Introduction
Aldehydes, ketones, and carboxylic acids are important classes of organic compounds that contain carbonyl groups. These compounds play a crucial role in organic chemistry and are frequently encountered in chemical reactions and synthesis. This study note will cover the structure, nomenclature, properties, and reactions of aldehydes, ketones, and carboxylic acids, as per the JEE Advanced Chemistry syllabus.
Aldehydes
Structure and Nomenclature
Aldehydes contain a carbonyl group ($C=O$) bonded to a hydrogen atom and an alkyl or aryl group. The general formula for aldehydes is $RCHO$, where $R$ is an alkyl or aryl group.
Nomenclature Rules
- Identify the longest carbon chain containing the carbonyl group.
- Replace the terminal "-e" of the corresponding alkane name with "-al".
- Number the chain starting from the carbonyl group (which is always carbon-1).
Example:
- $CH_3CHO$ is named ethanal.
- $C_6H_5CHO$ is named benzaldehyde.
Physical Properties
- Aldehydes typically have higher boiling points than hydrocarbons of similar molecular weight due to the polar carbonyl group.
- Lower aldehydes are more soluble in water due to hydrogen bonding with water molecules.
Chemical Reactions
Nucleophilic Addition Reactions
Aldehydes undergo nucleophilic addition reactions due to the electrophilic nature of the carbonyl carbon.
Example: $$ CH_3CHO + HCN \rightarrow CH_3CH(OH)CN $$ (ethanal reacts with hydrogen cyanide to form 2-hydroxypropanenitrile)
Oxidation
Aldehydes can be easily oxidized to carboxylic acids.
$$ RCHO + [O] \rightarrow RCOOH $$
Aldehydes can be distinguished from ketones using Tollen's reagent, which oxidizes aldehydes to carboxylic acids but does not react with ketones.
Ketones
Structure and Nomenclature
Ketones contain a carbonyl group ($C=O$) bonded to two alkyl or aryl groups. The general formula for ketones is $RCOR'$, where $R$ and $R'$ are alkyl or aryl groups.
Nomenclature Rules
- Identify the longest carbon chain containing the carbonyl group.
- Replace the terminal "-e" of the corresponding alkane name with "-one".
- Number the chain such that the carbonyl carbon gets the lowest possible number.
Example:
- $CH_3COCH_3$ is named propanone.
- $C_6H_5COCH_3$ is named acetophenone.
Physical Properties
- Ketones have higher boiling points than hydrocarbons of similar molecular weight due to the polar carbonyl group.
- Lower ketones are more soluble in water due to hydrogen bonding with water molecules.
Chemical Reactions
Nucleophilic Addition Reactions
Ketones also undergo nucleophilic addition reactions, although they are generally less reactive than aldehydes.
Example: $$ CH_3COCH_3 + HCN \rightarrow CH_3C(OH)(CN)CH_3 $$ (propanone reacts with hydrogen cyanide to form 2-hydroxy-2-methylpropanenitrile)
Reduction
Ketones can be reduced to secondary alcohols.
$$ RCOR' + 2[H] \rightarrow RCH(OH)R' $$
The reactivity of ketones is lower compared to aldehydes due to steric hindrance and electronic effects.
Carboxylic Acids
Structure and Nomenclature
Carboxylic acids contain a carboxyl group ($-COOH$), which consists of a carbonyl group ($C=O$) and a hydroxyl group ($-OH$) attached to the same carbon atom. The general formula for carboxylic acids is $RCOOH$, where $R$ is an alkyl or aryl group.
Nomenclature Rules
- Identify the longest carbon chain containing the carboxyl group.
- Replace the terminal "-e" of the corresponding alkane name with "-oic acid".
- Number the chain starting from the carboxyl carbon (which is always carbon-1).
Example:
- $CH_3COOH$ is named ethanoic acid.
- $C_6H_5COOH$ is named benzoic acid.
Physical Properties
- Carboxylic acids have higher boiling points than aldehydes and ketones of similar molecular weight due to the ability to form hydrogen bonds.
- They are typically soluble in water, especially lower carboxylic acids, due to hydrogen bonding.
Chemical Reactions
Acid-Base Reactions
Carboxylic acids can donate a proton ($H^+$), making them acidic.
$$ RCOOH \rightarrow RCOO^- + H^+ $$
Esterification
Carboxylic acids react with alcohols to form esters in the presence of an acid catalyst.
$$ RCOOH + R'OH \rightarrow RCOOR' + H_2O $$
A common mistake is to confuse esterification with saponification. Esterification forms esters from carboxylic acids and alcohols, whereas saponification is the hydrolysis of esters in the presence of a base.
Reduction
Carboxylic acids can be reduced to primary alcohols using strong reducing agents like lithium aluminium hydride ($LiAlH_4$).
$$ RCOOH + 4[H] \rightarrow RCH_2OH + H_2O $$
Conclusion
Aldehydes, ketones, and carboxylic acids are fundamental functional groups in organic chemistry. Understanding their structure, nomenclature, physical properties, and chemical reactions is crucial for mastering organic synthesis and reaction mechanisms. This study note provides a comprehensive overview, incorporating examples, tips, and common mistakes to aid in preparation for the JEE Advanced Chemistry exam.
