GEOMETRY AND ISOMERISM IN ORGANIC MOLECULES
One report per two persons or individual reports are fine. Three authors per report is not allowed, but
should this occur the grade would be reduced by one third.
Please read all the instructions. Simply skipping to the bold faced type has been known to cause excessive
confusion.
The Objective is to illustrate various aspects of isomerism, to see how the molecules appear in 3-D space, and to practice drawing and naming
them.
The Report must be a maximum of one regular sheet, double sided. You will have to write small. Perhaps try your answers on scrap paper
first and then rewrite them. This restriction is to help promote neatness, organization, and to make the marker's life a little easier.
Procedure
1. Build models of CH4, C2H6 and C3H8 and line them up side by side for a look.
Notice

There is only one way each of the above molecules can be assembled. Another way of saying this is that there is only one structural
isomer for each.

The two ends of ethane can spin like propellers around the C - C single bond. Try to make the ‘propellers’ spin. The actual molecule
spins like this too. Heat in the room provides the energy.

Parts of propane can spin around the C - C single bonds.

There is a way to spin the end carbons of propane so that two hydrogen atoms move very near to each other. Try this.

There is a way to spin the end carbons of propane so that the hydrogen atoms have a minimum crowding of each other. Try to find this
orientation (also called conformation).

Draw (copy) the 3-D representations for methane shown here: use perspective - make the hydrogen atoms that are further away from
you look smaller and make the bonds leading to them wider near you and thinner as they get further way. Now, draw ethane and
propane using this 3-D representation. Name each.
Methane
For PROCEDURES 2-10 do NOT try to draw perspective diagrams.
Instead, draw structural diagrams such as the ones here for 2-bromo pentane. Notice that the zigzag of the carbon backbone is evident.
Structural diagrams
C
C
C
C
C
or
Br
Br
2-bromo pentane
Double bonds can be shown in this type of diagram too. The diagram on the right is often favoured because of its simplicity.
C
C
C
C
C
C
C
C
OH
OH
OR
C
2. Build all the structural isomers of: C3H7Br
Number of isomers:
2
C4H10
2
C5H12 and C6H14
3
?
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Every isomer will have its own name. Draw and name each. (The smaller and neater you can draw these the better for you and also for
the person who marks your report!!!)
3. Build both geometric isomers of 1,2 - di bromo cyclobutane. (Cyclo means the carbons make a ring)
Notice

Br
H H
These two molecules have different properties and are different compounds because the two
bromine atoms are oriented differently in space.
Br

There is a cis- and a trans- form of this molecule

These isomers need to be drawn carefully. The orientation of the bromine atoms can be shown with the kind of figure on the right. The
two lines pointing up are meant to be coming out above the page and the other two are meant to be going below. One of the
compounds is drawn here.
Draw both molecules. Label them cis and trans.
4. Build cyclohexane. It is flexible and will take on several shapes or conformations as the C - C single bonds move.
Notice

Two conformations (positions) are quite stable.

Put your model into these two forms.

Heating this molecule causes it to flip back and forth between the two conformations.
Chair Form
Boat Form
Build cis - 1,4 - di fluoro cyclohexane. (cis - just means that both fluorine atoms are on the same side of the carbon ring.)
Notice

The fluorine atoms are so close to each other in the boat form that they nearly touch. This crowding requires energy, so at low
temperatures the chair form is more common.

Put your model into these two forms and watch the distance between the fluorine atoms change.
Draw 1,4 - di fluoro cyclohexane.
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5. There are many structural isomers of C8H16, but build only the ones that contain a six carbon ring. Since there are eight carbons in the
molecule, the ring will have to have side groups. Do not distinguish between either chair or boat forms, or cis- or trans- forms. In all,
there are five structural isomers.
Draw all five isomers, leaving perspective out of the diagram entirely. An example is shown.
6.
Build the six isomers of C4H8.
Hints:
 Two of them are saturated cyclo compounds.

The remaining four each have a double bond.

Two of these four unsaturated molecules are geometric isomers because they are cis - and trans - forms of the same structural isomer.
The last two are not geometric isomers.
Notice


Your model will not rotate around its C=C double bond. The actual molecule will not rotate around the double bond either. It is this
inability to rotate that gives rise to the cis - and trans - forms.
The bond angles around double bonds are 120o, and this can be drawn as shown at right.
C
C
Draw (but don’t bother to name) each isomer of C4H8.
C
H
C
or
H
H
H
7. Build hexanol, C6H13OH. This alcohol has many isomers, but build only the three that have six carbons in a straight chain.
Draw and name all three structural isomers of hexanol that have 6 carbons in a straight chain. You may draw either - O - H
An example of one of the names is 1 - hexanol.
or - OH.
8. Build the three saturated cyclo isomers of C3H4Br2. (Saturated means there are no double or triple bonds present.) Draw and name
each one, pointing out which two are geometric isomers.
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9. The following two compounds are structural isomers with very different chemical properties. When you build them, note the placement of
the oxygen atom in each case.
C2H5OH
ethanol
(an alcohol)
CH3OCH3
methoxy methane
(an ether)
Draw each molecule.
Challenge:
Each oxygen atom in the above compounds have two pairs of non-bonding electrons. Include these electron pairs on your diagram.
Now look at your models to judge whether the non-bonding electron pairs take up space (If they do they will cause the molecule to bend.)
One molecule is polar, while the other one is only slightly polar. Which is which? (Hint: compare the electronegativity difference in the
O-H bond versus the O-C bond. The greater the E.N. difference, the more polar the bond.)
10.
OPTIONAL: Build and draw the two isomers of bromo chloro fluoro methane. These are called optical isomers and they have
identical properties except that they affect polarized light differently. How do the two models differ?
Here is the structure of glucose, C6H12O6 . You may also wish to look it up in a text. Build and draw
both optical isomers (they are mirror images). One is called dextrose and the other is called levulose.
Enzymes in your body that can break down glucose do not work on levulose for the same reason that your
left hand doesn't fit very well into a right handed glove. Much of your body chemistry is similarly optically
active. In the making of drugs, it is often a matter of chance as to which mirror image gets made, and so
50% of the product, being the wrong form, is completely useless. If the drug can be produced biologically,
say by genetically engineered bacteria, then the all of the product will be the correct mirror image.
QUESTIONS
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1. State how many structural isomers are possible for each of the following. HINT: refer part 2 in your procedure where your built the
isomers. (Don't draw them.)
a) C2H6
b) C3H8
c) C4H10 d) C5H12 e) C6H14
2. Draw cyclopentane and pentane.
a) Can cyclopentane, C5H10, be considered an isomer of pentane?
State yes or no, and explain.
b) Can cyclopentane be considered an isomer of 2 - pentene? State yes or no, and explain.
3. Draw 1,1 - di chloro butane and 1,2 - di chloro butane. They are structural isomers.
a) How are they similar?
b) How are they different?
4. cis - 1,2 - di chloro cyclo butane and trans - 1,2 - di chloro cyclo butane are geometric isomers.
a) How are they similar?
b) How are they different?
THE END!
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