Question 8 of 26beginner🧠 RememberLong Answer5 marks

What is meant by the following terms? Give an example of the reaction in each case. (i) Cyanohydrin (ii) Acetal (iii) Semicarbazone (iv) Aldol (v) Hemiacetal (vi) Oxime (vii) Ketal (viii) Imine (ix) 2,4-DNP-derivative (x) Schiff's base

(i)
Answer

Cyanohydrin: Cyanohydrins are compounds containing both a cyano group (-CN) and a hydroxyl group (-OH) on the same carbon atom, formed by nucleophilic addition of hydrogen cyanide to the carbonyl group of aldehydes or ketones. For example, cyclohexanone forms cyanohydrin in good yield.

Reaction: R-CHO + HCN → R-CH(OH)CN

(ii)
Answer

Acetal: Acetals are gem-dialkoxy compounds formed when aldehydes react with alcohols in the presence of an acid catalyst. For example, acetaldehyde reacts with methanol to form acetaldehyde dimethyl acetal.

Reaction: CH₃CHO + 2CH₃OH → CH₃CH(OCH₃)₂ + H₂O

(iii)
Answer

Semicarbazone: Semicarbazones are derivatives of aldehydes and ketones formed by reaction with semicarbazide (NH₂NHCONH₂). Only one -NH₂ group in semicarbazide is involved in the formation of semicarbazones.

Reaction: R₂C=O + NH₂NHCONH₂ → R₂C=NNHCONH₂ + H₂O

(iv)
Answer

Aldol: Aldol is a β-hydroxy aldehyde or ketone formed by aldol condensation, where an aldehyde or ketone with α-hydrogen undergoes nucleophilic addition with another molecule of the same or different carbonyl compound.

Reaction: 2CH₃CHO → CH₃CH(OH)CH₂CHO

(v)
Answer

Hemiacetal: Hemiacetals are compounds containing both an alkoxy group and a hydroxyl group attached to the same carbon atom, formed by nucleophilic addition of an alcohol to a carbonyl group. For example, formaldehyde forms methyl hemiacetal with methanol.

Reaction: R-CHO + R'OH → R-CH(OH)OR'

(vi)
Answer

Oxime: Oximes are derivatives of aldehydes and ketones formed by reaction with hydroxylamine (NH₂OH). For example, cyclopropanone forms cyclopropanone oxime.

Reaction: R₂C=O + NH₂OH → R₂C=NOH + H₂O

(vii)
Answer

Ketal: Ketals are gem-dialkoxy compounds formed when ketones react with alcohols in the presence of an acid catalyst. For example, hexan-3-one forms ethylene ketal with ethylene glycol.

Reaction: R₂C=O + 2R'OH → R₂C(OR')₂ + H₂O

(viii)
Answer

Imine: Imines are compounds containing a carbon-nitrogen double bond (C=N), formed by reaction of aldehydes or ketones with primary amines.

Reaction: R₂C=O + R'NH₂ → R₂C=NR' + H₂O

(ix)
Answer

2,4-DNP-derivative: 2,4-DNP-derivatives are formed when aldehydes or ketones react with 2,4-dinitrophenylhydrazine, giving yellow, orange or red precipitates. For example, benzaldehyde forms 2,4-dinitrophenylhydrazone of benzaldehyde.

Reaction: R₂C=O + (NO₂)₂C₆H₃NHNH₂ → R₂C=NNHC₆H₃(NO₂)₂ + H₂O

(x)
Answer

Schiff's base: Schiff's bases are imines formed by the reaction of aldehydes with primary amines. They contain the characteristic C=N- group.

Reaction: R-CHO + R'NH₂ → R-CH=NR' + H₂O

Explanation

This question tests understanding of various derivatives of aldehydes and ketones. Each term represents a specific type of addition product formed when carbonyl compounds react with different nucleophiles like HCN, alcohols, ammonia derivatives (hydroxylamine, semicarbazide, 2,4-DNP), and amines. The context from Questions 8.5 and 8.18 provides specific examples like cyclopropanone oxime, acetaldehyde dimethyl acetal, semicarbazone of cyclobutanone, ethylene ketal of hexan-3-one, and methyl hemiacetal of formaldehyde.

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