Download New LS-VSEPR Modeling Lab

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Introduction and Purpose: Molecular compounds (and polyatomic ions) are formed when atoms
with similar electronegativities covalently bond with one another. In most cases, each atom
will form the requisite number of bonds in order to achieve a “stable” octet of electrons in its
outer valence shell (the Octet Rule). These molecules (or ions) will take on specific, stable
molecular geometries in a manner consistent with valence shell electron pair repulsion theory
(VSEPR). VSEPR states that pairs of electrons (bonding or non-bonding (lone) pairs) in an
atom’s valence shell will mutually repel each other and thereby move away from each other as
much as possible in order to minimize that repulsion and produce the most stable (lowest
energy) geometric arrangement around a given atom. This lab has a number of objectives:
· to give students additional experience with drawing correct Lewis structures
· to apply valence shell electron pair repulsion (VSEPR) theory for predicting correct
molecular geometries
· to predict the correct hybridizations, molecular polarities and resonance forms of
various molecules
· to construct and manipulate molecular models in order to better visualize various
types of molecular isomers (structural, geometric and rotational)
Materials: molecular modeling kits or Styrofoam spheres and toothpicks
Bond Polarity and Molecular Polarity
Bond Polarity
• No electronegativity difference between two atoms in a bond leads to a pure non-polar
covalent bond.
• A small electronegativity difference leads to a polar covalent bond.
• A large electronegativity difference leads to an ionic bond.
Polar bonds and polar molecules
In a simple molecule like HCl, if the bond is polar, so also is the whole molecule. What about
more complicated molecules?
In CCl4, each bond is polar with the more electronegative chlorines with partial negative
charges (denoted as δ–) and carbon with a partial positive charge (δ+).
COP=CON
The molecule as a whole, however, isn't polar. The center of positivity
(COP) is located on the carbon. And while the individual chlorines are
partially negative, given the geometry of the molecule, the center of
negativity (CON) is also on the carbon. With the COP and the CON at the
same point of the molecule (COP=CON), the molecule is nonpolar.
By contrast, CHCl3 is polar.
COP
The hydrogen at the top of the molecule is less electronegative than
carbon and so is slightly positive. This means that the COP is between
the C and the H. However, the CON lies between under the C and at the
center of the three chlorines. The COP≠CON so the molecule is polar.
CON
Part A — Ring Structures
Benzene, C6H6, consists of a ring of six carbon atoms. Draw the Lewis structure for benzene
and construct a model of this molecule.
Questions:
1. What is the geometry at each carbon atom?
2. Is benzene capable of exhibiting resonance? Why or why not?
3. Would the overall molecule be polar or non-polar?
Part B —Glycine– An Amino Acid
Amino acids are a class of biologically important organic molecules. These molecules contain
an amine group, NH2, a Bronsted-Lowry base (H+ acceptor) and a carboxylic acid group,
COOH, a Bronsted-Lowry acid (H+ donor). The structure of the carboxylic acid unit is similar
to the structure and reactivity of the carbonic acid molecule. The term amino acid usually
refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic
side-chain. In an α-amino acid, the amine group is covalently bonded the carbon atom (the αcarbon) immediately adjacent to the carboxylic acid unit. Glycine, the simplest α-amino acid,
has the following formula, H2NCH2COOH.
Draw the complete Lewis structure and construct a model for glycine. [Hint: The structure of
the carboxylic acid unit, –COOH, is similar in structure and reactivity to the carbonic acid
molecule, H2CO3]
Questions:
1. What is the geometry around each carbon atom? Around the nitrogen atom?
2. What is the hybridization for each carbon? For the nitrogen?
3. Can this molecule exhibit resonance? Explain.
4. Is the molecule polar or non-polar? Explain.
Because amino acids have both amine and carboxylic acid functional groups, these molecules
can act as both an acid and a base at the same time. At a particular pH (known as the
isoelectric point), the amine group will become protonated (having accepted a proton from the
carboxylic acid functionality). This form of the amino acid, with a protonated amine group
and a deprotonated carboxylate group, is known a zwitterion.
Draw the Lewis structure for the zwitterionic form of glycine, H3NCH2COO.
Questions:
1. What is the geometry around the nitrogen atom?
2. What is formal charge on the nitrogen?
3. What is the formal charge of each of the two oxygens?
Part C — Multiple Bonds, Geometries and Hybridizations
A hydrocarbon with the formula C5H6 has the following “skeleton” structure, showing only
the sigma bonds and without any regard for the actual geometry at each carbon:
H
C
C
H
H
H
C
C
C
H
H
Determine the number of electrons that should be in this molecule and draw the complete and
correct Lewis structure for this compound. Construct a molecular model with the geometry at
each carbon as determined using VSEPR.
Questions:
1. What is the geometry at each carbon atom?
2. What is the hybridization at each carbon atom?
Part D — Geometric Isomers
Draw the Lewis structure(s) and construct the model(s) for
C2H2Cl2
Questions:
1. How many different Lewis structures are possible? Draw and construct the models for
each.
2. What is the geometry at each carbon?
3. Which molecules are polar?
Part E — Rotational and Structural Isomers
Draw Lewis structures and make models for each of the following:
a. C2H6
b. C2H4Cl2
For (b), you should draw and construct both structural isomers.
Questions:
Examine the model for C2H6 carefully. Note that the central C–C bond can undergo “free”
rotation (these are called rotational isomers — “rotamers”). Looking along the central C–C
bond, it is possible to produce two rotamers, an eclipsed form and a staggered form.
Staggered
Eclipsed
1. Using VSEPR, what would be the best possible alignment of the bonds in each structure?
Explain your reasoning.
Now examine the rotamers for the isomeric (structural) form of C2H4 Cl2 in which one chlorine
is on each carbon atom.
2. Using the concept of electronic cloud repulsion, known as steric interaction, rank the four
rotamers depicted from highest energy (least stable) to lowest energy?
Cl
Cl
Cl
Cl
Syn
Eclipsed (1)
Cl
Cl
Anti
Cl
Eclipsed (2)
Cl
Highest energy
Lowest energy
Part F — Structural or Functional Isomers
Draw two different Lewis structures (structural or functional isomers — molecules with the
same chemical formulas but with different bond combinations that create different organic
“functional groups”) for
C2 H4 O
Questions:
1. Describe the geometry around the carbons in each.
2. Describe the hybridization around the carbons in each.
3. Draw the “electron cloud” structure for each.
Part G — Expanded Octets and Other Exceptions to the Octet Rule
Draw the Lewis structures and construct models for:
a. BCl3
b. PCl5
c. SF6
d. SF4
e. IF5
Questions:
1. What are the geometries and hybridizations of each?