Question

How HIO4 oxidise geminal alcohol

Answer 1

HIO4 does not oxidize geminal alcohols via its characteristic oxidative cleavage mechanism because the hydroxyl groups are on the same carbon, preventing the formation of the required cyclic intermediate involving adjacent carbons.

Answer 2

Here's the explanation of how HIO4 reacts with geminal alcohols, suitable for JEE/NEET level.

Periodic acid (HIO4) is a strong oxidizing agent specifically used for the oxidative cleavage of compounds having hydroxyl groups or carbonyl groups on adjacent carbon atoms (1,2-difunctionalized compounds). The reaction typically proceeds through a cyclic intermediate formed between the substrate and HIO4.

Examples of compounds oxidized and cleaved by HIO4:

1. Vicinal diols (1,2-diols): Cleaved into aldehydes or ketones.

   RCH(OH)-CH(OH)R' -> RCHO + R'CHO (if R, R' are H or alkyl)

   R2C(OH)-C(OH)R'2 -> R2CO + R'2CO

2. $\alpha$-hydroxy aldehydes or ketones: Cleaved into carboxylic acids and aldehydes/ketones.

   RCOCH(OH)R' -> RCOOH + R'CHO

3. $\alpha$-diketones: Cleaved into carboxylic acids.

   RCOCOR' -> RCOOH + R'COOH

Now, consider a geminal alcohol (geminal diol). A geminal diol has two hydroxyl groups on the same carbon atom (1,1-diol).

Example: A simple geminal diol like ethane-1,1-diol.

   OH
   |
R - C - H
   |
   OH

(This exists in equilibrium with RCHO + H2O)

The characteristic oxidative cleavage reaction of HIO4 requires the functional groups (-OH, C=O, etc.) to be on adjacent carbon atoms to form the cyclic periodate ester intermediate that leads to C-C bond cleavage.

In a geminal diol, the two hydroxyl groups are on the same carbon atom. There is no C-C bond between the carbons bearing the hydroxyl groups (because it's the same carbon). Therefore, the standard cyclic intermediate required for HIO4 cleavage cannot be formed.

Consequently, HIO4 does not oxidize or cleave simple geminal diols in the same way it oxidizes vicinal diols.

A geminal diol is typically in equilibrium with the corresponding carbonyl compound (aldehyde or ketone) and water:

   OH
   |
R - C - R'  <=>  R - C=O  +  H2O
   |                 |
   OH                R'

(where R or R' can be H)

While HIO4 is a strong oxidant, its primary and characteristic reaction is the oxidative cleavage of 1,2-difunctionalized compounds. It does not typically oxidize simple aldehydes or ketones to carboxylic acids under the standard conditions used for diol cleavage. Other oxidants like KMnO4 or CrO3 are commonly used for oxidizing aldehydes to carboxylic acids.

Therefore, HIO4 does not perform its characteristic oxidative cleavage on geminal diols because they lack the necessary structural feature (functional groups on adjacent carbons) to form the reactive cyclic intermediate. If a reaction involving HIO4 and a compound containing a geminal diol feature is observed, it is likely due to HIO4 reacting with another part of the molecule or the geminal diol being an unstable intermediate that converts to a carbonyl which might then react differently (though not typically cleaved by HIO4).

In summary: HIO4 does not oxidize geminal alcohols via its standard oxidative cleavage mechanism because the hydroxyl groups are on the same carbon, preventing the formation of the required cyclic intermediate involving adjacent carbons.

Explanation: HIO4's mechanism involves cleaving a C-C bond between two carbons bearing specific functional groups (like -OH). Geminal diols have two -OH groups on the same carbon, meaning there is no C-C bond between the carbons bearing the -OH groups to cleave.

The question asks "How HIO4 oxidise geminal alcohol". The most accurate answer is that it doesn't oxidize it via the characteristic oxidative cleavage mechanism. If a reaction does occur, it is not the typical HIO4 reaction involving C-C bond cleavage between the carbons bearing the hydroxyl groups.

Explanation of the solution:

HIO4 oxidizes compounds with functional groups on adjacent carbons (1,2-difunctionalized). It forms a cyclic intermediate leading to C-C bond cleavage. Geminal alcohols (1,1-diols) have hydroxyl groups on the same carbon. They lack adjacent carbons bearing these groups, preventing the formation of the necessary cyclic intermediate for oxidative cleavage. Therefore, HIO4 does not oxidize geminal alcohols via its characteristic cleavage reaction.