Aldehydes, Ketones & Carboxylic Acids

1. Among the following acids which has the lowest pK_a value?

CH3COOH

HCOOH

(CH3)2CHCOOH

CH3CH2COOH

HCOOH, also known as formic acid, has the lowest pKa value among the given acids. pKa is a measure of the acidity of a compound, with lower pKa values indicating stronger acids. Formic acid has a pKa value of around 3.75, while the other acids listed have higher pKa values. Therefore, HCOOH is the correct answer.

Explanation

HCOOH, also known as formic acid, has the lowest pKa value among the given acids. pKa is a measure of the acidity of a compound, with lower pKa values indicating stronger acids. Formic acid has a pKa value of around 3.75, while the other acids listed have higher pKa values. Therefore, HCOOH is the correct answer.

2. During reduction of aldehydes with hydrazine and potassium hydroxide, the first is formation of:

R-C≡N

R-CO-NH2

R-CH=NH

R-CH=N-NH2

The reduction of aldehydes with hydrazine and potassium hydroxide leads to the formation of R-CH=N-NH2. This is because hydrazine (N2H4) acts as a reducing agent and reacts with the aldehyde functional group (R-CHO) to form an imine (R-CH=NH). The imine then undergoes further reduction with hydrazine to form the desired product, R-CH=N-NH2, which is a hydrazone.

Explanation

The reduction of aldehydes with hydrazine and potassium hydroxide leads to the formation of R-CH=N-NH2. This is because hydrazine (N2H4) acts as a reducing agent and reacts with the aldehyde functional group (R-CHO) to form an imine (R-CH=NH). The imine then undergoes further reduction with hydrazine to form the desired product, R-CH=N-NH2, which is a hydrazone.

3. Which of the following reagents may be used to distinguish between phenol and benzoic acid?

Tollens’ reagent

Molisch reagent

Neutral FeCl3

Aqueous NaOH

Neutral FeCl3 can be used to distinguish between phenol and benzoic acid because it reacts differently with each compound. When phenol is added to neutral FeCl3, a violet color is formed due to the formation of a complex between phenol and FeCl3. On the other hand, when benzoic acid is added to neutral FeCl3, no color change occurs. Therefore, the presence of a violet color indicates the presence of phenol, while the absence of a color change indicates the presence of benzoic acid.

Explanation

Neutral FeCl3 can be used to distinguish between phenol and benzoic acid because it reacts differently with each compound. When phenol is added to neutral FeCl3, a violet color is formed due to the formation of a complex between phenol and FeCl3. On the other hand, when benzoic acid is added to neutral FeCl3, no color change occurs. Therefore, the presence of a violet color indicates the presence of phenol, while the absence of a color change indicates the presence of benzoic acid.

4. Which of the following acids on heating loses a molecule of H₂O to from and α, β-unsaturated acid?

CH3CHOHCOOH

HOCH2COOH

CH3CHOHCH2COOH

HOCH2CH2CH2COOH

When CH3CHOHCH2COOH is heated, it loses a molecule of H2O to form an α, β-unsaturated acid. This is because the molecule contains a hydroxyl group (OH) attached to a carbon atom adjacent to a carbonyl group (C=O). Upon heating, the hydroxyl group and a hydrogen atom from the adjacent carbon atom combine to form water (H2O), resulting in the formation of a double bond between the two carbon atoms. This double bond creates the α, β-unsaturated acid.

Explanation

When CH3CHOHCH2COOH is heated, it loses a molecule of H2O to form an α, β-unsaturated acid. This is because the molecule contains a hydroxyl group (OH) attached to a carbon atom adjacent to a carbonyl group (C=O). Upon heating, the hydroxyl group and a hydrogen atom from the adjacent carbon atom combine to form water (H2O), resulting in the formation of a double bond between the two carbon atoms. This double bond creates the α, β-unsaturated acid.

5. Which of the following compound will have the smallest pK_a value?

Benzoic acid

Formic acid

Acetic acid

Phenylacetic acid

Formic acid will have the smallest pKa value because it is the strongest acid among the given compounds. The presence of the electron-withdrawing group, the formyl group (-CHO), increases the acidity of formic acid compared to the other compounds. The formyl group can stabilize the negative charge on the conjugate base, making it easier to lose a proton and increasing the acidity of the compound. Therefore, formic acid will have the smallest pKa value.

Explanation

Formic acid will have the smallest pKa value because it is the strongest acid among the given compounds. The presence of the electron-withdrawing group, the formyl group (-CHO), increases the acidity of formic acid compared to the other compounds. The formyl group can stabilize the negative charge on the conjugate base, making it easier to lose a proton and increasing the acidity of the compound. Therefore, formic acid will have the smallest pKa value.

6. When CH₂ = CH–COOH is reduced with LiAlH₄, the compound obtained will be:

CH3–CH2–CHO

CH2–CH–CH2OH

CH3–CH2–COOH

When CH2 = CH–COOH is reduced with LiAlH4, the compound obtained will be CH2–CH–CH2OH. Reduction with LiAlH4 converts the carboxylic acid functional group (–COOH) into an alcohol functional group (–CH2OH), resulting in the formation of CH2–CH–CH2OH.

Explanation

When CH2 = CH–COOH is reduced with LiAlH4, the compound obtained will be CH2–CH–CH2OH. Reduction with LiAlH4 converts the carboxylic acid functional group (–COOH) into an alcohol functional group (–CH2OH), resulting in the formation of CH2–CH–CH2OH.

7. Identify the product Y in the following reaction sequence:

Pentane

Cyclobutane

Cyclopentane

Cyclopentanone

The given reaction sequence starts with pentane, which is a hydrocarbon with five carbon atoms. It then undergoes a series of reactions, resulting in the formation of cyclobutane, cyclopentane, and finally cyclopentanone. The product Y in this reaction sequence is cyclopentane, which is a cyclic hydrocarbon with five carbon atoms.

Explanation

The given reaction sequence starts with pentane, which is a hydrocarbon with five carbon atoms. It then undergoes a series of reactions, resulting in the formation of cyclobutane, cyclopentane, and finally cyclopentanone. The product Y in this reaction sequence is cyclopentane, which is a cyclic hydrocarbon with five carbon atoms.

8. Ozonolysis of an organic compound gives formaldehyde as one of the products. This confirms the presence of:

A vinyl group

An isopropyl group

An acetylene triple bond

Two ethylenic double bonds

Ozonolysis is a reaction that involves the cleavage of carbon-carbon double bonds. When ozonolysis of an organic compound gives formaldehyde as one of the products, it confirms the presence of a vinyl group. A vinyl group is a functional group consisting of a carbon-carbon double bond attached to an alkyl group. The formation of formaldehyde suggests that the organic compound contains a vinyl group, which is consistent with the given information.

Explanation

Ozonolysis is a reaction that involves the cleavage of carbon-carbon double bonds. When ozonolysis of an organic compound gives formaldehyde as one of the products, it confirms the presence of a vinyl group. A vinyl group is a functional group consisting of a carbon-carbon double bond attached to an alkyl group. The formation of formaldehyde suggests that the organic compound contains a vinyl group, which is consistent with the given information.

9. Which of the following combination of aldehydes gives cross Cannizzaro reaction?

CH3CHO, HCHO

C6C5CHO, CH3CHO

C6C5CHO, HCHO

All of these

The cross Cannizzaro reaction is a reaction where one aldehyde acts as the reducing agent and the other aldehyde acts as the oxidizing agent. In this reaction, the aldehyde that acts as the reducing agent is oxidized to a carboxylic acid, while the aldehyde that acts as the oxidizing agent is reduced to an alcohol. In the given options, only the combination of C6C5CHO and HCHO involves two different aldehydes, making it the correct combination for the cross Cannizzaro reaction.

Explanation

The cross Cannizzaro reaction is a reaction where one aldehyde acts as the reducing agent and the other aldehyde acts as the oxidizing agent. In this reaction, the aldehyde that acts as the reducing agent is oxidized to a carboxylic acid, while the aldehyde that acts as the oxidizing agent is reduced to an alcohol. In the given options, only the combination of C6C5CHO and HCHO involves two different aldehydes, making it the correct combination for the cross Cannizzaro reaction.

10. The dipole moment is the highest for

Trans-2-butene

1, 3-dimethylbenzene

Acetophenone

Ethanol

Acetophenone has the highest dipole moment because it has a polar carbonyl group (C=O) which creates a significant difference in electronegativity between the carbon and oxygen atoms. This leads to a separation of charge and the formation of a dipole moment. In contrast, trans-2-butene and ethanol do not have such polar groups, while 1,3-dimethylbenzene lacks any functional groups that would contribute to a dipole moment. Therefore, acetophenone has the highest dipole moment among the given options.

Explanation

Acetophenone has the highest dipole moment because it has a polar carbonyl group (C=O) which creates a significant difference in electronegativity between the carbon and oxygen atoms. This leads to a separation of charge and the formation of a dipole moment. In contrast, trans-2-butene and ethanol do not have such polar groups, while 1,3-dimethylbenzene lacks any functional groups that would contribute to a dipole moment. Therefore, acetophenone has the highest dipole moment among the given options.

11. Consider

The correct decreasing order of their reactivity towards hydrolysis is:

(ii) > (iv) > (i) > (iii)

(ii) > (iv) > (iii) > (i)

(i) > (ii) > (iii) > (iv)

(iv) > (ii) > (i) > (iii)

The correct order of reactivity towards hydrolysis is (ii) > (iv) > (i) > (iii). This means that compound (ii) is the most reactive towards hydrolysis, followed by compound (iv), then compound (i), and finally compound (iii).

Explanation

The correct order of reactivity towards hydrolysis is (ii) > (iv) > (i) > (iii). This means that compound (ii) is the most reactive towards hydrolysis, followed by compound (iv), then compound (i), and finally compound (iii).

12. If the enolate ion combines with the carbonyl group of an ester, we get:

Aldol

α, β-unsaturated ester

β-ketoaldehyde

Acid

When the enolate ion combines with the carbonyl group of an ester, the resulting product is a β-ketoaldehyde. This is because the enolate ion adds to the carbonyl carbon of the ester, creating a new carbon-carbon bond and forming a β-ketoaldehyde. This reaction is known as the Claisen condensation and is commonly used in organic synthesis to create β-ketoaldehydes.

Explanation

When the enolate ion combines with the carbonyl group of an ester, the resulting product is a β-ketoaldehyde. This is because the enolate ion adds to the carbonyl carbon of the ester, creating a new carbon-carbon bond and forming a β-ketoaldehyde. This reaction is known as the Claisen condensation and is commonly used in organic synthesis to create β-ketoaldehydes.

13. Which of the following on heating with aqueous KOH, produces acetaldehyde:

CH3CH2Cl

CH2Cl.CH2Cl

CH3CHCl2

CH3COCl

When CH3CHCl2 is heated with aqueous KOH, it undergoes a nucleophilic substitution reaction known as dehydrohalogenation. In this reaction, one chlorine atom is replaced by a hydroxyl group (-OH), resulting in the formation of acetaldehyde (CH3CHO).

Explanation

When CH3CHCl2 is heated with aqueous KOH, it undergoes a nucleophilic substitution reaction known as dehydrohalogenation. In this reaction, one chlorine atom is replaced by a hydroxyl group (-OH), resulting in the formation of acetaldehyde (CH3CHO).