EQUIVALENT (DBE) OR INDEX OF HYDROGEN DEFICIENCY (IHD)
1.10.2 Carbonium Ion and Carbenium Ion or Carbocation
There are intermediates in which a carbon bears a positive charge, but the formal covalency of the carbon atom is fi ve rather than three. The simplest example is the methanonium ion, CH≈ 5. Such pentacoordinated positive ions are called carbonium ions. The intermediates in which there is positive charge at a carbon atom which is trivalent are called carbenium ions or carbocations. For example, methyl cation (CH )≈ 3 and ethyl cation (CH CH )3≈ 2 , etc.
[N.B. The methanonium ion CH≈ 5 has a three-centre two-electron bond. It is not known whether this ion is a transition state or a true intermediate, but an ion CH5≈ has been detected by mass spectroscopy.
(2) Carbanions
Carbanions are a group of reactive intermediates carrying a negative charge on carbon atom possessing eight electrons in its valence shell. For example, CH @ 3 (methyl carbanion) and CH CH3 @ 2 (ethyl carbanion), etc. They are represented by symbol R①. Their reactivity is due to the presence of formal negative charge on the carbon atom.
Generation: Carbanions are generated by heterolytic fi ssion of a bond to carbon in which the bonding electron pair remains with the carbon atom.
The principal methods of generating carbanions are as follows:
(i) By an abstraction of an acidic hydrogen alpha to an electron-withdrawing group:
Compounds containing acidic hydrogen attached to carbon alpha to —NO2, —CN or C == O groups produce carbanions when treated with a strong base. For example:
(ii) By abstraction of hydrogen from terminal alkynes using a strong base: Terminal alkynes being acidic produce carbanion when treated with strong bases. For example:
H N: + H—C2 ∫∫C—CH3 C∫∫C—CH3 + NH3
– –
(iii) By metal halogen exchange: When organic halogen compounds are treated with strongly electropositive metals like Li, Na, etc. in an inert solvent, carbanions are obtained in the form of organometallic compounds. For example:
ether
3 3
CH — Br 2Li+ ổổổặCH L i LiBr @ ≈ +
ether
3 3
Ph C — Cl 2Na+ ổổổặPH C Na NaCl @ ≈ +
ether
Ph — Br 2Li+ ổổổặPh Li @ ≈ +LiBr
(iv) By decomposition of carboxylate ions: When metal carboxylates are heated, they undergo decarboxylation to yield carbanions. For example:
(v) By addition of an anion to multiple bond containing electron-withdrawing groups:
Carbanions are obtained when an alkene containing electron-attracting group undergoes nucleophilic attack by an anion. For example:
(vi) By addition of electrons to an unsaturated system: Unsaturated compounds may accept electrons from electropositive metals to generate carbanions. For example, when cyclooctatraene is treated with metallic sodium, it is converted to cyclooctatetraenyl dianion which is a stable aromatic system containing (4n + 2)p electrons, where n = 2.
Nomenclature: In naming a carbanion, the word ‘anion’ is added to the name of the alkyl or aralkyl group. For example, CH , (CH ) CH and C H CH @ 3 3 2 @ 6 5 @ 2 are named as methyl anion, isopropyl anion and benzyl anion, respectively.
Classifi cation: Carbanions are classifi ed as primary (1°), secondary (2°) and tertiary (3°) on the basis of the number of carbon atoms (one, two or three) directly bonded to the negatively charged carbon atoms. For example, ethyl anion (CH CH )3 @ 2 is a primary, isopropyl anion (Me CH)2 @ is a secondary and tert-butyl anion Me C3≈ is a tertiary carbanion.
Methyl anion (CH ) @ 3 with one carbon atom is a special case.
Structure: In simple carbanions, the negatively charged central carbon atom is sp3 hybridized; it is surrounded by three bonding electron pairs and one unshared pair of electrons occupying an sp3 orbital. Therefore, a carbanion is expected to have the tetrahedral shape. However, the shape is not exactly that of a tetrahedron and in fact, it is found to have the pyramidal shape just like ammonia. Since the repulsion between the unshared pair and any bonding pair is greater than the repulsion between any two bonding pairs, therefore, the angle between two bonding pairs (i.e., between two sp3-s bonds) is slightly less than the normal tetrahedral value of 109.5° and for this reason, a carbanion appears to be shaped like a pyramid with the negative carbon at the apex and the three groups at the corners of a triangular base.
The central carbon atoms in resonance-stabilized carbanions are, however, sp2 hybridized and hence they are planar. This is due to the fact that planarity is an essential criterion for resonance to occur. Allyl anion, for example, is a planar carbanion.
The negative carbon atom of the following conjugated anion is, however, not resonance- stabilized because according to the Bredt’s rule a double bond at the bridgehead position cannot be formed in bridged bicyclic compounds with small rings and for this reason, this carbon is sp3-hybridized. Its shape is pyramidal.
Stability: Any factor that tends to delocalize the negative charge must increase the stability of carbanions while any factor that tends to localize or intensify the negative charge must decrease the stability of carbanions.
[W = Electron-withdrawing group; it disperses the negative charge and thus,
stabilizes the carbanion]
[D = Electron-donating group; it intensifi es the negative charge and hence destabilizes the carbanion]
The stability of carbanions follows the order: Methyl anion (CH ) RCH @ 3 > @ 2 (primary or 1°)
> R CH2 @ (secondary or 2°) > R C3 @ (tertiary or 3°) because an alkyl group destabilizes a carbanion. Carbanions are stabilized mainly by –I and –R effects. Functional groups in the a position stabilize carbanions in the following order: —NO2 > —COR > —COOR >
—CN ~ —CONH2 > —X > —H.
The structural features responsible for the increased stability of carbanions are as follows:
(a) s character of the anionic carbon atom: An s orbital being closer to the nucleus than the p orbital in a given main quantum level possesses lower energy. An electron pair in an orbital having large s character is, therefore, more tightly held by the nucleus and hence of lower energy than an electron pair in an orbital having relatively small s character. Thus, a carbanion in which the anionic carbon is sp- hybridized (50 percent s character) is more stable than a carbanion in which the anionic carbon is sp2 hybridized (33.33 percent s character), which in turn is more stable than a carbanion in which the negative carbon is sp3-hybridized (25 percent s character). Therefore, the order of decreasing stability of carbanions is
(b) Inductive electron withdrawal: Substituents possessing electron-withdrawing inductive effects (–I) stabilize a carbanion by dispersing or delocalizing the negative charge. Examples of some carbanions experiencing such stabilizing effect are as follows: