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Aldehydes Ketones And Carboxylic Acids

Aldehydes Ketones And Carboxylic Acids

Aldehydes, Ketones and Carboxylic Acids- the functional groups are commonly termed carbonyl groups. They are characterised by the presence of a double bond between carbon and oxygen atoms. Aldehydes and ketones are the simplest carbonyl groups which oxidise to form carboxylic acids. 

Table of Contents 

  • Definition: Aldehydes and Ketones
  • Definition: Carboxylic Acids 
  • Properties of Aldehydes, Ketones and Carboxylic Acids
  • Practice Questions 
  • Frequently Asked Questions 

Definition: Aldehydes and Ketones

The carbonyl group >C=O has two unfilled valencies. Attach one hydrogen atom and one alkyl group to form the aldehyde. 

Replace the hydrogen atom with another or the same alkyl group to get ketone. Hence, aldehydes are the functional groups with the mandatory presence of hydrogen atoms in the carbonyl group, while ketones have two alkyl groups attached to the primary carbon. 

In the case of the same carbonyl groups, the ketone is said to be symmetrical, while in the case of different groups, the ketone will be tagged unsymmetrical. 

Definition: Carboxylic Acids 

The two valencies of carbon in the carbonyl group can be filled by alkyl and hydroxyl groups. The presence of these groups leads to the formation of carboxylic acids. A compound can have a single or double carboxylic group or multiple of them. They are known as monocarboxylic and dicarboxylic acids. The compounds with long aliphatic chains and monocarboxylic acids are termed fatty acids. 

Properties of Aldehydes, Ketones and Carboxylic Acids

Considering the properties of separately:

Aldehydes and Ketones: Physical Properties

  • The nature of aldehydes and ketones going up the carbon number follows physical characteristics as gas, liquid and solid. 
  • The boiling points are higher than ethers and hydrocarbons of the same molecular mass, aldehydes and ketones, but lower than alcohols due to lack of intermolecular hydrogen bonding 
  • The solubility in water is also dependent on the same property of hydrogen bonding. Hence, the lower aldehydes and ketones are soluble, while the lengthy alkyl chain molecules exhibit low solubility. 
  • Regardless of the carbon number, all aldehydes and ketones show solubility in organic solvents
  • The sharp pungent odours are observed in lower carbon number molecules while the increase in size is accompanied by loss of pungent smell and the presence of a fragrant smell 

Aldehydes and Ketones: Chemical Properties

  • They exhibit nucleophilic addition reactions due to the presence of a partial positive charge on Carbon. It is formed as an electronegative atom Oxygen attracts electrons towards itself, thus carrying a partial negative charge. 
  • Aldehydes have hydrogen, while ketones have both alkyl groups. This leads to steric hindrance making aldehydes more reactive than ketones. 
  • They are reduced to alcohols and, subsequently, hydrocarbons.
  • The oxidation leads to carboxylic acid formation due to the addition of oxygen atoms. However, ketones show the reaction under vigorous conditions, resulting in shorter carboxylic acids than their parent ketones due to cleavage. 
  • The alpha hydrogen of aldehydes and ketones exhibits acidic nature. 
  • Some other special reactions include the Cannizzaro reaction and Electrophilic substitution reaction (seen in aromatic aldehydes)

Carboxylic Acids: Physical Properties

  • The lower aliphatic carboxylic acids, from one to nine carbon-containing acids, are characterised by colourless liquid and unpleasant smell at room temperature. 
  • With increasing carbon, the solidification increases and odour decreases due to a decrease in volatility. 
  • The presence of more hydrogen bonding increases their boiling points to higher temperatures compared to their aldehyde, ketone and alcoholic counterparts. 
  • The bonds are so strong that vaporised form or aprotic solvents comprise them as dimers. 
  • The high solubility in water is seen in up to four carbon atoms, while the lower extent is seen with the increase in size due to heavy aliphatic groups leading to hydrophobic interactions. 
  • The aromatic compounds are insoluble in cold water. 

Carboxylic Acids: Chemical Properties

  • They exhibit acidic nature by breaking hydroxyl bonds that lead to the release of hydrogen gas in reaction with metals and alkalies. 
  • The cleavage reactions are seen in carboxylic bonds where cleaves bonds are: O-H and C-OH. 
  • They also undergo substitution reactions where hydrocarbons are present. 
  • The characteristic reaction for carboxylic acid is decarboxylation, characterised by the release of carbon dioxide. 

Practice Questions 

Q1. Which will show the highest reactivity in nucleophilic addition reaction? 

A. Ethanal
B. Butanone
C. Propanone
D. Propanal

Answer. A. Ethanal

Owing to its smallest size among other options. 

Q2. Which factor is responsible for the solubility of aldehyde or ketone in water? 

A. Ionic bond
B. Hydrogen bond
C. Covalent bond
D. Van der Waals force

Answer. B. Hydrogen bond

The Hydrogen bond between oxygen and hydrogen atoms of aldehyde or ketone and water is responsible for solubility. 

Q3. Hemiacetal is formed by which reaction between which compounds? 

A. Alcohol and Alkane
B. Alcohol and Aldehyde
C. Acid and Acid 
D. Alcohol and ketone

Answer. B. Alcohol and Aldehyde

Alcohol and aldehyde react to form intermediate hemiacetal which further leads to acetal formation. 

Frequently Asked Questions 

Q1. What is the mechanism of the esterification of carboxylic acids?
Answer: The carboxylic acids undergo a nucleophilic acyl substitution reaction, which begins with the protonation of oxygen. The partially positive charged carbon is attacked with nucleophiles leading to a tetrahedral intermediate. The proton transfer and rearrangement lead to ester formation. 

Q2. What are the substrates for the preparation of ketones?
Ketones are prepared from acyl chlorides, nitriles, benzene, or its substituted compounds. 

Q3. What is Gatterman-Koch's reaction?
The Gatterman-Koch reaction converts benzene into benzaldehyde in the presence of carbon monoxide, hydrochloric acid and anhydrous aluminium chloride or cuprous chloride.

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