Organic Model Exercise: An Adventure in Structure and Bonding
Dr. Gergens - Mesa College
Introduction
Organic chemistry is the study of compounds that contain the element of carbon; compounds that do not contain carbon
are termed inorganic. Carbon is singled out as a branch of chemistry because of the tremendous number of compounds it
forms. While there are about 200,000 known inorganic compounds, there are over 6 million known compounds of
carbon. While organic chemistry is the study of the compounds of carbon, biochemistry is the study of the chemistry
of living organisms. Organic compounds are found in all living organisms, foods (fats, proteins and carbohydrates),
fuels (petroleum), wood, paper, fabrics, plastics, dyes, paints, cosmetics, drugs, medicines, insecticides, herbicides,
soaps, and detergents.
Organic compounds can be classified according to their structural features. The structural features that make it
possible to classify compounds by reactivity are called functional groups.
In this experiment you will familiarize
yourself with the structures of functional groups shown in Chapter 3 of the McMurry text. The use hand-held models
will aid you in this task. Many studies have shown that tactile (touch) learning far outweighs visual absorption of this
sort of information.
Getting Started:
Start by reviewing Chapter 3 in your McMurry text. For this exercise, pay particular attention to the naming of the
general classification of functional groups. Your instructor will discuss structural isomerism, geometric isomerism,
and chirality for aliphatic and cyclic compounds covered in Chapter 3 in laboratory.
This project is composed of three parts: Part A - covalencey; Part B - functional group classification and their threedimensional representations through the use of hand held models; and Part C - working with structural, geometric,
and optical isomerism. While working on this exercise, you may hand draw the structures and answer questions in
pencil. However, your may be required to turn in your answers to the following questions as a word processed
document. Data sheets to word process this assignment are available for download from the web site at
http://homework.sdmesa.edu/dgergens/chem231L/structure_bonding/
You will use a model kit to build each functional group. This will greatly assist you in visually seeing the threedimensional perspective of organic compounds drawn on a two-dimensional piece of paper. For example, dimethylether
drawn below can be drawn with a 2-D and 3-D perspective. If drawn in 2-D, the molecule appears to be linear.
In actuality, the ether has a bent structure about the oxygen atom. If your were to make a molecular model of this
ether, you would quickly recognize the central oxygen atom as being bent in geometry and the carbons as tetrahedral.
To represent this molecule as having a three-dimensional structure, the tetrahedral methyl carbons are drawn with a
solid black wedge,
, and a hashed wedge
to represent a 3-D perspective of a bond in three-dimensional
space. In the 3-D structure, each black wedge shows bonded atoms pointing towards you, coming out of the page,
whereas each hashed wedge represents bonded atoms pointing away from you, going into the page. Each solid line
represents bonds lying in the plane of the page. Being able to look at compounds drawn on a two-dimensional piece of
paper and convert it to a 3-D perspective structure will be an important outcome of this exercise.
A.
H
H C
H
O
2-D
H
CH
H
Functional Group: ether
H
H
H
O
H
H
H
3-D
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Part A - Covalencey
The atoms in organic compounds are generally held together by covalent bonding. In contrast, inorganic compounds are
usually ionic, though they can have covalent bonds. Since carbon has four electrons in its outer shell, it forms four
covalent (shared) bonds. These may be single, double or triple bonds as long as the total number of bonds to carbon
equals four. Other elements, such as hydrogen and oxygen, are found in organic compounds, making the possibilities for
covalent combinations to become enormous. In the table below, are the most stable bonding modes for carbon, nitrogen,
and oxygen. For each bonding mode, give the angle, geometric name, and hybridization about the central atom. Word
process this image by using the drawing tool in Microsoft Word to add angles, geometric names, and hybridizations.
VSEPR = ________
________
________
________
________
Determine the angles between bonds, name the geometry about the central atom and give the its hybridization.
Ideal Geometries
H
H
H
bond angles
C
H
H
H
C
C
H
H
C
C H
:
N
N
O
C
O
H
geometric name
hybridization
Non-Ideal Geometries
:
:
:
N
H
H
N
N
:
H
bond angles
H
H
geometric name
hybridization
:
H
:
bond angles
:
O
H
:
H
O
C
H
geometric name
hybridization
On the next page, we will be using ball and stick models to demonstrate the versatility of bonding and therefore
structure in organic compounds. In general, the color correlation of the plastic spheres to elements is as follows:
C - carbon
H - hydrogen
O - oxygen
N - nitrogen
Cl - chlorine
Br - bromine
black
white
red
blue
green
red
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Part B - Functional Group Classification
Structural features make it possible to classify compounds by functional groups and the methods for the writing
chemical bonds are highly descriptive and useful. In this exercise, you will be asked to identify and use of a model kit
to build each functional group for the compounds below.
Identify each functional group. Construct molecular models for the compounds below. This will greatly assist you in
visually seeing the three-dimensional perspective of organic compounds. Notice the tetrahedral carbon has a three
dimensional array of atoms. For the moment, sketch a hand drawn 3D-chemical structure for each on this handout.
Afterwards, use a computer assisted chemical drawing program (i.e. ISIS) to redraw a three-dimensional
representation for each molecule and paste its structure into the table below.
A.
B.
H
H C
H
O
H
CH
H
Functional Group: ether
H
H
D.
C
Functional Group:
H
CH
H
Functional Group:
O
C
H
I.
H
H C
H
NH2
Functional Group:
OH
OH
Functional Group:
J.
Functional Group:
K.
H
HC
H
O
C
Functional Group:
H
O CH
H
C H
Functional Group:
H.
O
C
OH
Functional Group:
G.
C
H
F.
H
H C
H
Functional Group:
H
H C
H
C
H
E.
H
H C
H
C
H
H
O
H
H
Cl
H
O
H
C.
H
H C
H
L.
H
H C
H
Functional Group:
NH2
H
H C
H
O
C
H
Functional Group:
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Part C - Structural, Geometric, and Optical Isomerism.
In this laboratory activity, you will be examining molecular models of various organic compounds. You will pay
particular attention to the existence of isomers. Isomers are prevalent in organic compounds due primarily to carbon's
ability to make 4 bonds.
Constitutional isomerism. Two molecules with the same molecular formula but different structures are called isomers.
Using your model kit, construct a model of the requested isomer. Using either ISIS or ChemDraw, draw a threedimensional representation and paste its figure into the table below.
1. an alcohol of compound A.
2. an aldehyde of compound D
3. an ester of compound E.
Geometric cis- trans isomerism is a type of geometric isomerism that arises when two species have the same
molecular formulas but different structures. For example, square planar complexes, [PtBr 2 Cl 2 ]2– .
Br
2-
Cl
2-
cis & trans alkenes
H
Cl Pt Br
Br Pt Br
Cl
Cl
cis
chlorine atoms next to each other
on the same side
trans
C
CH3
chlorine atoms opposite to each other
on opposite sides
H
C
CH3
cis
H
CH3
C
CH3
C
H
trans
Alkenes have rigid double bonds that prevent rotation, giving rise to cis- and trans-isomers. Construct the cis and
trans 2-butenes above. Notice the restrictive rotation about a double bond can result in cis-trans geometric isomerism
in organic molecules; the methyl groups cannot interconvert.
Construct a molecular model and draw the three-dimensional representation for the three isomers of dibromoethene.
Using either ISIS or ChemDraw, draw each isomer and paste its figure into the table below.
cis
trans
neither cis or trans
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Geometric isomerism is also possible when there is a ring present.
A cycloalkane has two distinct faces.
If
substituents on a cyclic ring point toward the same side, they are cis. If they point toward opposite sides of the ring
they are trans. Note the geometric isomers of 1,2-dimethylcylcohexane cannot inter convert without breaking and
reforming bonds.
a.
Below are cis- and trans-isomers of 1,2-dimethylcylcohexane. Make models of these compounds to convince
yourself that cis- and trans-1,2-dimethylcylcohexane cannot inter convert by simple rotations about the bonds.
Also convince yourself that all three forms draw for cis and for trans
CH3
CH3
CH3
CH3
CH3
CH3
cis
trans
CH3
cis
Cl
CH3
Cl
trans
Cl
Cl
trans
cis
b.
Construct a molecular model for each isomer of 1,2-dibromocyclohexane. As seen for 1,2-dimethylcylcohexane
above, duplicate the three perspectives for 1,2-dibromocyclohexane and paste their images into each box.
c.
Can you construct a molecular model for an isomer of dibromocyclohexane the is neither cis or trans?
same three perspectives for this structural isomer and paste them into the box below.
Draw the
neither cis or trans
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Optical Isomerism - The Optically Active Tetrahedral Carbon
Optical isomerism is a type of isomerism that is frequently encounter throughout organic chemistry. It occurs because
of the tetrahedral nature of the bonding around a carbon atom.
Molecules may be chiral, or handed; think of your left and right hand. Chiral is derived from the Greek word cheir,
meaning hand. A carbon atom with four different atoms or groups attached to it is referred to as a chiral center,
meaning the carbon is without a plane of symmetry. A chiral center is asymmetric, just like your left and right hand.
A molecule containing such a carbon atom may show optical isomerism. Optical activity is exhibited by molecules that
have a nonsuperimposable mirror image.
Your hands are nonsuperimposable mirror images, and a pair of
nonsuperimposable mirror images are called enantiomers.
a.
Construct a molecule of 2-butanol. Notice the number two carbon, C2 , has four different groups attached to it—a
methyl, ethyl, hydroxyl, and hydrogen—thus a chiral carbon. Construct the mirror image of 2-butanol. Note that
both are isomers—actually stereoisomers (stereo meaning in space).
Me
Me
H
H
HO
b.
OH
Et
Et
Draw the three-dimensional representation using ISIS draw for each optical isomer of 2-butanol and 2-bromobutane
and paste the structure into box at the right. Note these molecules are nonsuperimposable.
CH3
CH3
CH3
CH2
HO
H
CH3
H
CH2
Br
c.
What are the tests or observations you can make on the structure of a molecule to determine whether it is chiral?
d.
Using your models, construct the optical isomers of carvone. You may have to look up the structure using
Chemfinder on the computer. Smell the authentic (-)- and (+)-carvones that are in the hood. What do they smell
like? Can you smell a difference between the odors spearmint and caraway? In the space below, draw the
isomers of carvone using ISIS Draw and identify which carvone corresponds to which scent.
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Name:_______________________________
Isomers of Octane Experiment
Chemistry 231L/232L
Dr. Gergens - SD Mesa College
Section:_____________ Date:____________
Write out the line-angle formula for all 18 isomers with the formula C8 H18 . Organize the isomers from
decreasing parent chain length starting with octane. The first three are done for you. Give all isomers
an I.U.P.A.C. name. Identify by letter, all equivalent carbon atoms. Use the first three examples as a guideline.
Note, 3-methylheptane has no equivalent carbon atoms because it is asymmetric due to its chiral center
marked '* '. Mark all asymmetric centers with a '* '. Finally, answer the questions on the next page.
a
*
a
b
Name:
c
d
octane
d
c
b
a
a
Name:
2-methylheptane
Name:
Name:
Name:
Name:
Name:
Name:
Name:
Name:
Name:
Name:
Name:
Name:
Name:
Name:
Name:
Name:
3-methylheptane
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Isomers of Octane - Additional Questions
Dr. Gergens - SD Mesa College
Answer the following questions. Use a molecular model kit to assist you.
1.
Give the total number of each isomer having the following parent chain name. octane ____, heptane ____,
hexane ____, pentane ____, butane ____, propane ____?
2.
How many alkanes have the ethyl— branch in its name ____. Give their IUPAC name:
3.
How many alkanes have the propyl— branch in its name ____. Give their IUPAC name:
4.
How many alkanes have the isopropyl— branch in its name ____. Give their IUPAC name:
5.
How many alkanes have the prefix di in their IUPAC name ____.
6.
How many alkanes have the prefix tri in their IUPAC name ____.
7.
How many alkanes have the prefix tetra in their IUPAC name ____.
8.
How many alkanes have only one chiral carbon center ____ .
A tetrahedral carbon atom that bears four
different substituents is called a chiral center. Give their IUPAC name:
9.
How many alkanes have two chiral centers ____. Give their IUPAC name:
10. Which alkane appears to have six equivalent methyl branches its structure? Give its IUPAC name:
11. Which alkane appears to have three equivalent ethyl branches in its structure. Give its IUPAC name:
12. How many alkanes contain both two equivalent methyl and two equivalent ethyl branches in its structure?
Give their IUPAC name:
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13. Which alkane appears to have two equivalent sec-butyl branches? (Hint: It is the one with two chiral centers
as well). Give its IUPAC name:
14. Which alkane appears to have two equivalent n-butyl branches in its structure? Give its IUPAC name:
15. Which alkane appears to have two equivalent isobutyl branches in its structure? Give its IUPAC name:
16. Which alkane appears to have two equivalent t-butyl branches in its structure? Give its IUPAC name:
17. Which alkane appears to have two equivalent n-propyl branches in its structure? Give its IUPAC name:
18. Which alkane appears to have two equivalent isopropyl branches in its structure? Give its IUPAC name:
19. Which chiral molecule appears to have a methyl, ethyl, n-butyl branch about its chiral center? Give its IUPAC
name:
20. Which chiral molecule appears to have a methyl, ethyl, t-butyl branch about its chiral center?
Give its IUPAC
name:
21. Which chiral molecule appears to have a methyl, propyl, isopropyl branch about its chiral center?
Give its
IUPAC name:
22. IUPAC stands for_________________________________________________________________
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