The carbohydrates may also be classified as either reducing or non-reducing sugars. All those carbohydrates which reduce Fehling’s solution and Tollens’ reagent are referred to as reducing sugars. All monosaccharides whether aldose or ketose are reducing sugars. In disaccharides, if the reducing groups of monosaccharides, i.e., aldehydic or ketonic groups, are bonded, these are non-reducing sugars, e.g., sucrose. On the other hand, sugars in which these functional groups are free are called reducing sugars, for example, maltose and lactose.
Which of the following statements is incorrect?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Non-reducing sugars reduce Fehling’s solution and Tollens’ reagent
The passage states that the carbohydrates which reduce Fehling’s solution and Tollens’ reagent are the reducing sugars. A non-reducing sugar such as sucrose has its aldehydic and ketonic groups bonded in the glycosidic linkage, so it has no free reducing group and does not reduce these reagents.
Option (1) All monosaccharides, whether aldose or ketose, are reducing sugars — A correct statement. The passage states this line in the same words.
Option (2) Lactose is found in milk — A correct statement. Lactose is the disaccharide known as milk sugar, made of β-D-galactose and β-D-glucose.
Option (3) Non-reducing sugars reduce Fehling’s solution and Tollens’ reagent — The incorrect statement. Reducing these reagents is what defines a reducing sugar; a non-reducing sugar does not reduce them.
Option (4) Glucose is an aldohexose — A correct statement. Glucose has six carbon atoms and an aldehyde group, so it is an aldohexose.