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SCH3U
Avital Stopnicki and Laura McLennan

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These compounds have the same molecular formula but are different in structure: they have different physical properties and behave differently:
These are naturally occurring isomers
R limonene S limonene
This drug causes serious birth defects when pregnant women take it during their first trimester. Known as a teratogen
This drug does not cause birth defects and has therapeutic uses eg. alleviates nausea and vomiting
Smells like lemon
Smells like orange
These isomers interact differently with receptors on the tongue, enabling a person to perceive different tastes.

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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
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)

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Organic compounds are often identified by their molecular formula (e.g. C each other.
6
H
14
), but the molecular formula alone does not tell you how the atoms in the compound are connected to
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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 .
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Since isomers have different properties, different uses can be made of the isomers

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Students will
B2. Investigate physical and chemical properties of elements and compounds, and use various methods to visually represent them
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* B2.3 build molecular models for a variety of simple organic compounds [PR, AI, C]
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* 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.
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.

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* Compounds that contain carbon, but not CO(g),
CO
2
(g), and ionic compounds containing carbon
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Organic compounds that contain only carbon and hydrogen atoms in their molecular structure

compounds that contain carbon (but not
CO, CO
2 or ionic carbon compounds)
Hydrocarbons organic compounds containing only carbon and hydrogen atoms
Aromatics contain aromatic rings e.g. benzene
Aliphatic straight chains and nonaromatic rings
Cyclic
Non-aromatic rings
Acyclic straight chains
Alkanes
Contain single bonds only between carbon atoms
C n
H
2n+2
Alkenes
Contain at least one double bond between carbon atoms
Alkynes
Contain at least one triple bond between carbon atoms

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# of C atoms
1
2
3
4
5
6
7
8
9
Alkane
C n
H
2n+2 methane
CH
4 ethane
CH
3
-CH
3 propane
CH
3
—CH
2
-CH
3 butane
CH
3
—(CH
2
)
2
-CH
3 pentane
CH
3
—(CH
2
)
3
-CH
3 hexane
CH
3
—(CH
2
)
4
-CH
3 heptane
CH
3
—(CH
2
)
5
-CH
3 octane
CH
3
—(CH
2
)
6
-CH
3 nonane
CH
3
—(CH
2
)
7
-CH
3
Alkene methene
Ethene propene butene pentene hexene heptene octene nonene
Alkyne methyne ethyne propyne butyne pentyne hexyne heptyne octyne nonyne
Alkyl Group
Prefix methyl-
-CH
3 ethyl-
-CH
2
-CH
3 propyl-
-(CH
2
)
2
-CH
3 butyl-
-(CH
2
)
3
-CH
3 pentyl-
-(CH
2
)
4
-CH
3 hexyl-
-(CH
2
)
5
-CH
3 heptyl-
-(CH
2
)
6
-CH
3 octyl-
-(CH
2
)
7
-CH
3 nonyl-
-(CH
2
)
8
-CH
3

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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
Structural Isomers
(aka Constitutional
Isomers)
Atoms are connected in a different order
Stereo- or
Geometric Isomers
Atoms are connected in the same order, but arranged differently in space

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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 :

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Structural isomers are not…molecules 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 methylgroup on carbon 2).

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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
* 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.

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.

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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.

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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.
CH
3 methyl group on carbon 2
CH – CH – CH – CH – CH
2
Longest chain of carbons is 6 carbons  parent chain is hexane
– CH
6
3
CH
2
-CH
3 ethyl group on carbon 3
Name of compound: 3-ethyl-2-methylhexane.

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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.

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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

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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

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Form a closed ring (non-aromatic)
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May be cycloalkanes (all single bonds) or cycloalkenes (at least one double bond)
CH
2
CH
2
CH
CH
2
CH
2
Cyclopentane
C
5
H
10
2
CH
CH
2
CH
CH
2
CH
2
Cyclopentene
C
5
H
8

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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-butene
CH
2
= CH – CH
2
– CH
3
2-butene
CH
3
– CH = CH – CH
3

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In addition to structural isomers, alkenes and alkynes can have stereo- or geometric isomers. Geometric isomers have their atoms connected in the same order, but arranged differently in space. Geometric isomers result from the fact that a double bond does not rotate.
Geometric Isomers of 2-butene:
H
3
C CH
3
H
3
C H
C C C C
H
H
(cis – substituent on the same side)
H CH
3
(trans– substituents on opposite sides)

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Students to use molecular model kits to investigate double and triple bonds, specifically that double bonds don’t 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)

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* 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.
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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?
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* Students will be evaluated on: accuracy of information (TI)
Making connections (A)
Communication and Presentation (C)

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Khan Academy video on Isomers http://www.youtube.com/watch?v=457xnJv80O0

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* Students will often miss the existence of geometric isomers.
They’ll write 2-butene as:
CH
3
-CH=CH-CH
3 and miss the cis- and trans- isomers. If there’s a carbon-carbon double bond, students should always draw the full structural diagrams.
H
3
C
C C
H
CH
3
H
H
3
C
H
C C
H
CH
3

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cis- and trans- isomers behave differently. Pharmaceuticals take advantage of different behaviours of geometric isomers.
Amphetamine and dextroamphetamine are stereoisomers of one another and have different effects on humans.
* Stereoisomers 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

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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.

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* http://antoine.frostburg.edu/cgibin/senese/tutorials/isomer/index.cgi
online tutorial to build structural isomers
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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
This site from ASCP discusses isomers at length through a symposium of medical researchers and doctors.