Chapter 16, Problem 16.21P

Interpretation Introduction

(a)

Interpretation:

To check whether the statement, “Aqueous solutions of amines are basic” is true or false.

Concept Introduction:

Lewis acids are the ones which can accept a pair of electrons. Lewis bases are the ones which donate a pair of electrons. So, a substance which can donate a pair of electrons can be termed as a base.

Explanation of Solution

The general formula of amines is $R-N{\text{H}}_2$. They have nitrogen in their formula. Nitrogen has a lone pair of electrons and is capable of donating electrons and hence can act as a Lewis base.

Let us consider its reaction with water.

From the reaction, it is clear that the nitrogen donates a pair of electrons to hydrogen thus, making a salt. Thus, aqueous solutions of amines are basic in nature.

Hence, the statement is True.

Interpretation Introduction

(b)

Interpretation:

To check whether the statement, “Aromatic amines such as aniline in general are weaker bases than aliphatic amines such as cyclohexanamine”.

Concept Introduction:

Lewis acids are the ones which can accept a pair of electrons. Lewis bases are the ones which donate a pair of electrons. So, a substance which can donate a pair of electrons can be termed as a base.

Explanation of Solution

Aniline is the benzene ring that is attached to amine group. The lone pair of electrons present on the nitrogen atom is in conjugation with the aromatic ring. Thus, the lone pair of electrons is less available to donate to a proton. Hence, aniline is less basic in nature in comparison with methyl amine.

The cyclohexylamine is the one where the cyclohexyl ring is attached to the amine group. Hence, it is similar to an alkyl group attached to amine group. The alkyl groups are electron donating in nature due to inductive effect. Hence, the basicity of the amine nitrogen increases due to the attached cyclohexyl ring. Hence, the compound is more basic.

Thus, cyclohexylamine is more basic than aniline. Hence, aromatic amines are weaker bases than aliphatic amines.

The given statement is True.

Interpretation Introduction

(c)

Interpretation:

To check whether the statement, “Aliphatic amines are stronger bases than inorganic bases such as NaOH and KOH” is true or false.

Concept Introduction:

Generally, the strength of a base is compared on the basis that how readily the base produces the hydroxide ion. This can be either by releasing the hydroxide ion which they readily have and may be because they take hydrogen ions from the water and thus they can produce hydroxide ion.

Explanation of Solution

Generally, the strength of a base is compared on the basis that how readily the base produces the hydroxide ion. This can be either by releasing the hydroxide ion which they readily have and may be because they take hydrogen ions from the water and thus they can produce hydroxide ion.

NaOH and KOH are the acids which readily give hydroxide ions when added to water. They dissociate completely. The reaction is not reversible and hence, they are strong acids.

Aliphatic amines with general formula $R-N{\text{H}}_2$, they do not dissociate completely in water. They do not have hydroxide ion within them. Hence, when react with water, they take the hydrogen ion from water and release hydroxide ions into the solution. The reaction is reversible; hence, the aliphatic amines are weaker bases.

Hence, the statement is False.

Interpretation Introduction

(d)

Interpretation:

To check whether the statement, “Water insoluble amines react with strong aqueous acids such as HCl to form water soluble salts” is true or false.

Concept Introduction:

Water has a property to ionize salts. This property helps in dissolution of salts in water.

Explanation of Solution

Water is an universal solvent. Its polarity and many other aspects make it an unique solvent. It also has the property to ionize. This property makes salts to be easily dissolved in water. For this reason, amines which are usually insoluble in water are made soluble by converting into hydrochloride salts.

Interpretation Introduction

(e)

Interpretation:

To check whether the statement, “If the pH of an aqueous solution of a primary aliphatic amine is adjusted to pH 2.0 by the addition of concentrated HCl, the amine will be present in solution almost entirely as its conjugate acid” is true or false.

Concept Introduction:

The nature of the compound depends on its similarity of the chemical environment. The balance of a reaction is according to Le Chatlier’s principle.

Explanation of Solution

The general formula of amines is $R-N{\text{H}}_2$. This reacts with water to give the amine salt. The amine is basic in nature and the amine salt is the conjugate acid. The reaction is a reversible reaction. That means if factors such as chemical environment supports the reactant, the reaction will be to the left side. If the chemical environment supports the product, the reaction will be to the right side.

Let us consider its reaction with water.

The reactant amine is basic in nature, and the conjugate acid is the product. Given that pH is 2.0. That means, it is on the acidic side. As the acidic environment is present, the species that will present is the amine salt.

Hence, the statement is True.

Interpretation Introduction

(f)

Interpretation:

To check whether the statement, “If the pH of an aqueous solution of a primary aliphatic amine is adjusted to pH 10.0 by the addition of NaOH, the amine will be present in solution almost entirely as the free base” is true or false.

Concept Introduction:

The nature of the compound depends on its similarity of the chemical environment. The balance of a reaction is according to Le Chatlier’s principle.

Explanation of Solution

The general formula of amines is $R-N{\text{H}}_2$. This reacts with water to give the amine salt. The amine is basic in nature and the amine salt is the conjugate acid. The reaction is a reversible reaction. That means if factors such as chemical environment supports the reactant, the reaction will be to the left side. If the chemical environment supports the product, the reaction will be to the right side.

Let us consider its reaction with water.

The reactant amine is basic in nature, and the conjugate acid is the product. Given that pH is 10.0 by addition of NaOH. That means, the pH is on the basic side which is because of the added NaOH. As the basic environment is present, the species that will present is the basic amine.

Hence, the statement is True.

Interpretation Introduction

(g)

Interpretation:

To check whether the statement, “For a primary amine, the concentrations of salt and basic amine will be equal when the pH of the solution is equal to the pK_b of the amine” is true or false.

Concept Introduction:

The nature of the compound depends on its similarity of the chemical environment. The balance of a reaction is according to Le Chatlier’s principle.

Explanation of Solution

The general formula of amines is $R-N{\text{H}}_2$. This reacts with water to give the amine salt. The amine is basic in nature and the amine salt is the conjugate acid. The reaction is a reversible reaction. That means if factors such as chemical environment supports the reactant, the reaction will be to the left side. If the chemical environment supports the product, the reaction will be to the right side.

Let us consider its reaction with water.

The reactant amine is basic in nature, and the conjugate acid is the product. At certain pH such as pKb the system is at equilibrium. pKb is the equilibrium dissociation constant pH. That means, the pH is so supporting that the forward and reverse reactions are at equal rate. That means, there will be forward and reverse reactions but that is not noticeable because they are at equal rate.

Hence, at this pH, the concentrations of salt and the basic salt will be equal. Hence, the statement is True.