Application of Hammond’s Postulate

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In the Classroom
204 Journal of Chemical Education • Vol. 78 No. 2 February 2001 • JChemEd.chem.wisc.edu
Background
The inquiry approach to college science teaching began
to take form in the mid 1980s and gained momentum in
the 1990s. This is succinctly reviewed by Jeanne Narum (1)
from Project Kaleidoscope. Our own activity-based approach
to learning and teaching has evolved over time and most
closely resembles that described by Krumsieg and Baehr (2)
and Hanson and Wolfskill (3), in which the instructor serves
as a facilitator in guided discovery learning. This format is
particularly well suited for engaging students in higher order
cognitive processes (4) that are characteristic of science. At
Seattle University, we find it pedagogically advantageous to
introduce activities into the chemistry lecture curriculum in
which students are given experimental observations and then
prompted, with a little guidance, to draw and articulate
conclusions for themselves.
At the beginning of the academic year, organic chemistry
students are asked to form small study groups. A formalized
program is also offered by the Learning Center at Seattle
University, in which facilitated study groups are aided with
experienced students as leaders. After thoroughly working
through and discussing guided discovery activities outside of
class, the student groups are asked to articulate and discuss
the concept(s) explored with the entire class during its
normally scheduled period. After students complete the
activity, they are assigned critical thinking and skill exercises
that require further application of these concepts to help
reinforce and solidify understanding. The following activity
is an example.
The analysis of the transition state associated with the
rate-determining step of organic reactions is used extensively
in organic chemistry to predict relative reaction rates and,
for kinetically controlled reactions, product distributions.
Applications of transition-state theory are relatively new to
students at the beginning of their organic chemistry course and
visualization of a chemical species with a lifetime of approxi-
mately one molecular vibration is sometimes difficult for them.
With the aid of potential energy/reaction progress diagrams, we
have developed an activity that guides students to the under-
standing and articulation of the relationship between selectivity
and reactivity in terms of Hammond’s postulate (5). Because of
its wide variety of applications, we find it effective to intro-
duce this postulate in the early stages of the course (6–10).
The activity is based on the mechanism of free radical
halogenation of alkanes. It focuses on the rate-determining (and
product-determining) step of the reaction (11–13) and requires
students to carry out a stepwise analysis of what happens
along the reaction progress coordinate from reactant to tran-
sition state to intermediate. The activity requires students to
integrate and apply prior knowledge into a new context. They
must consider statistical factors and potential energy changes
and estimate the extent of bond making and breaking in the
transition state.
The depth of coverage of this and other concepts and
the appropriateness of the exercises will obviously be dictated
by the backgrounds and academic goals of the students in
the organic chemistry class. The teaching/learning model
described in this article is used in a course in which the
student audience consists of chemistry and biology majors,
most of whom plan to continue their academic training
either in graduate school or in medical school. We have noted
that the guided discovery learning approach has the same
positive effects on student learning in chemistry courses that
are somewhat less rigorous.
At Seattle University, we began incorporating discovery
learning activities into the organic chemistry curriculum in
1993. At present, we utilize approximately 25 activities during
the year, including the one reported here. Earlier articles include
discussions of the efficacy of using learning teams for guided
discovery learning projects (3, and refs therein).
We, too, believe that the incorporation of these learning
activities into the curriculum promotes greater understanding,
enhances problem-solving abilities, and increases retention of the
material. Although the group activities require additional time
outside of class, students make this sacrifice with a positive
attitude. For example, although no specific questions were asked
about group activities on a recent fall quarter’s evaluation
form, almost 50% of the students made very positive comments
regarding the use of this and other group guided discovery
learning activities. We have also found that such exercises
enhance student understanding, thereby allowing instructors to
include more complex exam questions that require students
to apply concepts. (See the skill exercises included online.W )
While we have increased emphasis on the process of
science through the activities, we have not sacrificed content.
Performance has improved in the American Chemical Society
standardized exam in organic chemistry, the average percen-
tile score being 10 points higher than in the years before we
initiated the activities. The textbook, laboratory program and
instructor have been the same for the last 15 years, and during
this period the average GPA from our prerequisite general
chemistry courses has remained essentially constant. Others
have collected definitive data on the benefits of inquiry
learning (14 ) and more open-ended classroom discussion in
chemistry (15). Our observed improvement in final exam
Application of Hammond’s Postulate W
An Activity for Guided Discovery Learning in Organic Chemistry
J. E. Meany* and Vicky Minderhout
Department of Chemistry, Seattle University, Seattle, WA, 98122; *jmeany@seattleu.edu
Y. Pocker*
Department of Chemistry, University of Washington, Seattle, WA 98195
In the Classroom
JChemEd.chem.wisc.edu • Vol. 78 No. 2 February 2001 • Journal of Chemical Education 205
percentiles for organic chemistry is further evidence of the
merits of the type of active learning described in this paper.
Activity: Selectivity/Reactivity Principle in Terms
of Hammond’s Postulate
General Objectives
In this activity, students apply relevant principles of
thermochemistry and kinetics to the understanding of the
relationship between selectivity and reactivity. Recognizing
how enthalpies of reaction, ∆H, and activation energies, Ea,
influence the regiospecificity of an elementary reaction
demonstrates to students the application of Hammond’s
postulate.
Prerequisite Knowledge
1. Mechanism of halogenation of alkanes
2. Interpretation of potential energy/reaction progress
diagrams
3. Hammond’s postulate
4. Calculation of heat of reaction from bond dissociation
energies
5. Use of the Arrhenius equation
6. ∆G°, ∆H°, and ∆S° calculations and the qualitative
meaning of these terms
7. Collision and transition state theory
Learning Objectives
1. Given activation energies and the statistical availability
of reaction sites on the reactant, calculate the relative
rates of formation of primary and secondary alkyl free
radicals in terms of collision theory.
2. Describe how kinetic and thermochemical data can be
used to show that bromine atoms are more selective
than chlorine atoms in abstracting hydrogen atoms of
different classification.
3. Apply Hammond’s postulate in predicting transition
state structures for the rate-determining step of the
chlorination and bromination of propane.
4. Explain the relationship between selectivity and
reactivity in terms of Hammond’s postulate for the
reactions under consideration.
5. Relate entropy of activation to the probability factor
in collision theory.
6. Describe how kinetic and thermochemicaldata can be
used to assess the relative contributions by competing
reaction mechanisms.
Performance Criteria
Performance is evaluated on the basis of the student’s
ability to discuss and assess his or her performance in this
activity with the group, taking into account the student’s
success in recognizing and understanding (i) the chemical
dynamics associated with potential energy (enthalpy) vs
reaction progress diagrams and (ii) the structures of reactants,
transition states, and products in terms of Hammond’s
postulate. Learning is also measured in terms of the student’s
answers to the critical thinking exercises at the end of the project.
Relevant Information
Students are given pertinent kinetic and bond dissociation
data associated with the free radical halogenation of alkanes
at primary, secondary, and tertiary hydrogen sites (16, 17 ).
After making some simple calculations, each student group
constructs potential energy/reaction progress diagrams. They
then use Hammond’s postulate to approximately locate the
respective transition states along the reaction coordinates of the
potential energy/reaction progress diagrams, predict transition
state structures, and articulate a rationale for the greatly differ-
ent selectivities of bromination and chlorination. After this
qualitative examination, some quantitative exercises completed
outside of class illustrate some of the principles of thermo-
chemistry and reaction rate theories in connection with the
reaction in question.
Students are given activation energies and bond disso-
ciation energies, D (13, 14), for the elementary reactions
given below.
 Ea = 1 kcal/mol �
Cl� + CH3CH2CH2–H → CH3CH2CH2 + HCl (1)
 D = 98 kcal/mol D = 103 kcal/mol
∆H = ____ kcal/mol
 Ea = 0.5 kcal/mol
Cl� + CH3CHCH3 → CH3CHCH3 + HCl (2)
 | 
�
 H
D = 94.5 kcal/mol ∆H = ____ kcal/mol
 Ea = 13 kcal/mol �
Br� + CH3CH2CH2–H → CH3CH2CH2 + HBr (3)
D = 88 kcal/mol
∆H = ____ kcal/mol
 Ea = 10 kcal/mol
Br� + CH3CHCH3 → CH3CHCH3 + HBr (4)
 | 
�
 H ∆H = ____ kcal/mol
Plan
Students are asked to (i) form groups of three or four;
(ii) carry out the key and critical thinking exercises on a group
basis and prepare to discuss these exercises with the rest of
the class; and (iii) within each group, complete the skill
exercises outside of class.
Key Exercises
1. Calculate ∆H for elementary reactions 1–4 above,
using the bond dissociation energies, D, given.
2. Using the graph paper provided, draw potential energy/
reaction progress diagrams to scale for each reaction (one-half
space per kcal). In accordance with Hammond’s postulate,
approximate the position of the respective transition states
along the reaction progress axes. As a starting point, we have
arbitrarily assigned the reactants an enthalpy value of zero
(Figs. 1 and 2. NOTE: we have completed the graphs using
dotted lines and italics for the print publication only).
In the Classroom
206 Journal of Chemical Education • Vol. 78 No. 2 February 2001 • JChemEd.chem.wisc.edu
Critical Thinking Exercises
1. On the basis of Hammond’s postulate, draw the
structures of the transition states for the four reactions to
indicate the extent of bond making and breaking for each
elementary reaction.
2. Using the graphs you have generated and the transi-
tions states you have drawn, explain, in terms of Hammond’s
postulate, why bromine atoms are so much more selective than
chlorine atoms in abstracting hydrogen atoms of different
classifications.
Figure 2. Bromination: ∆H vs % reaction.
Figure was completed by authors for pub-
lication.
Figure 1. Chlorination: ∆H vs % reaction.
Figure was completed by authors for pub-
lication.
Skill Exercises
A number of exercises are included at this point to further
solidify the concepts involved. These exercises, which require
students to carry out simple calculations using collision theory
and transition state theories, are provided online.W
Acknowledgments
Special thanks to S.U. Chemistry professors Susan
Jackels, G. Spyridis, and Bernard Steckler for their signifi-
In the Classroom
JChemEd.chem.wisc.edu • Vol. 78 No. 2 February 2001 • Journal of Chemical Education 207
cant contributions to this paper. We are also indebted to Carol
Schneider, Director, S.U. Learning Center, for organizing and
supervising the Facilitated Study Group program. J.E.M.,
serving as the Arline F. Bannan Chair of Science and Math-
ematics at Seattle University, is indebted to the Thomas J.
Bannan Family.
WSupplemental Material
Background and prerequisite information about the free
radical halogenation of alkanes, the student version of the
activity including the blank graphs to be completed by students,
possible strategies for class discussion, and skill exercises and
their solutions are available in this issue of JCE Online.
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