Organic CHEM 291A

Chemistry 291A McCurdy

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Homework #3 Due Oct 20, 2014 – in (or before) class

Read and study Chapter 3, and 676-688 from Chapter 14 Karty; any

handouts/presentations Reminder: Don’t forget to do your electronic homework!!! Suggested problems from Karty (Not to be turned in for a grade):

Chapter 3: 3.28; 3.29; 3.33; 3.36; 3.39; 3.44; 3.49; 3.53; 3.54; 3.55 Learning Objectives. The student should be able to: a. Understand and use the rules of LCAO-MO theory to generate bonding and

antibonding molecular orbitals in simple molecules. d. Understand and use hybridization models to generate a picture of bonding in simple

organic molecules. Be able to identify the hybridization (and geometry) of atoms in a molecule; be able to identify the symmetry of a bond and to identify the hybrid or atomic orbitals that overlap to form that bond.

e. Understand the MO basis for resonance in organic molecules. Be able to use curved arrows to show electron movement in resonance structures

Group Problems due:

Note: Each person who particpated in group discussion of the problems must sign the homework to be turned in. If you did not participate in the group discussion on that problem set, then you may not sign the answer sheet.

1. Draw a linear combination of atomic orbitals that forms 2 moleclar orbitals (bonding and antibonding) resulting from the interaction of a 2p orbital and an s orbital on different nuclei oriented as shown below. What is the symmetry of the resulting bond? (sigma? pi?)

2. Consider the pi molecular orbitals of the molecule shown below.

a. Show the energies of the pi molecular orbitals and label them π1, π2, etc. Include a sketch of the MO’s, including phases. Show the arrangement of electrons for the ground state. (See Figures 14-11 and 14-12, for an example of what this means).

b. For the ground state, sketch the highest occupied molecular orbital, label it; and sketch the lowest unoccupied molecular orbital and label it. How many internuclear nodes do the HOMO and the LUMO have?

Chemistry 291A McCurdy

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3. Consider formaldehyde (H2CO). Explicitly sketch and label each orbital (orbitals include both atomic orbitals and hybrid orbitals) that overlaps to form a bond. Don’t forget to include the phase of the orbital using shading. (See Karty figures 3-14b, 3-17b for examples of this type of picture). Also sketch and label the orbitals occupied by the lone pairs of electrons on oxygen. 4.i. For the bond between b and c, indicate the symmetry of bond and the orbitals that are overlapping to form the bond. (for example, σCsp3+H1s) ii. For the bond between d and m, indicate the symmetry of bond and the orbitals that are overlapping to form the bond. iii. For the bond between d and k, indicate the symmetry of bond and the orbitals that are overlapping to form both bonds.

O

OH

O

NH2

a

b

c d

e

f g

h

i

j

k

m

5. For each molecule, circle all of the atoms that are required to be in the same plane. Since these are line drawings, you may have to draw in H atoms in order to circle them. Why are these atoms required to be in the same plane?

N

6. a. How many electons reside in MO’s of pi symmetry in the following molecule?

b. For each of the nonhydrogen atoms in the molecule below, provide the electronic

geometry, hybridization, and molecular geometry.

Chemistry 291A McCurdy

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C

O

NH2H3C 7. Draw a sketch of the hybrid orbital/atomic orbitals for the molecules below, including

orbitals used for bonds, lone pairs, and empty orbitals. Don’t forget to include the phase of the orbitals using shading. (See Karty figures 3-14b, 3-17b for examples of this type of picture). Label each hybrid or atomic orbital. Also draw the MO diagram (see Karty Figures 3-19, 3-25 for examples of MO diagrams). On the MO diagram, indicate the HOMO and the LUMO.

a. CH3CH2 +

b. CO