Problem 141 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following cannot be oxidized by bromine water? (a) D- glycerladehyde (b) D- arabinose (c) L- fructose (d) L- glucose

Step-by-Step Solution

Answer

L-fructose cannot be oxidized by bromine water.

Step 1: Understand Oxidation by Bromine Water

Bromine water (\( \text{Br}_2 (aq) \)) is often used to oxidize aldehydes to carboxylic acids. Ketones and secondary alcohols are generally resistant to oxidation by bromine water. Thus, compounds containing aldehyde groups can be oxidized, while ketones cannot.

Step 2: Determine Functional Groups in the Given Compounds

Identify the functional groups present in each compound: (a) D-glyceraldehyde contains an aldehyde group. (b) D-arabinose is an aldose, which means it has an aldehyde group. (c) L-fructose is a ketose, possessing a ketone group. (d) L-glucose is an aldose, that also has an aldehyde group.

Step 3: Analyze Which Compounds Can Be Oxidized

Since bromine water oxidizes aldehydes but not ketones, the compounds with aldehyde groups (D-glyceraldehyde, D-arabinose, and L-glucose) will be oxidized. L-fructose, having a ketone group, will not be oxidized by bromine water.

Step 4: Conclusion

L-fructose cannot be oxidized by bromine water due to the presence of a ketone group, which is resistant to oxidation under these conditions.

Key Concepts

Aldehyde and Ketone Chemistry

Aldehydes and ketones are two important classes of organic compounds. They are characterized by the presence of the carbonyl group, denoted as \( C=O \). What distinguishes aldehydes from ketones is the placement of this carbonyl group.
Aldehydes have the carbonyl group at the end of the carbon chain, flanked by at least one hydrogen atom. This positioning makes them reactive and prone to oxidation because the hydrogen atoms can be removed easily to form carboxylic acids. Common oxidizing agents for aldehydes include substances like bromine water.Ketones, on the other hand, have their carbonyl group flanked by carbon atoms. This internal placement makes them less reactive than aldehydes in oxidation reactions. The absence of a hydrogen atom directly attached to the carbonyl carbon means that ketones generally do not oxidize easily under mild conditions such as those provided by bromine water.
Understanding the fundamental differences between aldehydes and ketones is crucial in organic chemistry. It helps predict the outcomes of reactions, especially oxidation, as highlighted by the fact that aldehydes are susceptible to changes that ketones resist.

Bromine Water in Organic Chemistry

Bromine water is a mixture of bromine and water, often symbolized as \( \text{Br}_2 (aq) \). It is an insightful reagent in organic chemistry, primarily used to test for the presence of unsaturation (like in alkenes) or the oxidation of aldehydes to carboxylic acids.
The bromine molecule is highly electrophilic, meaning it is attracted to electron-rich sites. In the context of aldehydes, bromine water acts to oxidize the compound by facilitating the shift of electrons from hydrogen, allowing an aldehyde to become a carboxylic acid.
Because of its specificity, bromine water serves as a useful method to differentiate aldehydes from ketones in organic compounds. If a compound reacts with bromine water and forms a carboxylic acid, it indicates the presence of an aldehyde group.
The applications of bromine water extend beyond testing for aldehydes to understanding reaction mechanisms and transformations in organic synthesis.

Functional Groups in Organic Compounds

Functional groups are specific groupings of atoms within molecules. These groups define how molecules react chemically. Common examples are hydroxyl groups (\(-\text{OH}\)), carbonyl groups (\(C=O\)), and amine groups (\(-\text{NH}_2\)).
In the realm of organic compounds, functional groups are integral to predicting the behavior of molecules in reactions. For instance, the distinction between an aldehyde and a ketone lies in the functional group placement around the carbonyl group. This subtle difference influences their chemical properties, especially in reactions like oxidation.
In the original exercise, understanding the functional groups present in each compound allows us to predict which molecules bromine water will oxidize. Aldehyde functional groups, present at the terminal end of carbon chains, are reactive with bromine water.
Knowing the functional groups is essential for mastering organic chemistry reactions. They provide insights necessary for anticipating how molecules interact under different conditions and what transformation occurs.