Deck 3: Interchapter 1molecular Orbital Theory and Chemical Reactions

Question 1

Which of the following transition states will be allowed?

A)II only
B)II and IV
C)III only
D)I, II, and IV
E)I, II, III, and IV

Answer 1

I, II, and IV

Question 2

Draw the energy levels of the transition-state molecular orbitals based on the HOMO-LUMO interaction below.

Answer 2

Question 3

Use FMO theory to rationalize the different stereospecificity of S_N1 and S_N2 reaction mechanisms.

Answer 3

S_N2 reactions are stereospecific because the HOMO of the nucleophile (often a nonbonding orbital)must interact with the LUMO of the electrophile (often an antibonding molecular orbital)with the correct symmetry.This can occur only with inversion of stereochemistry.An S_N1 reaction proceeds via a planar carbocation intermediate,in which the LUMO is an empty p orbital.The HOMO of the incoming nucleophile can thus interact with either lobe of the p orbital,so a racemic mixture of enantiomers will be obtained.

Question 4

What is meant by the term frontier molecular orbital?

Question 5

Draw the HOMO-LUMO interaction that occurs in the transition state of the second step of the reaction shown below (that is,the bromide leaving).Label the HOMO and the LUMO.

Question 6

Frontier molecular orbital theory can be used to predict which of the following about a chemical reaction?

A)The kinetics of the reaction
B)The thermodynamics of the reaction
C)The stereospecificity of the reaction
D)The stability of the product of the reaction
E)More than one of the above

Question 7

According to frontier molecular orbital theory,a reaction is favorable if which of the following is true?

A)The energy gap between the HOMO and the LUMO is large.
B)The HOMO contains nonbonding electrons.
C)The reverse reaction is forbidden.
D)The energy gap between the HOMO and the LUMO is small.
E)The LUMO is at a lower energy than the HOMO.

Question 8

In the elementary reaction step shown below,identify the reactant that contributes the HOMO and the reactant that contributes the LUMO.

Question 9

According to FMO theory,an allowed transition state requires that the HOMO and the LUMO have which of the following?

A)Similar energies
B)Different energies
C)Similar symmetries only
D)Opposite symmetries
E)Similar energies and symmetries

Question 10

Draw the HOMO-LUMO interaction that stabilizes the transition state of the 1,2-hydride shift that would occur in the molecule below.

Question 11

Briefly explain why the HOMO and the LUMO must have similar energies in order for a reaction to occur.

Question 12

Identify the HOMO and the LUMO that participate in the following reaction.

A)HOMO = nb orbital on ^CNLUMO = σ* orbital on CH₃Br
B)HOMO = π orbital on ^CNLUMO = π* orbital on CH₃Br
C)HOMO = π orbital on CH₃BrLUMO = nb orbital on ^CN
D)HOMO = nb orbital on CH₃BrLUMO = π* orbital on ^CN
E)HOMO = π orbital on ^CNLUMO = σ* orbital on CH₃Br

Question 13

Would the transition state shown below lead to a successful reaction? Why or why not?

Question 14

Construct the MO energy-level diagram for acetone,and then use the diagram to determine the HOMO and the LUMO.

Question 15

Which of the following transition states are forbidden?

A)IV only
B)I and III
C)III only
D)III and IV
E)II and IV

Question 16

Which of the following would be expected to contribute the HOMO to a transition-state complex?

A)I only
B)I and III
C)I and II
D)II and III
E)II and IV

Question 17

Briefly explain what determines whether a particular transition state is "allowed" or "forbidden."