Question

Name the reagents used in the following reactions : (i) Oxidation of a primary alcohol to aldehyde (ii) Oxidation of a primary alcohol to carboxylic acid

Hint:

Remember: PCC is a mild oxidizing agent that stops oxidation at the aldehyde stage, whereas acidified potassium dichromate and potassium permanganate are strong oxidizing agents that convert primary alcohols into carboxylic acids.

Answer 1

Concept: Alcohols are oxygen-containing organic compounds containing one or more hydroxyl (\(-OH\)) groups attached to a saturated carbon atom. One of the most important chemical properties of alcohols is their ability to undergo oxidation. The product obtained on oxidation depends upon:

• The nature of the alcohol (primary, secondary or tertiary).

• The strength of the oxidizing agent used.

• The reaction conditions.
Primary alcohols are particularly important because they can be oxidized either partially to aldehydes or completely to carboxylic acids depending upon the reagent employed.

(i) Oxidation of a primary alcohol to aldehyde

Step 1: Understanding the transformation A primary alcohol contains the functional group: \[ RCH_2OH \] When subjected to controlled oxidation, it loses two hydrogen atoms and forms an aldehyde. For example, \[ CH_3CH_2OH \xrightarrow{[O]} CH_3CHO \] The oxidation must stop at the aldehyde stage and should not proceed further to the carboxylic acid stage.

Step 2: Selection of reagent For this purpose, a mild oxidizing agent is required. The most commonly used reagent is Pyridinium Chlorochromate (PCC). PCC selectively oxidizes primary alcohols to aldehydes without causing further oxidation. \[ \boxed{\text{PCC (Pyridinium Chlorochromate)}} \]

Answer: \[ \boxed{\text{PCC}} \]

(ii) Oxidation of a primary alcohol to carboxylic acid

Step 1: Understanding the transformation When a stronger oxidizing agent is used, the aldehyde produced initially is further oxidized into a carboxylic acid. \[ RCH_2OH \xrightarrow{[O]} RCHO \xrightarrow{[O]} RCOOH \] For example, \[ CH_3CH_2OH \xrightarrow{[O]} CH_3COOH \]

Step 2: Selection of reagent Strong oxidizing agents such as acidified potassium dichromate or acidified potassium permanganate are commonly used. \[ K_2Cr_2O_7/H_2SO_4 \] or \[ KMnO_4 \] These oxidizing agents convert the alcohol completely into the corresponding carboxylic acid.

Answer: \[ \boxed{\text{Acidified } K_2Cr_2O_7} \] (or acidified \(KMnO_4\))

Final Answers: \[ \boxed{ \text{(i) PCC} } \] \[ \boxed{ \text{(ii) Acidified } K_2Cr_2O_7 } \]