Download organic compounds in three dimensions

Austin Peay State University Department of Chemistry
CHEM 1021
ORGANIC COMPOUNDS IN THREE DIMENSIONS
(adapted from Blackburn et al., Laboratory Manual to Accompany World of Chemistry, 2nd ed., (1996)
Saunders College Publishing: Fort Worth)
Purpose:
To become familiar with organic molecules in three dimensions and to understand isomerism of simple
organic molecules.
Introduction:
Over 11 million of the more than 13 million known compounds are carbon compounds, and a separate
branch of chemistry, organic chemistry, is devoted to the study of them. Two important reasons why
there are so many organic compounds are (1) the ability of thousands of carbon atoms to be linked in
sequence with stable carbon-carbon bonds in a single molecule and (2) the occurrence of isomers. The
basic geometrics for organic compounds are illustrated below:
Tetrahedral
Trigonal Planar
Linear
Aromatic
There are four classes of hydrocarbons: alkanes, which contain C-C single bonds with a tetrahedral
arrangement around each carbon; alkenes, which contain one or more C=C double bonds with trigonal
planar geometry around the carbon atoms with double bonds; alkynes, which contain one or more C≡C
triple bonds with linear geometry around the carbon atoms with triple bonds; and the aromatics, which
consist of benzene, benzene derivatives, and fused benzene rings.
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Austin Peay State University Department of Chemistry
CHEM 1021
Structural Isomers:
Two or more compounds with the same molecular formula but different arrangements of atoms are
called isomers. Isomers differ in one or more physical or chemical properties such as boiling point,
color, solubility, reactivity, and density. Several different types of isomerism are possible for organic
compounds. These include structural isomers (straight-chain and branched-chain), cis- and transisomers, and optical isomers.
The simplest example of structural isomers is for C4H10 which can be either butane or methylpropane.
butane (C4H10)
methylpropane (C4H10)
In the alkene series, the possibility of locating the double bond between two different carbon atoms
adds additional structural isomers. Ethene and propene have only one possible location of the double
bond. However, the next alkene in the series, butene, has two possible locations for the double bond.
1-butene
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2-butene
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Austin Peay State University Department of Chemistry
CHEM 1021
Cis- and Trans- Isomers:
An important difference between alkanes and alkenes is the degree of flexibility of the carbon-carbon
bonds in the molecules. Rotation around single carbon-carbon bonds in alkanes occurs readily at room
temperature, but the carbon-carbon double bond in alkenes is strong enough to prevent free rotation
about the bond. This leads to the possibility of cis-trans isomers. Cis-trans isomerism in alkenes is only
possible when both of the double-bond carbon atoms have two different groups.
If two methyl groups replace two hydrogen atoms, one on each carbon atom of ethane (H2C=CH2), the
result is 2-butene, CH3CH=CHCH3. Experimental evidence confirms the existence of two compounds
with the same set of bonds. The difference in the two compounds is in the location in space of the two
methyl groups: the cis- isomer has two methyl groups on the same side in the plane of the double bond
and the trans- isomer has two methyl groups on the opposite sides of the double bond. The two
arrangements are distinguished from each other by the prefixes cis- indicating groups on the same side
and trans- indicating groups on opposite sides.
cis-2-butene
trans-2-butene
Optical Isomers:
One class of optical isomers are found for compounds in which four different groups are attached to a
given carbon atom as in CHXYZ. As can be seen from the images below, the resultant structures cannot
be superimposed on each other no matter how they are turned about:
mirror
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Austin Peay State University Department of Chemistry
CHEM 1021
Equipment:
molecular model kit
Procedure:
In this experiment you will make models of organic compounds using plastic balls to represent the
atoms (with different colored balls for different kinds of atoms) and plastic rods to represent the
chemical bonds. Using these as a guide, make the models called for and answer the questions. Also
consult your textbook for additional information about the different kinds of organic isomers and the
rules for naming organic compounds.
Waste Disposal and Cleanup: Without breaking any of the parts, disassemble all parts of the molecular
model kits and place them back in the container.
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Austin Peay State University Department of Chemistry
CHEM 1021
ORGANIC COMPOUNDS IN THREE DIMENSIONS DATA SHEET
Name:__________________________________
Partner: ________________________________
(1) Make a three-dimensional model of ethane and draw its structure below. Are all six hydrogen atoms
equivalent? Replace one hydrogen ball by a different colored plastic ball (representing Br). How many
isomers of ethyl bromide (bromoethane) are possible? ________ Draw the structure(s) below.
(2) Make a model of propane and draw its structure below. If you substitute one bromine atom for one
hydrogen atom, how many isomers are possible? Draw their structures and name them.
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(3) How many isomers are possible for an alkane with the formula C5H12? Draw all structures and name
each one.
(4) Make a model of 1,2-dibromoethene and draw its structure below. If cis-trans isomers are possible,
make models for them. Draw the structures below and name them.
(5) Make a model of 2-methyl-1-butene. How does this differ from 2-methyl-2-butene? Draw their
structures.
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CHEM 1021
(6) How many butenes (C4H8) are possible? Draw their structures and name them.
(7) Make a model for the molecule CHClBrF. Can the atoms be arranged in more than one way in three
dimensions? If so, what is the relationship of the different arrangements? (What is the relationship of
your face to the face you look at in the mirror every morning?) Draw structures of any arrangements
that are different.
(8) Make a model of 2-butene and draw its structure below. Are cis-trans isomers possible? If so, draw
the cis-trans isomers and name each one.
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CHEM 1021
(9) An alcohol is classified according to the kind of carbon atom which bears the –OH group. A carbon
atom is classified as primary, secondary, or tertiary according to the number of alkyl groups attached to
it. If the carbon atom which bears the –OH group has no or one alkyl group attached to it, the alcohol is
a primary alcohol. If two alkyl groups are attached, the alcohol is a secondary alcohol; if three alkyl
groups are attached, the alcohol is a tertiary alcohol.
primary alcohol
secondary alcohol
tertiary alcohol
In general, alcohols undergo reactions which are dependent on the type of alcohol used. For example,
primary alcohols are oxidized more readily than are secondary alcohols, and tertiary alcohols are
oxidized only under severe conditions. Other reactions show similar trends.
Make models of primary, secondary, and tertiary alcohols that have the formula C4H9OH. Draw their
structures below and name them.
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Austin Peay State University Department of Chemistry
CHEM 1021
ORGANIC COMPOUNDS IN THREE DIMENSIONS POST-LAB
Name:__________________________________
Partner: ________________________________
1. Why does 2-butene have cis- and trans- isomers but 1-butene doesn’t?
2. Draw the structural formula of 2-methyl-2-hexene.
3. Which of the following molecules can have optical isomers?
(a) CH2Cl2
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(b) H2NCH(CH3)COOH
(c) ClCH(OH)CH2Cl
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Austin Peay State University Department of Chemistry
CHEM 1021
ORGANIC COMPOUNDS IN THREE DIMENSIONS PRE-LAB
Name: ____________________________________
1. Why is C4H10 the simplest alkane that can have structural isomers?
2. Draw the cis- and trans- isomers for ClCH = CHCl. Why are cis- and trans- isomers not possible for
ClCH2CH2Cl?
3. Draw the optical isomers for CHClBrF. Why are the optical isomers not found for CH2ClBr?
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