Download Structure and Bonding in Organic Compounds

Structure and Bonding in Organic Compounds, Chem 101-E0E
While organic chemistry is a vast subject, we will concern ourselves with one fundamental
concepts that will form a solid foundation for those students who will be continuing with Chem
104 or Chem 150: an understanding of the structure and bonding in organic compounds.
This experiment is designed to acquaint you with structural aspects of organic chemistry through
the assembly of molecular models of a variety of organic compounds. We will investigate
structural aspects of a variety of hydrocarbons, as well as other functionalized (complicated)
organic compounds (ones that contain oxygen in addition to carbon and hydrogen), and give you
some additional practice in drawing formulas, predicting geometries, and identifying structural
isomers for organic compounds.
Procedure
You and your lab partner should obtain one molecular model kit, which includes a variety of
balls and sticks which are used to represent atoms and bonds, respectively.
Atoms
Black
Blue
Red
Purple
White
Short, gray
Long, gray
Carbon
Nitrogen
Oxygen
Halogens
Hydrogen
Bonds
For single bonds (join two atoms with one short
stick for a single bond; these will not bend)
For multiple bonds (join two atoms with two long
sticks for a double bond, 3 for a triple bond; these
will bend as needed)
Construct models of the organic compounds described below, and answer the questions
associated with each model. You may find it useful to draw an expanded structural formula (or
condensed structural formula) before you can start assembling a model for each molecule.
1. Start with methane, the simplest alkane: CH4. Draw the expanded structural formula of
methane below. Based on VSEPR theory, predict the geometry about the central C atom.
Structural formula
Geometry
Now, build a model of methane, CH4. In the space below, draw, to the best of your ability, the
three-dimensional structure of methane based on your model.
Structure of CH4
Note that the carbon atom in methane is tetrahedral (all of the H-C-H bond angles are ~109.5o),
and as a result, the molecule is non-planar (all of the atoms do not reside in one plane). This is
characteristic of the structures of all alkanes.
2. Now, consider the simplest alkene, ethylene: C2H4. Draw the expanded structural formula of
ethylene below. Based on VSEPR theory, predict the geometry about the two C atoms.
Structural formula
Geometries
Now, build a model of ethylene, C2H4. In the space below, draw, to the best of your ability, the
three-dimensional structure of ethylene based on your model.
Structure of C2H4
Does your model of ethylene confirm the geometries for each carbon atom that you predicted
from VSEPR theory?
Note that as a result of the geometries of the carbon atoms in ethylene, the entire molecule is
planar (all of the atoms do not reside in one plane), or flat. In addition, all of the bond angles for
each carbon that participates in the double bond are ~120o. This is characteristic of the structures
of all alkenes; the atoms that participate in the double bond (i.e., the two carbons), and the atoms
directly attached to those atoms (here, the four hydrogens), all occupy one plane, and the carbons
are trigonal planar.
3. Now, consider another alkene, butene: C4H8. There are several structural isomers of this
compound, and we will build models of all of them. Alkenes are hydrocarbons that contain at
least one double bond between two carbon atoms; butene has one such double bond. In the
spaces below, draw expanded structural formulas of two possible isomers of butene: one in
which the double bond is between the first and the second carbon in a four-carbon chain (isomer
“A”), and one in which the double bond is between the second and the third carbon is a fourcarbon chain (isomer “B”). Complete the formulas by including hydrogens in the appropriate
locations.
Isomer “A”
Isomer “B”
Assemble models of these two isomers. After assembling them, try to move individual carbon
atoms in each molecule by rotating around C-C (and C=C) bonds. In the two isomers, are all of
the carbon atoms necessarily in the same plane all of the time?
Isomer “A”:
Isomer “B”:
These two isomers have different names. Isomer “A” is named 1-butene, because the double
bond starts with carbon #1 in a 4-carbon chain. Isomer “B” is named 2-butene, since the double
bond starts at carbon #2 in the 4-carbon chain.
Now, hold your model of 2-butene such that the double bond is straight up-and-down. Are the
two –CH3 groups (called methyl groups) both to the left of the double bond, both to the right of
the double bond, or one on each side?
Rebuild your model so that both methyl groups are on the same side of the double bond. This
isomer of 2-butene is the cis isomer (the molecule is named cis-2-butene). Now, rearrange the
atoms so that one methyl group is on each side of the double bond; this is the trans isomer (the
molecule is trans-2-butene. Using your models as a guide, draw expanded structural formulas of
the cis and trans isomers of 2-butene below.
Cis-2-butene
Trans-2-butene
There is another structural isomer of C4H10. Rearrange the atoms and bonds in one of your 2butene models to identify this isomer. In the space below, draw an expanded structural formula
of this fourth isomer of C4H10.
4th C4H10 isomer
The name of this fourth isomer of C4H10 is 2-methyl-1-propene.
4. Let’s consider some more complicated organic molecules, specifically molecules that contain
oxygen. The first group that we’ll consider will be alcohols, molecules that contain at least one
-O-H group bonded to a single carbon. Using the models as a starting point, build two isomers of
the alcohol C3H8O, named propanol. After building the models, draw expanded structural
formulas of the two isomers you have built.
C3H8O isomer 1
C3H8O isomer 2
How do these two isomers differ?
We can use the same formula to illustrate another type of oxygen-containing organic compound,
ethers, which are organic compounds in which the oxygen is bonded to two carbon atoms, not
only to one as found in alcohols. Rearrange the atoms in one of your propanol models to
construct an ether. Draw the expanded structural formula of this ether in the space below.
C3H8O ether
5. Finally, let’s examine two other groups of oxygen-containing organic molecules, aldehydes
and ketones. Both of these groups of organic molecules contain a double bound between carbon
and oxygen. Both of these types of molecules may be illustrated with the formula C3H6O. Build
models of two isomers of C3H6O, both of which must contain the C=O unit. After you have built
these models, draw expanded structural formulas for the compounds below.
C3H6O isomer 1
C3H6O isomer 2
In your two models, is the oxygen bonded to the first carbon in the 3-carbon chain, or the second
carbon?
Isomer 1:
Isomer 2:
One of the compounds you have built is acetone (a component of fingernail polish remover), a
ketone, and the other is propionaldehyde, an aldehyde. These two compounds are different
because in one, the oxygen is doubly-bonded to the first (or last) carbon in the chain, while in the
other, the oxygen is doubly-bonded to a carbon (somewhere) in the middle of the chain of carbon
atoms. For your two isomers, which is the aldehyde, propionaldehyde, and which is the ketone,
acetone?
Isomer 1:
Isomer 2: