## Stepwise Explanation ### Step 1: Reaction with H₂CO/H⁺ (Formaldehyde and Acid) **Mechanism Name:** Kiliani–Fischer Synthesis (Chain Extension) #### Explanation: - **Step 4** shows a sugar (an aldose) reacting with formaldehyde (H₂CO) in acidic conditions. - The aldehyde group of the sugar reacts with formaldehyde via nucleophilic addition to form a new carbon–carbon bond, extending the carbon chain by one. - This creates a new aldose with one additional carbon atom, forming a mixture of epimers at the new carbon center. **Mechanism:** The mechanism involves: 1. Protonation of formaldehyde. 2. Nucleophilic attack by the sugar's aldehyde group. 3. Rearrangement to yield a new aldose (with an extra carbon). --- ### Step 2: Reaction with H₂CO/H⁺ (Same Mechanism) **Mechanism Name:** Kiliani–Fischer Synthesis (Chain Extension) #### Explanation: - **Step 5** repeats the same process: the newly formed sugar again reacts with formaldehyde under acidic conditions. - The chain is further extended by one carbon, producing another pair of epimers at the new carbon. **Mechanism:** This step follows the same Kiliani–Fischer synthesis mechanism as above, further lengthening the carbon backbone of the sugar. --- ## Final Answer Both steps are **Kiliani–Fischer Synthesis** (Chain Extension) reactions. Each step adds one carbon to the aldose sugar by reacting with formaldehyde in acidic conditions, resulting in a mixture of epimers at the new carbon center each time.