CUET UG Chemistry Booster Test -2 Nomenclature and Bond Nature

Methanal exists as a gas at room temperature, while ethanal is a volatile liquid. Lower aldehydes generally have a sharp pungent smell. Propanone is not a solid at room temperature; it is a liquid.

As molecular size increases, van der Waals forces become stronger. Aldehydes and ketones also show dipole-dipole interactions due to the polar carbonyl group, making higher members liquids or solids.

Increasing boiling point follows increasing intermolecular force strength: n-Pentane < Ethoxyethane < Butanal < Butan-1-ol

Butan-1-ol contains an –OH group and forms strong intermolecular hydrogen bonds. Butanal has dipole-dipole interactions but cannot form hydrogen bonds with itself.

Lower aldehydes are soluble in water because the carbonyl oxygen forms hydrogen bonds with water. However, solubility decreases as the alkyl chain length increases. Aldehydes are generally fairly soluble in organic solvents.

Propanone is a lower ketone and is miscible with water in all proportions. Increasing alkyl chain length reduces water solubility. Aldehydes and ketones are fairly soluble in organic solvents like ether. Lower aldehydes dissolve in water because they can form hydrogen bonds with water.

Lower aldehydes have sharp pungent odours. With increase in molecular size, the odour becomes less pungent and more pleasant or fragrant.

Formalin is a commercial aqueous solution of formaldehyde containing approximately 40% formaldehyde. It is commonly used to preserve biological specimens.

The passage states that the nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of the sp² hybridised orbitals.

During nucleophilic addition, carbonyl carbon changes from sp² to sp³ hybridisation. This produces a tetrahedral alkoxide intermediate, which later accepts a proton to give the neutral product.

p-Nitrobenzaldehyde is most reactive because the –NO₂ group withdraws electron density and increases the electrophilic nature of the carbonyl carbon. Benzaldehyde is less reactive than p-nitrobenzaldehyde. p-Tolualdehyde is still less reactive because the –CH₃ group donates electron density. Acetophenone is least reactive because ketones are more sterically hindered and electronically less reactive than aldehydes.

In benzaldehyde, the carbonyl group is conjugated with the benzene ring. Due to resonance, electron density is delocalised and the carbonyl carbon becomes less electrophilic. The polarity of the carbonyl group is also reduced compared to aliphatic aldehydes like propanal, making benzaldehyde less reactive toward nucleophilic addition.

In cyanohydrin formation, hydrogen cyanide reacts with aldehydes or ketones in the presence of a base. The base helps produce cyanide ion (CN⁻), which is a stronger nucleophile and attacks the electrophilic carbonyl carbon.

Most ketones do not readily form stable sodium hydrogensulphite addition products because the two alkyl groups around the carbonyl carbon create steric hindrance. This makes nucleophilic addition less favourable, so the equilibrium lies largely to the left.

Ketones react with ethylene glycol in the presence of dry HCl to form cyclic ketals known as ethylene glycol ketals. These are useful for protecting carbonyl groups.

In reactions of aldehydes and ketones with alcohols, dry HCl acts as an acid catalyst by protonating the carbonyl oxygen. This increases the electrophilic character of the carbonyl carbon, facilitating nucleophilic attack by alcohol molecules. In cyanohydrin formation, CN⁻ is the attacking nucleophile.

Aldehydes and ketones react with ammonia derivatives through nucleophilic addition followed by dehydration. The dehydration step converts the unstable intermediate into a stable compound containing the >C=N-Z linkage.

Aldehydes and ketones react with 2,4-dinitrophenylhydrazine (Brady's reagent) to form yellow, orange, or red crystalline derivatives known as 2,4-dinitrophenylhydrazones. This reaction is widely used for identification of carbonyl compounds.

Reduction Process — Reagent Clemmensen Reduction — Zn-Hg / HCl Wolff-Kishner Reduction — NH₂NH₂ / KOH NaBH₄ Reduction — Sodium borohydride LiAlH₄ Reduction — Lithium aluminium hydride

Fehling's solution oxidizes aliphatic aldehydes to the corresponding carboxylate ions in alkaline medium. During the reaction, Cu²⁺ ions are reduced to cuprous oxide (Cu₂O), which appears as a reddish-brown precipitate.