oxidation of aldehydes using Cr₂O₇^2–/H⁺ (i.e. K₂Cr₂O₇/H₂SO₄) to form carboxylic acids

In equations for organic redox reactions, [O] and [ H] should be used.
PAG7 (see also 6.3.1c)

nucleophilic addition reactions of carbonyl compounds with:

(i) NaBH₄ to form alcohols
(ii) HCN [i.e. NaCN(aq)/H⁺(aq)], to form hydroxynitriles (see also 6.2.4b)

the mechanism for nucleophilic addition reactions of aldehydes and ketones with NaBH₄ and HCN

For NaBH₄, the nucleophile can be considered as being the hydride ion, H^–, with subsequent protonation of the organic intermediate from H₂O.
For HCN, initial nucleophilic attack is from CN^– ions; subsequent protonation stage can be shown using H₂O or H⁺.

use of 2,4-dinitrophenylhydrazine to:

(i) detect the presence of a carbonyl group in an organic compound
(ii) identify a carbonyl compound from the melting point of the derivative

The equation for this reaction is not required.
Structure of derivative not required.
PAG7 (see also 6.3.1c)

use of Tollens’ reagent (ammoniacal silver nitrate) to:

(i) detect the presence of an aldehyde group
(ii) distinguish between aldehydes and ketones, explained in terms of the oxidation of aldehydes to carboxylic acids with reduction of silver ions to silver.

In equations involving Tollens’ reagent, [O] is acceptable.
PAG7 (see also 6.3.1c)