Wood Chemistry
Wood Chemistry
Why Study Monosaccharides?
As we discussed in lecture 1,
wood carbohydrates
(cellulose and hemicelluloses)
are made of various sugars
such as glucose, mannose,
xylose, etc. In order to
understand the properties
and reactions of the
carbohydrates, we need to
understand
monosaccharides.
Wood Chemistry
PSE 406/Chem E 470
Lecture 2:
Monosaccharides
D-glucose
PSE 406 - Lecture 2
Wood Chemistry
1
What is a Monosaccharide?
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H
l
Monosaccharides are
polyhydroxy aldehydes or
ketones (aldoses or ketoses)
of 3-9 carbons.
» Polyhydroxy means that there is
more than 1 hydroxyl group (OH)
on the carbon backbone.
» Aldehydes and ketones both
contain carbonyl groups.
l
Typically monosaccharides
contain either 5 or 6 carbons.
PSE 406 - Lecture 2
C
l
O
R
R
O
R
2
Glyceraldehyde
This 3 carbon sugar is the
simplest monosaccharide.
» It is an aldehyde.
» As is the case with all
monosaccharides, it contains an
asymmetric or chiral carbon.
This means there are 4 different
groups attached to this carbon
» Because of this, there are 2
different stereoisomers of this
structure known as
enantiomers .
Aldehyde
C
PSE 406 - Lecture 2
CHO
H
C
OH
CH2OH
Ketone
3
PSE 406 - Lecture 2
4
1
1
Glyceraldehyde
Representations
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Wood Chemistry
3 D Views of the Enantiomers
of Glyceraldehyde
1. Wedge and Dash
= Carbon
H
CHO
CHO
H
C
HO
OH
C
H
CH 2 O H
CH 2 OH
2. Fischer Formula
C
C HO
OH
HO
CH2 O H
C
H
C H2 O H
D-glyceraldehyde
= Hydrogen
HO
In the Fischer representation,
vertical lines indicate bonds
going into the paper while
horizontal lines indicate bonds
coming out of the paper.
C
PSE 406 - Lecture 2
CH2OH
CHO
HO
D Glyceraldehyde
5
(Not Found in Hemicelluloses or Cellulose)
CHO
H
H
C OH
Enantiomers
CH2OH
HO C
H
HO C
H
CH2OH
D-erythrose
L-erythrose
Diasteriomers
CHO
HO
H
C
H
C OH
CH2OH
D-threose
CHO
Enantiomers
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H C OH
HO C H
CH2OH
With 2 chiral
centers in the
molecule, there
are 4
sterioisomers;
two pairs of
enatiomers and
two pairs of
diasteriomers.
l
l
L-threose
PSE 406 - Lecture 2
7
H
L Glyceraldehyde
PSE 406 - Lecture 2
l
CHO
C OH
C
CH2OH
Four Carbon Sugars
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CHO
H
C
OH
CH2OH
L-glyceraldehyde
Notes
= Oxygen
CHO
H
CHO
CHO
OH
CH2OH
L-glyceraldehyde
D-glyceraldehyde
H
C
In this representation,
the dotted lines indicate
bonds going into the paper
the wedges indicate bonds
coming out of the paper.
6
Stereochemistry
Enantiomers are identical in physical properties
except that they rotate polarized light in opposite
directions. From a biochemical standpoint, they are
different.
Diastereoisomers possess different physical
properties
For n asymetric carbons there are:
» 2 n maximum possible number of sterioisomers
» 2 n-1 maximum possible number of enantiometric
pairs
» That means there are 2n-1 possible
diastereoisomer pairs
PSE 406 - Lecture 2
8
2
2
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Enatiomers of Erythrose
Wood Chemistry
l
CHO
CHO
HO
H
H
HO
H
H
CH 2 OH
OH
OH
CH2 OH
D Erythrose
L Erythrose
l
Are you a 3D type of Student?
How do chemical
representations
correlate with the
actual spatial
arrangement of the
molecule?
The representations
are a simplified (but
tricky way) of showing
the molecule.
We don’t care
about steriochemistry
CHO
HO
H
HO
H
CH 2 OH
PSE 406 - Lecture 2
Wood Chemistry
l
9
Simple Bob Method for 3D
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11
10
Simple Bob Method for 3D (II)
Check out the second carbon
down, in the 3d version the
carbons attached to this atom both
are going into the page. The OH
group and the H group attached to
this carbon are coming out of the
page and pointing to the left and
right respectively. Therefore, in
the Fischer formula these are
drawn to the left and right.
Remember, in the
Fischer formula, the
vertical lines always go
into the paper. You
don’t want your
molecule to be a circle
so in 3D the carbon
backbone alternates in
and out of the page.
PSE 406 - Lecture 2
PSE 406 - Lecture 2
PSE 406 - Lecture 2
CHO
HO
H
HO
H
CH 2 OH
12
3
3
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Simple Bob Method for 3D (III)
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Now if you go to the 3rd
carbon from the top, you
will notice that the carbon
above and below it are
coming out of the page;
this is unacceptable . To
fix this, we rotate the
molecule 180°.
PSE 406 - Lecture 2
Simple Bob Method for 3D (IV)
If you look at the molecule now
you will see that the OH and H
groups on the 3rd carbon now
stick out of the page and are on
the left and right side
respectively. Draw them into the
Fischer formula and you get
your final molecule.
It may seem a little tricky but it is
a fun thing to do at parties.
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PSE 406 - Lecture 2
CHO
HO
H
HO
H
CH 2 OH
14
4
4