Questions: Resonance in Organic Intermediates

Question 1

Bromination of an alkene via radical mechanism (using NBS) at an allylic position is highly favored. A student argues this is because the allylic carbon 'alternates rapidly between the two resonance structures.' What is the correct explanation?

• A. The student is correct: the intermediate rapidly interconverts between two structures, trapping bromine at either carbon
• B. The allylic radical is a single species with electron density delocalized simultaneously across both carbons — a resonance hybrid lower in energy than either individual structure
• C. Radical reactions are inherently non-selective, so allylic positions are not especially preferred
• D. The allylic position is preferred purely due to steric effects, not resonance stabilization

Answer: (B)

The most common resonance misconception is treating resonance structures as equilibrating species. The allylic radical is one real molecule whose electron density is spread across both carbons simultaneously — a hybrid lower in energy than either resonance structure alone. This delocalization thermodynamically stabilizes the intermediate, lowering the activation energy to form it. The radical does not 'bounce' between structures; it is a single entity with partial radical character at both allylic carbons.

Question 2

A benzylic primary substrate undergoes SN1 reaction readily, while a simple primary alkyl substrate under the same conditions does not. What accounts for this reactivity difference?

• A. Benzene rings are electron-withdrawing, destabilizing the nearby carbocation and forcing an alternative mechanism
• B. Benzylic carbocations can delocalize positive charge into the aromatic ring (ortho and para positions), providing resonance stabilization unavailable to primary alkyl carbocations
• C. The phenyl group provides steric bulk that prevents backside attack, making SN2 impossible and SN1 the only available path
• D. Primary benzylic substrates react via radical mechanisms, not SN1

Answer: (B)

A benzylic carbocation can delocalize positive charge across the aromatic ring — resonance structures with charge at the ortho and para positions provide four or more resonance contributors. This extensive delocalization dramatically lowers the energy of the intermediate. A simple primary carbocation has no such stabilization, making it essentially too unstable to form in SN1. In SN1 reactions, feasibility depends on carbocation formation, so any feature that stabilizes the carbocation dramatically accelerates the reaction.

Question 3

A molecule described by two resonance structures of equal energy is more stable than a molecule described by two resonance structures of unequal energy.

• T. True
• F. False

Answer: True

When resonance contributors are equivalent (equal energy), electron delocalization is maximized and stabilization is greatest — this is why benzene is exceptionally stable (six equivalent contributors). When contributors are unequal in energy, the hybrid resembles the lower-energy structure more closely, and the stabilization gained from delocalization is reduced. The closer in energy the contributors, the more equal their contribution to the hybrid, and the greater the resonance stabilization.

Question 4

More resonance structures can generally be drawn for a more stable molecule — so counting resonance structures is a reliable measure of stability.

• T. True
• F. False

Answer: False

While more equivalent, low-energy resonance contributors do increase stability, not all drawable resonance structures contribute meaningfully. High-energy structures — those with adjacent like charges, broken octets on electronegative atoms, or formal charge violations — contribute very little to the hybrid and provide almost no stabilization. You can draw many technically valid but energetically unfavorable resonance structures, inflating the apparent count without reflecting real stabilization. Quality of contributors matters, not raw quantity.

Question 5

What is a resonance hybrid, and why does the concept of resonance matter for predicting where a reaction intermediate will form in a molecule?

Model answer: A resonance hybrid is the actual electronic structure of a molecule — a single species with electron density distributed continuously across multiple atoms, not a mixture of alternating structures. The contributing resonance structures are bookkeeping tools showing which atoms share the electron density. For reaction intermediates, the position that can form the most stable hybrid — where the charge, radical, or empty orbital can be delocalized across the most atoms via low-energy contributors — is where the intermediate forms preferentially. This governs regioselectivity: reactions proceed through allylic or benzylic positions because those intermediates are resonance-stabilized hybrids lower in energy than their non-delocalized counterparts.

The practical import is that whenever you see a reaction site adjacent to a π system (double bond or aromatic ring), ask: would the intermediate at this position be resonance-stabilized? If yes, and if a competing non-stabilized pathway exists, the stabilized pathway dominates. This is why Markovnikov addition, NBS bromination, and SN1 at benzylic positions all follow from the same underlying principle.