Reactive Intermediates

Reaction intermediates are chemical species formed transiently during a reaction. The three main types of reactive intermediates are carbocations, carbanions, and free radicals. Carbocations and carbanions are generated through heterolytic bond fission, whereas free radicals are produced through homolytic bond fission.

1.0Introduction 

Reaction intermediates are transient chemical species that exist for mere seconds to a few seconds and play a crucial role in organic reactions. They are formed when covalent bonds within the substrate are broken. These intermediates are highly reactive and quickly transform into more stable molecules during a chemical process. Under typical conditions, they are fleeting and difficult to isolate. However, reaction intermediate compounds can be isolated and preserved for further study in rare situations such as low temperatures or matrix isolation.

2.0Carbocations 

Carbocations are key intermediates in various reactions, particularly in nucleophilic substitution reactions.

Structure of Carbocations

Generally, the positively charged carbon atom in carbocations is bonded to three other atoms and has no nonbonding electrons. It is sp² hybridized, resulting in a planar structure with bond angles of approximately 120°. Additionally, there is a vacant unhybridized p orbital, which, in the case of CH₃⁺, lies perpendicular to the plane of the C—H bonds.

Formation of Carbocations

Carbocations can be formed through two main mechanisms:

• Heterolytic Bond Cleavage by the Loss of a Leaving Group
• Addition of π Electrons to an Electrophile

An electrophile attacks an unsaturated point (double or triple bond), breaking the π bond and forming a carbocation.

                     $\mathrm{RCH}={\mathrm{CH}}_2+{\mathrm{H}}^{+}\to \mathrm{R}-{\mathrm{CH}}_2^{+}+{\mathrm{H}}_2\mathrm{C}={\mathrm{CH}}_2$

Stability of Carbocation

Resonance, hyperconjugation, and inductive effects influence the stability of alkyl carbocations. Alkyl groups exhibit an electron-releasing inductive effect, stabilizing the positively charged carbon by dispersing the positive charge. The more alkyl groups attached, the greater the dispersal of positive charge, leading to increased carbocation stability.

When a positive carbon is near a double bond, resonance spreads the charge over two atoms, increasing stability, like in allylic and benzylic cations.

Like in methoxymethyl cations, heteroatoms with lone pairs next to the cationic center also stabilize carbocations.

Cyclopropylmethyl cations are particularly stable due to conjugation with the cyclopropyl ring.

Overall, carbocation stability follows this order:(influenced by hyperconjugation)

                                    tertiary > secondary > primary > methyl 

 Vinyl cations, lacking resonance stability, are very unstable.

3.0Carbanion

A carbanion is a reaction intermediate in organic chemistry with a negative charge on a carbon atom. Carbanions form when an organic compound is treated with a very strong base. Carbanions are important in organic chemistry because they act as nucleophiles, donating electrons to other molecules. 

Structure of Carbanion

A carbanion has an unshared pair of electrons, making it a base. The central carbon atom in a carbanion is typically sp³ hybridized, with the unshared pair occupying one corner of a tetrahedron, giving it a pyramidal shape similar to amines. Carbanions can rapidly interconvert between two pyramidal forms.

Formation of Carbanions

Carbanions can be formed by:

Deprotonation: A base removes a proton from a carbon atom, leaving behind a carbanion

                    $\mathrm{R}-{\mathrm{CH}}_3+\text{ Base }\to \mathrm{R}-{\mathrm{CH}}_2^{-}+\mathrm{H}\text{-Base }$

Nucleophilic Addition: A nucleophile adds to an unsaturated carbon, breaking a π bond and forming a carbanion.

                    $\mathrm{R}-\mathrm{CH}={\mathrm{CH}}_2+{\mathrm{Nu}}^{-}\to \mathrm{R}-\mathrm{CH}-{\mathrm{CH}}_2^{-}$

Stability of Carbanion

A carbanion is a nucleophile whose stability and reactivity are determined by several factors:

• Inductive Effect: Electronegative atoms adjacent to the negatively charged carbon stabilize the carbanion.
• Hybridization: The more s-characters in the hybrid orbitals of the charge-bearing atom, the more stable the carbanion (e.g., sp-hybridized carbanions are more stable than sp³).
• Conjugation: Resonance effects, including aromaticity, can stabilize the carbanion by delocalizing the negative charge.

The general stability order from most stable to least stable is:

• Benzylic/Allylic > Methyl > Primary > Secondary > Tertiary > Vinylic
  

4.0Free radicals

Free radicals are species with one or more unpaired electrons, which gives them a net magnetic moment and makes them paramagnetic. They are typically detected by electron spin resonance (or electron paramagnetic resonance).

A notable example of a radical is the hydroxyl radical, with one unpaired electron on the oxygen atom. Two additional examples are triplet oxygen and triplet carbene, each possessing two unpaired electrons.

Structure and Geometry of Free Radicals

• Simple alkyl radicals generally have a planar (trigonal) structure with sp² bonding and the unpaired electron in a p orbital. 
• However, they can also adopt a pyramidal structure with sp³ bonding and the unpaired electron in an sp³ orbital.
• The planar structure is associated with the loss of stereochemistry at a chiral center, as the radical can be attacked from either face equally, producing enantiomers. 
• Unlike carbocations, radicals can form at bridgehead positions, indicating that both planar and pyramidal geometries are possible for free radicals.

Stability Of Free Radicals 

The stability of free radicals is influenced by three main factors: inductive effect, hyperconjugation, and resonance effect.

• Similar to carbocations, the stability of free radicals follows the order:

tertiary > secondary > primary. This is primarily due to hyperconjugation, where the odd electron is delocalized onto the β-hydrogens, enhancing stability.

• The vinyl and phenyl groups provide additional stability through resonance structures in allylic and benzyl radicals. For example, bond dissociation energies indicate that forming a benzyl radical from toluene requires 19 kcal/mol less energy than forming a methyl radical from methane.
• Triphenylmethyl radicals are stabilized by resonance and significantly by steric hindrance, which prevents dimerization

Uses of free radicals 

Free radicals are crucial in various chemical processes, including combustion, atmospheric chemistry, polymerization, plasma chemistry, and biochemistry. Their common uses include:

• Industrial Applications: Primarily used in polymerization reactions.
• Halogenation Reactions: Alkyl halides or aryl halides serve as precursors.
• Biological Functions: In our body, helps fight pathogens by damaging cell walls or disrupting metabolic processes.

5.0Carbenes

Carbenes are transient carbon intermediates that are neutral. They possess a carbon atom with a valency of two and two unshared valence electrons.

• Carbenes are represented as R-(C:)-R', R=C, or R₂C:,

Where R represents substituent groups or hydrogen atoms, carbenes can exist as either singlet or triplet species based on their electronic structure.

Methylene (CH₂ )is the simplest carbene.

Structure of Carbenes 

Carbenes are neutral and have six outer-shell electrons, including a nonbonding pair. They can be singlet or triplet and exhibit either linear or bent geometries.

Carbenes are short-lived, highly reactive intermediates due to their electron deficiency, making them electrophilic. Substitute groups influence their reactivity and electron structure. Electron-withdrawing groups increase their electrophilicity, while strong donor groups can make them nucleophilic.

Generation of Carbenes

Carbenes can be produced by thermal or photochemical decomposition of diazoalkanes. They can also be generated through α-elimination of a hydrogen halide from a haloform with a base, or by removing a halogen from a gem-dihalide using a metal.

Singlet and Triplet Carbenes

According to valence bond theory, carbenes have 𝑠𝑝² hybridized carbon atoms. The two nonbonding electrons in carbenes are placed in vacant orbitals. 

Both nonbonding electrons occupy the same orbital with opposite spins in singlet carbenes.

Conversely, based on Hund's law, the species is known as a triplet carbene when these electrons are placed in separate orbitals with parallel spins. Triplet carbenes can exhibit either a bent or linear structure.

6.0Arenium Ions

In organic chemistry, an arenium ion is a cyclohexadienyl cation that serves as a reactive intermediate in electrophilic aromatic substitution reactions.

• Experimental evidence shows that electrophiles attack the π system of benzene, forming a delocalized, non-aromatic carbocation known as an arenium ion or σ complex. 
• These ions are also referred to as sigma complexes. The smallest arenium ion is the benzenium ion (C₆H₇⁺), which is protonated benzene.

7.0Benzynes

 Benzynes are reactive intermediates in some nucleophilic aromatic substitutions. 

• They are derived from benzene by removing two hydrogen atoms and are typically depicted with a highly strained triple bond within the six-membered ring. 
• Benzyne intermediates have been observed spectroscopically and trapped in reactions.