Problem 133 - A Complete Resource Book in Chemistry for JEE Main

Question

1-propanol and 2-propanol can be best distinguished by (a) oxidation with \(\mathrm{KMnO}_{4}\) followed by reaction with Fehling solution (b) oxidation with acidic dichromate followed by reaction with Fehling solution (c) oxidation by heating with copper followed by reaction with Fehling solution (d) oxidation with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) followed by reaction

Step-by-Step Solution

Answer

Option (c) is best: heating with copper distinguishes 1-propanol by forming aldehyde for Fehling's test.

Step 1: Understand the Alcohol Types

1-propanol is a primary alcohol with the molecular structure as \( CH_3CH_2CH_2OH \), while 2-propanol is a secondary alcohol with the molecular structure \( CH_3CHOHCH_3 \). The position of the OH group affects their oxidation products.

Step 2: Consider Oxidation Outcomes

Primary alcohols (like 1-propanol) can be oxidized to aldehydes and then to carboxylic acids, while secondary alcohols (like 2-propanol) are oxidized to ketones, which cannot be further oxidized to carboxylic acids using mild conditions.

Step 3: Test with Fehling's Solution

Fehling's solution is used to identify aldehydes, as it gives a positive result (red precipitate) with aldehydes but not with ketones. Therefore, oxidized 1-propanol will give a positive result, while oxidized 2-propanol will not.

Step 4: Process Evaluation

The options \(a, b, c\) involve oxidation processes that will convert 1-propanol to a primary aldehyde or acid, which will react with Fehling's solution. Option \(c\) (oxidation by heating with copper) is the most practical for distinguishing since it directly allows for oxidation without strong acidic conditions or further reactants.

Key Concepts

Primary Alcohol

Primary alcohols are a type of alcohol where the hydroxyl group (OH) is attached to a carbon atom that is also connected to only one other carbon atom. This setup is quite significant, as it influences how the molecule can react chemically.
For example, 1-propanol is a primary alcohol, with the chemical structure of \( CH_3CH_2CH_2OH \). The presence of the OH group on a terminal carbon atom makes it more reactive in oxidation reactions.

• Primary alcohols like 1-propanol can be oxidized to form aldehydes and subsequently carboxylic acids.
• This makes primary alcohols very useful in organic synthesis and for analytical tests where distinguishing between different types of alcohol is necessary.

Secondary Alcohol

Secondary alcohols are characterized by a hydroxyl group bonded to a carbon atom that is further connected to two other carbon atoms. This alters the nature and extent of their chemical reactivity.
2-propanol, with the formula \( CH_3CHOHCH_3 \), is a secondary alcohol. This means that upon oxidation, it converts only to a ketone rather than further oxidation products.

• The inability of secondary alcohols to oxidize to carboxylic acids is due to the structure surrounding the OH group.
• Secondary alcohols are widely used in industrial and laboratory settings due to their specific reaction patterns.

Fehling's Solution

Fehling's solution is a chemical reagent used to differentiate between aldehyde and ketone functional groups. It is composed of two solutions that need to be mixed just prior to the test.

• Fehling's solution reacts specifically with aldehydes, causing a characteristic red precipitate to form, which is indicative of a positive test.
• The solution does not react with ketones, allowing for the differentiation between oxidized products of primary and secondary alcohols.

This makes it particularly useful in identifying the presence of aldehydes following the alcohol oxidation reaction, as in the case of 1-propanol turning into an aldehyde upon oxidation.

Aldehyde

An aldehyde is an organic compound containing a formyl group \(-CHO\). It is typically formed when a primary alcohol undergoes mild oxidation.
In the oxidation of 1-propanol, an aldehyde like propanal is first created before forming carboxylic acids upon further oxidation.

• Aldehydes are reactive and easily identified by reagents such as Fehling's solution.
• The reaction of aldehydes with Fehling's solution results in a positive test, important for distinguishing them from ketones.

This aspect of aldehydes is leveraged in tests designed to tell primary alcohols apart from secondary ones.

Ketone

Ketones are the products of oxidized secondary alcohols. They have the structure \( RC(=O)R' \), where the carbon atom of the carbonyl group is bonded to two other carbon atoms.
In the context of the exercise, when 2-propanol (a secondary alcohol) is oxidized, the result is acetone, a simple ketone.

• Unlike aldehydes, ketones do not react with Fehling's solution.
• This lack of reactivity helps in differentiating the oxidation products of primary and secondary alcohols.

Thus, the formation of ketones is a key step in identifying secondary alcohols during oxidation tests, as they do not proceed to form more complex carboxylic acids.