Aldehydes, Ketones & Carboxylic Acids

  • Concept Based Problems
  • Acidity order of carboxylic acids

Introduction

  • Aldehydes, ketones, and carboxylic acids are organic compounds containing the carbonyl group (C=O).
  • They have various applications in industry and daily life.
  • Understanding their properties and reactivity is crucial for mastering organic chemistry.

Aldehydes

  • Aldehydes have a carbonyl group attached to at least one hydrogen atom.
  • Examples: Formaldehyde (HCHO), Acetaldehyde (CH₃CHO)
  • Common reactions: oxidation, reduction, aldol condensation

Ketones

  • Ketones have a carbonyl group attached to two carbon atoms.
  • Examples: Acetone (CH₃COCH₃), Propanone (CH₃COCH₂CH₃)
  • Common reactions: nucleophilic addition, oxidation, reduction

Carboxylic Acids

  • Carboxylic acids have a carbonyl group attached to a hydroxyl group (-OH).
  • Examples: Formic acid (HCOOH), Acetic acid (CH₃COOH)
  • Common reactions: esterification, dehydration, decarboxylation

Concept of Acidity

  • Acidity is an important property of carboxylic acids.
  • It is determined by the stability of the corresponding carboxylate ion.
  • Factors affecting acidity: inductive effect, resonance, electronegativity

Acidity Order of Carboxylic Acids

  • Acidity order can be determined by comparing the stability of carboxylate ions.
  • Electron-withdrawing groups increase acidity.
  • Electron-donating groups decrease acidity.
  • Examples: CH₃COOH < ClCH₂COOH < BrCH₂COOH

Concept Based Problems

  1. Identify whether the following compound is an aldehyde, ketone, or carboxylic acid: CH₃CHO₂CH₃.
  1. Write the reaction mechanism for the oxidation of propanol to propanoic acid.
  1. Compare the acidity of benzoic acid and p-nitrobenzoic acid.

Example: Identification

  • CH₃CHO₂CH₃ has the structure CH₃-CO-O-CH₃.
  • It contains a carbonyl group attached to an oxygen atom, indicating that it is an ester.

Example: Oxidation Mechanism

  1. Propanol is oxidized to propanal using an oxidizing agent (e.g., chromium trioxide).
  1. Propanal is further oxidized to propanoic acid using the same or a different oxidizing agent.

Aldehydes

  • Aldehydes have a carbonyl group (-C=O) attached to at least one hydrogen atom.
  • They are often named by replacing the -e ending of the corresponding alkane with -al.
  • Examples: methanal (formaldehyde), ethanal (acetaldehyde)
  • Aldehydes are commonly used as reducing agents in various organic reactions.

Ketones

  • Ketones have a carbonyl group (-C=O) attached to two carbon atoms.
  • They are often named by replacing the -e ending of the corresponding alkane with -one.
  • Examples: propanone (acetone), butanone (methylethyl ketone)
  • Ketones have higher boiling points compared to aldehydes due to the absence of a hydrogen atom involved in intermolecular hydrogen bonding.

Carboxylic Acids

  • Carboxylic acids have a carbonyl group (-C=O) attached to a hydroxyl group (-OH).
  • They are often named by replacing the -e ending of the corresponding alkane with -oic acid.
  • Examples: methanoic acid (formic acid), ethanoic acid (acetic acid)
  • Carboxylic acids are weak acids and can be neutralized by bases to form salts.

Oxidation of Aldehydes and Ketones

  • Aldehydes can be oxidized to carboxylic acids by strong oxidizing agents such as potassium permanganate (KMnO₄) or chromic acid (H₂CrO₄).
  • Ketones, on the other hand, do not undergo oxidation under normal conditions.
  • This difference in reactivity is due to the presence of a hydrogen atom in aldehydes that can be easily oxidized.

Reduction of Aldehydes and Ketones

  • Aldehydes and ketones can be reduced to alcohols by using reducing agents such as lithium aluminum hydride (LiAlH₄) or sodium borohydride (NaBH₄).
  • In the reduction process, the carbonyl group (-C=O) is converted to a hydroxyl group (-C-OH).

Nucleophilic Addition Reactions of Aldehydes and Ketones

  • Aldehydes and ketones are reactive towards nucleophiles due to the presence of a polarized carbonyl group.
  • Nucleophiles attack the carbonyl carbon and form a new bond.
  • Examples of nucleophilic addition reactions: Grignard reaction, cyanohydrin formation, imine formation

Aldol Condensation

  • The aldol condensation is a reaction between two molecules of aldehyde or ketone, resulting in the formation of a β-hydroxyaldehyde or β-hydroxyketone.
  • The reaction involves the formation of an enolate ion as an intermediate.
  • Examples: the synthesis of dibenzalacetone, crossed aldol condensation

Esterification

  • Esterification is the reaction between a carboxylic acid and an alcohol, leading to the formation of an ester.
  • It is an equilibrium reaction and can be catalyzed by an acid or a base.
  • The reaction involves an acyl substitution mechanism.

Decarboxylation

  • Decarboxylation is the loss of a carbon dioxide molecule from a carboxylic acid.
  • It often occurs under high temperature or in the presence of a catalyst.
  • The reaction is commonly seen in the preparation of aromatic compounds such as benzene carboxylic acids.

Concept Based Problems

  1. Identify the functional group present in the following compound: CH₃CHO.
  1. Write the balanced chemical equation for the oxidation of propanal to propanoic acid.
  1. Predict the product(s) formed when acetic acid reacts with methanol.
  1. Explain the difference in reactivity between aldehydes and ketones towards oxidation.

Concept Based Problems

  • Determine the IUPAC name for the following compound: CH₃CH₂CHO.
  • Write the reaction mechanism for the reduction of propanone to propan-2-ol.
  • Compare the reactivity of aldehydes and ketones towards nucleophilic addition reactions.

Example: IUPAC Name

  • The compound CH₃CH₂CHO is an aldehyde.
  • To name it, we replace the -e ending of the corresponding alkane (ethane) with -al.
  • Therefore, the IUPAC name for this compound is ethanal.

Example: Reduction Mechanism

  • Propanone can be reduced to propan-2-ol using a reducing agent such as lithium aluminum hydride (LiAlH₄).
  • The carbonyl group (-C=O) is reduced to a hydroxyl group (-C-OH) in the presence of the reducing agent.

Reactivity of Aldehydes and Ketones

  • Aldehydes are more reactive than ketones towards nucleophilic addition reactions.
  • This is due to the presence of a hydrogen atom attached to the carbonyl carbon in aldehydes, which increases the electrophilicity of the carbon.
  • Ketones, lacking this hydrogen atom, are less reactive and require stronger nucleophiles or reaction conditions to undergo nucleophilic addition.

Grignard Reaction

  • The Grignard reaction involves the addition of an organomagnesium (Grignard) reagent to the carbonyl group of an aldehyde or ketone.
  • The reaction produces a new carbon-carbon bond.
  • Example: The reaction between propanal and a Grignard reagent (RMgX) forms a tertiary alcohol.

Cyanohydrin Formation

  • Cyanohydrin formation is the addition of hydrogen cyanide (HCN) to the carbonyl group of an aldehyde or ketone.
  • The reaction produces a cyanohydrin, which contains a hydroxyl group (-OH) and a cyano group (-CN) attached to the same carbon.
  • Example: The reaction between propanone and hydrogen cyanide forms a cyanohydrin.

Imines Formation

  • Imines are formed by the reaction of an aldehyde or ketone with a primary or secondary amine.
  • The reaction involves the removal of a water molecule to form a double bond between the carbon and the nitrogen.
  • Example: The reaction between benzaldehyde and methylamine produces an imine.

Acidic Nature of Carboxylic Acids

  • Carboxylic acids are weak acids, with the ability to donate a proton (H+) to a base.
  • The acidity of carboxylic acids is higher than that of alcohols due to the presence of two electron-withdrawing groups (-C=O and -OH).
  • Deprotonation of a carboxylic acid forms a carboxylate ion.

Esterification Mechanism

  • Esterification is the reaction between a carboxylic acid and an alcohol to form an ester.
  • The reaction involves the loss of a water molecule.
  • It can be catalyzed by an acid or a base, with an acid-catalyzed mechanism being most common.

Decarboxylation Mechanism

  • Decarboxylation is the removal of a carboxyl group (-COOH) from a carboxylic acid, resulting in the formation of carbon dioxide (CO₂) and an organic compound.
  • The reaction is often carried out under high temperature or in the presence of a catalyst, such as sodium or heat.
  • Examples: The decarboxylation of benzoic acid forms benzene.