Question

In Seliwanoff's method of detection of added sugar in milk, the sequence of reaction is:

• A. Sucrose + HCl \(\rightarrow\) Fructose \(\rightarrow\) 5-HMF \(\rightarrow\) Dehydration product of 5-HMF + Resorcinol \(\rightarrow\) Red colour
• B. Sucrose + HCl \(\rightarrow\) Glucose \(\rightarrow\) 5-HMF \(\rightarrow\) Dehydration product of 5-HMF + Resorcinol \(\rightarrow\) Red colour
• C. Sucrose + HCl \(\rightarrow\) Fructose \(\rightarrow\) Ketol \(\rightarrow\) Dehydration product of Ketol + Resorcinol \(\rightarrow\) Red colour
• D. Sucrose + HCl \(\rightarrow\) Glucose \(\rightarrow\) Aldol \(\rightarrow\) Dehydration product of Aldol + Resorcinol \(\rightarrow\) Red colour

Hint:

To remember Seliwanoff's reaction sequence:
- Sucrose is cleaved to release Fructose (ketose).
- Fructose is dehydrated by HCl to form 5-HMF.
- 5-HMF reacts with Resorcinol to form a cherry-red complex.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Seliwanoff's test is a colorimetric chemical reaction used to distinguish ketose sugars from aldose sugars.
In dairy chemistry, this test is used to detect the illegal adulteration of milk with cane sugar (sucrose).
While lactose (the natural sugar in milk) is an aldose and reacts slowly, sucrose contains fructose (a ketose) and reacts rapidly to produce a distinct red color.

Step 2: Detailed Explanation:
Let us analyze the sequence of chemical reactions that occur during this test:
-

Step 1: Hydrolysis:
When milk containing added sucrose is treated with concentrated hydrochloric acid (HCl) and heated, the sucrose (a disaccharide) is hydrolyzed into its monomeric components: glucose (an aldose) and fructose (a ketose):
\[ \text{Sucrose} \xrightarrow{\text{HCl, }\Delta} \text{Glucose} + \text{Fructose} \]
- Dehydration:
The freed fructose molecule undergoes rapid dehydration in the hot acid medium.
This dehydration removes three molecules of water, converting the fructose into 5-hydroxymethylfurfural (5-HMF):
\[ \text{Fructose} \xrightarrow{\text{HCl, }\Delta} \text{5-HMF} + 3\text{H}_2\text{O} \]
-

Step 2: Condensation and Color Development:
The 5-HMF then condenses with resorcinol (meta-dihydroxybenzene, the active reagent in Seliwanoff's reagent) to form a cherry-red colored complex.
Because aldoses like glucose or lactose dehydrate much more slowly under the same conditions, they do not produce this deep red color within the short heating time of the test (usually 5 minutes).
This specific sequence corresponds to Option (A).

Step 3: Final Answer:
The correct sequence of reaction is Sucrose + HCl \(\rightarrow\) Fructose \(\rightarrow\) 5-HMF \(\rightarrow\) Dehydration product of 5-HMF + Resorcinol \(\rightarrow\) Red colour.