A by-product of the alkaline hydrolysis of tristearin is a

Analysis

The question asks for the identity of the by-product formed during the alkaline hydrolysis of tristearin. Tristearin is a common fat (a triglyceride).

Concept Explanation

• Tristearin: This is a triglyceride formed from the reaction of glycerol (a trihydric alcohol) and stearic acid (a fatty acid). Its chemical structure consists of a glycerol backbone attached to three stearate groups.
• Alkaline Hydrolysis (Saponification): When a fat or oil is heated with an alkali (like NaOH or KOH), the ester bonds are broken. This process is known as saponification, which is the basis for soap making.
• The Reaction: $$\text{Tristearin} + 3\text{NaOH} \rightarrow 3\text{Sodium Stearate (Soap)} + \text{Glycerol}$$
• Glycerol: The chemical name for glycerol is propane-1,2,3-triol. Because it contains three hydroxyl (-OH) groups attached to different carbon atoms, it is classified as a trihydric alkanol.

Step-by-Step Solution

1. Identify the reactants: Tristearin (an ester of glycerol) and an alkali (e.g., NaOH).
2. Identify the products: The main product is soap (sodium stearate), and the by-product is glycerol.
3. Analyze the structure of the by-product: Glycerol has the formula $CH_2(OH)CH(OH)CH_2(OH)$.
4. Classify the by-product: Since it has three -OH groups, it is a trihydric alcohol (trihydric alkanol).

Conclusion

The by-product of the alkaline hydrolysis of fats like tristearin is glycerol, which is a trihydric alkanol. This matches option D.

Exam Tip

Remember the "Saponification Rule": Fat + Alkali $\rightarrow$ Soap + Glycerol. Whenever you see "alkaline hydrolysis of fats/oils," always look for glycerol or trihydric alkanol as the by-product.