Reaction Intermediates for IIT JEE Chemistry

Short lived (l0-6 seconds to a few seconds) and highly reactive fragments called reaction intermediates result from homolytic and heterolytic bond fission. The important reaction intermediates are free radicals, carbocations, carbanions, carbenes, nitrenes and benzyne.

1. Free Radicals:

Free radical is an atom or group of atoms having an odd or unpaired election. These result on account of homolytic fission of a covalent bond and are denoted by putting a dot (•) against the symbol of atom or group of atoms.

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Characteristics of Free Radicals 

(i) Free radicals are generally electrically neutral.

(ii) They are highly unstable.

(iii) They are short-lived and highly reactive on account of the presence of odd electrons. They readily try to pair up the odd electrons.

(iv) Free radicals are paramagnetic in nature.

(v) Free radicals are generally formed either in presence of UV/visible light orin presence of peroxides

Reactions Involving Free Radicals 

(i) Wurtz reaction giving alkanes.

(ii) Substitution reactions of alkanes.

(iii) Kolbe's electrolytic reaction giving alkanes, alkenes and alkynes.

(iv) Anti-Markownikoff's addition or peroxide effect or Kharasch effect.

Relative Stabilities of Free Radicals

The tertiary alkyl free radicals are most stable and methyl free radical is least stable, i.e., the free radical formed easily has greater stability.

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2. Carbocations (Carbonium ions):

When a covalent bond wherein carbon is connected to a more electronegative atom or group breaks down by heterolytic fission, the more electronegative atoms remove the pair of electrons while carbon loses its electron and thus attains a positive charge. Such organic ions carrying a positive charge on carbon atoms are known as carbocations or carbonium ions.

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Carbocations are named by adding the words 'carbocation' to the parent alkyl group. These are also termed as primary, secondary, tertiary, depending upon the nature of the carbon atom bearing positive charge.

Formation of Carbocations

(i) By heterolysis: They are formed by heterolysis of halogen compounds.

(ii) By protonation of alkenes or alcohols:

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(iii) By decomposition of diazo compounds:

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Characteristics of Carbocation

a) Carbon atom carrying positive charge has six electrons in its valence shell, i. e., 2 electrons less than octet. 

b) The positively charged carbon atom in the carbocation is in sp2 state of hybridization (trigonal planar). 

The three hybridized orbitals which lie in the same plane are involved in the formation of three σ-bonds with other atoms or groups while the hybridized p -orbital remains vacant. The carbocation has a planar structure.

Reactions Involving Carbocations

(i) Elimination reactions (EI) to form alkenes from alkyl halides and alcohols.

(ii) Electrophilic addition reaction of alkenes, alkynes and alkadienes.

(iii) SN 1 reactions of alkyl halides and diazonium salts

(iv) Molecular pinacol-pinacolone rearrangement

Structure of Carbocations

The only difference from free radicals which have odd electron

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Carbocations are very reactive as they have a carbon atom having a vacant p-orbital (6 electrons in valence shell). The positively charged carbon atom tries to complete its octet and hence, these ions react readily with those species which can release two electrons for the formation of fourth bond, i. e., they react with nucleophilic reagents. Usually the order of reactivity of any chemical species is reversed to that of its stability. Therefore, the order of reactivity of ions is:

Primary (1°) > Secondary (2°) > Tertiary (3°)

Stability of Carbocation: 

The stability of alkyl carbocations is influenced by resonance, hyperconjugation and inductive effects. An alkyl group has an electron releasing inductive effect. An alkyl group attached to the positively charged carbon of a carbocation tends to release electrons towards that carbon. In doing so it reduces positive charge on the carbon. In other words~ the positive charge gets dispersed as the alkyl group becomes somewhat positively charged itself. This dispersal of the charge stabilizes the carbocation. The number of more alkyl groups, the greater the dispersal of positive charge and, therefore, more the stability of carbocation is observed.

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Stability decreases as +I decreases (dispersal of positive charge decreases).

3. Carbanions:

When a covalent bond, in which carbon is attached to a lesser electronegative atom, breaks up by heterolysis the atom leaves without taking away the bonding pair of electrons and thus the carbon atom gains a negative charge because of an extra electron.

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Such organic ions which contain a negatively charged carbon atom are called carbanions. These are named for parent alkyl groups and add the word “carbanion”. These are also termed as primary, secondary and tertiary depending on the nature of the carbon atom bearing the negative charge.

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Organic compounds which possess labile or acidic hydrogen have the tendency to produce carbanions as in the case of reactive methylene compounds which lose proton in presence of sodium ethoxide (C2H5ONa).

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Characteristics of Carbanions

a. The carbon carrying the negative charge contains 8 electrons in the valence shell, i. e., and complete octet.

b. They are highly reactive because in them the carbon carrying the negative charge is electron rich and can donate its non-bonding pair of electrons to some other group for sharing. Hence, carbanions behave as nucleophiles and are readily attacked by electrophiles.

c. The negatively charged carbon is in a state of sp3 hybridization. The hybrid orbitals are directed towards the comers of a tetrahedron. Three hybrid orbitals form single covalent bonds with other atoms while the fourth hybrid orbital contains a lone pair of electrons. Thus, carbanions have a pyramidal structure similar to NH3 molecules.

d. Carbanions are diamagnetic.

Reactions Involving Carbanions

1. Aldol condensation of aldehydes having α-H atoms.

2. Cannizzaro's reaction of aldehydes without α-H atoms.

3. Perkin's reaction involves the formation of carbanions as intermediate.

4. Knoevenagel reaction involves the formation of carbanions' as intermediate.

Structure of Carbanions

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Stability of carbanions: 

The stability of carbanions is influenced by resonance, inductive effect and s-character of orbitals. Groups having –I effect decrease in the stability groups having - I effect increase the stability of carbanions.

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4. Carbenes:

Carbenes : CH2 (or methylene)are highly reactive, short-lived, diagonal-in geometry and neutral species in which a carbon atom has six electrons in the outer shell (electron deficient), out of which two constitute a lone pair and two are shared. So, they are divalent carbon species containing two unpaired electrons and possess no charge. Thus, in short, carbenes are sp2 as well as sp hybridized, neutral, transitory reaction intermediate containing a carbon atom with two bonds and two electrons.

(A) By the photochemical decomposition or pyrolysis of aliphatic diazo compounds or ketenes.

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(B) By the action of a base on a suitable polyhalogen compound.

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So, carbenes are related to carbanions through the α-elimination reaction.

Reactions of carbenes:

(a) Addition to alkenes: Formation of cycloalkane derivatives, e.g.

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Addition reaction of singlet carbenes with alkenes or alkynes is known as chelotropic addition.

(b) Insertion reactions: Carbenes are also used in its insertion between the C-H bond e.g

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(c) Ring expansion reactions: These involve the addition of a halogen carbene across a double bond followed by rearrangement.

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Carbenes have important synthetic applications and are the reactive intermediates in some well-known reactions like Carbylamine reaction, Reimer-Tiemann reaction, Wittig reaction and Wolff rearrangement, etc.

5. Nitrenes or Imidogenes

Nitrenes are the organic species having the general formula, R-:N:. These are the nitrogen similar to carbenes and neutral univalent nitrogen intermediates (with one bond and two non-bonded electron pairs, i.e., -N). These are defined as 'the electron deficient species in which nitrogen has a sextet of electrons (six electrons in the outermost shell).

They are highly reactive and act as strong electrophiles as they need a pair of electrons to complete the octet. The parent species is N-:H: (known as nitrene or imidogene or azene or imine). It is difficult to be formed because it tends to polymerise to (NH) n as soon as it is formed. Hence, substituted nitrenes have received much attention.

Nitrenes can exist in the singlet and triplet states just as in the case of carbenes with one of the covalent bonds replaced by nitrogen lone pair. The triplet state is the ground state and most nitrenes exist in this state. In general nitrenes obey Hund's rule and are ground state triplet with two degenerate sp-orbitals containing a single electron each.

Formation of Nitrenes

(a) The simplest nitrene (:N:-H) is formed when hydrazoic acid (N3H) is irradiated with UV light in aromatic solvents which forms a small amount of primary aromatic amines.

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(b) Alkyl, aryl and acyl nitrenes may be prepared by the photolysis of alkyl azides, alkyl isocyanates or acyl azides respectively.

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Applications

Nitrenes, have important synthetic applications particularly acyl nitrene has been proposed as possible intermediate in the Hofmann, Curtius and Lossen rearrangements.

6. Benzyne (1,2-Dehydrobenzeile or Aryne)

Benzyne is a neutral, highly reactive reaction intermediate in which the aromatic character has not been markedly disturbed. It is formed as an intermediate during nucleophilic substitution of aromatic compounds. It contains Carbon-Carbon triple bond in the benzene ring involving the formation of a new weaker C-C bond by sideways overlapping of Sp2 –hybridized orbitals of two adjacent carbon atoms.