Chemistry 211
Fall 2009
Structural Representations of Organic Compounds & The Learning Cycle
Introduction:
1. Structure-Function Relationships:
From many years of study, organic chemists have developed elaborate theories to explain the structures and the reactivity of molecules. The consensus
from their vast experience is that the behavior of molecules should be directly related to molecular structure.
Thus, we will begin our consideration of organic chemistry by looking at a range of organic molecular structures and at theories for explaining why
certain structures are more likely to react than are others. Although theories are generally expressed in technical terms, we should recognize that they
simply provide us with models or pictures to use in thinking about objects or processes that are actually invisible. Thus, structural models can be
thought of as:
MACROSCOPIC METAPHORS FOR MOLECULAR STRUCTURES.
2. Representations (Metaphors) for Molecular Structures
(See also, CGWW pp. 19-31 & Rev. Gen. Chem. & Math #'s 23->25)
a. Need for Representations:
Before we begin to develop theories to explain properties of molecular structures, we need to have methods for visualizing potential structures
and communicating them to others. Several methods for representing molecular structures are in general use, thus to understand theories that
have been developed and to communicate our own discoveries to others:
WE MUST BE ABLE TO WRITE, READ & UNDERSTAND ALL TYPES OF REPRESENTATIONS
FOR MOLECULAR STRUCTURES
Our class group activities are designed to assist you in gaining the ability to interpret and compare the various methods for representing molecular
structures. CGWW (pp. 19-31) gives a slightly different, but interesting discussion on why it is important to understand structures and how to deal
with them.
b. Two Dimensional Structural Representations:
Depending upon the aspect of an organic structure that is to be emphasized, organic chemists use various types of structural representations. You
are familiar with some of these representations, but are probably not facile with the use of all of them. Model 1 below illustrates three of the
common structural representation types:
(1.) Lewis "Dot" Structures
(2.) Dash Structures
(3.) Bond-Line Structures:
Learning Goals:
1. To correctly interpret most often used organic structural representations.
2. To correctly convert one type of structural representation into another.
3. To recognize and label the elements of a Learning Cycle Activity.
Structural Representations
2
Activity
1. Working together as a group, use the information available in Table 1 to fill in the missing representations. For each structure you draw be prepared to
explain how your group devised the structure from the information in Table 1.
Model 1
Table 1: Often Used Representations of Organic Molecules
Lewis "Dot"
H
H
H
:
:
:
H
H : C : C : C :H
:
:
:
H
H
H
H
H
H
C
C
C
H
H
H
:
:
H
H
C
:
::
:
: :
H
C
:
H
:
:
H
H
H
C
C
H
H
H
H
H
O
C
H
C
C
H H
H
O
:
:
H
:
:
H
H
:
:
H : C : C :O:H
H
: :
H
C
H
:
H
C
H
H
H
:O :
:
::
:
H
H : C :C : C :H
H
H
H :O : H
H :O : H
CH3 CH3
Bond-Line
Dash
H
E
The is an exploration type questions in that its answer needs only pattern matching and no conclusion about extrapolating the process to new
situations.
3
Structural Representations
:
2. Draw a bond-line structure for the following compound:
H
H
H
H
N
H
H
N
:
H
C
C
C
H
H
H
H
Explain how you used your experience with Table 1 to devise your
structure.
Note that N-H bonds are included (drawn in red above) and there
must be an explicitly drawn bond between the center carbon
position and the N (drawn in red above).
I-A
This requires rules to be derived from the data in Table 1 and then
applied to a new situation. This is the point when the concept (rules
for drawing Bond-Line structures) must be invented
3. Answer the following questions about the structure below and explain
how your responses were derived:
3 H's
[1.] How many carbon atoms are there in
the compound?
2 H's
There
are 6 carbon atoms, which are
3 H's
1H
3 H's illustrated by the ends of each line.
2 H's
[2.] How many hydrogen atoms are attached
to each of the carbon atoms?
Explain how you used your experience with Table 1 to devise your
answers.
The number of hydrogen atoms attached to each atom is determined by
subtracting the number of bonds present around each carbon position
from 4 (total number of bonds needed for carbon). The numbers of
hydrogen atoms for the carbon atoms are given in the diagram above.
I-A
As with question 2. New rules about numbers of bond to C must be
invented and applied to a new situation.
4. Sometimes structure types are mixed in order to emphasize particular aspects of a structure. Note also that lone pair electrons are not always included if
they are not important to the discussion.
a. Consult as a group and then draw a bond-line structure below each mixed structure below.
b. For each mixed type structure, circle the part of the structure that is being emphasized.
O
H
H
H
C
O
C
CH3
H
CH2
C
H
O
O
In each case, the extra atoms, not usually shown in a Bond-line structure focus attention on that particular part of the structure and
would be used to focus a discussion.
A This is an application of the concepts invented in questions 2 & 3.
Structural Representations
4
5. As a group, decide which type of structure is best described by the each of the following statements and explain your reasoning:
a. Provides the most concrete information about the structure. (Requires the least interpretation.)
In terms of the maximum amount of explicit concrete information, either Lewis Dot or Dash structures are best.
Explain
They use different symbols (lines vs. versus dots for bonding electrons) but both explicitly include information on all atoms and electrons in
the molecule.
b. Is the quickest to draw.
Bond-line structures are the best here.
Explain
They require the fewest pen strokes to create. However, they require more interpretation than any of the other types of structures.
Three Dimensional Structure Representations: Molecular Models:
Actual three-dimensional representations of three-dimensional structures.
(See the instruction manual for your molecular model kit. Atoms are represented by variously colored and shaped plastic pieces and bonds are represented by various plastic rods.)
will have the opportunity to use your models and some computer representations of molecules during the first lab period exercise.
Reading Assignments: Rev. Gen. Chem. & Math #'s 23->25, CGWW pp. 19-31
The Learning Cycle
Model 2
POGIL activities are written using the “Learning Cycle”, which enhances learning through:
1. Exploration (E) of a model; using guiding questions to examine data and develop evidence from the model.
2. Invention of a concept (I) through making sense of the evidence from the model to draw out the concepts.
3. Application (A) of the new concepts to new situations through higher order thinking skills.
Figure 1
LEARNING CYCLE2,3
(Karplus, Piaget, Abraham)
inductive
E
Exploration
2
3
deductive
I
A
Concept Invention
Application
(Term Introduction)
Karplus and Thier. A New Look at Elementary school Science. Chicago rand McNally (1967)
Piaget, J. J. Res. Sci. Teach. 1964.2.176
1. What does the E stand for in Figure 1? Exploration
2. What might be introduced during the Concept Invention portion of the Learning Cycle? Terms generally used for the concept.
3. What type of questions would be found at the deductive end of the Learning Cycle? Applications
You
5
Structural Representations
4. Go back to the “Structural Representations of Organic Compounds” section and label each question with an E for Exploration, an I for concept
Invention or an A for Application. It is possible that some questions could be E & I or I & A.
See comments in Green below each question above.
5. How do your designations of questions in the activity correspond to sequence presented in Figure 1?
The generally progress from E -> I -> A.
Reflector’s Report Discussion
As a group, summarize what you believe to be the most important points of the two sections of this activity, Structural Representations of Organic
Compounds and The Learning Cycle. Your reflector will summarize your conclusions in the group electronic reflector’s report for this activity.
Structural Representations of Organic Compounds:
Ability to convert one structural representation type to another
The Learning Cycle
An appreciation of the general logic of POGIL activities starting with data and progressing along the path from E -> I -> A.
Strategy Analyst’s Report Discussion
As a group, analyze 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.
The sequence of experiencing a learning cycle activity before being introduced to the Learning Cycle concept was an exploration.
Then the analysis of the structure drawing activity was an application of the learning cycle terms introduced after the exploration.