Organic Chemistry

Section 22.11 Stability of Glucose 937

The -position is to the right in a Fischer

projection, down in a Haworth

projection, and axial in a chair

conformation.

A

PROBLEM 20

4-Hydroxy- and 5-hydroxyaldehydes exist primarily in the cyclic hemiacetal form. Give

the structure of the cyclic hemiacetal formed by each of the following:

a. 4-hydroxybutanal c. 4-hydroxypentanal

b. 5-hydroxypentanal d. 4-hydroxyheptanal

PROBLEM 21

Draw the following sugars, using Haworth projections:

a. -D-galactopyranose b. -D-tagatopyranose c. -L-glucopyranose

PROBLEM 22

D-Glucose most often exists as a pyranose, but it can also exist as a furanose. Draw the Ha-

worth projection of -D-glucofuranose.

22.11 Stability of Glucose

Drawing glucose in its chair conformation shows why it is the most common aldohex-
ose in nature. To convert a Haworth projection into a chair conformation, start by
drawing the chair so that the back is on the left and the footrest is on the right. Then
place the ring oxygen at the back right-hand corner and the primary alcohol group in
the equatorial position. (It would be helpful here to build a molecular model.) The pri-
mary alcohol group is the largest of all the substituents, and large substituents are
more stable in the equatorial position because there is less steric strain in that position
(Section 2.13). Because the OH group bonded to C-4 is trans to the primary alcohol
group (this is easily seen in the Haworth projection), the C-4 OH group is also in the
equatorial position. (Recall from Section 2.14 that 1,2-diequatorial substituents are
trans to one another.) The C-3 OH group is trans to the C-4 OH group, so the C-3 OH
group is also in the equatorial position. As you move around the ring, you will find that
all the OH substituents in -D-glucose are in equatorial positions. The axial positions
are all occupied by hydrogens, which require little space and therefore experience lit-
tle steric strain. No other aldohexose exists in such a strain-free conformation. This
means that -D-glucose is the most stable of all the aldohexoses, so it is not surprising
that it is the most prevalent aldohexose in nature.

HO H O

OH

HOH

H

CH 2 OH

H

HO H

CH 2 OH

O

OH

OH

OH

OH

HO

CH 2 OH

HO

HO

HO

O

Fischer projection Haworth projection chair conformation

• D-glucose

1 2 3 4 5 6

6

5

41

3 2 1

4 5

6

3 2

HO H O

OH

HOH

H

CH 2 OH

H

H OH

CH 2 OH

O

OH

OH OH

HO OH

CH 2 OH

HO

HO

HO

O

• D-glucose

Fischer projection Haworth projection chair conformation

1 2 3 4 5 6

6

5

41

3 2 1

4 5

6

3 2

b

b

a

b a a

The -position is to the left in a Fischer

projection, up in a Haworth projection,

and equatorial in a chair conformation.

B