Aldehydes, Ketones and Carboxylic Acids: Overview, Questions, Preparation

Aldehydes, Ketones and Carboxylic Acids 2021

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Updated on Apr 17, 2021 07:55 IST

A carbonyl functional group, C=O, is introduced into aldehydes and ketones. Both are organic compounds of -CHO and RC(=O)R’ structures, where R and R’ are, respectively, carbon-containing substituents.

Table of Contents
  1. What are Aldehydes?
  2. What are Ketones?
  3. Occurrence of Aldehydes and Ketones
  4. Uses of Ketones and Aldehydes
  5. Aldehydes and Ketones in Class 12
  6. Illustrated Examples
  7. FAQs on Aldehydes and Ketones

What are Aldehydes?

In aldehydes, one hydrogen atom is bound to the carbonyl group and either a second hydrogen atom or a hydrogen group that may be an alkyl group or one that comprises a benzene ring.

What are Ketones?

The carbonyl group of ketones has two groups of hydrocarbons attached to it. These may be either benzene rings or alkyl groups found in them. Ketone does not have a carbonyl group attached to the hydrogen atom.

The methanoyl or formyl group is also called aldehydes and ketones. There are 2 remaining bonds in this group’s carbon atom that may be occupied by aryl or alkyl or substituents. The compound is a ketone if none of these substituents is hydrogen. If at least one is hydrogen, then an aldehyde is the compound.

Occurrence of Aldehydes and Ketones

Aldehydes and ketones are commonly used in nature in conjunction with other functional groups. They are mainly present in microorganisms or plants and substances such as cinnamaldehyde (cinnamon bark), vanillin (vanilla bean), Citra (lemongrass), helminthosporium (a fungal toxin), carvone (spearmint and caraway), camphor (camphor trees).

Uses of Ketones and Aldehydes

Aldehyde Uses 

  • For the manufacture of acetic acid and pyridine derivatives, acetaldehyde is used to a great degree.
  • In perfumes, beauty products, and colourants, benzaldehyde is used. It is applied and often used as a bee repellent to give almond flavour to food items.

Ketone Uses

  • Acetone is used in the kitchen as a nail polish remover and paint thinner.
  • It is used for chemical peeling and acne procedures in medicine.

Aldehydes and Ketones in Class 12

This concept is taught in chapter ‘Aldehydes, Ketones And Carboxylic acids.’ You will learn about the structures, preparation, and different reactions of aldehydes and ketones. The chapter carries 28 marks.

Illustrated Examples

1. Do silver mirror tests cover ketones? 

Solution: An aldehyde is oxidised into the resulting carboxylic acid by Tollens' reagent. Tollens' reagent does not oxidise ketones, so preparing a ketone in a glass test tube with Tollens’ reagent does not result in a silver mirror.

2. Is formaldehyde a type of aldehyde? 

Solution: It is the simplest of the (R-CHO) aldehydes.

3. Are aldehydes electrophilic? 

Solution: Aldehydes are a broad class of electrophilic carbonyl compounds which replace the carbonyl carbon atom with at least one hydrogen atom.

FAQs on Aldehydes and Ketones

Q: How are ketones and aldehydes different?

A:  A hydrogen atom is bound to an aldehyde’s carbonyl carbon, while that of a ketone is attached to two alkyl or aryl groups. The aldehyde C-H bond makes them readily oxidisable (they are strong reducing agents).

Q: Why are aldehydes more responsive than ketones to nucleophilic substitutions?

A:  Steric hindrance during substitution reactions is given by the two alkyl/aryl groups in ketones. Since the hydrogen atom is comparatively thin, there is hardly any steric impedance to it. This is the main explanation for the greater sensitivity of aldehydes to nucleophilic substitutions.

Q: Why are the ketones’ boiling points greater than those of aldehydes?

A:  Two electron-donating R-groups in ketones are more polar than aldehydes because of their presence. The greater boiling points of ketones are compensated for by the dipole moments that result from this polarity.

Q: Can ketones be analysed by Fehling’s solution?

A:  To differentiate aldehyde and ketone functional groups, Fehling’s solution may be used. In Fehling’s solution, the substance to be checked is applied, and the mixture is cooked. Aldehydes are oxidised, giving a positive outcome, but because they are alpha-hydroxy ketones, ketones do not react.

Q: Are aldehydes poisonous for humans?

A:  Aldehydes, originating from both natural and anthropogenic causes, are carbonyl compounds occurring ubiquitously in the environment. Therefore, since aldehydes are reactive species, they usually are poisonous to the body.

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