Resonance: Patterns, Charges, and Allyl Carbocations

Flashcards cover resonance basics: what resonance is, rules and arrows for drawing resonance, key players (carbocations, carbanions), allyl systems, patterns to recognize, and the concept of resonance hybrids and induction.

What is resonance in organic chemistry?

Delocalization of electrons across overlapping orbitals (pi bonds and lone pairs) with the real structure being a resonance hybrid of multiple contributing structures.

Can sigma bonds participate in resonance delocalization?

No. Resonance involves pi bonds and lone pairs; sigma bonds do not delocalize in resonance.

What does curved-arrow notation represent in resonance?

Movement of electrons: double-headed arrows indicate movement of two electrons, single-headed arrows indicate movement of one electron; arrows show how electrons flow from electron-rich to electron-poor areas.

What is the octet rule constraint in resonance for second-row elements?

Second-row elements (C, N, O, F) should not exceed eight electrons in their valence shell during resonance structures.

What is a carbocation and its typical hybridization?

A positively charged carbon atom; typically sp2 hybridized with an empty p orbital, making it electrophilic.

What is a carbanion and its typical hybridization?

A negatively charged carbon atom; typically sp3 hybridized with a lone pair, making it nucleophilic.

What is an allyl carbocation?

A carbocation directly adjacent to a double bond (allylic position); the positive charge is delocalized across the allyl system via resonance, with involved carbons typically sp2.

What is a resonance structure?

One valid Lewis structure that contributes to the overall description of a molecule; multiple resonance structures describe delocalization.

What is a resonance hybrid?

The actual molecule is a weighted average (hybrid) of all valid resonance structures, with electron density delocalized across the system.

Are resonance structures isomers (constitutional or stereoisomers)?

No. They are not differentconnectivities or spatial orientations; they are multiple contributing forms of the same molecule.

What is inductive effect in resonance context?

Electron density is pulled toward more electronegative atoms (e.g., oxygen), creating partial charges that influence reactivity alongside resonance.

Name Pattern 1 commonly seen in resonance patterns.

A lone pair adjacent to a pi bond (allylic lone pair) that can delocalize into the pi system.

Name Pattern 2 commonly seen in resonance patterns.

A lone pair adjacent to a carbocation, allowing delocalization of charge through the allylic/π system.

Name Pattern 3 commonly seen in resonance patterns.

A pi bond between two atoms with different electronegativities, which can participate in electron delocalization.

What is conjugation in the context of resonance?

A system where p orbitals on adjacent atoms overlap (conjugated pi bonds), allowing delocalization of electrons across multiple atoms (e.g., benzene).

Where do electrons move during resonance (in general)?

From electron-rich regions toward electron-poor regions; pi electrons and lone pairs move, while hydrogens and sigma bonds usually do not move in resonance.