Chemistry 212

Introduction to the Structure of Organic Molecules:

LAB ACTIVITY

1. Introduction:

Sugar (Carbohydrate) Structures:

There are many biomolecules that seem to have the same order of connection of atoms, but exhibit different reactivities or functions in nature. Carbohydrates (sugars) provide particularly spectacular examples of this variation in behavior.

e.g. Glucose, mannose and galactose all can be described by the structural formula shown below, however, glucose is the major energy source for the vast majority of living organisms while mannose and galactose are not metabolized by many organisms.

2. References:

CGWW: Ch. 16

3. Background:

1. Stereospecificity of Reactions:
   

 

Lactic Acid Source	Reaction Conditions	Maximum Yield of Pyruvic Acid
Chemical Synthesis	Chemical Oxidation	100 %
Chemical Synthesis	Muscle Extract	50 %
Muscle	Chemical Oxidation	100 %
Muscle	Muscle Extract	100 %

2. Recall chiral atoms from the first laboratory exercise:

 

CH3Cl	one structure	CH2ClBr	one structure
CHCl2F	one structure	CHClBrF	two mirror image structures

-> Configurational isomers-- Chiral Compounds

As we discussed in class and in lab, configurational isomerism (optical isomerism) results from differences in the directions from one substituent on a carbon atom to another substituent on the same carbon atom. Molecules that can exist as two non-superimposable mirror image structures are known as chiral compounds. The term chiral is from Greek meaning handed. One cause of this type of isomerism is a carbon atom that has four different substituents. Atoms of this type are one kind of chiral atom (also known as centers of chirality, stereogenic atoms or stereocenters). Chiral atoms have configurations such that interchange of any two ligands produces a new stereoisomer. Recall that interchanging any two of the atoms of the R-isomer above produces its mirror image (S-isomer).

Lactic Acid (see above) has one chiral atom. Thus it has two possible configurational isomers and they are mirror images, enantiomers.

4. Explorations:
   

Lactic Acid Configurational Isomers

Carbohydrates

1. Recognizing Chiral Atoms
   

1. Lactic Acid (See section C. 1) has one chiral atom.  Thus it has two possible configurational isomers.  Note that the three carbon carbohydrate above, glyceraldehyde, also has a chiral carbon atom and can exist as two configurational isomers.  Mark the chiral atom in the glyceraldehyde structure above with an asterisk and draw dash-wedge structures of its two configurational isomers. 

2. Also, the four carbon carbohydrate above can exist as configurational isomers.  How many chiral carbon atoms does the four carbon carbohydrate have?  Indicate each chiral carbon atom with an asterisk. 

3. How many configurational isomers can share the four carbon carbohydrate structure?  Explain your reasoning.  Draw dash-wedge structures for all of the configurational isomers of the four carbon carbohydrate. 

2. Drawing and Using Fischer Projections: See also CGWW pp. 395.

As you may have noted in devising structures in 2., when a structure can have multiple stereoisomers, representing them on paper can be cumbersome. Fischer Projections, are relatively simple two dimensional representations for three dimensional structures and are particularly useful for representing the configurations of atoms in molecules with multiple chiral atoms.

Fischer Projections require that the molecule be oriented in a specific manner so that a flat representation can give three-dimensional information.

Conventions for Fischer Projection Structures

1. Conventions for Drawing Fischer Projection Structures
   

1. The carbon chain is oriented vertically and bending away from the observer. In this orientation, the side substituents are projecting toward the observer. (Reorientation of a wedge-dash wedge structure to the Fischer Projection orientation is illustrated below. )

A->B requires a 180? rotation around the C2-C3 bond, B->C requires a rotation of the entire structure in space to orient the carbon chain vertically with the C2 and C3 substituents projecting toward the observer and C1 & C4 in the back. D illustrates the convention for the Fischer Projection representation of C.

2. The carbon structure is represented by a vertical line and bonds to side substituents are represented by horizontal lines. Thus the Fischer Projection structure is the projection of the three dimensional structure onto a two dimensional surface.

3. In comparing structures for superimposability, you must be aware of the assumed orientation of the carbon chain.

2. Using Fischer Projections:
   

Below are given the Fischer Projections of D-glucose, D-mannose, and D-galactose as well as L-glucose. L-Glucose is the enantiomer (non-superimposable mirror image) of D-glucose.

1. How many chiral atoms are there in each of these structures? Using the numbering system illustrated for D-glucose, indicate which atoms are chiral. How did you recognize them?

2. Using the numbering system illustrated for D-glucose, explain how the four structures differ.

3. Defining and Recognizing Types of Stereoisomers
   

1. CHIRAL COMPOUNDS:

1. Examples of CHIRAL COMPOUNDS:

These compounds are CHIRAL

These compounds are NOT CHIRAL (achiral)

2. Definition:  On the basis of the above structures define, as specifically as possible, the characteristics required for a compound to be chiral.  Explain the logic used to devise your definition.
   

3. Application:  Identify all of the chiral compounds in the table below.  Briefly explain the logic of your classifications.
   

(a.)	(b.)	(c.)
(d.)	(e.)	(f.)

2. MESO COMPOUNDS:

1. Examples of MESO COMPOUNDS:

These compounds are MESO

These compounds are NOT MESO

2. Definition:  On the basis of the above structures define, as specifically as possible, the characteristics required for a compound to be MESO.  Explain the logic used to devise your definition.
   

3. Application:  Identify all of the MESO compounds in the table below.  Briefly explain the logic of your classifications.
   

(a.)	(b.)	(c.)
(d.)	(e.)	(f.)

4. Defining and Recognizing Relationships Among Stereoisomers: 

Explorations of term that define relationships among configurational isomers: ENANTIOMERS, DIASTEREOMERS & EPIMERS See also CGWW  pp. 382-396.

1. ENANTIOMERS:

1. Examples of ENANTIOMERS (Enantiomeric Pairs of Compounds): 
   

These pairs of structures are ENANTIOMERS

These pairs of structures are NOT ENANTIOMERS

2. Definition:  On the basis of the above structures define, as specifically as possible, the relationship between the members of a pair of enantiomers.  Explain the logic used to devise your definition.
   

3. Application:   Identify all of the all pairs of compounds that are enantiomers.  Briefly explain the logic of your classifications.
   

(a.)	(b.)	(c.)
(d.)	(e.)	(f.)

2. DIASTEREOMERS:

1. Examples of DIASTEREOMERS:   (Diastereomeric Sets of Compounds)

These sets of structures are DIASTEREOMERS

These sets of structures are NOT DIASTEREOMERS

2. Definition:  On the basis of the above structures define, as specifically as possible, the relationship between the members of a set of diastereomers.  Explain the logic used to devise your definition.
   

3. Application:    Identify all of the all pairs of compounds that are diastereomers.  Briefly explain the logic of your classifications. 
   

(a.)	(b.)	(c.)
(d.)	(e.)	(f.)

3. EPIMERS

1.     Examples of EPIMERS (pairs of epimeric compounds).

These pairs of structures are EPIMERS

These pairs of structures are NOT EPIMERS

2. Definition:  On the basis of the above structures define, as specifically as possible, the relationship between the members of a set of epimers.  Explain the logic used to devise your definition.
   

3. Application:    Identify all of the all pairs of compounds that are diastereomers.  Briefly explain the logic of your classifications. 
   

(a.)	(b.)	(c.)
(d.)	(e.)	(f.)

5. General Applications:

a. Enantiomer of	b. Diastereomer of	c. C3 epimer of
d. Meso diastereomer of	e. Chiral diastereomer of	f. Enantiomer of

2.    Is it possible for a structure to have more than one enantiomer?  Explain using an example.


3.    Is it possible for a structure to have more than one diastereomer?  Explain using an example.


4.    Is it possible for a structure to have more than one epimer?  Explain using an example.



5.    Is it possible for a structure to be chiral and meso?  Explain using an example.



6.    Is it possible for a structure with one chiral atom to be a meso compound?  Explain using an example.

Stereochemistry Lab Activity - Out of Class Applications:

Stereochemistry Lab Activity - Summary of Class Discussion: 

Nomenclature of Molecules with Chiral Atoms - Out of Class Applications: