Question

Which of the following statements is not true about glucose? (A) It is an aldohexose. (B) On heating with red \(\mathrm{P} / \mathrm{HI}\), it forms \(\mathrm{n}\)-hexane. (C) It is present in furanose form. (D) It does not give 2,4 -DNP test.

Short answer

The correct answer is option (D), as glucose does give a positive 2,4-DNP test because it contains an aldehyde functional group. The other statements are true about glucose.

Step-by-step solution

Statement A: It is an aldohexose.

Glucose is an aldohexose, meaning it is a six-carbon monosaccharide containing an aldehyde functional group. This statement is true.

Statement B: On heating with red \(\mathrm{P} / \mathrm{HI}\), it forms \(\mathrm{n}\)-hexane.

When glucose is heated in the presence of red phosphorus (\(\mathrm{P}\)) and hydroiodic acid (\(\mathrm{HI}\)), it produces \(\mathrm{n}\)-hexane as one of the products. This reaction is called the "Wolff-Kishner Reduction", in which the aldehyde functional group in glucose is reduced to \(\mathrm{n}\)-hexane. So, this statement is true.

Statement C: It is present in furanose form.

Glucose can exist in two forms, furanose, and pyranose. Furanose is a ring structure containing four carbon atoms and one oxygen atom, while pyranose has five carbon atoms and one oxygen atom. In aqueous solution, less than 0.3% of \(\alpha\)-d-Glucose exists in the furanose form. Therefore, this statement is true but relatively minor compared to the pyranose form.

Statement D: It does not give 2,4 -DNP test.

The 2,4-dinitrophenylhydrazine (2,4-DNP) test is used to detect the presence of carbonyl compounds, such as aldehydes and ketones. Since glucose is an aldohexose containing an aldehyde functional group, it will give a positive 2,4-DNP test. Thus, this statement is false. Since statement D is not true about glucose, the correct answer is option (D).

Key concepts

Aldohexose

Aldohexoses are a type of monosaccharide, which is simply a single sugar unit. They contain six carbon atoms, hence the 'hex' in their name, and possess an aldehyde group at one end, which is indicated by the 'aldo'. Glucose is the most prominent example of an aldohexose and is a key player in energy metabolism within our bodies. Its structure allows it to undergo a variety of chemical reactions because of the reactive aldehyde group. This group reacts with other molecules easily, which is why aldohexoses like glucose play crucial roles in biological systems.

In organic chemistry, glucose demonstrates classic properties of an aldohexose through reactions involving its aldehyde group. Because of its importance in biological processes and industrial applications like food production and bioethanol fuel, understanding the reactivity of glucose's aldehyde group is a fundamental lesson for students.

Wolff-Kishner Reduction

The Wolff-Kishner Reduction is a chemical reaction that is used to remove the oxygen from a carbonyl group, converting it into a methylene (-CH2-) group. This reaction is particularly useful because it provides a means to reduce aldehydes and ketones to alkanes, which are saturated hydrocarbons. This reduction is carried out by using hydrazine (H2NNH2) and a strong base, typically in the presence of heat.

When applied to glucose, the Wolff-Kishner Reduction reduces the aldehyde group to a methylene group, ultimately converting the glucose into n-hexane. This reaction is essential for students to understand as it demonstrates the transformation of a functionalized molecule—a sugar in this case—into a simple hydrocarbon, showcasing the versatility and power of chemical reactions in modifying molecular structures.

Furanose and Pyranose Forms

Glucose, like other sugars, can cyclize to form ring structures known as furanose and pyranose. The terms 'furanose' and 'pyranose' originate from the similarity these structures share with the organic compounds furan and pyran, respectively. A furanose ring contains four carbon atoms and one oxygen atom, resembling the structure of furan, while a pyranose ring has five carbon atoms and one oxygen atom, reflecting the structure of pyran.

The formation of these rings is a result of the reaction between the aldehyde or ketone group of the sugar with a hydroxyl group on the same molecule. In aqueous solutions, glucose is predominantly found in the pyranose form, which is a six-membered ring resembling a hexagon. The furanose form is less common for glucose and involves a five-membered ring, like a pentagon. The ability of sugars to exist in different ring forms is significant because it influences their chemical reactivity and interactions with other biomolecules.

2,4-DNP Test

The 2,4-Dinitrophenylhydrazine (2,4-DNP) test is a widely used chemical test to detect the presence of carbonyl groups in aldehydes and ketones. This test involves the reaction of the carbonyl compound with 2,4-DNP, which results in the formation of a yellow, orange, or red precipitate known as a 2,4-dinitrophenylhydrazone. It's a qualitative test that provides a visual indication that a carbonyl group is present in a molecule.

Because glucose contains an aldehyde group, it would respond positively to the 2,4-DNP test. This reaction is crucial for students to learn, as it arms them with a practical tool for identifying aldehyde or ketone functionality in unknown substances. Given the importance of such functional groups in organic chemistry and biochemistry, understanding how to use the 2,4-DNP test effectively is an important skill in a student's repertoire.