CH₃CH₂COO⁻Na⁺ $\stackrel{NaOH,+?}{\underset{Heat}{⟶<!--\; ⟶\; -->}}$CH₃CH₃+ Na₂CO₃.

Consider the above reaction and identify the missing reagent/chemical.

The correct answer is: CaO

The reaction given is:

CH₃CH₂COO⁻Na⁺ $\stackrel{NaOH,+?}{\underset{Heat}{⟶}}$ CH₃CH₃ + Na₂CO₃

Here, sodium propanoate (the sodium salt of propanoic acid) is heated with NaOH and an unknown reagent to produce ethane (CH₃CH₃) and sodium carbonate (Na₂CO₃). We need to identify the missing reagent.

Step 1: Identify the type of reaction.
This reaction involves the removal of a carboxyl group (−COO⁻) from a carboxylic acid salt, producing a hydrocarbon with one fewer carbon atom. This is known as decarboxylation.

Step 2: Recall the Duma Reaction.
The Duma reaction is a classic decarboxylation reaction in which the sodium salt of a carboxylic acid is heated with soda lime to yield an alkane containing one carbon less than the original acid, along with sodium carbonate as a by-product.

The general reaction is:

RCOONa + NaOH $\stackrel{CaO,\Delta }{⟶}$ R−H + Na₂CO₃

Step 3: What is soda lime?
Soda lime is a mixture of NaOH (sodium hydroxide) and CaO (calcium oxide / quicklime). In the question, NaOH is already shown above the arrow. Therefore, the missing reagent must be CaO, which together with NaOH constitutes soda lime.

Step 4: Why is CaO needed?
NaOH alone is hygroscopic and tends to become sticky and fuse into a hard mass when heated, making the reaction impractical. CaO acts as a drying agent and bulking agent, keeping the mixture porous and allowing proper heating. It ensures the reaction proceeds smoothly and efficiently.

Step 5: Apply to the given reaction.
Sodium propanoate (3 carbons) undergoes decarboxylation with soda lime (NaOH + CaO) upon heating:

CH₃CH₂COONa + NaOH $\stackrel{CaO,\Delta }{⟶}$ CH₃CH₃ + Na₂CO₃

The product ethane (CH₃CH₃) has 2 carbons — exactly one carbon less than propanoic acid — confirming successful decarboxylation.

Why not the other options?

• B₂H₆ (Diborane) — This is a reducing agent used in hydroboration reactions, not in decarboxylation.
• Red Phosphorus — Used in Hell–Volhard–Zelinsky (HVZ) reactions for α-halogenation of carboxylic acids, not relevant here.
• DIBAL-H — A selective reducing agent used to reduce esters to aldehydes, unrelated to decarboxylation.

Therefore, the missing reagent is CaO.