CUET Preparation Today
Aldehydes and ketones are two important classes of organic compounds which are widely used in the synthesis of a variety of other organic compounds. Due to the polarity of the carbonyl group, they undergo nucleophilic addition reactions. Broadly speaking, these addition reactions are of two types- one in which simply the addition of the nucleophilic reagent occurs across \(>C=O\) bond and the other in which addition is followed by the elimination of a molecule of water. A number of ammonia derivatives such as hydroxylamine, hydrazine, phenylhydrazine, 2,4-dinitrophenylhydrazine, and semicarbazide belong to the second category of reactions. These derivatives are used for the identification and characterization of aldehydes/ketones. Both types of nucleophilic addition reactions are influenced by steric and electronic factors. In general, aliphatic aldehydes are more aromatic aldehydes. The aliphatic aldehydes reduce both Fehling's solution and Tollen's reagent but aromatic aldehydes reduce only Tollen's reagent. Like aldehydes, aromatic ketones are less reactive than aliphatic ketones towards nucleophilic addition reactions. Phenols and carboxylic acids are acidic in nature. Both dissolve in \(NaOH\) solution and turn blue litmus red. However carboxylic acids are much stronger acids than phenols and decompose \(NaHCO_3\) with the evolution of \(CO_2\) but phenols do not. Both electron-donating and electron-withdrawing substituents influence the acid strength of aliphatic as well as aromatic acids. Unlike all other aliphatic acids, formic acid has reducing properties and also does not show reactions of the alkyl group. |
Which of the following compounds most readily undergoes nucleophilic addition reaction? |
Formaldehyde Acetone Acetaldehyde Di-tert-butyl ketone |
Formaldehyde |
The correct answer is (1) Formaldehyde. Among the given options, formaldehyde \((HCHO)\) is the compound that most readily undergoes nucleophilic addition reactions. This is primarily due to the electron-deficient nature of its carbonyl carbon atom, which makes it highly susceptible to nucleophilic attack. Formaldehyde possesses a highly polarized carbonyl group \((C=O)\) bond, with the oxygen atom bearing a partial negative charge \((δ^-)\) and the carbon atom bearing a partial positive charge \((δ^+)\). This polarization facilitates nucleophilic attack on the carbonyl carbon, as the nucleophile \((Nu^-)\) is attracted to the electron-deficient carbon atom. Compared to other carbonyl compounds, formaldehyde exhibits the highest degree of polarization due to the electron-withdrawing effect of the two hydrogen atoms attached to the carbonyl carbon. These hydrogen atoms effectively pull electron density away from the carbonyl carbon, further enhancing its electron deficiency and increasing its reactivity towards nucleophiles. Consequently, formaldehyde readily undergoes nucleophilic addition reactions with a wide range of nucleophiles, including hydrides, alkoxides, amines, and nitriles. These reactions lead to the formation of various derivatives, such as alcohols, ethers, imines, and cyanohydrins. In contrast, acetone \((CH_3COCH_3)\), acetaldehyde (CH_3CHO), and di-tert-butyl ketone \((CH_3)_3CCOCH_3)\) are less reactive towards nucleophilic addition reactions due to the presence of electron-donating alkyl groups attached to the carbonyl carbon. These alkyl groups, particularly the tert-butyl groups in di-tert-butyl ketone, provide electron density to the carbonyl carbon, reducing its electron deficiency and making it less susceptible to nucleophilic attack. Therefore, the correct answer is 1. Formaldehyde. |
