(i) 2,4-DINITROPHENYLHYDRAZONE OF BENZALDEHYDE

Step 1 - Write the reaction.
2,4-Dinitrophenylhydrazine (Brady's reagent) reacts with the carbonyl group, eliminating
a molecule of water:
$$C_6H_5CHO + H_2N-NH-C_6H_3(NO_2)_2 \xrightarrow{H^+} C_6H_5CH=N-NH-C_6H_3(NO_2)_2 + H_2O$$

Step 2 - Structure of the product.
$$\textbf{C}_6\textbf{H}_5\textbf{-CH=N-NH-C}_6\textbf{H}_3\textbf{(NO}_2\textbf{)}_2$$
i.e. a phenyl ring joined by $CH=N-NH-$ to a benzene ring bearing $-NO_2$ groups at
positions 2 and 4. It is an orange-red crystalline solid, used to detect and characterise
carbonyl compounds (its sharp melting point identifies the parent aldehyde/ketone).

(ii) INCREASING ORDER OF REACTIVITY TOWARDS HCN

Step 1 - The two governing factors.
Addition of HCN is a NUCLEOPHILIC ADDITION at the carbonyl carbon. Reactivity increases
when the carbonyl carbon is (a) MORE ELECTROPHILIC and (b) LESS STERICALLY CROWDED.

Step 2 - Electronic effect.
Alkyl groups release electrons $(+I)$, which reduces the positive charge on the carbonyl
carbon and makes it less electrophilic.
Acetaldehyde $(CH_3CHO)$ has ONE alkyl group.
Acetone $((CH_3)_2CO)$ has TWO alkyl groups.
Di-tert-butyl ketone $(((CH_3)_3C)_2CO)$ has TWO BULKY tert-butyl groups.

Step 3 - Steric effect.
The two very bulky tert-butyl groups in di-tert-butyl ketone physically BLOCK the
approach of the $CN^-$ nucleophile, making it by far the least reactive.

Step 4 - Write the order.
$$\textbf{Di-tert-butyl ketone} < \textbf{Acetone} CH-OH \xrightarrow{CrO_3} >C=O$$

Step 3 - Write the reaction and the answer.
$$\text{Cyclohexanol} \xrightarrow{CrO_3} \textbf{Cyclohexanone}$$
$$\textbf{'X'} = \text{Cyclohexanone, a six-membered ring with a } C=O \text{ group}$$