SCH3U Avital Stopnicki and Laura McLennan * Hydrocarbons: Isomers
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SCH3U Avital Stopnicki and Laura McLennan. * How can compounds with identical molecular formulas have distinctly differing properties? The answer lies.
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Slide 1
SCH3U Avital Stopnicki and Laura McLennan
Slide 2
* How can compounds with identical molecular formulas have
distinctly differing properties? The answer lies in the unique
structure of each compound. Structure is the key to unlocking the
mystery of how chemicals behave. Compounds with the same molecular
formula but different structures are called isomers.
Slide 3
This drug causes serious birth defects when pregnant women take
it during their first trimester. Known as a teratogen These
compounds have the same molecular formula but are different in
structure: they have different physical properties and behave
differently: This drug does not cause birth defects and has
therapeutic uses eg. alleviates nausea and vomiting R limoneneS
limonene These are naturally occurring isomers Smells like lemon
Smells like orange These isomers interact differently with
receptors on the tongue, enabling a person to perceive different
tastes.
Slide 4
Think about this question! This will be researched and
discussed at the end of the unit, and we will come back to answer
why isomers are important. (specifically in the food and drug
industry) Look at these two molecules, they have the same molecular
formula. Can you explain why they might behave differently or have
different properties? Teaching Strategies: Brainstorm ideas on the
board, have students discuss in groups
Slide 5
* Organic compounds are often identified by their molecular
formula (e.g. C 6 H 14 ), but the molecular formula alone does not
tell you how the atoms in the compound are connected to each other.
* Compounds with the same molecular formula but the atoms connected
in a different order or orientated differently in space will have
different chemical and physical properties. The connectivity of the
atoms in a compound and the orientation of those atoms in space
determines the physical and chemical properties of that compound. *
Compounds which have the same molecular formula but different
molecular structures are called isomers. * Since isomers have
different properties, different uses can be made of the
isomers
Slide 6
Students will B2. Investigate physical and chemical properties
of elements and compounds, and use various methods to visually
represent them * *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 * B2.7 write chemical formulae and name the compounds
using the International Union of Pure and Applied Chemistry (IUPAC)
nomenclature system. * These expectations come from SCH4U
curriculum documents. The new curriculum documents do not include
hydrocarbons or isomers as part of the expectations, however, it is
indeed introduced in the grade 11 curriculum and is included in
both McGraw and the newer Nelson texts.
Slide 7
Organic compounds are * Compounds that contain carbon, but not
CO(g), CO 2 (g), and ionic compounds containing carbon Hydrocarbons
are * Organic compounds that contain only carbon and hydrogen atoms
in their molecular structure
* Isomers are molecules that have the same molecular formula
but a different arrangement of the atoms in space, and different
properties (e.g. melting point) Isomers Stereo- or Geometric
Isomers Structural Isomers (aka Constitutional Isomers) Atoms are
connected in a different order Atoms are connected in the same
order, but arranged differently in space
Slide 11
* Same molecular formula, but atoms are connected in a
different order Example: C 4 H 10 might be 2-methylpropane: (aka
isobutane) or might be butane:
Slide 12
* Structural isomers are notmolecules that are in apparently
different arrangements which result from the molecule rotating as a
whole or from a portion of the molecule rotating about a particular
bond or bonds These are all 2-methylbutane (a straight chain of 4
carbons, with a methyl- group on carbon 2).
Slide 13
To find all possible structural isomers of a given molecular
formula: * Use molecular model kits to have students investigate
isomers and non-isomers of hydrocarbons. The teacher should
demonstrate and students should investigate how rotation around a
single bond does not create a new isomer. * Online interactive
isomer builder:
http://antoine.frostburg.edu/cgi-bin/senese/tutorials/isomer/index.cgi
http://antoine.frostburg.edu/cgi-bin/senese/tutorials/isomer/index.cgi
* Card game: distribute cards - one per student - with different
hydrocarbons (formula or drawing) on each card. Students are
instructed to find their classmate(s) with cards containing isomers
of their card. Once found, the group has to figure out which
structures are true isomers and which are simply the same
structure, drawn differently.
Slide 14
Teaching Strategy: Have students find and draw all possible
isomers of a given molecular formula using molecular models or
online applet. Differentiation: Provide the expanded molecular
formula (set out here) as scaffolding to struggling students. For
advanced students, do not tell them the total number of possible
isomers.
Slide 15
The IUPAC naming system provides a unique name for each
compound. The name provides the structure and the structure
provides the IUPAC name of the compound. In other words, if you
know one, you can find the other. IUPAC name compound
structure
Slide 16
1. Find the longest continuous chain this is the parent chain
that gives the hydrocarbon its name. 2. Number the carbons in the
parent chain. Start numbering at the end closest to the first
branch. 3. Name the branches, in alphabetical order, using the
numbered carbons of the parent chain to identify the branch
location. 1 2 3 4 5 6 CH 3 CH 2 -CH 3 CH 3 CH CH CH 2 CH 2 CH 3
Longest chain of carbons is 6 carbons parent chain is hexane methyl
group on carbon 2 ethyl group on carbon 3 Name of compound:
3-ethyl-2-methylhexane.
Slide 17
* Students will often fail to find the longest carbon chain and
will name the parent chain as the longest straight chain in the
provided diagram. CH 3 CH 2 CH CH 2 CH 2 CH 3 CH 2 CH 2 CH 2 CH 3
Many students will name this compound 3-buytlhexane, when its
proper name is 3-propylheptane. * Teaching Strategy: proper
modeling by the teacher and reinforcement of this concept is
necessary. Students will need practice in correctly naming
compounds.
Slide 18
* Replay the card game activity, but add in cards with names of
hydrocarbons. Students with structure cards must find their match
with the correct name card. * Role play activities where students
must be the carbon and form a hydrocarbon by holding hands. The
rest of class names the hydrocarbon. (Consider having two students
back to back act as a single carbon since carbon bonds four times
and a single student only has two hands.) * Pen and paper
worksheets naming and drawing hydrocarbon isomers
Slide 19
* Alkenes have one or more double bonds between two carbon
atoms e.g. ethene * Alkynes have one or more triple bonds between
two carbon atoms e.g. ethyne
Slide 20
* Form a closed ring (non-aromatic) * May be cycloalkanes (all
single bonds) or cycloalkenes (at least one double bond)
Cyclopentane C 5 H 10 Cyclopentene C 5 H 8 CH 2 CH
Slide 21
* The location of the double or triple bond affects the
properties of the compound. The IUPAC name tells you the location
of the double or triple bond in the compound. Structural Isomers of
butene (C 4 H 8 ): 1-butene2-butene CH 2 = CH CH 2 CH 3 CH 3 CH =
CH CH 3 Note: The parent chain is the longest chain which contains
the double or triple bond.
Slide 22
* In addition to structural isomers, some compounds can have
stereo- or geometric isomers. Geometric isomers have their atoms
connected in the same order, but arranged differently in space.
Geometric isomers in alkenes result from the fact that a double
bond does not rotate. Geometric Isomers of 2-butene: H CH 3 C C
H3CH3CH3CH3CH H3CH3CH3CH3C C C HH cis-2-butene (cis substituent on
the same side) trans-2-butene (trans substituents on opposite
sides)
Slide 23
* Students to use molecular model kits to investigate double
and triple bonds, specifically that double bonds dont twist or
rotate propene * Lab activity Reactivity of alkanes and alkenes
identifying saturated and unsaturated compounds in fats and oils
(McGraw-Hill, pp. 554-555)
Slide 24
* Students can further investigate the properties of saturated
and unsaturated compounds in various fats and oils, including
making a comparison of geometric isomers, the unsaturated cis- and
trans- fatty acids, using both print and electronic resources. *
Students will explain, based on their research and in understanding
the structure of the compounds, why unsaturated fats are said to be
healthier than saturated fats. They will connect this to the
question asked at the beginning of the unit: Why are isomers
important in the food and drug industry? * Teacher to provide
suitable resources * Students will be evaluated on: accuracy of
information (TI) making connections (A) communication and
presentation (C)
Slide 25
* Khan Academy video on Isomers
http://www.youtube.com/watch?v=457xnJv80O0
Slide 26
* Students will often miss the existence of geometric isomers.
Theyll write 2-butene as: CH 3 -CH=CH-CH 3 and miss the cis- and
trans- isomers. If theres a carbon-carbon double bond, students
should always draw the full structural diagrams. H CH 3 C C
H3CH3CH3CH3CH H3CH3CH3CH3CC C H H
Slide 27
* cis- and trans- isomers behave differently. Pharmaceuticals
take advantage of different behaviours of geometric isomers.
Amphetamine (a psychostimulant) and dextroamphetamine (used to
treat ADHD) are stereoisomers of one another and have different
effects on humans. * Structural isomers have different physical
properties: butane melting point -140 C boiling point -1 C
2-methypropane melting point -159.6 C boiling point -11.7 C
Slide 28
* Natural rubber from the rubber tree is a polymer of
isopropene, in the cis configuration. It is flexible and elastic. *
trans-1,4-isopolypropene, better known as gutta percha, is found in
tropical trees of Southeast Asia. It has similar, but not identical
properties, to its stereoisomer natural rubber, but is harder and
tougher.
Slide 29
* http://antoine.frostburg.edu/cgi-
bin/senese/tutorials/isomer/index.cgi online tutorial to build
structural isomers http://antoine.frostburg.edu/cgi-
bin/senese/tutorials/isomer/index.cgi * Schmidt, H.J. (1995).
Student Misconceptions Looking for a Pattern. Science Education,
vol. 81, Issue 2, pp.123-135. * American Society of Consultant
Pharmacists
http://www.cmecorner.com/macmcm/ascp/ascp2002_01.htm