PINACOL AND SEMIPINACOL REARRANGEMENTS

PINACOLONE REARRANGEMENT ::

                                          
                                                                                                     PINACOLS ARE DIETERTIARY 1,2 - DIOLS . THE SIMPLEST MEMBER OF THIS CLASS IS:

CH3-C(OH)(CH3)-C(OH)(CH3)-CH3

WHEN PINACOL IS HEATED WITH DILUTE OR MODERATELY CONCENTERATED H2SO4 , A REARRANGEMENT REACTION TAKES PLACE WHICH LEADS TO THE FORMATION  OF KETONE  CALLED PINACOL AND THE REARRANGEMENT IS KNOWN AS PINACOL PINACOLON REARRANGEMENT .

CH-C(OH)(CH3)-C(OH)(CH3)-CH3 --------> CH3-C(CH3)(CH3)-COO-CH3

SEMIPINACOL  REARRANGEMENT ::

                                                                                                               SEMIPINACOL REARRANGEMENT IS GIVEN BY THOSE ALCOHOLS WHICH HAVE A VERY GOOD LEAVING GROUP AT BETA - CARBON . THE LEAVING GROUP MAY BE HALO GROUP (I) AND A DIAZO GROUP . BEST RESULT IS ALWAYS OBTAINED BY TOSYL GROUP .  TOSYL GROUP IS A BULKY GROUP AND HENCE TOSYLATION REACTION AT LESS HINDERED BY HYDROXYL GROUP .PINACOL AND SEMIPINACOL  , BOTH ARE REGIOSELECTIVE BUT REGIOSELECTIVITY IS IN OPPOSITE DIRECTION .


R-C(OH)(R)-C(OH)(R')-H  ------------------------->  R-CH(R)-COO-R



AUTHOR ::::::::::::::    YASHVEER VAIS