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Reactive
Intermediates

Explore the three key reactive intermediates in organic chemistry: carbocations, carbanions, and free radicals: their structure, stability, and role in reaction mechanisms.

Organic Chemistry 1

What Are Reactive Intermediates?

Reactive intermediates are short-lived, high-energy species that form during the course of a chemical reaction. They are too unstable to be isolated, but they determine how a reaction proceeds and what products form.

\(\text{C}^+\) Carbocation
Carbon with only 6 electrons: electron-deficient
\(\text{C}^-\) Carbanion
Carbon with 8 electrons including a lone pair
\(\text{C}\cdot\) Radical
Carbon with 7 electrons: one unpaired electron

Carbocations: The Electron-Hungry Carbon

\(\text{C}^+\)
\(\text{R}\)
\(\text{R}\)
\(\text{R}\)
empty p orbital
\(sp^2\) hybridized : trigonal planar
+

Carbanions: Carbon with a Lone Pair

\(\text{C}^-\)
\(\text{R}\)
\(\text{R}\)
\(\text{R}\)
lone pair
−
\(sp^3\) hybridized : pyramidal geometry

Free Radicals: The Unpaired Electron

\(\text{C}\cdot\)
\(\text{R}\)
\(\text{R}\)
\(\text{R}\)
half-filled p orbital
\(sp^2\) hybridized : trigonal planar

Why Stability Matters

The more stable a reactive intermediate is, the more easily it forms and the faster the reaction proceeds through that pathway. Stability determines which products are favored.

1
Hyperconjugation: Alkyl groups donate electron density into an empty or partially empty orbital, stabilizing the intermediate.
2
Induction: Electron-donating groups (EDG) stabilize positive charges; electron-withdrawing groups (EWG) stabilize negative charges.
3
Resonance: Delocalization of charge or the unpaired electron across adjacent \(\pi\) systems dramatically increases stability.

Carbocation Stability Order

Less stable
More stable
\(\text{CH}_3^+\)
Methyl
(0 alkyl)
\(1^\circ\)
Primary
(1 alkyl)
\(2^\circ\)
Secondary
(2 alkyl)
\(3^\circ\)
Tertiary
(3 alkyl)
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<
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Carbanion Stability Order

Less stable
More stable
\(3^\circ\)
Tertiary
\(2^\circ\)
Secondary
\(1^\circ\)
Primary
\(\text{CH}_3^-\)
Methyl
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<
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Radical Stability Order

Less stable
More stable
\(\text{CH}_3\cdot\)
Methyl
\(1^\circ\)
Primary
\(2^\circ\)
Secondary
\(3^\circ\)
Tertiary
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<
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Key Distinction: Charge vs. No Charge

Carbocation
Positive charge
\(sp^2\), planar
6 electrons
Electrophile
Stabilized by EDGs
Carbanion
Negative charge
\(sp^3\), pyramidal
8 electrons
Nucleophile / Base
Stabilized by EWGs
Radical
Neutral (no charge)
\(sp^2\), planar
7 electrons
Neither nuc. nor elec.
Stabilized by alkyl groups

Where Do They Appear?

1
Carbocations form in \(\text{S}_{\text{N}}1\) and \(\text{E}1\) reactions, electrophilic additions to alkenes (Markovnikov), Friedel-Crafts alkylation, and acid-catalyzed dehydration.
2
Carbanions appear in aldol reactions, Grignard reagent chemistry, deprotonation of \(\text{C-H}\) acids, Michael additions, and Wittig reactions.
3
Radicals arise in halogenation of alkanes, anti-Markovnikov addition of \(\text{HBr}\) (with peroxides), polymerization, and combustion reactions.

How to Identify and Rank Stability

1
Identify the intermediate type. Check the formal charge: positive = carbocation, negative = carbanion, unpaired electron = radical.
2
Count the degree of substitution. Count how many carbon groups (not H atoms) are attached to the carbon bearing the charge or radical.
3
Check for resonance. Is the charge/radical adjacent to a \(\pi\) bond, aromatic ring, or heteroatom? Resonance dramatically boosts stability.
4
Consider inductive effects. For carbanions, look for nearby EWGs (\(-\text{NO}_2, -\text{CN}, -\text{C}=\text{O}\)). For carbocations, look for EDGs.
5
Apply the correct trend. Carbocations and radicals: \(3^\circ \gt 2^\circ \gt 1^\circ \gt \text{methyl}\). Carbanions: \(\text{methyl} \gt 1^\circ \gt 2^\circ \gt 3^\circ\).

Example 1: Carbocation Ranking

Problem: Rank the following carbocations from least to most stable:
(A) \(\text{CH}_3^+\)   (B) \((\text{CH}_3)_2\text{CH}^+\)   (C) \((\text{CH}_3)_3\text{C}^+\)   (D) \(\text{CH}_3\text{CH}_2^+\)

Least stable
Most stable
(A) methyl
(D) \(1^\circ\)
(B) \(2^\circ\)
(C) \(3^\circ\)
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Example 2: Resonance Stabilization

Problem: Which carbocation is more stable: a tertiary carbocation or a secondary allylic carbocation?

Tertiary \(\text{C}^+\)
\(\text{C}^+\)
\(\text{CH}_3\)
\(\text{CH}_3\)
\(\text{CH}_3\)
Only hyperconjugation
vs.
\(2^\circ\) Allylic \(\text{C}^+\)
\(\text{C}^+\)
C
C
\(\leftrightarrow\)
C
C
\(\text{C}^+\)
Charge delocalized
over two carbons
Hyperconjugation + resonance
\(2^\circ\) Allylic can rival or exceed \(3^\circ\)

Example 3: Carbanion with EWG

Problem: Why is the carbanion \(\text{CH}_2^-\text{NO}_2\) much more stable than \(\text{CH}_3\text{CH}_2^-\)?

\(\text{CH}_3\text{CH}_2^-\)
Primary carbanion with no EWG. Negative charge is localized on one carbon. No resonance stabilization.
\(\text{CH}_2^-\text{NO}_2\)
Adjacent \(-\text{NO}_2\) group is a strong EWG. The negative charge is delocalized by resonance into the nitro group's \(\pi\) system.

Common Mistakes

✕ Same Stability for All Three
Carbocations and radicals follow the same substitution trend (\(3^\circ \gt 2^\circ \gt 1^\circ\)), but carbanions are opposite. Don't apply one rule to all three.
✕ Ignoring Resonance
An allylic or benzylic intermediate at lower substitution can be more stable than a higher-substituted one without resonance. Always check for adjacent \(\pi\) bonds.
✕ Confusing Hybridization
Carbocations and radicals are \(sp^2\) (planar). Carbanions are usually \(sp^3\) (pyramidal). Don't assign all intermediates the same geometry.

Summary Comparison Table

Property Carbocation Carbanion Radical
Charge Positive (+) Negative (−) Neutral (0)
Valence Electrons 6 8 7
Hybridization \(sp^2\) \(sp^3\) \(sp^2\)
Geometry Trigonal planar Pyramidal Trigonal planar
Stability Trend \(3^\circ \gt 2^\circ \gt 1^\circ \gt \text{methyl}\) \(\text{methyl} \gt 1^\circ \gt 2^\circ \gt 3^\circ\) \(3^\circ \gt 2^\circ \gt 1^\circ \gt \text{methyl}\)
Character Electrophile Nucleophile Neither
Stabilized By EDGs, resonance EWGs, resonance Alkyl groups, resonance

Question 1 of 5

Which reactive intermediate has an empty p orbital and is \(sp^2\) hybridized with a positive formal charge?

Question 2 of 5

Rank the following carbocations from most stable to least stable:
I. \((\text{CH}_3)_3\text{C}^+\)   II. \(\text{CH}_3^+\)   III. \((\text{CH}_3)_2\text{CH}^+\)

Question 3 of 5

Which statement about carbanion stability is correct?

Question 4 of 5

A secondary allylic carbocation is often comparable in stability to a tertiary carbocation. What is the primary reason?

Question 5 of 5

How many valence electrons does a free radical carbon possess, and what is its hybridization?