Question

Which of the following describe(s) pyridinium chlorochromate \((P C C) ?\) An oxidant that can form aldehydes from primary alcohols An oxidant that can completely oxidize primary alcohols I. \(\quad\) An oxidant that can completely oxidize secondary alcohols (A) I only (B) I and II only (C) II and III only (D)?, II, and III

Short answer

(B) I and II only

Step-by-step solution

- Understand the role of Pyridinium Chlorochromate (PCC)

Pyridinium Chlorochromate (PCC) is a reagent commonly used in organic chemistry. It is primarily used to oxidize primary alcohols to aldehydes and secondary alcohols to ketones. It is important to note that PCC does not typically completely oxidize primary alcohols to carboxylic acids.

- Evaluate Statement I

Statement I suggests that PCC can completely oxidize secondary alcohols. Recall that PCC oxidizes secondary alcohols to ketones, not completely oxidizing them further. Therefore, Statement I is incorrect.

- Evaluate Statement II

Statement II suggests that PCC can form aldehydes from primary alcohols. This is true, as PCC's main function is to oxidize primary alcohols to aldehydes without further oxidation to carboxylic acids.

- Check Context of Complete Oxidation in Statement III

Statement III is similar to II but suggests complete oxidation of primary alcohols. As established, PCC does not completely oxidize primary alcohols to carboxylic acids but stops at aldehydes.

- Determine the Correct Answer

The only accurate statement about the function of PCC is Statement II, which asserts that PCC can oxidize primary alcohols to aldehydes.

Key concepts

Oxidation of Primary Alcohols

When we talk about the oxidation of primary alcohols, we focus on converting these alcohols to aldehydes or carboxylic acids. A primary alcohol is a molecule where the carbon bearing the -OH group is attached to only one other carbon atom. Under mild oxidation conditions, primary alcohols are typically oxidized to aldehydes. More aggressive oxidation converts them to carboxylic acids.

Pyridinium Chlorochromate (PCC) is a reagent that plays a key role here. PCC specifically oxidizes primary alcohols to aldehydes. Unlike strong oxidizing agents like potassium permanganate (KMnO4) or Jones reagent (H2CrO4), PCC stops the oxidation process at the aldehyde stage. This is highly useful in organic synthesis, as it allows for selective oxidation without further transforming the aldehyde to carboxylic acid.

For example:
- Methanol (a primary alcohol) can be selectively oxidized by PCC to formaldehyde (an aldehyde).
- Ethanol can be oxidized by PCC to acetaldehyde.

Always remember, although PCC is great for forming aldehydes from primary alcohols, it does not completely oxidize them to carboxylic acids.

Oxidation of Secondary Alcohols

Secondary alcohols behave differently during oxidation compared to primary alcohols. In secondary alcohols, the carbon atom carrying the -OH group is connected to two other carbon atoms. The oxidation of secondary alcohols results in the formation of ketones.

Pyridinium Chlorochromate (PCC) is also instrumental in oxidizing secondary alcohols. PCC converts secondary alcohols to ketones without further oxidation. This makes it a very controlled and selective reagent in organic chemistry.

Here are some examples:
- Isopropanol (a secondary alcohol) is oxidized by PCC to acetone (a ketone).
- Cyclohexanol is oxidized by PCC to cyclohexanone.

It’s crucial to note that PCC does not lead to complete oxidation beyond the ketone stage. It halts the oxidation at the ketone level, preventing the formation of carboxylic acids or other products. This controlled oxidation makes PCC a valuable reagent for organic chemists looking to synthesize ketones.

Organic Chemistry Reagents

In organic chemistry, reagents are substances or compounds added to a system to cause a chemical reaction or test if a reaction occurs. Some reagents are specific to certain types of reactions, providing a level of control over the outcome.

Pyridinium Chlorochromate (PCC) is an essential reagent used widely for oxidation processes. PCC is particularly favored because it allows for selective oxidation:

- It oxidizes primary alcohols to aldehydes without further oxidation to carboxylic acids.
- It oxidizes secondary alcohols to ketones without further reaction.

Other major oxidizing agents include potassium permanganate (KMnO4), which is a strong oxidizer and often used for complete oxidation to carboxylic acids, and Jones reagent (H2CrO4), similar in use to KMnO4 but with slightly milder conditions. PCC, though, is specifically valued for its moderate strength and selectivity, as it provides chemists with precise control over their oxidation reactions.

Understanding the characteristics and applications of different reagents is crucial in organic chemistry. Proper selection of a reagent, like PCC, according to the desired end-product, significantly affects the efficiency and outcomes of chemical syntheses.