Carbohydrates Form Cyclic Hemiacetals equilibrium lies far to the right cyclic hemiacetals that have 5-membered rings are called furanose forms

25.6

Recall from Section 17.8

R"OH C

•• O•• R

R'

R"O C O•• H ••

•• ••

R

R' Product is a hemiacetal.

Cyclic Hemiacetals

Aldehydes and ketones that contain an OH group elsewhere in the molecule can undergo intramolecular hemiacetal formation.

The equilibrium favors the cyclic hemiacetal if the ring is 5- or 6-membered.

C O

R

OH

C

OH R

equilibrium lies far to the right

cyclic hemiacetals that have 5-membered rings are called furanose forms

Carbohydrates Form Cyclic Hemiacetals

CH O

CH₂OH 1

2

3

4

H OH O

1

2 3

stereochemistry is maintained during cyclic hemiacetal formation

D-Erythrose

CH O

CH₂OH 1

2

3

4

H OH O

1

2 3

4 H

H

OH

OH

H H H

OH OH

D-Erythrose

1

2

3

turn 90°

D-Erythrose

1

2 3

4 1

2

3

D-Erythrose

move O into

position by rotating about bond

between carbon-3 and carbon-4

1

2 3

D-Erythrose

1

2 3

4 1

2 3

D-Erythrose

1

2 3

4 close ring by _{hemiacetal formation}

D-Erythrose

1

2 3

4 1

2 3

stereochemistry is variable at anomeric carbon; two diastereomers are formed

D-Erythrose

CH O

CH₂OH 1

2

3

4

H OH O

1

2 3

4 H

H

OH

OH

H H H

OH OH

H

D-Erythrose

H OH O

1

2 3

4

H H H

OH OH

H OH

H O

1

2 3

4

H H H

OH OH

H

D-Ribose

CH O

CH₂OH

H OH

H OH

H OH

1

2

3

4

5

D-Ribose

CH O

CH₂OH

H OH

H OH

H OH

1

2

3

4

5

need C(3)-C(4) bond rotation to put OH in proper orientation to close 5-membered ring

CH O H

H

H CH₂OH

OH OH

HO

1

2 3

4

D-Ribose

CH O H

H

H CH₂OH

OH OH

HO

1

2 3

4

5

CH O H

H H

HOCH₂

OH OH

OH

1

2 3

D-Ribose

CH₂OH group becomes a substituent on ring

CH O H

H H

HOCH₂

OH OH

OH

1

2 3

D-Ribose

CH₂OH group becomes a substituent on ring

CH O H

H H

HOCH₂

OH OH

OH

1

2 3

4 5

HOCH₂

H OH

O

1

2 3

4

H H OH OH

H

5

25.7

cyclic hemiacetals that have 6-membered rings are called pyranose forms

Carbohydrates Form Cyclic Hemiacetals

H OH

O

1

2 3

4

5

CH O CH₂OH

1

2

3

4

D-Ribose

CH O CH₂OH

1

2

3

4

5

H H

H OH OH OH

D-Ribose

CH O H

H

H CH₂OH OH OH

HO

1

2 3

4

5

CH O CH₂OH

1

2

3

4

5

H H

H OH OH OH

D-Ribose

CH O H

H

H CH₂OH OH OH

HO

1

2 3

4

5

D-Ribose

CH O H

H

D-Glucose

CH O CH₂OH

1

2

3

4

5

H HO

H OH H OH H OH

6

D-Glucose

CH O CH₂OH

1 2 3 4 5 H HO H OH H OH H OH

6

OH _{CH O} H OH H _CH 2OH OH H HO 1 2 3 4 5 6 H

D-Glucose

CH O H

H _CH

2OH

OH H

HO

1 2

3 4

5

6

H

OH OH

D-Glucose CH O H OH H _CH 2OH OH H HO 1 2 3 4 5 6 H OH CH O H OH H HOCH₂ OH H HO 1 2 3 4 5 6

H OH

D-Glucose CH O H OH H HOCH₂ OH H HO 1 2 3 4 5 6

H OH

b-D-Glucopyranose

D-Glucose

b-D-Glucopyranose

H OH

O

1

2 3

4

OH H

HO

H OH

H

H HOCH2

5 6

D-Glucose b-D-Glucopyranose H OH O 1 2 3 4 OH H HO H OH H H HOCH₂ 5 6 OH H OH H HO HO H H H HOCH₂ O

all substituents are equatorial in b-D-glucopyranose

D-Glucose

a-D-Glucopyranose

OH H

OH H

HO HO

H

H H HOCH₂

O

OH group at anomeric carbon is axial in a-D-glucopyranose

1

b-D-Glucopyranose

H OH OH

H HO

HO

H

H H HOCH₂

O

Figure 25.5

CH O

CH₂OH

H OH

H OH

H OH

Figure 25.5

OH H

OH H

H HO

H

OHH

O

b-D-Ribopyranose (56%)

H

HO

a-D-Ribopyranose (20%)

H OH OH

H H

H

OHH

O

1

H

Figure 25.5

HOCH₂

H OH

O

H H OH OH

H

b-D-Ribofuranose (18%)

HOCH₂

OH H

O

H H OH OH

H

a-D-Ribofuranose (6%) The a and b-furanose forms comprise 24% of

25.8

Mutarotation

Mutarotation is a term given to the change in the observed optical rotation of a substance with time.

Glucose, for example, can be obtained in either its a or b-pyranose form. The two forms have different physical properties such as

melting point and optical rotation.

Mutarotation of D-Glucose

a-D-Glucopyranose

OH H

OH H

HO HO

H

H H HOCH2

O

1

b-D-Glucopyranose

H OH OH

H HO

HO

H

H H HOCH₂

O

1

Mutarotation of D-Glucose

a-D-Glucopyranose

OH H

OH H

HO HO

H

H H HOCH₂

O

1

b-D-Glucopyranose

H OH OH

H HO

HO

H

H H HOCH₂

O

1

Initial: [a]_D +18.7° Initial: [a]_D +112.2°

Mutarotation of D-Glucose

a-D-Glucopyranose

OH H

OH H

HO HO

H

H H HOCH₂

O

1

b-D-Glucopyranose

H OH OH

H HO

HO

H

H H HOCH₂

O

1

Initial: [a]_D +18.7° Initial: [a]_D +112.2°

Mutarotation of D-Glucose

a-D-Glucopyranose

OH H

OH H

HO HO

H

H H HOCH₂

O

1

b-D-Glucopyranose

H OH OH

H HO

HO

H

H H HOCH₂

O

1

Explanation: After being dissolved in water, the