Experiment 3: MOLECULAR MODELS
Purpose: This laboratory exercise is to help you become skilled at determining which
structures are isomers and which are identical to each other. Structures written on paper are
often misleading as to whether they represent true isomers or the same compound viewed
from a different angle. By constructing models with the ball-and-stick model kit provided,
you will learn to visualize the three-dimensional nature of some organic compounds.
Introduction:
The observable physical and chemical properties of a substance depend on two factors: the
composition and the structure. Composition refers to which elements are present and how
many atoms of each element are present in a substance. Structure refers to the spatial
arrangements of these atoms.
The molecular formula of a substance provides us with the elements and the number of
atoms of each element in the substance. Thus, it reflects its composition. There are,
however, many instances where two or more different substances exist having the same
formula. For example, ethyl alcohol and dimethyl ether share the same molecular formula,
C2H6O, and hence the same composition. Yet these are two unique, different compounds,
each having its own characteristic physical and chemical properties. The reason these two
compounds are different is that they have different structures. Molecules with the same
molecular formula, but different structures are called isomers.
H H
H C C O H
H
H
H C O C H
H H
H
ethyl alcohol (or ethanol)
H
dimethyl ether
Isomerism
There are three types of isomers that you will come across this semester: structural
isomers, geometric isomers and optical isomers. The first two you are likely to have
come across in your previous chemistry course. Optical isomers are introduced here and
will be discussed more thoroughly later in Chem 146.
Structural Isomers: These are substances with the same molecular formula but differ in
which atom is attached to which. They are sometimes referred to as constitutional
isomers. Ethyl alcohol and dimethyl ether (shown above) are structural isomers.
H H
H
C H
C C
H
H
H
propene (C3H6)
cyclopropane (C3H6)
H
H
C
C
H
C
H
H
Propene and cyclopropane (shown above) also fit under structural isomers.
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EXPERIMENT 3: MOLECULAR MODELS
Geometric Isomers: These are isomers that do not differ in which atoms are attached to
which, but differ in the 3-dimensional arrangements that give rise to cis- and transisomers. They are associated with compounds with either double bonds or a ring as
shown in the examples below.
Cl
Cl
H
C C
C C
H
H
cis-dichloroethene
H
Cl
trans-dichloroethene
H
H
Br
C
C
Cl
H
H
C
Br
H
H
Br
trans-1,2-dibromocyclopropane
C
C
H
C
H
Br
cis-1,2-dibromocyclopropane
Optical Isomers: These are isomers that also do not differ in which atoms are attached
to which, but differ in 3-dimensional arrangements other than the cis/trans isomers
described above. The type of optical isomers you will be studying in the latter portion of
Chem 146 are the enantiomers. They are isomers that are mirror images of each other
that are not superimposable, such as your right hand and left hand. The word chiral is
derived from the Greek word for hand. Superimposable means that you cannot lay one
structure on top of the other and have them look identical. For example, transdichloroethene is superimposable on its mirror image and therefore its mirror image is
not its enantiomer. Part of this exercise will introduce you to recognizing chiral carbons
and enantiomers.
For some structures, it may not be obvious as to whether they are truly isomers or merely
representations of the same compound viewed from a different angle. Remember from your
previous chemistry course that the Lewis structures are not meant to show the correct shapes
necessarily. Often we do need to know the 3-dimensional orientations of the bonds and
atoms. This is where organic chemistry students often find it useful to construct models of
the compounds they are studying.
There are several important points to remember in determining whether two structures are
isomers or identical.
 If you can convert one structure to the other by rotating the entire molecule upside
down or flip it from right to left, the two structures are NOT isomers, but considered
identical. However, remember that you must be thinking in 3-dimensions!
 Carbon with four bonds is tetrahedral in shape, and does not exist as a flat cross:
H
H
Cl
C Cl
H
Lewis structure that
does not show correct shape
of dichloromethane
H
C
Cl
Cl
3-dimensional sketch
showing the correct shape
of dichlormethane
EXPERIMENT 3: MOLECULAR MODELS
47

Single bonds are generally free to rotate. If you can rotate a bond to convert one
structure to another, then they are NOT isomers, but considered identical.
 If you have to break one or more bonds to convert one structure to the other, they are
isomers.
Examine all the isomers described earlier and verify that indeed none of them can be
converted into each other without breaking some bonds.
Expanded versus Condensed Structural Formulas
An expanded structural formula is one that shows all the atoms and bonds. It certainly
gives more information than the molecular formula which shows only the number of
atoms for each element; however, for large molecules, it gets rather cumbersome to write
all the bonds out. Hence we often find it desirable to give a condensed structural
formula. This is an important format that you should understand fully. We write out
each C, followed by the number of H atoms attached to it. Thus butane is shown as
follows.
H H H H
expanded structural formula
H C C C C H
H H H H
condensed structural formula
CH3CH2CH2CH3
and isobutane is shown as follows:
H H H
expanded structural formula
H C C C H
H
H
H
C H
H
condensed structural formula
CH3CHCH3
|
CH3
Procedure:
Work in pairs to build molecular models and discuss the possibilities of isomers and the
polarity of each isomer. However, each student is expected to write and turn in his/her
own Data Table.
In this exercise we will limit ourselves to simpler species: neutral molecules with no
formal charges. For such cases, carbon forms four bonds, hydrogen and the halogens
form one each, oxygen forms two and nitrogen forms three. The color code is as
shown:
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EXPERIMENT 3: MOLECULAR MODELS
carbon
nitrogen
= black
= blue or gray
single bond = wooden peg
hydrogen
chlorine
= yellow
= green
oxygen
double bond = 2 springs
= red
triple bond = 3 springs
Follow the procedure below carefully and write your answers on the Report Page provided:
1. Isomers of C4H10
You and your partner are to each build one of the following. Compare your two models
and determine whether they are isomers or identical.
H
H H H H
H CH
H H
H C C C C H
H C C C H
H H H H
H H H
1A
1B
Build models for the following and determine whether they are isomers or identical.
H
H H H H
H C H
H
H
H C C C C H
H C C C H
H H H H
H H H
1A
1C
How many structural isomers does C4H10 have? Give the condensed structural formula
for each.
2. Isomers of C3H8O
Build models for each of the three structural isomers of C3H8O and give the condensed
structural formula for each.
Alcohols are organic compounds with an oxygen atom bonded to C on one side and H
on the other: C−O−H The −OH functional group is referred to as a hydroxyl group.
Ethers are compounds with the oxygen atom bonded to two carbon atoms, one on each
side:
C−O−C
Examine your three isomers of C3H8O. Label each of your structural formulas as an
alcohol or an ether.
EXPERIMENT 3: MOLECULAR MODELS
49
3. Isomers of C4H10O
Give the expanded structural formula of all the isomers of C4H10O that are alcohols.
Give the expanded structural formula of all the isomers of C4H10O that are ethers.
Label them as alcohols or ethers. Build models if you are not sure whether they are
isomers.
4. Isomers of C3H6O
Give the expanded structural formula of all the isomers of C3H6O. In order to fit this
formula, they must have either a double bond or a ring. The double bond can be either
C=C or C=O. The ring can be various sizes and could include O as part of the ring.
Keep in mind that C can have only four bonds and O can have only 2 bonds. A double
bond is counted as 2 bonds.
Helpful hint: 4 of the isomers have a C=C bond.
2 of the isomers have a C=O bond.
3 of the isomers have a ring.
5. Structure of Acetic Acid
There are several ways of writing the formula for acetic acid. You probably first learned
it as HC2H3O2, placing the acidic H at the front of the formula. In organic chemistry it is
more useful to write it as a condensed structural formula: CH3COOH or CH3CO2H.
It is important to note that the functional group –COOH or −CO2H consists of one O
atom connecting to the C with a double bond, and the other O atom is bonded to the
O
same C as −OH:
Note that the −OH is no longer considered an
C O H
alcohol functional group. It is now part of the larger functional group −COOH, known
as a carboxyl functional group. A compound with this carboxyl group is classified as an
O
organic carboxylic acid.
Fill in the missing lone pairs.
CH3 C O H
(1)
(2)
Build a model of acetic acid and draw a 3-dimensional sketch of the molecule.
What is the geometry and bond angle at C1? at C2? at the singly bonded O?
6. Optical Isomers
In the following exercise, with the help of your partner, build models of each of the
following to determine whether it can have isomers. Note that there is only one C and
we are varying the number of different groups attached to it. If it has isomers, they
would be ones known as optical isomers and the carbon atom is said to be a chiral
carbon. Remember that isomers are ones that cannot be converted into each other
without breaking one or more bonds. Do not merely draw Lewis structures. You must
build models to prove that certain ones have isomers.
Be particularly aware of the possibility of enantiomers (Review discussion in the
Introduction section).
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EXPERIMENT 3: MOLECULAR MODELS
Of the ones for which you can find isomers, draw 3-dimensional sketches to show how
they are isomers.
a) Build models to determine whether CH4 can have isomers. Do not merely draw
Lewis structures. You must build models.
b) Repeat for CH3Cl. (In this and subsequent formulas, Cl is chlorine.)
c) Repeat for CH2Cl2.
d) Repeat for CH2ClBr.
e) Repeat for CHClBrI. (C with H, chlorine, bromine, and iodine bonded to it.)
f) How many different groups must be attached to a C atom in order to have optical
isomers? Of the five formulas above, which has a chiral carbon?
7. The following structures have the formula C6H12O2. Some of the C and H atoms are not
shown. Answer the questions on the Report Page. Build models as necessary.
Hint: This is tricky!
OH OH
H
C C
H
H
H
H
OH
OH H
C C
C C
C C
OH OH
OH H
H
OH
6A
6B
6C
6D
Pre-Lab Questions: (To be turned in at the beginning of the lab.)
1. Based on the discussion on isomerism in the Introduction section of this experiment,
determine for each pair of structures below, what type of isomers are shown below
(structural isomers? geometric isomers? not isomers?)
a)
H
H
H
C C C
H C C C H
b)
H
H
H
OH
H
H
C C
C C
H
H
HO
H
H
2. a) Convert the following expanded structural formula into a condensed structural
formula:
H
H
H
H
C
H H
H C C C C OH
H H H H
b) Convert the following condensed structural formula into an expanded structural
formula for the trans-isomer:
CH3
CH3CH2CH=CHCHCH3
Post-Lab Question: Give the expanded structural formula of all the isomers of C2H4O.
EXPERIMENT 3: MOLECULAR MODELS
Report Page
51
Name: __________________________
Partner’s Name:__________________________
1. Isomers of C4H10
Structures 1A and 1B are…. (Circle one) isomers or identical ?
Structures 1A and 1C are…. (Circle one) isomers or identical ?
How many structural isomers does C4H10 have? Ans. ____
Condensed structural formula of all the structural isomers:
2. Isomers of C3H8O
Condensed structural formulas of the 3 structural isomers. Label each one as an alcohol
or ether.
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EXPERIMENT 3: MOLECULAR MODELS
Name: __________________________
3. Isomers of C4H10O
Expanded structural formula of all the isomers that are alcohols:
Expanded structural formula of all the isomers that are ethers.
4. Isomers of C3H6O
Expanded structural formulas of the 4 isomers with C=C bond.
EXPERIMENT 3: MOLECULAR MODELS
53
Name: __________________________
4. Isomers of C3H6O (continued)
Expanded structural formulas of the 2 isomers with a C=O bond:
Expanded structural formulas of the 3 isomers with a ring:
5. Structure of Acetic Acid
Fill in the missing lone pairs to give every atom
(other than H) an octet.
3-dimensional sketch of the model of acetic acid:
Geometry (linear? bent? tetrahedral?)
O
CH3 C O H
(1)
(2)
Bond Angle
C1
_____________________________
________
C2
_____________________________
________
Singly bonded O
_____________________________
________
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EXPERIMENT 3: MOLECULAR MODELS
Name: __________________________
6. Optical Isomers
a) Does CH4 have isomers?
Circle one: Yes or No
b) Does CH3Cl have isomers?
Circle one: Yes or No
c) Does CH2Cl2 have isomers?
Circle one: Yes or No
d) Does CH2ClBr have isomers?
Circle one: Yes or No
e) Does CHClBrI have isomers?
Circle one: Yes or No
f) How many different groups must be attached to a C atom in order to have optical
isomers?
Ans.____
Of the 5 formulas given, which has/have a chiral carbon? Ans. _______________
Of the ones for which you can find isomers draw 3-dimensional sketches to show how
they are enantiomers.
7. Which are isomers of C6H12O2? Circle all that applies: 6A 6B 6C 6D
(Do not include any that are identical to another.)
OH OH
H
H
H
OH
OH H
C
C
C
C
C
C
C
C
H
H
OH OH
OH H
H
OH
6A
6B
6C
6D
Which are enantiomers of each other (nonsuperimposable mirror images)?
Circle as many as applies: 6A and 6B
6A and 6C 6C and 6D
6B and 6C are isomers of each other. What type of isomers are they?
(Review the discussion of Isomerism in the Introduction section of this experiment.)