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Chemistry: An Introduction to General, Organic, and Biological Chemistry

Thirteenth Edition

Chapter 13

Carbohydrates

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13.3 Fischer Projections of Monosaccharides

Glucose, galactose, and fructose are the most important monosaccharides.

Learning Goal Identify or draw the D and L configurations of the Fischer projections for common monosaccharides.

Fischer Projections

A Fischer projection used to represent carbohydrates

places the most oxidized group at the top

shows chiral carbons as the intersection of vertical and horizontal lines

shows the

on the horizontal

intersecting line

places the

at the bottom of the Fischer

projection, which is not chiral

D and L Notations

In a Fischer projection, the

on the

chiral carbon farthest from the carbonyl group determines an L or a D isomer

left is assigned the letter L

right is assigned the letter D

D and L Isomers of Monosaccharides

Learning Check 1

In the following Fischer projection of mannose, identify the monosaccharide as a D or an L isomer.

Solution 1 (1 of 2)

In the following Fischer projection of mannose, identify the monosaccharide as a D or an L isomer.

Analyze the Problem	Given	Need	Connect
Analyze The Problem	Fischer projection of mannose	Identify as D or L mannose	chiral carbon farthest from carbonyl group

Solution 1 (2 of 2)

In the following Fischer projection of mannose, identify the monosaccharide as a D or an L isomer.

Step 1 Number the carbon chain, starting at the top of the Fischer projection.

Step 2 Locate the chiral carbon farthest from the top of the Fischer projection. The chiral carbon farthest from the top is carbon 5.

Step 3 Identify the position of the

as D or L. The

is on the right of

carbon 5, which makes it D-mannose.

Learning Check 2

In the following Fischer projection,

A. identify the number of chiral centers.

B. identify the monosaccharide as a D or an L isomer.

Solution 2

In the following Fischer projection,

A. identify the number of chiral centers.

B. identify the monosaccharide as a D or an L isomer.

Glucose and Fructose (1 of 2)

D-Glucose is

found in fruits, corn syrup, and honey

an aldohexose with the formula C6H12O6

known as dextrose and blood sugar in the body

a building block in sucrose, lactose, maltose, and in polysaccharides such as cellulose and glycogen

D-Fructose, obtained from sucrose, is

a ketohexose with the formula C6H12O6

the sweetest of the carbohydrates, twice as sweet as sucrose

Glucose and Fructose (2 of 2)

The sweet taste of honey comes from the monosaccharides D-glucose and D-fructose.

Galactose

D-Galactose

is an aldohexose, C6H12O6

is obtained from the disaccharide lactose

has a similar structure to glucose

except for the

on carbon 4

In a condition called galactosemia, an enzyme needed to convert galactose to glucose is missing.

Chemistry Link to Health: Hyperglycemia and Hypoglycemia (1 of 3)

In the body,

glucose has a normal blood level of 70–90 milligrams/deciLitre a glucose tolerance test measures blood glucose for several hours after ingesting glucose

Chemistry Link to Health: Hyperglycemia and Hypoglycemia (2 of 3)

Diabetes mellitus can cause hyperglycemia, which

occurs when the pancreas is unable to produce sufficient quantities of insulin

allows glucose levels in the body fluids to rise as high as 350 milligrams/deciLitre of plasma

Symptoms of diabetes include

thirst and excessive urination

increased appetite and weight loss

In older adults, diabetes is sometimes a consequence of excessive weight gain.

Chemistry Link to Health: Hyperglycemia and Hypoglycemia (3 of 3)

When a person is hypoglycemic,

the blood glucose level rises and then decreases rapidly to levels as low as 40 milligrams/deciLitre

low blood sugar may occur as a result of an overproduction of insulin by the pancreas

symptoms may appear, such as dizziness, general weakness, and muscle tremors

a diet may be prescribed that consists of several small meals high in protein and low in carbohydrates

Learning Check 3

Draw the Fischer projections of D-fructose and L-fructose.

Solution 3

Draw the Fischer projections of D-fructose and L-fructose.

D-Fructose

L-Fructose

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