Maharashtra Board Class 12 Chemistry Chapter 12 Aldehydes Ketones And Carboxylic Acids PDF Download

Aldehydes, Ketones and Carboxylic Acids

12.1 Introduction

In the previous chapter, you learnt about the organic compounds which contain carbon-oxygen single bond. In this chapter, we are going to study the organic compounds containing carbon-oxygen double bond (\(>C=O\)) called carbonyl group, which is one of the most important functional group in organic chemistry.

Can you recall?

Draw the structures of the following compounds and classify them on the basis of C-O single bond and C = O double bond present in them: Ethyl alcohol, acetaldehyde, o-nitrophenol, Diethyl ether, isopropyl alcohol, acetone.

What are carbonyl compounds?

Both aldehydes and ketones contain a carbon-oxygen double bond (\(C=O\)) as their functional group. Therefore they are called carbonyl compounds. In aldehydes, carbonyl carbon is bonded to at least one hydrogen apart from an alkyl or aryl group. The functional group of aldehydes, therefore, is -CHO which is called formyl group or aldehydic carbonyl group. On the other hand in ketones, carbonyl carbon is bonded to two alkyl or aryl groups either identical (R = R) or different (R ≠ R'). It is called ketonic carbonyl group. The functional group of carboxylic acids is -COOH called carboxyl group. Due to the -OH group bonded to (\(>C=O\)) group, carboxylic acids are distinct from aldehydes and ketones.

12.2 Classification of Aldehydes, Ketones and Carboxylic Acids

Aldehydes, ketones and carboxylic acids are classified as per the nature of carbon skeleton bonded to (\(>C=O\)).

12.2.1 Classification of Aldehydes

Aldehydes are classified as aliphatic and aromatic aldehydes.

a. Aliphatic Aldehydes: The compounds in which the -CHO group (formyl group) is attached directly to sp³ hybridized carbon atom that is saturated carbon atom are called aliphatic aldehydes. (Exception: Formaldehyde, H-CHO is also classified as aliphatic aldehyde though -CHO group is not attached to any carbon). For example:

\(CH_3 - CHO\) (Acetaldehyde)

\(CH_3 - CH_2 - CHO\) (Propionaldehyde)

General formula: \(R - CHO\) (R = H or alkyl group)

b. Aromatic Aldehydes: The compounds in which -CHO group is attached directly to an aromatic ring are called aromatic aldehydes. For example: Benzaldehyde, Salicylaldehyde, p-Nitrobenzaldehyde.

Use your brain power: Classify the following as aliphatic and aromatic aldehydes.

Teacher's Note

Aldehydes are found in many things around us. For example, vanilla flavour comes from vanillin which is an aldehyde. Many perfumes also contain aldehydes that give them a nice smell.

Exam Trick

Remember: Aldehydes always have -CHO at the end of their chain. Just like a surname at the end of a name, -CHO comes at the end of an aldehyde chain.

Points to Remember

Aldehydes have a carbonyl group (-CHO) bonded to carbon and hydrogen.
Aldehydes are more reactive than ketones towards nucleophilic attacks.
Formaldehyde is a gas at room temperature.
Aromatic aldehydes are less reactive than aliphatic aldehydes.
The -CHO group must always be at the end of the carbon chain.

12.2.2 Classification of Ketones

Ketones are classified as aliphatic and aromatic ketones.

a. Aliphatic Ketones: The compounds in which \(>C=O\) group is attached to two alkyl groups are called aliphatic ketones.

General formula: \(R - CO - R'\) (Where R, R' = alkyl group, identical or different)

On the basis of types of alkyl groups bonded to carbonyl carbon, aliphatic ketones are further classified as simple and mixed ketones.

i. Simple or Symmetrical Ketones: The ketones in which both the alkyl groups bonded to carbonyl carbon are identical, are called simple ketones or symmetrical ketones. For example:

\(H_3C - C - CH_3\) (Acetone / Dimethyl ketone)

ii. Mixed or Unsymmetrical Ketones: The ketones in which two alkyl groups bonded to carbonyl carbon are different, are called mixed ketones or unsymmetrical ketones. For example:

\(H_3C - C - CH_2 - CH_3\) (Ethyl methyl ketone)

b. Aromatic Ketones: The compounds in which a \(>C=O\) group is attached to either two aryl groups or one aryl and one alkyl group are called aromatic ketones. For example: Acetophenone, Benzophenone.

Use your brain power: Classify the followings as simple and mixed ketones. Benzophenone, acetone, butanone, acetophenone.

Teacher's Note

Ketones are used to make many things. For example, acetone is used as a nail polish remover. Many medicines also contain ketone groups in their structure.

Exam Trick

Remember: In ketones, the carbonyl carbon is always in the middle of the chain, not at the end. This is the main difference between aldehydes and ketones.

Points to Remember

Ketones have a carbonyl group in the middle of the carbon chain.
Simple ketones have two identical alkyl groups attached to the carbonyl carbon.
Mixed ketones have two different alkyl groups attached to the carbonyl carbon.
Ketones are less reactive than aldehydes towards nucleophilic attacks.
Ketones do not get oxidized easily by mild oxidizing agents.

12.2.3 Classification of Carboxylic Acids

Carboxylic acids are classified as aliphatic and aromatic carboxylic acids.

a. Aliphatic Carboxylic Acids: The organic compounds in which carboxyl (-COOH) group is bonded to an alkyl group are called aliphatic carboxylic acids or fatty acids. (Exception: Formic acid, H-COOH is also classified as aliphatic carboxylic acid though -COOH group is not attached to any carbon). For example:

\(H_3C - COOH\) (Acetic acid)

\(H_3C - CH_2 - COOH\) (Propionic acid)

Carboxylic acids are widely distributed in nature; they are found in both the plants and animals. L-lactic acid is present in curd, citric acid is found in citrus fruit (Lemons). Acetic acid is the key ingredient of vinegar.

b. Aromatic Carboxylic Acids: These are the compounds in which one or more carboxyl groups (-COOH) are attached directly to the aromatic ring. For example: Benzoic acid, Phthalic acid, Acetyl salicylic acid (Aspirin).

Depending on the number of -COOH groups present carboxylic acids are classified as mono, di, tri carboxylic acids and so on.

Teacher's Note

Carboxylic acids are found in many food items we eat. For example, citric acid in lemons gives them a sour taste. Acetic acid in vinegar is used in cooking.

Exam Trick

Remember: Carboxylic acids always have -COOH at the end. The -COOH group is the carboxyl group and it makes these acids strong.

Points to Remember

Aliphatic carboxylic acids have the -COOH group bonded to an alkyl group.
Aromatic carboxylic acids have the -COOH group bonded directly to an aromatic ring.
Formic acid is the simplest carboxylic acid.
Carboxylic acids are stronger acids than aldehydes and ketones.
The -COOH group is always at the end of the carbon chain.

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