Aldehydes Ketones And Carboxylic Acids – NEET UG ChemistryPractice Questions & PYQs

Nucleophilic addition-elimination reactions involving carbonyl compounds typically proceed through an initial nucleophilic attack on the carbonyl carbon followed by the expulsion of a water molecule to yield a new double bond. Reagents such as ammonia, hydroxylamine, and phenylhydrazine follow this two-step pathway when reacting with $\mathrm{CH}_3\mathrm{CHO}$ to form respective condensation products alongside $\mathrm{H}_2\mathrm{O}$. Conversely, the reaction of acetaldehyde with $\mathrm{NaHSO}_3$ is strictly a nucleophilic addition process that yields a stable bisulfite adduct, $\mathrm{CH}_3 - \underset{\mathrm{H}}{\underset{\vert}{\overset{\mathrm{OH}}{\overset{\vert}{\mathrm{C}}}}} - \mathrm{OSO}_2 \mathrm{Na}$, without eliminating any water molecules.

Heating a carboxylic acid salt with sodalime, composed of $\mathrm{NaOH}$ and $\mathrm{CaO}$, initiates a decarboxylation process that eliminates the carboxyl functional group. This chemical transformation successfully drives off carbon dioxide and yields a hydrocarbon product containing one fewer carbon atom than the original reactant.

Intermolecular forces such as dipole-dipole interactions and hydrogen bonding dictate the boiling points of organic compounds by determining the energy required to overcome attractions between molecules in the liquid state. Because aldehydes and ketones feature a polar carbonyl group, they experience dipole-dipole attractions that require more thermal energy to disrupt than the weak van der Waals forces holding non-polar hydrocarbons together. At the same time, because aldehydes and ketones lack the capability for intermolecular hydrogen bonding, their boiling points remain significantly lower than those of alcohols with comparable molecular masses.

When acetone and 2-pentanone react in the presence of dilute $NaOH$ followed by heating, they undergo a cross-aldol condensation. Because both reactant molecules contain $\alpha$-hydrogens, their respective enolates can react with either their own kind or the other carbonyl compound, resulting in a mixture of multiple self-aldol and cross-aldol products. Evaluating all feasible combinations of these nucleophilic additions shows that specific alternative structures, such as the one presented in option B, cannot be produced through any valid reaction pathway.

Carbonyl compounds participate in nucleophilic addition and condensation reactions where specific reagents yield distinct functional derivatives. Hydrogen cyanide ($HCN$) adds across the carbonyl double bond to form cyanohydrins, matching item (a) with (iv), whereas alcohols react with these systems to produce acetals, linking item (b) with (iii). Primary amines ($RNH_2$) condense with aldehydes and ketones to generate Schiff's bases, connecting item (c) with (ii), and hydroxylamine ($NH_2OH$) reacts to yield oximes, pairing item (d) with (i).

Formyl chloride ($HCOCl$) is an unstable compound that decomposes into carbon monoxide ($CO$) and hydrogen chloride ($HCl$), making direct Friedel-Crafts formylation of benzene impossible. Because this reactive intermediate cannot exist long enough to electrophilically attack the aromatic ring, this specific reaction pathway fails to yield benzaldehyde. Synthesis of benzaldehyde instead requires viable industrial routes such as the Gattermann-Koch reaction, which employs a stable pressurized mixture of carbon monoxide and hydrogen chloride alongside an anhydrous aluminum chloride catalyst.