Chapter 16, Problem 22P

17-30 Answer true or false.

(a) The reduction of an aldehyde always gives a primary alcohol.

(b) The reduction of a ketone always gives a secondary alcohol.

(c) The oxidation of an aldehyde gives a carboxylic acid.

(d) The oxidation of a primary alcohol gives a ketone.

(e) Tollens’ reagent can be used to distinguish between an aldehyde and a ketone.

(f) Sodium borohydride, NaBH₄, reduces an aldehyde to a primary alcohol.

(g) The addition of one molecule of alcohol to the carbonyl group of a ketone gives a hemiacetal.

(h) The reaction of an aldehyde with two molecules of alcohol gives an acetal, plus a molecule

of water.

(j) The formation of hemiacetals and acetals is reversible.

(j) The cyclic hemiacetal formed from 4-hydroxy-pentanal has two stereocenters and can exist as

a mixture of 2² = 4 stereoisomers.

Interpretation Introduction

(a)

Interpretation:

Answer true or false for the following statement.

The reduction of an aldehyde always gives an alcohol.

Concept Introduction:

Reduction is the addition of a hydrogen atom to a compound.

Answer to Problem 22P

The given statement is true.

Explanation of Solution

In aldehydes, the functional group is $\text{- CHO}$. Hydrogen is added to the double bond of the carbonyl carbon of the aldehydes and forms a $\text{- HO}$ group are formed at the terminal carbon chain.

Oxidation of aldehydes forms primary alcohol. For example, reduction of pentanal gives 1-pentanol.

Therefore, the given statement is true.

Interpretation Introduction

(b)

Interpretation:

Answer true or false for the following statement.

The reduction of ketones always gives a secondary alcohol.

Concept Introduction:

Aldehydes and ketones contain carbonyl group as a functional group. Reduction is the addition of a hydrogen atom to a compound.

Answer to Problem 22P

Therefore, the given statement is true.

Explanation of Solution

Reduction of ketones, hydrogen is added to the carbonyl carbon and gives secondary alcohol.

Therefore, the given statement is true.

Interpretation Introduction

(c)

Interpretation:

Answer true or false for the following statement.

The oxidation of aldehydes gives a carboxylic acid.

Concept Introduction:

Aldehydes and ketones contain carbonyl group as a functional group. Oxidation is the process in which oxygen is added to the molecule.

Answer to Problem 22P

The given statement is true.

Explanation of Solution

Oxidation of aldehydes, oxygen is added to the carbonyl group and form carboxylic acid.

Therefore, the given statement is true.

Interpretation Introduction

(d)

Interpretation:

Answer true or false for the following statement.

The oxidation of primary alcohol gives ketone.

Concept Introduction:

Aldehydes and ketones contain carbonyl group as a functional group. Oxidation is the process in which oxygen is added to the molecule.

Answer to Problem 22P

The given statement is false.

Explanation of Solution

Oxidation of primary alcohol first converted into aldehyde and followed by carboxylic acids.

Therefore, the given statement is false.

Interpretation Introduction

(e)

Interpretation:

Answer true or false for the following statement.

Tollen’s reagent can be used to distinguish between an aldehyde and a ketone.

Concept Introduction:

Tollen’s reagent is a mild oxidizing agent.

Tollen’s reagent is reacts with the aldehyde group of a compound. The carbonyl group acts as a reducing agent and reduces the silver ion of the Tollen’s regent to silver metal.

Answer to Problem 22P

The given statement is true.

Explanation of Solution

Aldehydes undergo oxidation easily, even with mild oxidizing agents and give carboxylic acid.

Ketone does not undergo oxidation with mild oxidizing agent due to selectivity of aldehyde and ketone for oxidation is used to differentiate an aldehyde and a ketone. Therefore, aldehyde and ketones are distinguished by using Tollen’s regent.

Therefore, the given statement is true.

Interpretation Introduction

(f)

Interpretation:

Answer true or false for the following statement.

Sodium borohydride, ${\text{NaBH}}_{\text{4}}$ reduces an aldehyde to a primary alcohol.

Concept Introduction:

Aldehydes and ketones contain carbonyl group as a functional group. The aldehydes undergoes a reduction reaction where in the addition of hydrogen occurs at the double bond between the oxygen and the carbon of the carbonyl group present in the aldehyde.

Answer to Problem 22P

Therefore, the given statement is true.

Explanation of Solution

${\text{NaBH}}_{\text{4}}$ behaves as a source of the hydride ion, ${\text{H}}^{\text{-}}$ during the reaction the hydride ion is added to the carbonyl carbon and forms an alkoxide ion as an intermediate followed by alcohol.

Therefore, the given statement is true.

Interpretation Introduction

(g)

Interpretation:

Answer true or false for the following statement.

The addition of one molecule of alcohol to the carbonyl group of a ketone gives a hemiacetal.

Concept Introduction:

Aldehydes and ketones contain carbonyl group as a functional group. In Hemiacetal, carbon is added to hydroxyl group and alkoxy group.

Answer to Problem 22P

The given statement is true.

Explanation of Solution

When a molecule of alcohol is added to a ketone, the hydrogen of the alcohol is added o the oxygen atom of the carbonyl group, while the alkoxy group of the alcohol is added to the carbon of the carbonyl group. Hence ketone forms hemiacetal when the reacts with one molecule of alcohol.

Therefore, the given statement is true.

Interpretation Introduction

(h)

Interpretation:

Answer true or false for the following statement.

The reaction of an aldehyde with two molecules of alcohol gives an acetal, plus a molecule of water.

Concept Introduction:

Aldehydes and ketones contain carbonyl group as a functional group. In acetal, carbon is bonded to two alkoxy groups.

Answer to Problem 22P

The given statement is true.

Explanation of Solution

Two molecules of an alcohol are added to an aldehyde to form hemiacetal. And it is unstable and it reacts with another molecule of an alcohol in the presence of an acid forms acetal and water.

Therefore, the given statement is true.

Interpretation Introduction

(i)

Interpretation:

Answer true or false for the following statement.

The formation of hemiacetals and acetal is reversible.

Concept Introduction:

Aldehydes and ketones contain carbonyl group as a functional group. In Hemiacetal, carbon is added to hydroxyl group and alkoxy group. In acetal, carbon is added to a two alkoxy groups.

Answer to Problem 22P

The given statement is true.

Explanation of Solution

By using large excess of alcohol hemiacetals and acetals are reversible by the removal of water molecules. To shift the equilibrium towards the left side by the hydrolysis of the acetal to the aldehyde or ketone and alcohol.

Therefore, the given statement is true.

Interpretation Introduction

(j)

Interpretation:

Answer true or false for the following statement.

The cyclic hemiacetal formed from 4-hydroxypentanal has two stereocentres and can exists as a mixture of ${\text{2}}^{\text{2}}\text{= 4}$ stereoisomers.

Concept Introduction:

A stereocentre is a carbon atom with four different groups bonded to it. The stereocentres of molecule can be calculated by using ${\text{2}}^n$ formula.

n= stereocentre.

Answer to Problem 22P

The given statement is true.

Explanation of Solution

The cyclic hemiacetal; obtained from 4-hydroxypentanal has two stereocenters.

4- hydroxypentanal has two stereocenters, therefore it have 4 stereoisomers.

Therefore, the given statement is true.