Chemistry 211
Fall 2011
Double Bond Equivalents
(Structural Information Obtainable from the Molecular Formulas)
(CGWW pp. 74-76)
Introduction:
In previous activities, you saw that some molecular formulas could represent structures that contain rings and/or multiple bonds while others require
structures with only single bonds. In the Isomers activity, you saw that molecular formulas give much less information about a compound than do any of
the types of structural representations. However, there is a relationship between the molecular formula and the possibilities of having multiple bonds
or rings in structures. Knowing which molecular formulas allow or require rings or multiple bonds in their structures should simplify the process of
devising possible structures. This activity explores how the numbers and kinds of atoms in the molecular formula can provide some general structural
information that dictates the number of possible rings and/or multiple bonds in its isomers.
Learning Objectives:
1. To predict the number of rings and or multiple bonds required for a structure with a given molecular formula.
2. To quickly determine the number of hydrogen atoms present in a molecule from its bond-line structure.
3. To isolate variables and solve problems with symbolic data.
Exploration:
1. Note that all of the structures in Figure 1 have the same number of carbon atoms, however, they differ in the arrangement of the atoms, the number of
rings or multiple bonds.
Figure 1: Structures of a group of hydrocarbon molecules with seven carbon atoms:
2. Write the molecular formula for each molecule in Figure 1 under the structure of the molecule. Briefly describe the process you used to determine the
molecular formula.
3. Based on your results from 2, what is the maximum number of hydrogen atoms that can be present in a 7 carbon hydrocarbon? What is the quantitative
relationship between this number and the number of carbons in the molecules? Provide your warrant.
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Double Bond Equivalents
4. How many rings or double bonds are there in a 7-carbon hydrocarbon with the maximum number of hydrogen atoms? Circle or highlight these
molecules above.
5. Organic molecules with the maximum number of hydrogen atoms are referred to as “saturated” because they are saturated with hydrogen. From the
relationship between carbon and hydrogen atoms developed in 3, predict the number of hydrogen atoms in saturated hydrocarbons with:
2 carbons atoms
5 carbon atoms
25 carbon atoms
103 carbon atoms
Show calculations:
6. Re-draw the structures from Figure 1. below in groups with common molecular formulas.
7. Make a quantitative judgment of the effect of presence of a ring or a multiple bond on molecular formulas of 7 carbon hydrocarbons. Be a specific as
you can. Provide your warrant.
8. How did the grouping of structures required in 6 assist you in your group’s analysis in 7?
9. Devise a method for using molecular formulas to predict, without drawing structures, the numbers of rings or multiple bonds in 7 carbon
hydrocarbons.
Double Bond Equivalents
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10. Can your method be easily modified to work for hydrocarbons with fewer than 7 carbon atoms? More than 7 carbon atoms? Demonstrate how or why
not.
11. Below are listed molecular formulas for three hydrocarbons.
C10H20
C6H14
C9H16
Use your method developed in 10 to predict, without drawing structures, the number of rings and/or multiple bonds that must be present in molecules
with these molecular formulas. Provide your warrant.
12. Use the method developed in 10 to write the molecular formulas of the following structures without counting all of the atoms. Which atoms must be
counted? Which ones can be determined from your relationship? Provide your warrant.
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Double Bond Equivalents
13. Below are structures for several additional molecules with 7 carbon atoms but which also contain heteroatoms (Atoms other than carbon or
hydrogen).
Figure 2
O
O
Cl
O
O
I
Cl
H
N
N
Br
H
Br
O
H
O
H
Cl
H2N
Figure 2: Structures of a group of molecules with seven carbon atoms and one other heteroatom:
Re-draw the structures in groups with the same heteroatom present, write the molecular formula for each molecule under its structure and make a note
of the number of rings and double bonds in each.
14. After comparing the groups created in 13 with the hydrocarbon groups in 6 above:
a. Determine the effect that adding a halogen atom to a 7 carbon hydrocarbon has on the number of hydrogen atoms present in the molecule?
Provide your warrant.
b. Determine the effect that adding a nitrogen atom to a 7 carbon hydrocarbon has on the number of hydrogen atoms present in the molecule?
Provide your warrant.
Double Bond Equivalents
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c. Determine the effect that adding an oxygen atom to a 7 carbon hydrocarbon has on the number of hydrogen atoms present in the molecule?
Provide your warrant.
d. How did the grouping of structures required in 13 assist in your analyses in a -> c?
15. Devise a general approach for determining the number of rings or multiple bonds in any molecule that contains one or more N, O or halogen atom from
its molecular formula, without drawing any structures. Provide your warrant.
16. Below are listed four new molecular formulas.
C5H9N
C7H14O
C15H29Cl
C9H16NOBr
Without drawing any structures, what can you predict about the number of rings or multiple bonds that must be present in molecules with each of these
molecular formulas? Provide your warrant.
17. Use the method developed in 15 to write the molecular formulas of the following structures without counting all of the atoms. Which atoms must be
counted? Which ones can be determined from your relationship from 15 above? Show your calculations and provide your warrant.
O
NH
C
O
C
Cl
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Double Bond Equivalents
18. Reflector’s Report Discussion:
Identify the most important concepts you learned from this activity:
What questions remain?
19. Strategy Analyst’s Report Discussion:
Look back at the process used to reach your method in 15. There were three data organizing steps in these analyses. (Steps 2, 6 & 13) Comment on the
value of each operation as a general problem solving strategy.
Double Bond Equivalents- Out of Class Applications:
A. Reading: CGWW pp. 74-76
B. Activities:
1. Use the method developed in step 15 of the Double Bond Equivalents activity to predict, without drawing structures, how many rings or multiple
bonds might be present in molecules with the following molecular formulas. Show your calculations.
C6H10
C8H18O
C12H22
C11H23NO2
C15H24
C20H35ClO
2. Use the relationship developed in Step 15 of the Double Bond Equivalents activity to write the molecular formulas of the following structures
without counting all of the atoms.
O
N
H
N
NH
O
H
H