Explain the following terms: (a) Cyanohydrin (b) Hydrazone

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Q. Explain the following terms:

(a) Cyanohydrin

(b) Hydrazone

Chhattisgarh CgbseCGBSE Intermediate Board 2022Subjective· 3mImportance★★★★★

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Concept understanding — Carbonyl Compound Derivatives of Ammonia (Hydrazine, Semicarbazide, Phenylhydrazine, Hydroxylamine)

Why Carbonyl Compounds React With "Ammonia Derivatives"

You already know that aldehydes and ketones are attacked by nucleophiles at their electrophilic carbonyl carbon. A whole family of nitrogen nucleophiles — all built by replacing one or more hydrogens of ammonia (NH₃) — reacts this way, and each gives a crystalline, easily purified, sharp-melting-point derivative. This made them classic tools (before modern spectroscopy) for identifying an unknown aldehyde or ketone. But exams don't just ask "which reagent forms which derivative" — they also test whether you know each reagent's exact structural formula, because it's easy to misremember one.

The Four Standard Derivatives, and Their Correct Formulas

\ce{H2N-NH2}

Watch out

A very common exam trap: writing phenylhydrazine as $\ce{C6H5-NH-NH-C6H5}$ — i.e., putting a phenyl group on BOTH nitrogens. That structure would actually be 1,2-diphenylhydrazine (hydrazobenzene), a completely different compound. Real phenylhydrazine has exactly one phenyl group and one free –NH₂ left to attack the carbonyl carbon.

The Reaction Pattern (Nucleophilic Addition–Elimination)

All four reagents follow the same two-step logic at the carbonyl carbon:

\ce{R2C=O + H2N-Nu -> R2C(OH)(NHNu) -> R2C=N-Nu + H2O}

The nitrogen's lone pair (nucleophile) first adds across the C=O π bond (nucleophilic addition), and the resulting unstable intermediate then eliminates a water molecule to form a stable C=N bond — so the overall process is an addition–elimination (condensation) reaction, not a simple addition.

Why This Matters …

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Cyanohydrins come from HCN addition to C=O; hydrazones come from hydrazine condensation with C=O.

Cyanohydrin: When hydrogen cyanide (HCN) adds across the polar C=O bond of an aldehyde or ketone (nucleophilic addition, CN− attacking the electrophilic carbonyl carbon), the product is an α-hydroxy nitrile called a cyanohydrin: $R_2C=O + HCN \rightarrow R_2C(OH)CN$ Example: acetone + HCN → acetone cyanohydrin, $(CH_3)_2C(OH)CN$ . Cyanohydrins are useful intermediates because the nitrile can later be hydrolysed to a carboxylic acid, extending the carbon chain by one carbon.
Hydrazone: When a carbonyl compound reacts with hydrazine ( $NH_2-NH_2$ ), the carbonyl oxygen is replaced by $=N-NH_2$ with loss of a water molecule (a condensation reaction): …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2024Set ANNUAL1 mark
$Q. Write chemical name of NH2-NHCONH2.$
›Reveal solutionSolution

NH2-NHCONH2 is semicarbazide, a hydrazine derivative widely used to form solid, characterisable semicarbazone derivatives of aldehydes and ketones.

The structure NH2-NH-CO-NH2 consists of a hydrazine unit (H2N-NH-) attached to a urea-like carbonyl (-CO-NH2); this compound is called semicarbazide. …

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CBSE 2022Set E1 markMCQ
$Q. When formaldehyde reacts with NH3, then which of the following compounds is formed ?$
(a) Formaldehyde ammonia
(b) Hexamethylene tetramine
(c) Formalin
(d) Methylamine
›Reveal solutionSolution

Formaldehyde reacts with ammonia to form hexamethylenetetramine (urotropine).

6 HCHO + 4 NH3 -> (CH2)6N4 + 6 H2O

The product, hexamethylenetetramine (also called urotropine), is a cage-like compound used as a urinary antiseptic and in the manufacture of the explosive RDX.

…

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CBSE 2019Set ANNUAL1 mark
$Q. Draw structure of cyclopropanone oxime.$
›Reveal solutionSolution

An oxime forms when a ketone's C=O reacts with hydroxylamine (NH₂OH) to give C=N–OH. Cyclopropanone oxime is this applied to the three-membered ring ketone cyclopropanone.

Cyclopropanone is the smallest cyclic ketone: a three-membered carbocyclic ring, $(CH_2)_2C=O$ , i.e. two –CH₂– groups and one carbonyl carbon closing the ring.

An oxime is formed by the condensation of a ketone (or aldehyde) with hydroxylamine, with loss of water:

$R_2C=O + H_2N-OH \rightarrow R_2C=N-OH + H_2O$

Applying this to cyclopropanone: the ring carbonyl carbon's C=O is converted to C=N–OH, while the ring itself (the two –CH₂– groups completing the three-membered ring) is retained.

…

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CBSE 2018Set ANNUAL1 markMCQ
$Q. Chemical formula of urotropine is:$
(a) (CH2)4N3
(b) (CH2)5N4
(c) (CH2)6N4
(d) (CH2)3N3
›Reveal solutionSolution

Urotropine (hexamethylenetetramine) is the cage-like condensation product of formaldehyde and ammonia, formula (CH2)6N4.

Urotropine, also called hexamethylenetetramine or methenamine, is formed by the condensation of six molecules of formaldehyde (an aldehyde) with four molecules of ammonia:

$6HCHO + 4NH_3 \rightarrow (CH_2)_6N_4 + 6H_2O$

…

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Q.Explain the following terms:

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