Sciencemadness Discussion Board

oxidation of sugar

CuReUS - 20-10-2014 at 05:02

recently i had an idea of making tertiary alcohols from glucose
because a tertiary alcohol was needed for another reaction
here is that thread from organic chemistry section
http://www.sciencemadness.org/talk/viewthread.php?tid=15171

now the idea is to oxidise all the secondary alcohols to ketones and the primary alcohol to carboxylic acid.as glucose contains a lactone ring,you will end up with 4 keto and 2 carboxylic acid groups
that will be treated with methyllithium to convert the keto and the carboxylic acids to tertiary alcohols

the problem is with the oxidation
i tried using KMnO4 in neutral solution of sugar
initally the color was pink
after 2 hours ,the color changed to red
after 4 hours the color was very pale brown
finally after 12 hours from the start,the solution was colorless
and MnO2 was settled at the bottom

i think that the glucose(actually i used sucrose ,but that will finally get hydrolysed to give glucose right?)
has become aldonic acid ,with the hydroxy groups remaining intact

so can anyone suggest better oxidising agents that can carry out the reaction without causing too much decarboxylation:(

pampyrozec - 21-10-2014 at 04:30

How did you come to the conclusion that
Quote:
the glucose has become aldonic acid ,with the hydroxy groups remaining intact

? Permanganate in neutral environment is actually one of the wildest oxidizing agents. It cleaves vicinal diols towards carboxylic acids with no mercy. Having a long chain of vininalities, this hardly stops before carbon dioxide. (Read also page 342 in Chemistry of The Carbohydrates).


[Edited on 21-10-2014 by pampyrozec]

CuReUS - 22-10-2014 at 06:30

i thought it was aldonic acid because i saw it in a paper
i cant find it now but i will upload it later:(

actually i thought that permanganate in neutral solution would be a weaker oxidising agent as Mn changes from +7 to +4 whereas in acidic or basic conditions ,it goes from +7 to +2,which means that it gets reduced more which in turn means that it oxidises your substrate more

i think it cleaves vicinal diols only when heat is supplied ,not in cold and dilute solution(i used 2 crystals for 1 tablespoon of sugar)

even if it converts to gluconic acid ,only one carbon will be lost as carbon dioxide

so now i am going to run the reaction again but i am going to connect a bent tube from the reaction bottle to a bottle of lime water,if any carbon dioxide is formed,it will form a ppt

also in the book you mentioned ,chemistry of carbohydrates ,i saw that they talk about oxidation of polyols to ketoses,i will check that out ,thanks:)