Isomers

chem.moravian.edu

Isomers

Chemistry 211 Fall 2009

Isomers

Introduction:

The Same or Not the Same, How Can We Tell

In order to appreciate the relationships between the organic structures and their reactivity, it is critical to be able to interpret all types of written structures

accurately. You investigated the relationships among the common types of structural representations in the previous class activity. This activity is

designed to help you recognize similarities and differences in variously represented organic structures and understand the elements of structures that

lead groups of atoms to yield different molecules. Try to complete all parts of this activity without using your model kits, but when necessary build

models to confirm your initial conclusions. For any analysis that required use of models, take your conclusion from the models and try to see how you

could have reached it using drawn structures. Being able to work with 2-dimensional drawn structures is much more efficient than building models of

structures.

Learning Goals:

1. To quickly identify isomeric organic structures by focusing on the structural elements that lead to creating the possibility for isomerism.

2. To distinguish between Constitutional isomers and Stereoisomers.

3. To quickly identify Constitutional isomers by focusing on the structural elements that lead to creating the possibility for constitutional isomerism.

4. To quickly identify Stereoisomers by focusing on the structural elements that lead to creating the possibility for stereoisomerism.

Exploration:

Model 1

Figure 1 Figure 2

Constitutional

isomers

A B C

G H

C

I

C 7 H C 6 H 6 H 10

14 16

D E F

C

C 7 H 7 H 14 C 5 H 10

14

J K L

These structures are all isomers of each other

To be the same, two structures must be superimposable.

None of these structures is an isomer of any other structure in this group.

1. What do the structures in Figure 1 have in common Explain.

• They all contain only C & H and they have the same numbers of H & C atoms. – The same Molecular Formula C 6 H 12

• They all have 1 C=C double bond.


Isomers 2

2. What differences are there among structures in Figure 1 Explain.

• The arrangements of atoms are different such that their structures cannot be superimposed.

• For A, D, E, F and C the shapes of the molecules are different.

• For A & B where the shapes seem to be the same, the bonding sequence C=C and C-H is different.

3. Is any structure in Figure 1 the same as any other structure in that figure Explain. (To be the same, two structures must be superimposable upon each

other.)

• As indicated in 2., all of the structures are different because they cannot be superimposed.

4. What do the structures in Figure 2 have in common Explain.

• They all contain only C & H.

5. What differences are there among structures in Figure 2 Explain.

• They have different molecular formulas.

• Most have different numbers of C=C bonds.

6. Is any structure in Figure 2 the same as any other structure in that figure Explain. (To be the same, two structures must be superimposable upon each

other.)

• Since they have different numbers of atoms (different molecular formulas), they cannot be superimposed and cannot be the same.

7. Based on the data in Model 1:

• What characteristic(s) does(do) isomeric structures have in common Explain your reasoning.

o They have the Same Molecular Formula C 6 H 12

• How are isomeric structures different Explain your reasoning.

o Their structures cannot be superimposed.

8. Are structures M & N isomers Are structures O & P isomers Explain using your argument from 7. above.

H O M

O

N

O

N

H

P

N

Isomers: They have the same

molecular formula (C 4 H 10 ) and cannot

be superimposed. They meet both

criteria for isomers.

Not Isomers: They have different

molecular formulas – cannot be isomers


Isomers 3

9. Structures A, B, C, F and K from Model 1 are Constitutional isomers. Based on this information how can you recognize if two drawn structures are

Constitutional isomers Explain your reasoning.

• All structures have the same molecular formula, C 6 H 12

• They cannot be superimposed because the orders of attachment of their atoms are different – They have different connectivity of their atoms.

NOTE: D & E are not included. They have the same atom connectivity as A & C respectively. (See 11. Below.)

10. Are structures M & N Constitutional isomers Are structures O & P Constitutional isomers Explain using your argument from 7. above.

M & N meet the criteria for constitutional isomers, same As Indicated in 8 above, O & P have different

molecular formula but not superimposable because they molecular formulas and cannot be isomers of any

have different atom connectivities.

kind.

Model 2

H

H

H C C

C C

C

H H

H

H H

Q

H

H

H

H

H

C

C

H

R

H

C

H

C

H

C

H

H

H

H

S

Cl

Cl

H

T

H

Cl

Cl

H

11. Stereoisomers.

The following pairs of structures are stereoisomers

Model 1: A & D

Model 1: C & E

Model 2: Q & R

Model 2: S & T

Based on this information how can you recognize if two drawn structures are stereoisomers Explain your reasoning.

• Within each pair of structures, both molecules have the same molecular formula and the same atom connectivity. But they are isomers since

they cannot be superimposed. In each case they differ in the directions their atom are displayed in space because of an inability of one or

more bonds to rotate freely. For A & D, C & E and Q & R it is lack of rotation around a C=C bond that keeps the structures from being

superimposed. With S & T it is the ring structure that restricts rotation around its C-Cs that makes them not superimposable.

• So molecules with the same molecular formula and the same connectivity of their atoms can be stereoisomer if they contain a C=C or ring that

causes portions of the structure to point in different directions.


Isomers 4

Application:

Consider the representations below and determine if:

• Which structures are the same Explain the process you used to reach your conclusions.

1. To be the same, molecules must have the same molecular formula (See below). So S is different from the rest.

2. To be the same, molecules need to have the same connectivity of their atoms. Numbering through the C=C toward the nearest Cl we see that

R & V have the Cl’s attached to the same carbon atoms in the ring (4 & 5) and T & U both have Cls at carbons 3 & 4. So R & V could be the

same and T & U could be the same.

3. Finally, to be the same the structures must be superimposable. R & V have the Cl atoms arrayed differently with respect to the ring (R top

and bottom and V both on the bottom. Since the ring restricts rotation around its C-C bonds, R & V cannot be superimposed and are

different. However, T & U both have the Cls arrayed on opposite sides of the ring and can be superimposed. So T & U represent the same

molecule.

• Which structures are the Constitutional isomers Explain the process you used to reach your conclusions.

1. To be isomers of any kind molecules must have the same molecular formula. So S cannot be an isomer of any of the others.

2. To be constitutional isomers, molecules must also have different connetivities of their atoms. Based on the numbering of the structures, T

(and of course U – same structure) differs in connectivity from R & V, so R & V are constitutional isomers of T (U).

• Which structures are Stereoisomers Explain the process you used to reach your conclusions.

From the discussion above, R & V are the only two compounds that have the same molecular formula, same atom connectivity but are not

superimposable. The structural difference is due to the directions that the Cls point in relation to the plane of the ring. So R & V are

Stereoisomers.

1

2

6 H

Cl

H Cl H

1 2 H

Cl

H

Cl

H

Cl

4 5

4 3

6 3

H

5 2

4

Cl

5

3

H

Cl

6 1

Cl

R

S

T

U

C 6 H 8 Cl 2 C 5 H 6 Cl 2 C 6 H 8 Cl 2 C 6 H 8 Cl 2 C 6 H 8 Cl 2

Reflector’s Report Discussion

As a group, summarize what you believe to be the most important points in this activity. Your reflector will summarize your conclusions in the group

electronic reflector’s report for this activity.

See the Learning Goals.

Strategy Analyst’s Report Discussion

As a group, use the items below to assist you in analyzing the sequence of the models and questions in this activity. Note how the logic outlined in the

activity helped your group reach the conclusions summarized in the Reflector’s Report Discussion. Your strategy analyst will summarize your

conclusions in the group electronic strategy analyst’s report for this activity.

• Exploration of Model 1 to recognize the structural requirements for isomerism.

• Exploration of Model 1 to recognize the structural characteristics that lead to Constitutional isomerism.

• Exploration of Models 1 & 2 to recognize the structural characteristics that lead to Stereoisomerism.

Consider how the data in each case led to formation of definitions and a process for accomplishing the learning goals.

1

2

6 5

4

3

V

Cl

Cl

H

H

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