Question

To convert 2-butanone into propionic acid (a) \(\mathrm{NaOH}+\mathrm{Nal} / \mathrm{H}^{+}\) (b) Fehling's test (c) \(\mathrm{I}_{2} / \mathrm{NaOH} / \mathrm{H}^{+}\) (d) Tollen's reagent

Short answer

(c) \\(\mathrm{I}_{2} / \mathrm{NaOH} / \mathrm{H}^{+}\\) is the correct reagent for converting 2-butanone to propionic acid.

Step-by-step solution

Identify the Functional Group in 2-butanone

2-butanone is a ketone, which means it has a carbonyl group (C=O) attached to two alkyl groups. It's important to note that ketones cannot be directly oxidized to acids without breaking a carbon-carbon bond.

Introduce Suitable Oxidizing Agents

To convert a ketone like 2-butanone to a carboxylic acid such as propionic acid, we need to first cleave one of the carbon-carbon bonds. Among the given choices, the use of \(\mathrm{I}_{2} / \mathrm{NaOH} / \mathrm{H}^{+}\) is a suitable method known as the Haloform reaction. This will convert the ketone to a carboxylic acid.

Mechanism of the Haloform Reaction

In this reaction, the methyl ketone group in 2-butanone reacts with iodine in the presence of a base (NaOH) to form a carboxylate salt and iodoform (CHI₃). The carboxylate salt can then be acidified to yield propionic acid (CH₃CH₂COOH).

Final Reaction Step

By adding a dilute acid \(\mathrm{H^{+}}\), the carboxylate ion is protonated to form the carboxylic acid, propionic acid. The overall transformation involves the conversion of the terminal methyl group adjacent to the carbonyl into a carboxylic acid.

Key concepts

Haloform Reaction

The haloform reaction is a fascinating chemical process where a methyl ketone is transformed into a carboxylic acid. This occurs due to the action of halogens (like iodine) in the presence of a base, typically sodium hydroxide (NaOH). The haloform reaction is particularly noted for converting methyl ketones because they contain a structure in which this reaction can efficiently occur.

Here's how it works:

• The ketone's methyl group is attacked by iodine, leading to the formation of a triiodomethyl compound.
• This intermediate compound is then attacked by hydroxide ions, breaking down into iodoform (CHI₃).
• The remaining part of the molecule becomes a carboxylate ion, a precursor to carboxylic acids.

The iodoform that results is what typically gives this reaction its yellow precipitate, a useful visual cue in laboratory settings. Ultimately, when the carboxylate ion is acidified with \( \text{H}^+ \), a carboxylic acid is formed. This reaction is particularly well-known for dealing with ketones like 2-butanone and converting them into smaller carboxylic acids.

Oxidation of Ketones

Unlike alcohols, ketones cannot be straightforwardly oxidized to carboxylic acids. This is due to the nature of the carbonyl group's position in ketones, where it is bonded to two alkyl groups. To convert a ketone into an acid, one must break a carbon-carbon bond, which is not typical of oxidation reactions involving non-terminal carbonyls.

In the exercise provided, 2-butanone is transformed into propionic acid not by direct oxidation but through the haloform reaction, as mentioned earlier. The haloform process can be viewed as a workaround method since it involves the cleaving of a carbon-carbon bond – a step that regular oxidation does not accomplish directly with ketones.

This unique transformation solidifies the role of specific reagents like \( \text{I}_2 / \text{NaOH} / \text{H}^+ \), which are adept at turning methyl ketones into acids by circumventing ordinary oxidation limitations.

Functional Groups

Functional groups are specific groups of atoms within molecules responsible for the characteristic chemical reactions of those molecules. In organic chemistry, understanding these groups is crucial because they define the behavior and reactivity of different compounds.

For example, in 2-butanone, the functional group is the carbonyl group (C=O). This group classifies 2-butanone as a ketone, which significantly influences its chemical reactions. Knowing that 2-butanone is a ketone informs the chemist that it will not behave the same as an alcohol or an aldehyde, both of which have different functional groups.

Because functional groups determine the types of reactions that compounds can undergo, recognizing the presence and type of these groups is foundational in planning chemical transformations, such as converting 2-butanone to propionic acid through a haloform reaction. In essence, the ability to identify functional groups allows chemists to predict reaction pathways and outcomes effectively.

Mastery of functional groups not only helps in predicting reactions but also in designing syntheses for various chemical processes, especially in organic synthesis and industrial applications.