Carbocations and Carbanions are some of the most important topics in General Organic Chemistry for Jee Mains & Advance.
Carbocations are the key intermediates in several reactions and particularly in nucleophilic substitution reactions.
Structure of Carbocations : Generally, in the carbocations the positively charged carbon atom is bonded to three other atoms and has no nonbonding electrons. It is sp2 hybridized with a planar structure and bond angles of about 120°. There is a vacant unhybridized p orbital which in the case of CH3+ lies perpendicular to the plane of C—H bonds.
Stability of Carbocations: There is an increase in carbocation stability with additional alkyl substitution. Thus one finds that addition of HX to three typical olefins decreases in the order (CH3)2C=CH2 greater CH3—CH = CH2 greater CH2 = CH2. This is due to the relative stabilities of the carbocations formed in the rate determining step which in turn follows from the fact that the stability is increased by the electron releasing methyl group (+I), three such groups being more effective than two, and two more effective than one.
Stability of carbocations 3°greater 2° greater 1° greater CH3+
Electron release : Disperses charge, stabilizes ion.
Further, any structural feature which tends to reduce the electron deficiency at the tricoordinate carbon stabilizes the carbocation. Thus when the positive carbon is in conjugation with a double bond, the stability is more. This is so, because due to resonance the positive charge is spread over two atoms instead of being concentrated on only one. This explains the stability associated with the allylic cation. The benzylic cations are stable, since one can draw canonical forms as for allylic cations.
The benzyl cation stability is affected by the presence of substituents on the ring. Electron donating p-methoxy and p-amino groups stabilize the carbocation by 14 and 26 kcal/mole, respectively. The electron withdrawing groups like p-nitro destabilize by 20 kcal/mol.
A hetero atom with an unshared pair of electrons when present adjacent to the cationic centre strongly stabilizes the carbocation. The methoxymethyl cation has been obtained as a stable solid CH3O+CH2SbF6-. Cyclopropylmethyl cations are even more stable than the benzyl cations. This special stability is a result of conjugation between the bent orbitals of the cyclopropyl ring and the vacant p orbital of the cationic carbon. That the carbocations are planar is shown by the fact that these are difficult or impossible to form at bridgeheads, where they cannot be planar.
The stability order of carbocation is explained by hyperconjugation. In vinyl cations
(CH2 = C+H), resonance stability lacks completely and these therefore are very much less stable.
Structure of Carbanions: A carbanion possesses an unshared pair of electron and thus represents a base. The best likely description is that the central carbon atom is sp3 hybridized with the unshared pair occupying one apex of the tetrahedron. Carbanions would thus have pyramidal structures similar to those of amines. It is believed that carbanions undergo a rapid interconversion between two pyramidal forms.
There is evidence for the sp3 nature of the central carbon and for its tetrahedral structure. At bridgehead a carbon does not undergo reactions in which it must be converted to a carbocation. However, the reactions which involve carbanions at such centres take place with ease, and stable bridgehead carbanions are known. In case this structure is correct and if all three R groups on a carbanions are different, the carbanion should be chiral. All reactions therefore, which involve the formation of chiral carbanion should give retention of configuration. However, this never happens and has been explained due to an umbrella effect as in amines. Thus the unshared pair and the central carbon rapidly oscillate from one side of the plane to the other.
Stability and Generation of Carbanions: The Grignard reagent is the best known member of a broad class of substances, called organometallic compounds where carbon is bonded to a metal lithium, potassium sodium, zinc, mercury, lead, thallium – almost any metal known. Whatever the metal it is less electronegative than carbon, and the carbon metal bond like the one in the Grignard reagent highly polar. Although the organic group is not a full-fledged carbanion – an anion in which carbon carries negative charge, it however, has carbanion character. Or organometallic compounds can serve as a source from which carbon is readily transferred with its electrons. On treatment with a metal, in RX the direction of the original dipole moment is reversed (reverse polarization).
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