Question

Among the following compounds, the one that responds positively to both lodoform test and Fehling'stests (A) propionaldehyde (B) acetaldehyde (C) acetone (D) ethyl alcohol

Short answer

None of the given compounds (A) propionaldehyde, (B) acetaldehyde, (C) acetone, and (D) ethyl alcohol possess both methyl ketone and aldehyde functional groups. Therefore, no compound responds positively to both Iodoform test and Fehling's test.

Step-by-step solution

Understand the compound structures

First, let's understand the chemical structures of the given compounds to check if they possess the required functional groups. (A) propionaldehyde: \(CH_3 CH_2 CHO\) (B) acetaldehyde: \(CH_3 CHO\) (C) acetone: \(CH_3 CO CH_3\) (D) ethyl alcohol: \(CH_3 CH_{2} OH\)

Check for the presence of methyl ketone group

Now, let's see which of the compounds have the methyl ketone (CH3C=O) group: (A) In propionaldehyde, there is no ketone group present. (B) In acetaldehyde, there is no ketone group present. (C) In acetone, there is a methyl ketone group present. (D) In ethyl alcohol, there is no ketone group present. Only acetone (C) has a methyl ketone group.

Check for the presence of aldehyde group

Now let's check for the presence of an aldehyde group (R-CHO): (A) In propionaldehyde, there is an aldehyde group present. (B) In acetaldehyde, there is an aldehyde group present. (C) In acetone, there is no aldehyde group present. (D) In ethyl alcohol, there is no aldehyde group present. Propionaldehyde (A) and acetaldehyde (B) have an aldehyde group.

Identify the compound fulfilling both requirements

We are looking for a compound with both a methyl ketone group and an aldehyde group. None of the compound has both functional groups. Therefore, no compound responds positively to both Iodoform test and Fehling's test.

Key concepts

Aldehyde Group Detection

The functional group known as an aldehyde contains a carbon atom bonded to a hydrogen atom and double-bonded to an oxygen atom (\[\text{R-CHO}\] where R represents a hydrocarbon chain or hydrogen). This group is quite reactive due to the presence of the carbonyl group (C=O), which makes it easy to detect.

A common method used to detect aldehydes in organic compounds is Fehling's test. This test involves the use of Fehling's solution, which is a mixture of copper(II) sulfate, potassium sodium tartrate, and sodium hydroxide. When an aldehyde is added to Fehling's solution and heated, it reduces the copper(II) ion to copper(I), causing a reddish precipitate of copper(I) oxide to form. This indicates a positive result.

• Propionaldehyde (\[\text{CH}_3\text{CH}_2\text{CHO}\]) is an example of a compound that gives a positive Fehling's test.
• Acetaldehyde (\[\text{CH}_3\text{CHO}\]) also reacts positively with Fehling's solution.

Aldehydes are distinct in their reactions because ketones, another class of carbonyl compounds, do not respond to Fehling's test, which reinforces the specificity of this detection method.

Methyl Ketone Group

A methyl ketone contains a ketone group directly bonded to a methyl group. The general formula for a methyl ketone is \[\text{RCOCH}_3\] where R can be any alkyl or aryl group. One key feature of methyl ketones is the presence of the carbonyl group (C=O) adjacent to a methyl group, which gives them distinct chemical properties.

Methyl ketones are identifiable through the iodoform test, which specifically targets compounds with the methyl ketone configuration. In this test, a methyl ketone is treated with iodine in the presence of a base, usually sodium hydroxide. If a methyl ketone exists, the compound gets oxidized to form iodoform (\[\text{CHI}_3\]), which is a yellow precipitate that indicates a positive test.

• Acetone (\[\text{CH}_3\text{COCH}_3\]) contains a methyl ketone group and will give a positive result in the iodoform test.

Methyl ketones are different from simple aldehydes as they do not give positive results with tests like Fehling's, highlighting their unique reactivity in organic chemistry.

Iodoform Test and Fehling's Test

Iodoform and Fehling's tests are classic assays in organic chemistry used to detect specific functional groups. Each test has distinct chemical requirements, prioritizing different components of a molecule for detection.

**Iodoform Test:** This test is targeted towards identifying methyl ketones, alcohols with the structure \[\text{RCH(OH)CH}_3\], and certain secondary alcohols. The presence of a methyl ketone leads to the formation of the yellow iodoform precipitate when the compound is treated with iodine and a base. It's a particularly useful test for distinguishing methyl ketones from other carbonyl-containing compounds. Despite its limited scope, it’s a go-to for identifying common laboratory chemicals like acetone.

**Fehling's Test:** Unlike the iodoform test, Fehling's is primarily designed to spot aldehydes. In this reaction, aldehydes reduce the blue copper(II) ions to red copper(I) oxide, indicating the presence of an aldehyde group. Common compounds that test positive include simple aldehydes like acetaldehyde.

Together, these tests highlight the diverse chemical environment in an organic compound and underscore the specificity of traditional chemical tests for functional group identification. Although no compound can fulfill both tests due to their stringent structural needs, understanding each test's purpose and mechanism is pivotal in organic analysis.