Test: Diols & Polyols - NEET MCQ

• The transformation involves the conversion of an alkene to a diol.
• Performic acid (peroxyformic acid) is used for this conversion, which adds hydroxyl groups across the double bond in a syn-addition manner.
• Option C utilizes performic acid followed by hydrolysis, which effectively yields a syn-diol.
• The reaction mechanism involves the formation of an epoxide intermediate, which is then opened by water to form the syn-diol.

Greater the number of hydroxy groups, greater the solubility in water. Between (II) and (III), (III) is slightly less soluble due to greater hydrophobic alkyl fraction.

At planar carbonyl carbons, hydride ion attacks from both sides giving both stereomers. Hence, the product diols would be all stereomers of 2, 3 - butanediol.

• Ethylene glycol reacts with phosphorus halides (PCl₃ and PBr₃) and thionyl chloride (SOCl₂) to form dihalides by substitution reactions.
• PI₃ does not react similarly because it is unstable and decomposes easily, particularly in solution, making it unsuitable for dihalide formation.
• The reactivity and stability of phosphorus halides decrease from PCl₃ to PI₃, with PI₃ being the least stable.
• Thus, PI₃ does not effectively form dihalides with ethylene glycol.

Both oxirane hydrolysis and halide substitution (S_N2) take place.

Ethylene glycol on heating with concentrated H₂SO₄ gives 1 , 4-dioxane

(a) Alkaline KMnO₄ gives syn, vicinal diol with alkene.
(b) Here, oxirane is formed as the major product.
(c) Epoxidation followed by hydrolysis gives diol.
(d) Dihalide formed in the first step undergoes S_N2 reaction with NaHCO₃ giving diol.

(a) Vicinal diols, on heating with concentrated H₂SO₄ gives dioxane as the major product.



(d) Diols do not give dioxane in alkaline medium.

Syn vicinal diols undergo oxidative cleavage with HIO₄ giving diols, Option (b) and option (c) are anti diol and have restricted rotation.

It is a Pinacol-pinacolone rearrangement.

(I) on deprotonation gives (a) while (II) on deprotonation gives (b).

Pinacol-pinacolone rearrangement

Deprotonation of (I) gives (a) while deprotonation of (II) gives (c).

• Vicinal diols which is not tertiary, prefer to form dioxane while tertiary diols prefer to undergo Pinacol-pinacolone rearrangement.
• The diol is not tertiary, hence more likely to form dioxane.

Vicinal diols which is not tertiary, prefer to form dioxane while tertiary diols prefer to undergo Pinacol-pinacolone rearrangement.
It is tertiary diol, undergo Pinacol-pinacolone rearrangeme.

Vicinal diols which is not tertiary, prefer to form dioxane while tertiary diols prefer to undergo Pinacol-pinacolone rearrangement.

(i) It is a symmetrical, 3° diol, gives (CH₃)₂CO with HIO₄, a single pinacolone (CH₃)₃C —COCH₃ and a single cyclic a mine :

(ii) With HIO₄ gives (CH₃)2CO and (C₆H₅)₂CO. (ii) is unsymmetrical, 3° diol, gives more than one pinacolone in rearrangement reaction. However it gives a single cyclic amine.

(iii) with HIO₄ gives a single ketone-acetophenone. It gives two different pinacolone in rearrangement reaction.

(iv) It is unsymmetrical, 3° diol, gives two different ketones with HIO₄ and more than one pinacolone in rearrangement reaction.

• Oxidation with KMnO4 is effective because:
• Primary alcohols are oxidized by KMnO4 to form carboxylic acids.
• Diols, especially vicinal diols, can be oxidized to form dicarboxylic acids or cleaved into carboxylic acids and ketones.
• Lucas Test is more suitable for distinguishing between primary, secondary, and tertiary alcohols, not diols.
• Reaction with NaH and Iodoform Test are not suitable for distinguishing between a primary alcohol and a diol.
• Therefore, method B is the best choice.

• OsO₄ (osmium tetroxide) is used to directly convert alkenes into vicinal diols through syn-dihydroxylation, where hydroxyl groups are added to adjacent carbon atoms on the same side of the double bond.
• KMnO₄ in a basic medium can also achieve this reaction, but in an acidic medium, it often leads to oxidative cleavage.
• Cl₂/H₂O forms halohydrins, not diols.
• H₂SO₄ is not used for direct conversion to diols.