Chapter 13, Problem 13.25P

13-25 Answer true or false.

(a) Phenols and alcohols have in common the presence of an —OH group.

Phenols are weak acids and react with strong bases to give water-soluble salts.

1. The pK„ of phenol is smaller than that of acetic acid.

Autoxidation converts an R—H group to an R—OH group.

A carbon radical has only seven electrons in the valence shell of one of its carbons, and this carbon bears a positive charge.

(f, A characteristic of a chain initiation step is conversion of a nonradical to a radical.

Autoxidation is a radical-chain reaction.

A characteristic of the chain propagation step is reaction of a radical and a molecule to form a new radical and a new molecule.

Vitamin E and other natural antioxidants function by interrupting the cycle of chain propagation steps that occurs in autoxidation.

Interpretation Introduction

(a)

Interpretation:

To analyse whether the given statement: Phenols and alcohols have in common the presence of an −OH group,is true or not.

Concept Introduction:

Phenols are the compounds which have a hydroxyl group (—OH) bonded directly to a benzene ring. Phenol is a weak acid and pK_a value for phenol is 10. Phenols are oxidized to quinone which is in turn easily reduced to hydroquinone.

Phenols act as antioxidants. Autoxidation is an oxidation reaction that requires only oxygen and no other reactants. It takes place in molecules containing carbon-carbon double bonds. Auto oxidation is a radical chain process. It takes place in different steps. In the first step, a non-radical compound changes to a radical form. For example, an oxygen atom is a di-radical. A radical reacts with oxygen to form a new radical. This new radical reacts with another molecule to form a new auto-oxidation product and a carbon radical compound.

Answer to Problem 13.25P

Phenols and alcohols have in common the presence of an −OH group. Thus, the statement is true.

Explanation of Solution

The alcohols are represented by a general formula: R-OH. In alcohols, the r is an alkyl group. The phenols are represented by a general formula: R-OH In phenol, the r is an aromatic benzene ring. An example of alcohol is methanol compared to phenol, as shown in the figure below:

Therefore, the given statement is true.

Interpretation Introduction

(b)

Interpretation:

To analyse whether the given statement: Phenols are weak acids and react with strong bases to give water-soluble salts, is true or not.

Concept Introduction:

Phenols are the compounds which have a hydroxyl group (—OH) bonded directly to a benzene ring. Phenol is a weak acid and pK_a value for phenol is 10. Phenols are oxidized to quinone which is in turn easily reduced to hydroquinone.

Phenols act as antioxidants. Autoxidation is an oxidation reaction that requires only oxygen and no other reactants. It takes place in molecules containing carbon-carbon double bonds. Auto oxidation is a radical chain process. It takes place in different steps. In the first step, a non-radical compound changes to a radical form. For example, an oxygen atom is a di-radical. A radical reacts with oxygen to form a new radical. This new radical reacts with another molecule to form a new auto-oxidation product and a carbon radical compound.

Answer to Problem 13.25P

Phenols are weak acids and react with strong bases to give water-soluble salts. Thus, the statement is true.

Explanation of Solution

Phenol is a weak acid. When it reacts with a strong base like sodium hydroxide NaOH, it forms water-soluble salt, or sodium phenoxide. Therefore, the given statement is true.

Interpretation Introduction

(c)

Interpretation:

To analyse whether the given statement: The ${\text{pK}}_{\text{a}}$ of phenol is smaller than that of acetic acid, is true or not.

Concept Introduction:

Phenols are the compounds which have a hydroxyl group (—OH) bonded directly to a benzene ring. Phenol is a weak acid and pK_a value for phenol is 10. Phenols are oxidized to quinone which is in turn easily reduced to hydroquinone.

Phenols act as antioxidants. Autoxidation is an oxidation reaction that requires only oxygen and no other reactants. It takes place in molecules containing carbon-carbon double bonds. Auto oxidation is a radical chain process. It takes place in different steps. In the first step, a non-radical compound changes to a radical form. For example, an oxygen atom is a di-radical. A radical reacts with oxygen to form a new radical. This new radical reacts with another molecule to form a new auto-oxidation product and a carbon radical compound.

Answer to Problem 13.25P

The $p{K}_{\text{a}}$ of phenol is much greater than that of acetic acid. Thus, the statement is true.

Explanation of Solution

The strength of an acid is measured by the acid dissociation constant K_a.

Consider this reaction.

The acid dissociation constant is given by the formula.

$$

$K_{\text{a}}\text{=}\frac{{\text{[H}}^{\text{+}}{\text{]+ [A}}^{\text{-}}\text{]}}{\text{[ΗΑ]}}$

The pK_a is the logarithmic measurement of the acid dissociation constant. We know that the pK_a value for phenol is 10 and for acetic acid is 4.7. Hence, the pK_a value of phenol is much greater than the pK_a value of acetic acid. Therefore, the given statement is false.

Interpretation Introduction

(d)

Interpretation:

To analyse whether the given statement: Autoxidation converts an R-H group to an R-OH group, is true or not.

Concept Introduction:

Phenols are the compounds which have a hydroxyl group (—OH) bonded directly to a benzene ring. Phenol is a weak acid and pK_a value for phenol is 10. Phenols are oxidized to quinone which is in turn easily reduced to hydroquinone.

Phenols act as antioxidants. Autoxidation is an oxidation reaction that requires only oxygen and no other reactants. It takes place in molecules containing carbon-carbon double bonds. Auto oxidation is a radical chain process. It takes place in different steps. In the first step, a non-radical compound changes to a radical form. For example, an oxygen atom is a di-radical. A radical reacts with oxygen to form a new radical. This new radical reacts with another molecule to form a new auto-oxidation product and a carbon radical compound.

Answer to Problem 13.25P

Autoxidation converts an R-H group to an R-OOH group.Thus, the statement is false.

Explanation of Solution

Autoxidation is an oxidation reaction that requires only oxygen and no other reactants. It takes place in molecules containing carbon-carbon double bonds. In auto-oxidation, the r — H group changes to an ROOH group and not to an ROH group. Therefore, the given statement is false.

Interpretation Introduction

(e)

Interpretation:

To analyze whether the given statement: A carbon radical has only seven electrons in the valence shell of one of its carbons, and this carbon bears a positive charge, is true or not.

Concept Introduction:

Phenols are the compounds which have a hydroxyl group (—OH) bonded directly to a benzene ring. Phenol is a weak acid and pK_a value for phenol is 10. Phenols are oxidized to quinone which is in turn easily reduced to hydroquinone.

Phenols act as antioxidants. Autoxidation is an oxidation reaction that requires only oxygen and no other reactants. It takes place in molecules containing carbon-carbon double bonds. Auto oxidation is a radical chain process. It takes place in different steps. In the first step, a non-radical compound changes to a radical form. For example, an oxygen atom is a di-radical. A radical reacts with oxygen to form a new radical. This new radical reacts with another molecule to form a new auto-oxidation product and a carbon radical compound.

Answer to Problem 13.25P

A carbon radical has only seven electrons in the valence shell of one of its carbons, and this carbon may bear a positive charge or negative charge of no charge.Thus, the statement is false.

Explanation of Solution

Radicals are atoms, molecules, or ions with unpaired electrons or untilled electrons in their valence shells. A radical may have a positive charge, a negative charge, or no charge. A carbon radical does not carry any charge on it. Therefore, the given statement is false.

Interpretation Introduction

(f)

Interpretation:

To analyze whether the given statement -A characteristic of a chain initiation step is conversion of a nonradical to a radical, is true or not.

Concept Introduction:

Phenols are the compounds which have a hydroxyl group (—OH) bonded directly to a benzene ring. Phenol is a weak acid and pK_a value for phenol is 10. Phenols are oxidized to quinone which is in turn easily reduced to hydroquinone.

Phenols act as antioxidants. Autoxidation is an oxidation reaction that requires only oxygen and no other reactants. It takes place in molecules containing carbon-carbon double bonds. Auto oxidation is a radical chain process. It takes place in different steps. In the first step, a non-radical compound changes to a radical form. For example, an oxygen atom is a di-radical. A radical reacts with oxygen to form a new radical. This new radical reacts with another molecule to form a new auto-oxidation product and a carbon radical compound.

Answer to Problem 13.25P

A characteristic of a chain initiation step is conversion of a nonradical to a radical. Thus, the statement is true.

Explanation of Solution

The characteristic of any chain initiation is the conversion of any non-radical to a radical. In a chain initiation process, hydrogen is converted to a hydrogen radical and carbon is converted to a carbon radical. For Example,

Therefore, the given statement is true.

Interpretation Introduction

(g)

Interpretation:

To analyze whether the given statement -Autoxidation is a radical-chain reaction, is true or not.

Concept Introduction:

Phenols are the compounds which have a hydroxyl group (—OH) bonded directly to a benzene ring. Phenol is a weak acid and pK_a value for phenol is 10. Phenols are oxidized to quinone which is in turn easily reduced to hydroquinone.

Phenols act as antioxidants. Autoxidation is an oxidation reaction that requires only oxygen and no other reactants. It takes place in molecules containing carbon-carbon double bonds. Auto oxidation is a radical chain process. It takes place in different steps. In the first step, a non-radical compound changes to a radical form. For example, an oxygen atom is a di-radical. A radical reacts with oxygen to form a new radical. This new radical reacts with another molecule to form a new auto-oxidation product and a carbon radical compound.

Answer to Problem 13.25P

Autoxidation is a radical-chain reaction. Thus, the statement is true.

Explanation of Solution

Auto oxidation is a radical chain process. It takes place in different steps. In the first step, a non-radical compound changes to a radical form. For example, an oxygen atom is a diradical. A radical reacts with oxygen to form a new radical. This new radical reacts with another molecule to form a new auto-oxidation product and a carbon radical compound. Therefore, the given statement is true.

Interpretation Introduction

(h)

Interpretation:

To analyze whether the given statement -A characteristic of the chain propagation step is reaction of a radical and a molecule to form a new radical and a new molecule, is true or not.

Concept Introduction:

Phenols are the compounds which have a hydroxyl group (—OH) bonded directly to a benzene ring. Phenol is a weak acid and pK_a value for phenol is 10. Phenols are oxidized to quinone which is in turn easily reduced to hydroquinone.

Phenols act as antioxidants. Autoxidation is an oxidation reaction that requires only oxygen and no other reactants. It takes place in molecules containing carbon-carbon double bonds. Auto oxidation is a radical chain process. It takes place in different steps. In the first step, a non-radical compound changes to a radical form. For example, an oxygen atom is a di-radical. A radical reacts with oxygen to form a new radical. This new radical reacts with another molecule to form a new auto-oxidation product and a carbon radical compound.

Answer to Problem 13.25P

A characteristic of the chain propagation step is reaction of a radical and a molecule to form a new radical and a new molecule. Thus, the statement is true.

Explanation of Solution

A chain propagation step takes place in different steps. Autoxidation is an example of a chain propagation reaction. In the first step, a nonradical compound is changed to a radical form. Oxygen is a di-radical. A radical reacts with oxygen to form a new radical, which reacts with another radical and forms a carbon radical and a chain radical. Therefore, the given statement is true.

Interpretation Introduction

(i)

Interpretation:

To analyze whether the given statement -Vitamin E and other natural antioxidants function by interrupting the cycle of chain propagation steps that occurs in autoxidation, is true or not.

Concept Introduction:

Phenols are the compounds which have a hydroxyl group (—OH) bonded directly to a benzene ring. Phenol is a weak acid and pK_a value for phenol is 10. Phenols are oxidized to quinone which is in turn easily reduced to hydroquinone.

Autoxidation is an oxidation reaction that requires only oxygen and no other reactants whereas the substance which inhibits oxidation is known as antioxidant.

Answer to Problem 13.25P

Vitamin E and other natural antioxidants function by interrupting the cycle of chain propagation steps that occurs in autoxidation. Thus the statement is true.

Explanation of Solution

Vitamin E is a stable radical. It breaks the chain propagation steps in auto-oxidation and prevents the formation of destructive hydro peroxides. Therefore, the given statement is true.