Introduction:
NaSO3H, also known as sodium bisulfite, can be used as a reagent to distinguish between 3-pentanone and 2-pentanone. This is because the two compounds have different reactivities towards NaSO3H, leading to the formation of different products.

Reaction:
When NaSO3H is added to 3-pentanone, it undergoes a reaction known as a nucleophilic addition. This reaction involves the addition of the bisulfite ion (HSO3-) to the carbonyl group of the ketone, forming a stable addition product.

In contrast, when NaSO3H is added to 2-pentanone, it undergoes a reaction known as an enolization. This reaction involves the formation of an enol intermediate, which then reacts with the bisulfite ion to form a different product.

Distinguishing 3-pentanone and 2-pentanone:
By comparing the products formed from the reactions of NaSO3H with 3-pentanone and 2-pentanone, it is possible to distinguish between the two ketones.

If the product obtained from the reaction with NaSO3H is a stable addition product, it indicates that the ketone is 3-pentanone. On the other hand, if the product obtained is an enol-bisulfite product, it indicates that the ketone is 2-pentanone.

Conclusion:
NaSO3H can be used as a reagent to distinguish between 3-pentanone and 2-pentanone based on their different reactivities towards the reagent. This method is useful in organic chemistry for identifying and characterizing different compounds.