Sodium borohydride ($$\text{HOMO} = -0.7\ \text{eV})$$

Trimethylamine ($$\text{HOMO} = -5.4\ \text{eV})$$

Question 1

Which of the following pairs of atomic orbitals are orthogonal to each other?

Accepted Answer

A $$p_x$$ and a $$p_y$$ orbital on the same atom

Question 2

According to molecular orbital theory, which feature most increases a molecule's reactivity?

Accepted Answer

Poorly overlapping atomic orbitals

Question 3

Which compound is the most reactive electron donor based on the provided MO energies?

Accepted Answer

Sodium borohydride ($$	ext{HOMO} = -0.7\ 	ext{eV})$$

Question 4

Which statement best explains why the reaction rate of iodoethane with NaSEt in $$	ext{Et}_2	ext{O} is $$faster than that of fluoroethane?

Accepted Answer

The leaving group ability of iodide is greater than that of fluoride

Question 5

Which molecular orbital is orthogonal to the $$\pi_{CC}$$ molecular orbital in ethylene?

Accepted Answer

$$\sigma_{CC}$$ molecular orbital

Question 6

Which of the following is the best nucleophile based on its molecular orbital energies?

Accepted Answer

Trimethylamine ($$	ext{HOMO} = -5.4\ 	ext{eV})$$

Question 7

Which factor most stabilizes a carbocation intermediate in a reaction?

Accepted Answer

Hyperconjugation from adjacent C–H bonds

Question 8

In the context of acid/base chemistry, which factor most increases the stability of a conjugate base?

Accepted Answer

The negative charge is delocalized over several atoms

Question 9

Which of the following best describes a stereospecific reaction?

Accepted Answer

A reaction in which the stereochemistry of the starting material determines the stereochemistry of the product

Question 10

Which principle of frontier molecular orbital theory explains why reactions with a smaller HOMO-LUMO gap proceed faster?

Accepted Answer

The energy match between the HOMO of the nucleophile and the LUMO of the electrophile facilitates electron transfer

Question 11

Which of the following compounds is the most reactive electron acceptor based on its LUMO energy?

Accepted Answer

tert-Butyl cation ($$	ext{LUMO} = -6.8\ 	ext{eV})$$

Question 12

Which of the following best describes the role of the leaving group in an $$	ext{S}_N2$$ reaction?

Accepted Answer

It must be able to accept electrons into a low-energy orbital

Question 13

Which molecular orbital interaction is most important in the nucleophilic attack of NaSEt on bromoethane?

Accepted Answer

Overlap of the HOMO of NaSEt with the LUMO of bromoethane

Question 14

Which of the following statements about the stability of carbocations is correct?

Accepted Answer

Carbocations are stabilized by resonance and hyperconjugation

Question 15

Which of the following is a key requirement for a molecule to act as an electrophile in a reaction?

Accepted Answer

It must have a low-energy unoccupied molecular orbital

Question 16

Which of the following best rationalizes the difference in pKa values between phenol and p-nitrophenol?

Accepted Answer

p-Nitrophenol has a lower pKa because the nitro group stabilizes the conjugate base by resonance and inductive effects

Question 17

In the context of the tetrodotoxin molecule shown in the image, which functional group is most likely to participate in hydrogen bonding with water?

Accepted Answer

The hydroxyl groups

Question 18

Which of the following statements about stereoselectivity and stereospecificity is correct?

Accepted Answer

All stereospecific reactions are stereoselective, but not all stereoselective reactions are stereospecific

Question 19

Which of the following best explains why the $$	ext{S}_N2$$ reaction mechanism is stereospecific?

Accepted Answer

The nucleophile attacks from the opposite side of the leaving group, leading to inversion of configuration

Molecular Orbital Theory and Reactivity in Organic Chemistry: Study Guide

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