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Conversion of 1,2-glycols pinacol to ketone or aldehyde pinacolone by means of acid is called pinacol-pinacolone rearrangement. Its mechanism is as follows-. Note:Migration of alkyl group and elimination of -OH by the proton catalyst take place simultaneously.
The migrating group migrates within the molecule. When a mixture of two different pinacols are heated in the present of acid , no cross product is formed. Share on Facebook. Pinacol-Pinacolone Rearrangement source : you tube. Main features of Pinacol-Pinacolone Rearrangement — 1.
Stability of the carbonium ion — when two -OH groups are different then -OH group will be removed which products more stable carbonium ion. Example: 2. Migratory aptitude of the group- a The migrating group may be alkyl,aryl or even hydrogen atom,then migrating atom is Hydrogen atom.
Example1: Example2: b When each of the carbonatoms of glycol has an alkyl and an aryl group,then preferentially aryl group migrates.
Example: c When one carbon atom possesses two alkyl group and other aryl group then alkyl group migrates to form more stable carbonium ion. Example: 5 Transmigration — The migrating group migrates to the trans side of the leaving group. Recent Posts Rank the following atoms and molecules based on which would effuse the fastest. Saroj Bhatia. Next Article Equilibrium constant. Like our Facebook Page. Chemistry Online Guru.
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The pinacol rearrangement was the first molecular rearrangement identified as such by early chemists. The defining example of a pinacol rearrangement is shown in the following diagram. Pinacol itself is produced by magnesium reduction of acetone, probably by way of a ketyl intermediate. Since the diol is symmetrical, protonation and loss of water takes place with equal probability at either hydroxyl group. A 1,2-methyl shift generates an even more stable carbocation in which the charge is delocalized by heteroatom resonance.
30.2: Pinacol Rearrangement
The pinacol—pinacolone rearrangement is a method for converting a 1,2-diol to a carbonyl compound in organic chemistry. The 1,2-rearrangement takes place under acidic conditions. The name of the rearrangement reaction comes from the rearrangement of pinacol to pinacolone. This reaction was first described by Wilhelm Rudolph Fittig in of the famed Fittig reaction involving coupling of 2 aryl halides in presence of sodium metal in dry ethereal solution.