0.4 Vsepr: molecular shapes and isomerism  (Page 3/3)

• • C 3 – rotation by 120 ^o
  • C 4 – rotation by 90 ^o
  • C ∞ (infinity rotation axis) – rotation of any amount will result in an indistinguishable orientation
• Does the molecule have an inversion center (i) ?

Determine which of these symmetry elements are present in your assigned molecules. All of the columns of the table on the report form should be filled out. If you have any difficulty determining whether such symmetry elements are present in the molecules you are assigned, your TA can provide examples of each symmetry element.

Extra credit points can be earned by indicating in the table how many of each symmetry element are present for each molecule (i.e. How many mirror planes are present?).

  2. Mirror Images

Using the model kits, build models which are the mirror images of the models you were assigned to build (b, c, d, e, f, g, h, i and j) in Part 1. With the two mirror images in hand, try to place the models on top of one another, atom for atom.

If you can do this, the model and its mirror image are superimposable mirror images of one another. If not, the molecule and its mirror image form nonsuperimposable mirror images. Nonsuperimposable mirror images are also known as enantiomers .

For each compound, decide whether the mirror image is superimposable or nonsuperimposable. Can you make a generalization about when to expect molecules to have nonsuperimposable mirror images?

  3. Isomers

In this exercise you will build models of compounds which are structural and/or geometrical isomers of one another.

Make the following models:

A. Structural Isomers

1. Make a model(s) of C ₂ H ₅ Cl. How many different structural isomers are possible?
2. Make a model(s) of C ₃ H ₇ Cl. How many different structural isomers are possible?
3. Make a model(s) of C ₃ H ₆ Cl ₂ . How many different structural isomers are possible?

 B. Geometrical Isomers

1. Make a model(s) of C ₂ H ₃ Cl. How many different structural and geometrical isomers are possible?
2. Make a model(s) of C ₂ H ₂ Cl ₂ . How many different structural and geometrical isomers are possible?
3. Make a model(s) of cyclobutane (C ₄ H ₈ ). HINT : cyclo = ring of C atoms
4. Now make dichlorocyclobutane (C ₄ H ₆ Cl ₂ ) by replacing two H atoms on cyclopropane with Cl atoms. How many different structural and geometrical isomers are possible for dichlorocyclobutane? You may wish to make a couple of cyclobutane molecules so that you can compare the structures. Do any of the isomers have nonsuperimposable mirror images?

C. Aromatic Ring Compounds

1. Make a model of benzene, C ₆ H ₆ . Even though your model will contain alternating double and single bonds, remember that in the real molecules of benzene all the C-C bonds are equivalent. What symmetry elements does benzene possess?
2. Make a model(s) of chlorobenzene, C ₆ H ₅ Cl. How many different structural and geometrical isomers are possible?
3. Make a model(s) of dichlorobenzene, C ₆ H ₄ Cl ₂ . How many different structural and geometrical isomers are possible?
4. Make a model(s) of trichlorobenzene, C ₆ H ₃ Cl ₃ . How many different structural and geometrical isomers are possible?

4. Hypervalent Structures

Hypervalent compounds are those that have more than an octet of electrons around them. Such compounds are formed commonly with the heavier main group elements such as Si, Ge, Sn, Pb, P, As, Sb, Bi, S, Se, Te, etc. but rarely with C, N or O. Refer to the rules for Electron Domain theory in order to assign Lewis structures to the following molecules. Describe possible isomeric forms and the bond angles between the atoms. How many lone pairs of electrons are present on the central atom of each molecule, if any? (** Your book will be very useful in aiding you with these structures **)

1. PF ₅
2. PF ₃ Cl ₂
3. SF ₄
4. XeF ₂
5. BrF ₃