Osazone Formation: A Symphony of Sugar Chemistry




Uploaded by lamechivanes on 13.12.2010

Transcript:

We’ll finish our study of monosaccharide reactions
with osazone formation.
And while this reaction was of historical importance
for the elucidation of monosaccharide structures,
it’s no longer useful um, that way.
However, the chemistry is very closely related
to chemistry that takes place in food science
where amines combine with sugars
to make a variety of different ah, sugar derivatives.
And so I still like to include this reaction
and the study of its mechanisms.
Starting with D-glucose, D-glucose in the pyranose form,
in the presence of three equivalents of
the molecule of phenylhydrazine whose structure is shown here.
Two nitrogens are bound together.
We end up making the derivative known as the osazone
that’s shown there which has two hydrozone functionalities.
Phenylhydrazones at C1 and C2.
There’s also the third equivalent of phenylhydrazine
that ends up making aniline, that’s the structure shown here
– and then a molecule of ammonia.
You can notice in this reaction
and our mechanism will have to explain it,
that there’s an oxidation state change at carbon 2.
We had one carbon-hydrogen bond
and in the product, the osazone has none
and so at carbon 2
there’s been an oxidation reaction taking place.

As far as the initial steps of the mechanism,
I won’t work out all of these details,
but you should be able to write
the formation of phenylhydrazone.
Phenylhydrazone comb- is the reaction in which
phenylhydrazine combines with the carbonyl –
in this case the aldehyde – to make the hydrozone derivative.
And that reaction mechanism is very closely related
to imine formation and I would encourage you
to just stop the recorder
and go ahead and make sure you can write the ah, steps
involved in the transformation
of the aldehyde into the hydrazone.
Because that’s really the first step
in osazone formation, starting with the
ah, pyranose form of glucose,
we can open it up into the open chain form.
Then the carbonyl at position #1 can generate
the phenylhydrozone derivatives seen here.
At this point,
we have an acidic hydrogen at the α position
and that hydrogen can tautomerized through
a process that looks very much like
that base promoted isomerization of glucose.
And so, what we’re going to do - general base,
general acid, catalyzed process of tautomerization,
we can then ah, turn that around
and make the carbonyl at position 2
by protonating the position, carbon 1 to make uh,
the carbon 1 now picking up a second hydrogen.
And so, we have a- a CH2 at carbon 1
and a carbonyl at position 2,
still having the nitrogen bound to carbon 1.
I know the mechanism is kind of complicated,
but if you keep going,
what you can do is make another hydrazone
at position 2 with a- a second equivalent of hydrazine
– and that’s exactly what happens.
So, these tautomerization reactions are followed by
a second hydrozone formation
to generate this intermediate that’s shown here.
What happens next is a base ah, promoted process
of fragmentation at position 1.
And you can see what’s going on there,
we’re basically breaking that weak nitrogen-nitrogen bond
as our leaving group.
It’s a type of β elimination, where we’re generating now
a new carbon-nitrogen double bond
with loss of ah, the molecule of aniline.
And so here’s where our aniline is formed
in- in this particular step.
So, ah, at that point,
we now have the possibility of making a molecule of ammonia
that can happen by hydrolysis
of that carbon-nitrogen bond on position #1.
And that’s exactly what happened,
we basically have an imine functional group
which loses ammonia
and makes a new carbonyl at position 1.
That carbonyl can now take and make a hydrazone
with another equivalent of hydrazine.
Hydrazone formation at carbon 1
concludes the reaction mechanism
and generates the osazone of glucose.