CBSE Class 12 Chemistry Chapter 24: Aldehydes, Ketones, and Carboxylic Acids NCERT Solutions

Cyanohydrin: 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 formed by the nucleophilic addition of hydrogen cyanide (HCN) to aldehydes or ketones, typically catalyzed by a base like NaCN.

Reaction:

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

This reaction is a crucial step in synthesizing various organic molecules, as the cyano and hydroxyl groups can be further transformed.

Acetal: An acetal is a functional group characterized by a carbon atom bonded to two alkoxy (-OR) groups and two other carbon atoms (or one carbon and one hydrogen). They are essentially gem-dialkoxy alkanes. Acetals are formed from aldehydes (not ketones, which form ketals) by reaction with two equivalents of a monohydric alcohol in the presence of an acid catalyst, usually dry HCl.

Reaction:

The formation proceeds in two steps: first, a hemiacetal is formed, which then reacts with another molecule of alcohol to yield the acetal.

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

Acetals are stable in basic conditions but are hydrolyzed back to the aldehyde and alcohol in acidic conditions, making them useful protecting groups for aldehydes.

Semicarbazone: 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.

Reaction:

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

Semicarbazones are typically crystalline solids and are often used to identify and characterize aldehydes and ketones due to their distinct melting points.

Aldol: An aldol is a beta-hydroxy aldehyde or beta-hydroxy ketone. The term 'aldol' is a portmanteau of 'aldehyde' and 'alcohol', reflecting its structure. It is formed through the aldol condensation reaction, which involves the joining of two aldehyde or two ketone molecules (or one of each) in the presence of a dilute base or acid catalyst. The reaction requires the presence of an alpha-hydrogen atom in at least one of the carbonyl compounds.

Reaction (Example with Propanal):

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

The product shown is 3-hydroxy-2-methylpentanal, an aldol.

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

Reaction:

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

Hemiacetals are generally unstable and exist in equilibrium with the starting aldehyde/ketone and alcohol. They readily react with a second molecule of alcohol to form acetals (from aldehydes) or ketals (from ketones).

Oxime: An oxime is a compound formed by the reaction of an aldehyde or ketone with hydroxylamine (NH2OH). The reaction involves the nucleophilic addition of hydroxylamine to the carbonyl carbon, followed by the elimination of a water molecule, resulting in a C=N-OH functional group.

Reaction:

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

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

Ketal: A ketal is a functional group similar to an acetal, but it is derived from a ketone. It features a carbon atom bonded to two alkoxy (-OR) groups. Ketals are formed by the reaction of ketones with two equivalents of an alcohol in the presence of an acid catalyst.

Reaction:

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

Like acetals, ketals are stable in basic conditions and are used as protecting groups for ketones.

Imine: An imine, also known as a Schiff base when derived from a primary amine, 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 (RNH2), with the elimination of a water molecule.

Reaction:

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

Imines are important intermediates in organic synthesis and are found in various biological systems.

2,4-DNP-derivative: This refers to the derivative formed when an aldehyde or ketone reacts with 2,4-dinitrophenylhydrazine (2,4-DNP). The reaction is a condensation process similar to the formation of imines or semicarbazones, yielding a 2,4-dinitrophenylhydrazone.

Reaction:

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

These derivatives are often brightly colored (yellow, orange, or red) and precipitate out, making them extremely useful for the qualitative detection and characterization of carbonyl compounds.

Schiff's base: A Schiff base is a specific type of imine where the nitrogen atom is bonded to an alkyl or aryl group (derived from a primary amine). It is formed by the condensation reaction between an aldehyde or ketone and a primary amine (RNH2), with the elimination of water.

Reaction:

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

Schiff bases are important in coordination chemistry and catalysis, and they play roles in biological processes.