Organic Molecules Lab I: Alkanes
Chapter 20
Directions: Answer each question in complete sentences or with a drawing or graph as is
appropriate.
General notes:
a. Please count the parts in the packet provided BEFORE and AFTER each day’s activities. Your table
will be responsible for any pieces missing from your packet. See the sheet on the outside of Model Set
box for atom and bond inventory and what each represents.
b. Springs: Whenever possible, use the wooden pegs for bonds. The springs will be used in the next
labs, but are not needed for this lab. Please do not distend the springs, for they quickly become unusable.
PRELAB QUESTIONS:
Write your name, date and period at the top of this paper. Refer to Chapter 20 in your textbook for the
following questions.
1.Define Organic Chemistry:
2. Define Hydrocarbon:
a. What type of bonds does carbon form:
b. How many bonds does carbon form:
3. Alkanes: Hydrocarbons in which there are only _________ bonds resulting in each C atom being bonded to
___ other atoms.
4.
a. What are saturated hydrocarbons?
b. Are alkanes saturated hydrocarbons? Why?
5. Name a use or a place found for:
methane:
propane:
butane:
octane:
6. Define allotropes. Give examples of allotropes of carbon.
ALKANES
Objective: What is the general structure of an alkane?
a. Construct a model that will represent each of the following molecules. For each, bond the C
atoms in a row. CH4, C2H6, C3H8, C4H10.
7. Then, draw a 2-D structural formula of each molecule using dashes for the covalent bonds
and “C” and “H” for the atoms, as partially shown in the image above. (Hint: Use 90º angles
whenever possible.)
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CH4:
C3H8:
C2H6
C4H10:
8. Bonding in Alkanes:
a. How many H atoms are bonded to each “end” C atom?
b. How many H atoms to each “central” C atom (those C’s not on an end)?
9. If a fifth C atom were added to C4H10, how many H atoms would also have to be added? ______
Once you have made a prediction, construct the model and confirm.
10. Study the formulas of the four alkanes listed above. There is a mathematical relationship between the number
of C atoms and the number of H atoms in an alkane. Fill in the blank: if n is the number of C atoms in an
alkane; the general formula for alkanes would be CnH__.
11. a. Label your drawings in Question #6 with the correct name.
b. Come up with another word that has the same prefix and related meaning to hydrocarbons with five to ten
carbons. (This may require some research on your part; you can only name one polygon!)
Pentane:
Octane:
Hexane:
Nonane:
Heptane:
Decane:
c. Using your formula derived in Question #9, predict the formula for the following six alkanes.
Pentane:
Octane:
Hexane:
Nonane:
Heptane:
Decane:
d. Using your knowledge of atomic mass and the formulae you have used above, determine (and show your work
for) which alkane has an atomic mass of:
a. 30 amu
b. 58 amu
c. 100 amu
e. The process of vaporization requires each molecule to overcome the electrostatic attractions of nearby
molecules. Use the following table to identify what happens to the boiling point as the hydrocarbon chain gets
longer?
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Alkane Name
Butane
Decane
Ethane
Heptane
Hexane
f.
# C atoms
4
10
2
7
6
Boiling Point (ºC)
- 0.5
174.0
-88.6
98.4
Alkane Name
Methane
Nonane
Octane
Pentane
Propane
# C atoms
1
9
8
5
3
Boiling Point (ºC)
- 161.7
150.8
125.7
36.1
Based on the above table and the trends in boiling points of alkanes, predict the boiling points of hexane and
propane. Show your work.
Rules for Naming Alkanes using IUPAC System
I.
Find the longest continuous chain of carbon atoms, and use the name of this chain as the base name of the
compound. The longest chain may not always be written in a straight line, as seen in the following
example.
2
1
CH3—CH-CH3
|
CH2—CH2—CH2—CH3
3
4
5
6
2-methylhexane
Alkanes can be written as structural formula for 2-methylhexane on the left or as condensed formula
(C7H17) for the same molecule.
11. Because this compound has a chain of six C atoms, it is named as a substituted hexane. Groups attached to
the main chain are called ________________________ because they are substituted in place
of an H atom on the main chain.
12. Number the carbon atoms in the _______________ chain, beginning with the end of the
chain that is nearest to a substituent. In our example we number the C atoms from the upper
left because that places the CH3 substituent on the second C atom of the chain; if we number
from the lower right, the CH3 would be on the fifth C atom. The chain is numbered from the
end that gives the lowest number for the substituent position.
13. Name and give the location of each substituent group. A substituent group that is formed by removing
an H atom from an alkane is called an _________ group. Alkyl groups are named by replacing the –ane
ending of the alkane name with –yl. For example, the methyl group, CH3, is derived from _________,
CH4. Likewise, the ethyl group, C2H5, is derived from ____________, C2H6. The name 2-methylhexane
indicates the presence of a methyl, CH3 group on the second carbon atom of a hexane (six carbon) chain.
When two or more substituents are present, list them in alphabetical order. When there are two or more of the
same substituent, the number of substituents of that type is indicated by a prefix: di- (two), tri- (three), tetra- (four),
penta- (five), and so forth. Notice how the following example is named:
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CH3
|
CH3—5CH—6CH2
|
4
CH—3CH—CH2CH3
|
|
CH3 2CH—CH3
|
1
CH3
3-ethyl-2,4,5-trimethylheptane
14. Make and draw the following branched chain alkane: 2,3, dimethylhexane.
Cycloalkanes
Alkanes can from not only branched chains, but rings or cycles as well. Alkanes with this form of structure are called
cycloalkanes. Cycloalkanes structures are sometimes drawn as simple polygons in which each corner of the polygon
represents a CH2 group.
Carbon rings containing fewer than five carbon atoms are strained because the C—C—C bond angle in the smaller
rings must be less than the 109.5° tetrahedral angle. The amount of strain increases as the rings get smaller. In
cyclopropane, which has the shape of an equilateral triangle, the angle is only 60°; this molecule is therefore much
more reactive than its straight-chain analog, propane.
15. Draw the structures of cyclobutane, cyclohexane and methylcyclohexane.
Unsaturated Hydrocarbons (Alkenes and Alkynes)
1. Define the following:
a. Unsaturated Hydrocarbon:
b. Alkene:
c. Alkyne:
2. Construct a model that will represent each of the following alkene molecules: C2H4 and C3H6. (Use springs for
the two C—C bonds.)
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3. Draw a 2-D structural model of each molecule:
C2H4:
C3H6:
4. a. If a fourth C atom were added to C3H6, how many H atoms would also have to be added?
Once you have made a prediction, construct the model and see. (Keep the four C atoms in a chain, and keep only
one double bond.) Then, draw the structural model for the 4-carbon alkene.
5. Study the formulae of the alkenes above. There is a mathematical relationship between the number of C atoms
and the number of H atoms in an alkene. Fill in the blank: if n is the number of C atoms in an alkane; the
general formula for alkenes would be CnH__.
6. Construct a model of the alkane C3H8 (propane) and compare it to the model of C3H6 (propene); how does the
ability of the carbon chain to rotate in an alkane compare with the rotation in an alkene?
Naming Alkenes IUPAC Way:
7. The naming of alkenes is based on the longest continuous chain of carbon atoms that contain the double bond.
The name given to the chain is the name of the corresponding alkane by changing the ending from –ane
to –___________. For ex. a four carbon alkene will be called _________________.
8. The location of double bond is indicated by a prefix number that designates the number of carbon atom that is
part of double bond and is nearest to the end of chain. Draw the structure of 2-Methylpropene, 3-Hexaene
2-Methylpropene:
3-Hexene:
Alkynes
Alkynes are unsaturated hydrocarbons containing one or more C=C. The simplest alkyne is C2H2, acetylene. The
general formula of alkynes are CnH2n-2.
Naming of Alkynes
Alkynes are named by identifying the longest continuous chain in the molecule containing the triple bond and
modifying the ending of the name as –yne.
9. Draw 2- butyne
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Isomers
Answer the following questions:
1. Define isomers.
2. Compare and contrast structural and geometric isomers giving example of each.
3. Construct a “straight-chained” (all C atoms in a row) model of butane. Then, construct a second model of C4H10;
however, make this a “branched” model by building a 3-carbon chain, then bonding the 4th C atom onto the second
(middle) C atom of the 3-carbon chain. Draw structural models of these two isomers of butane. (Remember to use 90º
angles whenever possible.)
4. Compare the two different models of C4H10:
a. How are the formulae of the two molecules related?
Is there any way to look at the straight-chain butane from a different angle (or twist or turn the bonds)
and have it look like the branched model of C4H10?
 If so, then the two molecules are the same, i.e., identical.
 If not, the molecules are isomers of each other: they have different structures but the same
molecular formula.
b. Are the straight-chained and branched butanes isomers or identical?
c. If so, which type of isomers are they?
d. Compare your left hand, your right hand, and your partner’s right hand. Which 2 are (nearly)
identical? Which 2 are isomers? Explain. (Sketches may help!) This type of isomers are called as
“Optical Isomers” (Isomers that are mirror images of each other)
5.
How many isomers can you construct for propane?
6. For pentane how many can you construct? Draw them.
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Aromatic Hydrocarbons
Define the following terms:
1. Aromatic Hydrocarbon
2. Give two properties and two uses of aromatic hydrocarbons.
3. Aromatic Hydrocarbons contain a carbon ring. (C6H6). Draw the structure of benzene ring, both in condensed
form and structural formula.
4. Describe the bonding between carbon atoms in benzene.
5. What is the difference between cyclohexane and benzene?
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