Teaching a concept:
Hydrocarbon Isomers
SCH4U - Organic Chemistry
André Gagnier and Nathan Hickey
University of Toronto – OISE - EAQ1330Y - Science Senior
Janine Extavour
July 21, 2011
Expectations:
B3. demonstrate an understanding of the structure, properties, and chemical behaviour of
compounds within each class of organic compounds.
B2.3 build molecular models for a variety of simple organic compounds [PR, AI, C]
B3.5 explain the concept of isomerism in organic compounds, and how variations in the properties
of isomers relate to their structural and molecular formulae
Background Information
Isomers are chemical products that have the same formulas but that have different molecular
structures and thus behave differently.
There are two main types of isomers covered in these activities:
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isomers of alkanes (constitutional isomers)
isomers of alkenes (constitutional isomers and stereoisomers)
Constitutional isomers: isomers differ in the order in which the atoms are connected together so
they contain different functional groups and / or bonding patterns (e.g. branching).
Stereoisomers (geometric only): contain the same functional groups and branching patterns, they
differ only in the arrangement of atoms in space. Stereoisomers across a double bond are usually
designated with the prefixes cis- or trans-.
Teaching a concept: Hydrocarbon Isomers
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Advance Preparation
Before beginning this part of the organic chemistry unit, we are assuming that the students are
familiar with the following concepts and procedures:
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They must be fully acquainted to the nomenclature of organic chemistry.
Students need to know how many bonds various atoms can make within a hydrocarbon
molecule (C=4, O=2, H and halogen =1)
Students know how to represent molecules using drawings.
Students must remember that molecules are 3D structures.
Students will already have experience using molecular models.
Students will already have experience in lab safety techniques.
Required Materials
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Molecular models
Propanol
Isopropanol
Electronic thermometer (probeware)
Ring stand
Bunsen burner
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Hot plates
Beakers
Graduated cylinders
Electronic balance
Iron rings and wire gauze
Student Difficulties
PROBLEM
SOLUTION
Students may think that changing the shape (ie.
rotating a C-C bond) means a new isomer is
created.
Using molecular models, show that for it to be
an isomer, bonds need to be broken and
replaced. Rotations do not create isomers.
Understanding that cis- and trans- geometric
isomers are, in fact 2 different substances.
Using pictures and molecular models to allow
students to see the 3D configuration.
Students think that isomers are always members
of the same class of compound. (eg. the student
can think that any isomer of an alcohol must
also be an alcohol, when in fact it could be an
ether).
Using inquiry-based discovery, students will
have to construct all the possible isomers. Using
pictures of 2 molecules (alcohol and ether) and
asking students to count the atoms. They will
realize that the formulae are the same.
Teaching a concept: Hydrocarbon Isomers
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Lesson/Activity Sequence
Day.
Activity
1.1
Concept
Introductory activity (hook):
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1.2
Discussion of concepts:
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1.3
Introduce vocabulary (Isomer, constitutional isomer)
Discuss characteristics through question and answer
Different isomer = different substance
Thus isomers have different characteristics from each other.
Online isomer construction activity:
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2.1
Identify the Isomers Challenge
Introduced as a game (hook)
Students are separated into groups and given an empirical formula of a
simple alkane (eg. C6H14) and asked to identify with molecular models and
with hand-drawn images as many different molecules as they can that fit that
formula.
Time limit is given
The word “isomer” is not used
A small example can be demonstrated on the board if necessary
Adaptations:
o more time
o # of molecules is provided
Class discusses their answers and students are invited to share in front of the
class.
The students are questioned to determine whether they can recognize when
two compounds are the same or different. Use of the molecular kits.
Assessment/
Evaluation
Inquiry-based
learning
Assessment via
questioning
Cooperative
learning
Discussion in between
answers
Brainstorming
Class discussion
Assessment based on
Q+A
Socratic episode
Q+A
Computer
simulation
Assessment of learning
(homework)
Isomer construction set (click to access the site)
Homework:
o Using the site as a guide, make a 2-column table. Column A: # of
carbons, Column B: # of isomers.
o Complete the table up to an alkane with 10 carbon atoms
Pre-laboratory discussion
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Teaching/
Learning
Strategies
Discussion: review yesterday’s notes and work
Introduce set of isomers on board
propanol - isopropanol
Have students build the molecules with models.
Isomers have different characteristics
Teaching a concept: Hydrocarbon Isomers
Class discussion
Q+A
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2.2
Laboratory (groups of 2)
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3.1
Compare results
Discuss questions
Write possible answers on the board
Use molecular models to hypothesize why isopropanol boils faster.
Do structure/ polarity play a role?
Molecular modeling: Alkenes
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Laboratory
Laboratory report
Class discussion
Assessment based on
Q+A
Investigating boiling point and density of propanol and isopropanol
Using a simple Bunsen burner setup and an electronic thermometer,
determine the BP of each compound.
Using a balance and a graduated cylinder, determine the density.
Analysis questions:
o Why does propanol have a higher BP? Density?
o What are the applications of these compounds?
o Do these applications exploit their isomeric differences?
Discussion: return on laboratory results
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3.2
Hypothesize: which would have higher boiling point? Density?
All students (and teacher) will have molecular models. (if not, in groups)
Students follow teachers instructions and will repeat exactly what he/she
does
Single bonds can rotate
Double bonds cannot! Just try!
We can end up with 2 possibilities:
Q+A
Brainstorming
Modeling
Simulations
Socratic episode
Support these models with pictures:
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These are stereoisomers
Discussion of isomer applications
o see Applications section
Teaching a concept: Hydrocarbon Isomers
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Homework: Problem set (found on the Wiki)
3.3
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“Isomerism exercises.doc”
Adaptations
Q’s 1 and 2 only
heterogeneous grouping
Bonus question:
Can alkynes have isomers? Why or why not?
Paper and pencil
Asessment OF
learning
(exercises)
Problem set
Teaching Ideas and Adaptations
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This is a very visual lesson, auditory learners will have difficulty. However, tactile learners
will have the benefits of the molecular model kits, despite the fact that these take up more
time.
Images and drawings made by both the teacher and the students.
Inquiry-based strategies for students to discover isomers of given formulas on their own.
(how many can you find?)
Possible differentiation strategies for isomer-drawing activities
o giving the # of isomers for a particular formula
o leading students with a few examples for a given formula
o (Advanced) Asking students to find trends between the # of isomers and the # of
carbons in alkanes.
Laboratory activity: Investigating boiling points of different isomers.
As a side mini-lesson, we could lead a discussion touching the etymology of “iso”, “cis”,
“trans” and “tert” with the goal of providing context to the nomenclature.
o This has direct applications to fatty acids found in food.
Problem sets: Homework assignment reviewing the day’s lesson
o Finding and identifying the isomers for a given formula
o Naming the isomers
o Finding and identifying isomers containing different functional groups.
Post exercises on the Wiki for students to do at home.
o We could also open one page up to their editing, so that they can have a type of
discussion room to post questions and help each other out.
Evaluation Procedures
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Assessment FOR learning
o Questioning based on answers given during isomer identification challenge
Assessment AS learning
o Group-based work during isomer identification challenge and online activity
Assessment OF learning
Teaching a concept: Hydrocarbon Isomers
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Homework: Problem sets
Laboratory: Properties of different isomers.
Applications and Societal Issues or Implications
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Cis- and trans- fatty acids in foods
o trans-fatty acids are generally associated with health risks including coronary heart
disease
o The mechanisms through which these compounds contribute to these health risks are
under investigation
Octane rating of gasoline
o Simply, this rating can be used as a rough measure of the isomeric content in the fuel
Chiral drug industry
o Exploiting isomers in pharmaceuticals to gain better efficacy.
o Not to be discussed in depth as it includes classes of isomers outside the scope of the
class
Annotated Internet Addresses/References
These sites would be added to class wiki to help students with their homework.
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http://www.flatworldknowledge.com/pub/basics-general-organic-and-bio/421680#web421674
o (This site has general information on organic chemistry and is very simple and straightforward)
http://chemed.chem.purdue.edu/genchem/topicreview/bp/1organic/organic.html
o (A more advanced site with information on organic chemistry)
http://antoine.frostburg.edu/cgi-bin/senese/tutorials/isomer/index.cgi
o (A web-based isomer tutorial. This site contains the virtual isomer construction set)
www.a-levelchemistry.co.uk/
o (The isomerism exercises were obtained from this site)
Teaching a concept: Hydrocarbon Isomers
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