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Hydrocarbons: Organic… but not Organismal
Constructing Molecular Models
PRE-LAB DISCUSSION
The ability of carbon to form four covalent bonds with other atoms makes possible a wide
variety of organic molecules. Organic molecules are found in both living and nonliving things.
Organic molecules found in nonliving things are called hydrocarbons. Hydrocarbons are the simplest
organic compounds because they contain only carbon and hydrogen. Because carbon atoms also bond
easily to each other, there are numerous types of hydrocarbon molecules of varying lengths and shapes.
Organic molecules found in living things are called macromolecules. Macromolecules –
carbohydrates, lipids, proteins, and nucleic acids – contain other atoms, like oxygen, in addition to
carbon and hydrogen. An organic molecule’s shape depends on the arrangement of carbon atoms that
make up the backbone of the molecule. Carbon chains can form single, double, and triple bonds, and
ring and loop shapes. The three-dimensional shape of the molecule is critical in defining its properties
and its function in living organisms.
GOALS
By the end of this activity you should…
a. appreciate that molecules have a three dimensional shape.
b. understand that the shape of a molecule is determined by the composition of the molecule.
c. be able to explain the unique bonding properties of carbon.
d. describe the difference between a molecular formula and a structural formula.
e. start becoming familiar with the functional groups of biomolecules.
PROBLEM
Formulate a hypothesis to answer the question: How do carbon atoms join to form organic
molecules?
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MATERIALS
Molecular model kit
Flash card templates
Index cards
Tape or glue
PROCEDURE
Part A: The Hydrocarbons of Nonliving Things
1. Methane, or natural gas, is a hydrocarbon that has a molecular formula of CH4. Molecular
formulas tell the type of atoms and the number of each atom found in a molecule. Using the
molecular model kit, construct a methane molecule and write its structural formula in Data
Table 1. A structural formula shows the type and arrangement of atoms in a molecule, with
bonds between two atoms being represented by lines in between the element symbols.
2. Construct another model of a methane molecule. Combine the two methane molecules to form
ethane, removing atoms if necessary. Write the molecular and structural formulas for ethane in
Data Table 1.
3. Construct a third methane molecule. Add the methane molecule to the ethane molecule to form
propane, removing atoms if necessary. Write the molecular and structural formulas of propane
in Data Table 1.
4. Ethene has a molecular formula of C2H4. Construct a model of ethene, then write its structural
formula in Data Table 1.
5. Construct another molecule of propane. Remove two atoms of hydrogen, then reattach the third
carbon to form propene. Write the molecular and structural formulas for propene in Data Table 1.
6. Ethyne has a molecular formula of C2H2. Construct a model of ethyne, then write its structural
formula in Data Table 1.
7. Propyne has a molecular formula of C3H4. Construct a model of propyne and write its
structural formula in Data Table I.
8. Methane, ethane and propane are all alkanes – a group of hydrocarbons that have all single
bonds. Ethene and propene are alkenes, or hydrocarbons that have a double bond. Ethyne and
propyne are alkynes, or hydrocarbons that have a triple bond. Repeating steps 1 to 7, construct
two additional molecules of alkanes, alkenes and alkynes, each with either five or six carbons.
Record the molecular and structural formulas of each molecule in Data Table 2.
Data Table 1: Molecular and Structural Formulas of Hydrocarbons
Hydrocarbon
Molecule
Molecular
Formula
Methane
CH4
Ethane
Propane
Ethene
Propene
Ethyne
Propyne
Structural
Formula
Data Table 2: Molecular and Structural Formulas of Alkanes, Alkenes, and Alkynes
Hydrocarbon
Molecule
Alkanes
Alkenes
Alkynes
Molecular
Formula
Structural Formula
Analysis Questions
1. What atoms had to be removed in order to join together the two methane molecules? _______
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2. In what way is the methane model that you constructed different from the structural formula
that you recorded in your data table? _____________________________________________
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3. Why is it important for biochemists to use structural formulas rather than molecular formulas?
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4. When forming a propane molecule, what atoms had to be removed from the ethane molecule in
order to add the third carbon? ___________________________________________________
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5. In the ethene molecule, how many electrons are being shared? ________________________
What type of bond is formed between the two carbon atoms? __________________________
6. When you changed the propane molecule to a propene molecule, what type of bond did you
have to use to attach the third carbon? _____________________________________________
Part B. Introduction to the Functional Groups of the Biomolecules
1. Organic molecules that are found in living cells are more complex than hydrocarbons. These
molecules contain groups of atoms that are known as functional groups. Study the functional
groups in Figure 1 and, using handouts provided, construct a flash card for each group.
Figure 1.
Functional Groups
Carbonyl
(Aldehyde)
Phosphate
Carbonyl
(Ketone)
Hydroxyl
(alcohol)
Sulfhydryl
Carboxyl
Amino
2. Carbohydrates, commonly called sugars and starches, are important energy sources for living
things and also are structural components of plants. Glucose, which has the molecular formula
C6H12O6, is the sugar that provides the cells of your body with energy. Fructose, another
simple sugar, has the same molecular formula as glucose. Because glucose and fructose have
the same molecular formula but different structural formulas, they are called isomers. Study
the formulas of glucose and fructose shown in Figure 2 and construct a model of each
molecule.
Figure 2.
Analysis Questions
1. What is the specific functional group in glucose? ____________________________________
2. What is the specific functional group in fructose? ___________________________________
3. Explain the term isomer in your own words and provide an example from the lab. _________
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4. Glucose and fructose have the same molecular formula, but a different structure. Because of
this, the human body responds to these sugars in very different ways. Use the internet or your
textbook to research the difference between glucose and fructose in the human diet. What
function does each compound serve in the body? How do cells respond differently to glucose
and to fructose? MAKE SURE YOUR ANSWER IS PHRASED IN YOUR OWN WORDS!
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