Reaction Intermediates in Organic Chemistry

In organic chemistry, a reaction intermediate is a short-lived, high-energy species that is formed during the transformation of reactants into products. These intermediates are not usually isolated, but they play a crucial role in understanding reaction mechanism.

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Types of Organic Reaction Intermediates

1. Carbocations (Carbonium Ions)
2. Carbanions
3. Free Radicals
4. Carbenes
5. Nitrenes

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1. Carbocations (R⁺)

Carbocations are positively charged carbon species formed when a leaving group departs, leaving behind a carbon with only 6 electrons.

• Stability Order: 3° > 2° > 1° > methyl
• Hybridization: sp² (trigonal planar)
• Example: CH₃–C⁺H–CH₃ (secondary carbocation)

2. Carbanions (R^–)

Carbanions are negatively charged carbon species with a lone pair of electrons.

• Stability Order: methyl > 1° > 2° > 3°
• Hybridization: sp³ (pyramidal)
• Example: CH₃–

3. Free Radicals (R•)

Free radicals are species with an unpaired electron. They are neutral and highly reactive.

• Stability Order: 3° > 2° > 1° > methyl
• Hybridization: sp² or in some cases sp³
• Example: CH₃• (methyl radical)

4. Carbenes (:CR₂)

Carbenes are neutral species with two non-bonding electrons on divalent carbon atoms.

• Types: Singlet (paired) and Triplet (unpaired)
• Example: :CH₂ (methylene)

5. Nitrenes (:NR)

Nitrenes are nitrogen analogs of carbenes, formed during reactions involving azides or isocyanates.

• Structure: Neutral with a lone pair and an empty orbital.
• Example: :NH

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Key Features of Intermediates

• High reactivity and short life span
• Stabilized by resonance or inductive effects
• Observed through indirect evidence (e.g. spectroscopy)
• Essential in determining reaction mechanisms

Note: The stability of intermediates influences reaction pathways and rates. For example, SN1 reactions proceed via stable carbocations.

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Quiz: Reaction Intermediates

1. Which intermediate has a carbon atom with a positive charge?
2. What is the hybridization of a carbocation?
3. Which of the following is the most stable free radical?
   a) CH₃•    b) CH₃CH₂•    c) (CH₃)₂CH•    d) (CH₃)₃C•
4. Which intermediate has an unpaired electron but is neutral?
5. Name the nitrogen analog of a carbene.

Answers:

1. Carbocation
2. sp²
3. d) (CH₃)₃C•
4. Free radical
5. Nitrene