For primary amines, this reaction generates a diazonium salt, which can decompose, especially upon warming, to release nitrogen gas.

Secondary amines, under similar conditions, may form N-nitrosoamines, but these are generally more stable and do not release nitrogen gas readily. Example Application: In our exercise, the amine treated with nitrous acid, when gradually warmed, would result in the evolution of gas, implying it's a primary amine reacting to form a diazonium salt and confirming the previous findings from the benzenesulfonyl chloride test.

Aromatic Amine

Aromatic amines, such as aniline and its derivatives, are compounds in which one or more amine groups are directly connected to an aromatic ring. These compounds often display unique reactivity due to the delocalized pi electrons of the aromatic system.

For instance, aromatic amines tend to be less basic than their aliphatic counterparts because the electron-rich nature of the aromatic ring delocalizes the lone pair of electrons on the amine nitrogen. Example Application: The problem suggests that the unknown amine is aromatic because the molecular formula allows for an aromatic ring and sufficient carbons to satisfy the given structure, which indicates the presence of an aromatic system.

Primary and Secondary Amines

Differentiating between primary and secondary amines is vital in organic chemistry. A primary amine has one alkyl or aromatic substituent attached to the nitrogen atom, while a secondary amine has two. Each has distinct chemical behaviors and reactivity patterns.

Chemical tests like benzenesulfonyl chloride and solubility in sodium hydroxide can help differentiate them. Additionally, their reaction with nitrous acid can confirm the classification. Example Application: Based on the test results from the exercise, we can establish that the amine in question is a primary aromatic amine since the precipitate formed with benzenesulfonyl chloride dissolved in sodium hydroxide and reacting with nitrous acid would release a gas upon warming.