*
SCH3U
Avital Stopnicki and Laura McLennan
*
*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.
*
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
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
S limonene
Smells like
orange
These
isomers interact
differently with receptors
on the tongue, enabling a
person to perceive different
tastes.
*
Look at these two
molecules, they have the
same molecular formula.
Can you explain why
they might behave
differently or have
different properties?
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)
Teaching Strategies:
Brainstorm ideas on the
board, have students
discuss in groups
*
* Organic compounds are often identified by their molecular
formula (e.g. C6H14), 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
*
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
* These expectations come from SCH4U
and Applied Chemistry (IUPAC)
curriculum documents. The new curriculum
documents do not include hydrocarbons or
nomenclature system.
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.
*
Organic compounds are…
* Compounds that contain carbon, but not CO(g),
CO2 (g), and ionic compounds containing carbon
Hydrocarbons are …
* Organic compounds that contain only carbon and
hydrogen atoms in their molecular structure
Organic Compounds
compounds that contain carbon (but not
CO, CO2 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
Acyclic
straight chains
Cyclic
Non-aromatic rings
Alkanes
Contain single bonds
only between carbon
atoms
CnH2n+2
Alkenes
Contain at least
one double bond
between carbon
atoms
Alkynes
Contain at least
one triple bond
between carbon
atoms
*
# of C Alkane
atoms CnH2n+2
Alkene
Alkyne
Alkyl Group
Prefix
1
methane
CH4
methene
methyne
methyl-CH3
2
ethane
CH3-CH3
Ethene
ethyne
ethyl-CH2-CH3
3
propane
CH3—CH2-CH3
propene
propyne
propyl-(CH2)2-CH3
4
butane
CH3—(CH2)2-CH3
butene
butyne
butyl-(CH2)3-CH3
5
pentane
CH3—(CH2)3-CH3
pentene
pentyne
pentyl-(CH2)4-CH3
6
hexane
CH3—(CH2)4-CH3
hexene
hexyne
hexyl-(CH2)5-CH3
7
heptane
CH3—(CH2)5-CH3
heptene
heptyne
heptyl-(CH2)6-CH3
8
octane
CH3—(CH2)6-CH3
octene
octyne
octyl-(CH2)7-CH3
9
nonane
CH3—(CH2)7-CH3
nonene
nonyne
nonyl-(CH2)8-CH3
*
* 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
*
* Same molecular formula, but atoms are connected in a
different order
Example: C4H10
might be
2-methylpropane:
(aka isobutane)
or might be
butane:
*
* 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).

*
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.
*
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
*
1.
2.
3.
Find the longest continuous chain – this is the parent chain that
gives the hydrocarbon its name.
Number the carbons in the parent chain. Start numbering at the
end closest to the first branch.
Name the branches, in alphabetical order, using the numbered
carbons of the parent chain to identify the branch location.
CH3
methyl group
on carbon 2
Longest chain of
carbons is 6 carbons 
parent chain is hexane
CH3 – CH – CH – CH2 – CH2 – CH3
1
2
3
4
CH2 -CH3
5
6
ethyl group on
carbon 3
Name of compound: 3-ethyl-2-methylhexane.
*
* 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.
CH3 – CH2 – CH – CH2 – CH2 – CH3
CH2 – CH2 – CH2 – CH3
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.
*
* 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
*
* 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
*
* Form a closed ring (non-aromatic)
* May be cycloalkanes (all single bonds) or cycloalkenes (at
least one double bond)
CH
CH2
CH2
CH2
CH2
CH2
Cyclopentane
C5H10
CH2
CH2
CH
CH2
Cyclopentene
C 5 H8
*
* 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 (C4H8):
1-butene
2-butene
CH2 = CH – CH2 – CH3
CH3 – CH = CH – CH3
Note: The parent chain is the longest chain which
contains the double or triple bond.
*
* 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:
H3C
CH3
C
C
C
H
H
cis-2-butene
(cis – substituent on the same side)
H
H3C
C
H
CH3
trans-2-butene
(trans– substituents on opposite side
*
* 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)
*
* 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 transfatty 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)
*
* Khan Academy video on Isomers
http://www.youtube.com/watch?v=457xnJv80O0
*
* Students will often miss the existence of geometric isomers.
They’ll write 2-butene as:
CH3-CH=CH-CH3
and miss the cis- and trans- isomers. If there’s a carbon-carbon
double bond, students should always draw the full structural
diagrams.
H3C
CH3
C
H
H3C
C
H
H
C C
H
CH3
*
* 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
*
* 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.
*
* http://antoine.frostburg.edu/cgi-
bin/senese/tutorials/isomer/index.cgi online tutorial to
build structural isomers
* 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