Carbonyls: Aldehydes and Ketones

Question 1

Define carbonyl compounds.

Answer 1

Carbonyl compounds are organic compounds that contain a carbon-oxygen double bond (C=O). They can be classified as either aldehydes or ketones.

Question 2

Describe the difference between aldehydes and ketones based on the position of the C=O bond.

Answer 2

Aldehydes have the C=O bond at the end of the carbon chain, while ketones have the C=O bond in the middle of the carbon chain.

Question 3

How do you name an aldehyde?

Answer 3

An aldehyde is named by taking the name of the longest carbon chain and adding the suffix ‘-al’.

Question 4

How do you name a ketone?

Answer 4

A ketone is named by taking the name of the longest carbon chain and adding the suffix ‘-one’.

Question 5

Explain the solubility of smaller carbonyls in water.

Answer 5

Smaller carbonyls are soluble in water because they can form hydrogen bonds with water molecules.

Question 6

What type of intermolecular forces are present in pure carbonyl compounds?

Answer 6

Pure carbonyl compounds cannot hydrogen bond to themselves; they are instead attracted by permanent dipole forces.

Question 7

Compare the reactivity of the C=O bond to the C=C bond in alkenes.

Answer 7

The C=O bond is stronger than the C=C bond and does not undergo addition reactions easily.

Question 8

What is the significance of the electronegativity of oxygen in carbonyl compounds?

Answer 8

Oxygen’s higher electronegativity polarizes the C=O bond, making the carbon atom positively charged and attracting nucleophiles.

Question 9

Describe the oxidation process of aldehydes.

Answer 9

Aldehydes can be oxidized to carboxylic acids using oxidizing agents like potassium dichromate in the presence of dilute sulfuric acid.

Question 10

What happens to ketones during oxidation?

Answer 10

Ketones do not oxidize under normal conditions.

Question 11

Identify a common oxidizing agent used for oxidizing alcohols and aldehydes.

Answer 11

Potassium dichromate (K2Cr2O7) is a common oxidizing agent used for oxidizing alcohols and aldehydes.

Question 12

What is the observation when potassium dichromate is reduced during the oxidation of aldehydes?

Answer 12

The orange dichromate ion (Cr2O7^2-) reduces to the green Cr^3+ ion.

Question 13

How can aldehydes be tested for their presence?

Answer 13

Aldehydes can be tested using Fehling’s solution or Tollen’s reagent.

Question 14

What is the full equation for the oxidation of aldehydes using potassium dichromate?

Answer 14

3CH3CHO + Cr2O7^2- + 8H+ → 3CH3CO2H + 4H2O + 2Cr^3+.

Question 15

What is the role of heat under reflux in the oxidation of aldehydes?

Answer 15

Heat under reflux provides the necessary energy for the oxidation reaction to occur effectively.

Question 16

Describe the reduction of carbonyls using NaBH4.

Answer 16

NaBH4 (sodium tetrahydridoborate) reduces carbonyls to alcohols, converting aldehydes to primary alcohols and ketones to secondary alcohols.

Question 17

How do aldehydes and ketones differ in their reduction products?

Answer 17

Aldehydes are reduced to primary alcohols, while ketones are reduced to secondary alcohols.

Question 18

Define the role of nucleophilic hydride ions in the reduction of carbonyls.

Answer 18

Nucleophilic hydride ions (:H-) from reducing agents like NaBH4 are attracted to the positive carbon in the C=O bond, facilitating the reduction.

Question 19

What is the mechanism of nucleophilic addition in carbonyl reduction?

Answer 19

The mechanism involves nucleophilic hydride ions attacking the carbonyl carbon, followed by protonation from water or a weak acid.

Question 20

Explain the process of catalytic hydrogenation for carbonyls.

Answer 20

Catalytic hydrogenation reduces carbonyls using hydrogen gas and a nickel catalyst under high pressure.

Question 21

Provide an example equation for the reduction of propanone.

Answer 21

CH3COCH3 + H2 → CH3CH(OH)CH3.

Question 22

What is the observation when aldehydes react with Fehling’s solution?

Answer 22

Aldehydes cause blue Cu2+ ions in solution to change to a red precipitate of Cu2O.

Question 23

Describe the reaction of aldehydes with Tollens’ reagent.

Answer 23

Aldehydes are oxidized by Tollens’ reagent to form a carboxylic acid, resulting in the formation of a silver mirror inside the test tube.

Question 24

What is the active substance in Tollens’ reagent?

Answer 24

The active substance in Tollens’ reagent is the complex ion

Question 25

How do ketones respond to Fehling's solution?

Answer 25

Ketones do not react with Fehling's solution and show no change.

Question 26

What are the conditions required for the reduction of carbonyls using NaBH4?

Answer 26

The reduction using NaBH4 typically occurs at room temperature and pressure in aqueous ethanol.

Question 27

What happens to copper(II) ions in Fehling's solution during the oxidation of aldehydes?

Answer 27

Copper(II) ions are reduced to copper(I) oxide (Cu2O) during the oxidation of aldehydes.

Question 28

List the reagents used for the reduction of carbonyls.

Answer 28

The reagents include NaBH4, LiAlH4 for reduction, and Tollens' reagent or Fehling's solution for oxidation.

Question 29

What is the significance of the silver mirror test?

Answer 29

The silver mirror test indicates the presence of aldehydes, as they reduce Tollens' reagent to form metallic silver.

Question 30

Describe the reaction that occurs when hydrogen cyanide is added to carbonyls.

Answer 30

The reaction involves the addition of hydrogen cyanide (HCN) to carbonyl compounds, resulting in the formation of hydroxynitriles.

Question 31

Define the reagents used in the addition of hydrogen cyanide to carbonyls.

Answer 31

The reagents used are potassium cyanide (KCN) and dilute sulfuric acid (H2SO4).

Question 32

How does the nucleophilic addition mechanism work in the formation of hydroxynitriles?

Answer 32

The nucleophilic addition mechanism involves the CN- ions from KCN attacking the carbonyl carbon, followed by the addition of H+ ions from sulfuric acid.

Question 33

What are the conditions required for the reaction of hydrogen cyanide with carbonyls?

Answer 33

The reaction occurs at room temperature and pressure.

Question 34

Explain the significance of using KCN or NaCN over HCN in this reaction.

Answer 34

KCN or NaCN provide a higher concentration of CN- ions as they completely ionize, while HCN is a weak acid and only partially ionizes.

Question 35

What happens to the optical activity of the product formed from the nucleophilic addition of HCN to unsymmetrical carbonyls?

Answer 35

The reaction results in the formation of a racemate, leading to no optical activity due to the equal chance of forming both enantiomers.

Question 36

How is the naming of hydroxy nitriles determined?

Answer 36

In naming hydroxy nitriles, the CN group becomes part of the main carbon chain.

Question 37

What is the general reaction formula for the addition of HCN to a ketone?

Answer 37

The general reaction formula is: CH3COCH3 + HCN → CH3C(OH)(CN)CH3.

Question 38

What is the role of sulfuric acid in the nucleophilic addition mechanism?

Answer 38

Sulfuric acid supplies the H+ ions needed in the second step of the nucleophilic addition mechanism.

Question 39

Illustrate the outcome of the nucleophilic addition of HCN to aldehydes and ketones.

Answer 39

The outcome is the formation of a racemic mixture due to the planar nature of the carbonyl group being attacked from both sides.