Question 1

Which of the following resonance structures is MOST stable for the acetate ion ($$\mathrm{CH_3CO_2^-}$$)?

Accepted Answer

A structure where the negative charge is delocalized equally between both oxygen atoms.

Question 2

Why is the heat of hydrogenation for conjugated dienes lower than expected compared to unconjugated dienes?

Accepted Answer

Because conjugation stabilizes the molecule, resulting in less energy released upon hydrogenation.

Question 3

Which of the following is a criterion for a lone pair to participate in conjugation?

Accepted Answer

The lone pair must be adjacent to a pi-system and reside in a p-orbital.

Question 4

Given the molecule 1,3-butadiene, what is the hybridization of all carbon atoms?

Accepted Answer

All carbons are sp$$^2$$ hybridized.

Question 5

Which of the following statements best explains why sigma bonds are generally stronger than pi bonds?

Accepted Answer

Sigma bonds result from end-to-end overlap of orbitals, maximizing electron density between nuclei.

Question 6

Which of the following molecules is conjugated?

Accepted Answer

1,3-butadiene

Question 7

A resonance structure violates the octet rule on a carbon atom. What effect does this have on the stability of the structure?

Accepted Answer

It decreases the stability because carbon prefers a complete octet.

Question 8

Which of the following best describes the stabilization energy due to conjugation in 1,3-butadiene, based on heats of hydrogenation?

Accepted Answer

It is approximately 3 kcal/mol less exothermic than expected for two isolated double bonds.

Question 9

Which resonance structure is LEAST stable?

Accepted Answer

A structure with a positive charge on a carbon atom and a non-octet center.

Question 10

In an amide linkage, why is rotation around the C-N bond restricted?

Accepted Answer

Because resonance gives the C-N bond partial double-bond character.

Question 11

Which of the following is NOT a preference when evaluating resonance structures?

Accepted Answer

Maximize charge separation

Question 12

What is the hybridization of the nitrogen atom in an amide group that participates in conjugation?

Accepted Answer

sp$$^2$$

Question 13

Which of the following statements about conjugated systems is TRUE?

Accepted Answer

Conjugated systems have electron density delocalized over three or more connected p-orbitals.

Question 14

A lone pair on nitrogen is adjacent to a pi-bond. What must happen for the lone pair to participate in conjugation?

Accepted Answer

The lone pair must reside in a p-orbital, so nitrogen rehybridizes to sp$$^2.$$

Question 15

Which of the following best explains why the heat of hydrogenation is always exothermic?

Accepted Answer

Because forming sigma-bonds releases more energy than breaking pi-bonds consumes.

Question 16

Which molecule would have the highest stabilization energy due to conjugation?

Accepted Answer

1,3-butadiene

Question 17

Which of the following is TRUE about the geometry of conjugated systems?

Accepted Answer

Conjugated systems are generally planar to allow p-orbital overlap.

Question 18

Which of the following statements about resonance and orbital rehybridization is correct?

Accepted Answer

Atoms may rehybridize from sp$$^3 to sp$^2 to $$allow lone pairs to participate in conjugation.

Question 19

Which of the following is a consequence of partial double-bond character in amide linkages?

Accepted Answer

Restricted rotation and increased energy barrier for bond rotation.

Resonance, Conjugation, and Stability in Organic Molecules

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