CBSE Class 12 Chemistry Chapter 12: Aldehydes, Ketones and Carboxylic Acids - NCERT Solutions

A cyanohydrin is an organic compound that contains a hydroxyl (-OH) group and a cyano (-CN) group attached to the same carbon atom. The general formula is RR'C(OH)CN, where R and R' can be hydrogen, alkyl, or aryl groups. They are synthesized by the nucleophilic addition of hydrogen cyanide (HCN) to aldehydes or ketones. The reaction is typically catalyzed by a base, such as sodium cyanide (NaCN), which generates the cyanide ion (CN⁻) nucleophile.

The reaction is as follows:

$$RR\text{'}C\; =\; O\; +\; HCN\; \backslash xrightarrow\{NaCN\}\; RR\text{'}C(OH)CN$$

Cyanohydrins are valuable synthetic intermediates in organic chemistry.

An acetal is a functional group characterized by two alkoxy groups (-OR) attached to the same carbon atom. It is formed when an aldehyde reacts with two equivalents of a monohydric alcohol in the presence of an acid catalyst, typically dry hydrogen chloride (HCl). The reaction proceeds through a hemiacetal intermediate.

The overall reaction is:

$$RCHO\; +\; 2\; R\text{'}OH\; \backslash xrightarrow\{HCl\}\; RCH(OR\text{'})\_2\; +\; H\_2O$$

Where RCHO is an aldehyde and R'OH is an alcohol. Acetals are stable in basic conditions but are hydrolyzed back to the aldehyde and alcohol in acidic conditions.

A semicarbazone is a derivative of an aldehyde or ketone formed by the condensation reaction between the carbonyl compound and semicarbazide (NH₂NHCONH₂). The reaction involves the nucleophilic addition of the primary amine group of semicarbazide to the carbonyl carbon, followed by the elimination of a water molecule.

The general reaction is:

$$R\_2C=O\; +\; NH\_2NHCONH\_2\; \backslash rightarrow\; R\_2C=NNHCONH\_2\; +\; H\_2O$$

Semicarbazones are often crystalline solids with characteristic melting points, which aids in the identification and characterization of aldehydes and ketones.

An aldol is a β-hydroxy aldehyde or a β-hydroxy ketone. It is formed through the aldol condensation reaction, which involves the joining of two aldehyde or ketone molecules (or one of each) in the presence of a dilute base or acid catalyst. In this reaction, one molecule provides an enolate ion (acting as a nucleophile), which attacks the carbonyl carbon of another molecule (acting as an electrophile).

For example, the aldol condensation of propanal:

$$2\; CH\_3CH\_2CHO\; \backslash xrightarrow\{Base\}\; CH\_3CH\_2CH(OH)CH(CH\_3)CHO$$

The product shown is 3-hydroxy-2-methylpentanal. The term 'aldol' itself is a portmanteau of 'aldehyde' and 'alcohol', reflecting the functional groups present in the product.

A hemiacetal is an intermediate compound formed during the reaction of an aldehyde or ketone with an alcohol. It contains both a hydroxyl group (-OH) and an alkoxy group (-OR) attached to the same carbon atom. Hemiacetals are formed by the addition of one molecule of alcohol to the carbonyl group.

The formation of a hemiacetal from an aldehyde is represented as:

$$RCHO\; +\; R\text{'}OH\; \backslash rightleftharpoons\; RCH(OH)OR\text{'}$$

Hemiacetals are generally unstable and can readily revert to the starting aldehyde/ketone and alcohol, or react further with another molecule of alcohol to form an acetal.

An oxime is a compound formed by the reaction of an aldehyde or ketone with hydroxylamine (NH₂OH). The reaction is a nucleophilic addition followed by dehydration, resulting in the formation of a C=N-OH functional group.

The general reaction is:

$$R\_2C=O\; +\; NH\_2OH\; \backslash rightarrow\; R\_2C=NOH\; +\; H\_2O$$

Oximes are often crystalline solids and are used in the characterization of carbonyl compounds.

A ketal is a functional group similar to an acetal, but it is derived from a ketone. It is characterized by two alkoxy groups (-OR) attached to the same carbon atom, which was originally the carbonyl carbon of the ketone. Ketals are formed by the reaction of a ketone with two equivalents of an alcohol in the presence of an acid catalyst.

The general reaction is:

$$RR\text{'}C=O\; +\; 2\; R\text{'}\text{'}OH\; \backslash xrightarrow\{HCl\}\; RR\text{'}C(OR\text{'}\text{'})\_2\; +\; H\_2O$$

Ketals are stable under basic conditions and are often used as protecting groups for ketones in organic synthesis.

An imine is a compound containing a carbon-nitrogen double bond (C=N). It is formed by the condensation reaction between an aldehyde or ketone and a primary amine. The reaction involves the nucleophilic addition of the amine to the carbonyl group, followed by the elimination of a water molecule.

The general reaction is:

$$R\_2C=O\; +\; R\text{'}NH\_2\; \backslash rightarrow\; R\_2C=NR\text{'}\; +\; H\_2O$$

Imines are also known as Schiff bases when derived from primary amines.

A 2,4-Dinitrophenylhydrazine (2,4-DNP) derivative is formed when an aldehyde or ketone reacts with 2,4-dinitrophenylhydrazine. This reaction is a common test for the presence of a carbonyl group. The reaction proceeds via nucleophilic addition followed by dehydration, yielding a brightly colored precipitate (often yellow, orange, or red) of 2,4-dinitrophenylhydrazone.

The general reaction is:

$$R\_2C=O\; +\; C\_6H\_3(NO\_2)\_2NHNH\_2\; \backslash rightarrow\; R\_2C=NNHC\_6H\_3(NO\_2)\_2\; +\; H\_2O$$

The formation of a precipitate indicates a positive test for aldehydes or ketones, and the melting point of the derivative can help identify the specific carbonyl compound.

A Schiff's base is a type of imine formed by the condensation reaction between an aldehyde or ketone and a primary amine. It is characterized by the presence of a C=N double bond. The term 'Schiff's base' is often used interchangeably with 'imine', especially when the amine is a primary amine.

The general reaction is:

$$R\_2C=O\; +\; R\text{'}NH\_2\; \backslash rightarrow\; R\_2C=NR\text{'}\; +\; H\_2O$$

Schiff bases are important in coordination chemistry and as intermediates in various organic reactions.