Question

A monofunctional organic nitrogen compound dissolves in aqueous hydrochloric acid but not in aqueous sodium hydroxide. What kind of organic compound is it?

Short answer

The organic compound is likely a primary, secondary, or tertiary amine.

Step-by-step solution

Understanding the Solubility in Acids and Bases

Compounds that are soluble in aqueous hydrochloric acid (HCl) but not in aqueous sodium hydroxide (NaOH) often have basic characteristics. The fact that the compound dissolves in acid suggests that it can react with the acid, forming a salt and possibly water in a neutralization reaction. However, being insoluble in a base (like NaOH) suggests that it cannot react with or be deprotonated by the base.

Identifying the Functional Group

Organic nitrogen compounds with these properties typically contain functional groups that can accept protons (act as a base) and are referred to as amines. Therefore, a mono-functional organic nitrogen compound that behaves in this manner is likely an amine.

Determining the Type of Amine

Since the compound is monofunctional (containing one functional group), it is likely a primary, secondary, or tertiary amine. Considering that it does not dissolve in NaOH, we can generally rule out quaternary ammonium compounds as they would be soluble. Thus, it is probably a primary, secondary, or tertiary amine.

Key concepts

Understanding Amines

Amines are a fundamental class of organic nitrogen compounds characterized by the presence of a nitrogen atom connected to one or more alkyl or aryl groups. They are derivatives of ammonia, where one or more hydrogen atoms have been replaced by carbon-containing groups, known as alkyl or aryl groups.

Amines can be categorized based on the number of alkyl or aryl groups attached to the nitrogen:

• Primary amines have one carbon-containing group attached to the nitrogen.
• Secondary amines have two carbon-containing groups.
• Tertiary amines have three carbon-containing groups.
• Quaternary ammonium compounds have four carbon-containing groups; however, they also possess a positively charged nitrogen atom, which makes their properties distinct from the other three types.

These compounds play a vital role in biochemistry and pharmaceuticals, giving rise to various behaviors and reactions depending on their structure, which influences properties such as solubility, acidity, and basicity.

Solubility in Acids and Bases

Understanding the solubility of organic compounds in acids and bases is crucial in recognizing their chemical nature and behavior. A compound's solubility can provide insight into its functional groups and reactivity with other substances.

Organic nitrogen compounds, particularly amines, tend to be basic because the lone pair of electrons on the nitrogen atom can accept a proton (H+). This basic nature makes them soluble in acids like hydrochloric acid (HCl) because they can react to form ammonium salts. Conversely, when introduced to a base such as sodium hydroxide (NaOH), amines do not dissolve as readily, because there is no proton available from the base for the amine to accept. Instead, they would require an acid to proceed with a neutralization reaction.The solubility trend observed in acids and not in bases, is indicative of a substance with a basic functional group, such as an amine, which corroborates the analysis from the step-by-step solution provided for identifying the organic compound in the original exercise.

Neutralization Reaction

A neutralization reaction is a type of chemical reaction where an acid and a base react to form water and a salt. It is central to the concept of acid-base chemistry and is crucial in the study of amines' reactivity and solubility.

When an amine, which acts as a base, encounters an acid, they neutralize one another. Let's consider the example of a monofunctional amine reacting with hydrochloric acid:

• The amine group (NH2, NH, or N) accepts a proton (H+) from the acid.
• This leads to the formation of an ammonium ion (NH3+, NH2+, or NH+), which pairs with the accompanying anion from the acid, resulting in an ammonium salt.
• Depending on the specific amine and reaction conditions, water might also form as a byproduct.

Such a reaction exemplifies why amines are soluble in acids — they engage in a neutralization process that turns them into soluble salts. However, in the presence of a strong base like NaOH, no such proton exchange can occur, hence there's no neutralization, and the amine remains largely insoluble.