Lecture 24- Carbohydrates

General

Organic Chemistry Two credits Second Semester 2009

King Saud bin Abdulaziz University for Health Science

Reference Book: Organic Chemistry: A Brief Course, by Robert C. Atkins and Francis A. Carey Third Edition

Instructor: Rabih O. Al-Kaysi, PhD.

Lecture 24

Chapter 15

Carbohydrates

Classification of Carbohydrates

Classification of Carbohydrates

monosaccharide disaccharide oligosaccharide polysaccharide

Monosaccharide

is not cleaved to a simpler carbohydrate on hydrolysis glucose, for example, is a monosaccharide

Disaccharide is cleaved to two monosaccharides on hydrolysis these two monosaccharides may be the same or different C12 H22 O11 + H2O

sucrose (a disaccharide)

Disaccharide is cleaved to two monosaccharides on hydrolysis these two monosaccharides may be the same or different C12 H22 O11 + H2O

sucrose (a disaccharide)

C6H12 O6 + C6H12 O6

glucose (a monosaccharide)

fructose (a monosaccharide)

Higher Saccharides oligosaccharide: gives three or more monosaccharide units on hydrolysis polysaccharide: yields more than 10 monosaccharide units

Some Classes of Carbohydrates No. of carbons

Aldose

Ketose

4

Aldotetrose

Ketotetrose

5

Aldopentose

Ketopentose

6

Aldohexose

Ketopentose

7

Aldoheptose

Ketoheptose

8

Aldooctose

Ketooctose

Just for general knowledge, will not be tested on

Fischer Projections and D-L Notation

Fischer Projections

Fischer Projections

Fischer Projections of Enantiomers

Fischer Projections of Enantiomers

Enantiomers of Glyceraldehyde

CH H

D

O OH

CH2OH (+)-Glyceraldehyde

CH HO

L

O H

CH2OH (–)-Glyceraldehyde

The Aldotetroses

An Aldotetrose 1

H H

CH

2 3

4

O OH OH

CH2OH

stereochemistry assigned on basis of whether configuration of highest-numbered stereogenic center is analogous to D or L-glyceraldehyde Just for general knowledge, will not be tested on

An Aldotetrose 1

H H D

CH

2 3

4

O OH OH

CH2OH

stereochemistry assigned on basis of whether configuration of highest-numbered stereogenic center is analogous to D or L-glyceraldehyde Just for general knowledge, will not be tested on

An Aldotetrose 1

H H

CH

2 3

4

O OH OH

CH2OH

D-Erythrose Just for general knowledge, will not be tested on

The Four Aldotetroses

CH

O

CH

O

CH

H

OH HO

H

HO

H

OH HO

H

H

CH2OH D-Erythrose

CH2OH L-Erythrose

O H

CH H

OH H

OHHO CH2OH

D-Threose

O

CH2OH L-Threose

Just for general knowledge, will not be tested on

Aldopentoses and Aldohexoses

The Aldopentoses There are 8 aldopentoses. Four belong to the D-series; four belong to the L-series. Their names are ribose, arabinose, xylose, and lyxose.

The Four D-Aldopentoses

CH

O

CH

O

CH

O

CH

O

H

OH HO

H

H

OH HO

H

H

OH

H

H

OH

H

OH HO OH H

H HO OH H

H OH

CH2OH D-Ribose

CH2OH D-Arabinose

CH2OH D-Xylose

CH2OH D-Lyxose

Just for general knowledge, will not be tested on

Aldohexoses There are 16 aldopentoses. 8 belong to the D-series; 8 belong to the Lseries. Their names and configurations are best remembered with the aid of the mnemonic described in Section 25.5.

L-Aldohexoses

There are 8 aldohexoses of the L-series. H They have the HO same name as H their mirror image except the prefix is H L- rather than D-.

CH

O

CH

OH

HO

H OH

H

OH CH2OH

HO HO

O H OH H H

CH2OH

D-(+)-Glucose L-(–)-Glucose Just for general knowledge, will not be tested on

Cyclic Forms of Carbohydrates: Furanose Forms

Recall R

R C

O •• + R"OH ••

R'

••

R"O ••

C R'

Product is a hemiacetal.

••

O ••

H

Carbohydrates Form Cyclic Hemiacetals 1

CH

O

2

4

O

4

3

OH

3

1 2

H

CH2OH

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

D-Erythrose 1

H H

CH

2 3

4

O OH

HH

O

4

OH CH2OH

H

H

1

3

OH

OH

2

OH

stereochemistry is maintained during cyclic hemiacetal formation

H

D-Ribose 1

CH

O

2

OH

H

3

OH

H

4

OH

H

5 CH OH 2 2

furanose ring formation involves OH group at C-4

D-Ribose 1

H

CH

2

O OH

CH2OH H H

H

H

3

OH

4

H

4

OH

HO

5 CH OH 2

5

3

1

CH

2

OH

OH

2

need C(3)-C(4) bond rotation to put OH in proper orientation to close 5-membered ring Just for general knowledge, will not be tested on

O

D-Ribose

5

HOCH2 OH H

4

H

3

OH

H 2

OH

CH2OH H H

H

1

CH

5

O

4

HO

3

1

CH

2

OH

OH

Just for general knowledge, will not be tested on

O

D-Ribose

5

HOCH2 OH H

4

H

3

OH

H

1

CH

O

2

OH

CH2OH group becomes a substituent on ring Just for general knowledge, will not be tested on

D-Ribose 5

5

HOCH2 OH H

4

H

3

OH

H 2

OH

1

CH

O

HOCH2 H O H 4

H

3

OH

2

OH 1

H

OH

β -D-Ribofuranose CH2OH group becomes a substituent on ring

Cyclic Forms of Carbohydrates: Pyranose Forms

Carbohydrates Form Cyclic Hemiacetals 1

CH

O 5

2 3 4 5

OH

O

1

4 3

2

H

CH2OH

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

D-Ribose

1

CH

O

H

2

OH

H

3

OH

H

4

OH

5

CH2OH

pyranose ring formation involves OH group at C-5

Just for general knowledge, will not be tested on

D-Ribose

1

CH

O

H

2

H

3

OH

H

4

OH

5

OH

CH2OH

5

CH2OH H H

H 4

HO

3

1

CH

O

2

OH

OH

pyranose ring formation involves OH group at C-5

Just for general knowledge, will not be tested on

D-Ribose

5

CH2OH H H

H 4

HO

3

1

CH

O

2

OH

OH

pyranose ring formation involves OH group at C-5

Just for general knowledge, will not be tested on

D-Ribose

H H 4

HO

5

H H 3

OH

O

OH

H

1

2

H

OH

5

CH2OH H H

H 4

HO

3

1

CH

2

OH

OH

β -D-Ribopyranose

Just for general knowledge, will not be tested on

O

D-Glucose

1

CH

O

H

2

OH

HO

3

H

H

4

OH

H

5 6

OH

CH2OH

pyranose ring formation involves OH group at C-5

D-Glucose

6

6

HOCH2

HOCH2 OH H 5 H 4 OH H HO

3

H CH 1

2

H

OH

O

4

HO

5

H OH 3

H

O

OH

H

1

2

H

OH

β -D-Glucopyranose Just for general knowledge, will not be tested on

D-Glucose

6

6

HOCH2

HOCH2

H 4

HO

5

H OH 3

H

O H 2

H

H

1

4

OH

HO

OH

α -D-Glucopyranose

5

H OH 3

H

O

OH

H

1

2

H

OH

β -D-Glucopyranose

Just for general knowledge, will not be tested on

D-Glucose

H HOCH2 H HO HO H

H

H HOCH2 H

O OH

1

OH

H

β -D-Glucopyranose

HO HO H

H

O OH

1

H

OH

α -D-Glucopyranose

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

Just for general knowledge, will not be tested on

Ketoses

Ketoses Ketoses are carbohydrates that have a ketone carbonyl group in their open-chain form. C-2 is usually the carbonyl carbon.

Examples CH2OH

CH2OH

CH2OH

O

O

O

H

OH

H

H

OH

HO

CH2OH

OH H CH2OH

HO

H

H

OH

H

OH CH2OH

D-Ribulose

L-Xyulose

D-Fructose

Deoxy Sugars

Deoxy Sugars Often one or more of the carbons of a carbohydrate will lack an oxygen substituent. Such compounds are called deoxy sugars.

Examples CH

O

CH

O

H

H

H

OH

H

OH

H

OH

H

OH

HO

H

CH2OH

HO

H CH3

2-Deoxy-D-ribose

6-Deoxy-L-mannose

Amino Sugars

Amino Sugars An amino sugar has one or more of its oxygens replaced by nitrogen.

Example HOCH2 HO HO

O OH NH

O

C CH3

N-Acetyl-D-glucosamine

Example OH H3C HO

O NH2

L-Daunosamine

Branched-Chain Carbohydrates

Branched-Chain Carbohydrates Carbohydrates that don't have a continuous chain of carbon-carbon bonds are called branched-chain carbohydrates.

Examples CH H HO

O OH

CH3 H3C

CH2OH CH2OH D-Apiose

HO

OH

O NH2

L-Vancosamine

Glycosides

Glycosides Glycosides have a substituent other than OH at the anomeric carbon. Usually the atom connected to the anomeric carbon is oxygen.

Example HOCH2 HO HO

O OH

OH D-Glucose Linamarin is an O-glycoside derived from D-glucose.

HOCH2 HO HO

O

CH3 OCC

OH

CH3

N

Disaccharides

Disaccharides Disaccharides are glycosides. The glycosidic linkage connects two monosaccharides. Two structurally related disaccharides are cellobiose and maltose. Both are derived from glucose.

Maltose and Cellobiose HOCH2 HO

HOCH2 O α 1

O O

4

OH

Maltose

OH HO OH HO Maltose is composed of two glucose units linked together by a glycosidic bond between C-1 of one glucose and C-4 of the other. The stereochemistry at the anomeric carbon of the glycosidic linkage is α . The glycosidic linkage is described as α (1,4)

Maltose and Cellobiose HOCH2 HO

HOCH2 O β 1

O O

4

OH

Cellobiose

OH HO OH HO Cellobiose is a stereoisomer of maltose. The only difference between the two is that cellobiose has a β (1,4) glycosidic bond while that of maltose is α (1,4).

Maltose and Cellobiose

Maltose

Cellobios e

Just for general knowledge, will not be tested on

Cellobiose and Lactose HOCH2 HO

HOCH2 O β 1

O O

4

OH

Cellobiose

OH HO OH HO Cellobiose and lactose are stereoisomeric disaccharides. Both have β (1,4) glycosidic bonds. The glycosidic bond unites two glucose units in cellobiose. It unites galactose and glucose in lactose.

Cellobiose and Lactose HOCH2 HO

HOCH2 O β 1

O O

4

OH

Lactose

OH HO OH HO Cellobiose and lactose are stereoisomeric disaccharides. Both have β (1,4) glycosidic bonds. The glycosidic bond unites two glucose units in cellobiose. It unites galactose and glucose in lactose.

Polysaccharides

Cellulose Cellulose is a polysaccharide composed of several thousand D-glucose units joined by β (1,4)-glycosidic linkages. Thus, it can also be viewed as a repeating collection of cellobiose units.

Cellulose

Four glucose units of a cellulose chain.

Starch Starch is a mixture of amylose and amylopectin. Amylose is a polysaccharide composed of 100 to several thousand D-glucose units joined by α (1,4)-glycosidic linkages.