Reactive Intermediates

 

Table of Content

Carbocations Carbanions Free Radicals Carbenes Arenium Ions Benzynes

Synthetic intermediate are stable products which are prepared, isolated and purified and subsequently used as starting materials in a synthetic sequence. Reactive intermediate, on the other hand, are short  lived and their importance lies in the assignment of reaction mechanisms on the pathway from the starting substrate to stable products. These reactive intermediates are not isolated, but are detected by spectroscopic methods, or trapped chemically or their presence is confirmed by indirect evidence.

• Carbocations

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>CH₃—CH = CH₂ > 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°> 2° > 1° > 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. 

• Carbanions

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). 

Acetylene is ionized on treatment with amide ion in liquid ammonia to form a sodium acetylide; this has a little covalent character and may be regarded as a true carbanion. This property is used in making substituted alkynes. The stability order of carbanions points to their high electron density. Alkyl groups and other electron – donating groups in fact destabilize a carbanion. The order of stability is the opposite of that for carbocations and  free radicals, which are electron deficient and are stabilized by alkyl groups. Based on this stability order it is easy to understand that carbanions that occur as  intermediates in organic reactions are almost always bonded to stabilizing  groups. An imporant method of preparation thus involves a loss of proton from a haloform to afford a stabilized carbanion. Another factor which leads to stability is resonance e.g., a carbonyl group stabilizes an adjacent carbanion via resonance e.g., a carbonyl group stabilizes an adjacent carbanion via overlap of its pi bond with the nonbonding electrons of the carbanion. Carbanions derived from carbonyl compounds are often called enolate anions. Among the other functional groups which exert a strong stabilizing effect on carbanions are nitro and cyano groups. The second row elements, particularly phosphorus and sulphur stabilize the adjacent carbanions. A very important nucleophilic carbon species constitute the phosphorus and sulphur ylide. The preparation of ylide is a two stage process, each state of which belongs to a familiar reaction type: nucleophilic attack on an alkyl halide, and abstraction of a proton by a base.

The phosphorus ylide have hybrid structure, and it is the negative charge on carbon i.e, the carbanion character of ylide which is responsible for their  characteristic reactions. The sulphur atoms stabilize carbanions. When a double or triple bond is located a to the carbanionic carbon the ion is stabilized by resonance as in the case of benzylic type carbanions .

Properties of Carbenions: Carbanions are nucleophilic and basic and in this behaviour these are similar to amines, since the carbanion has a negative charge on its carbon, to make it a powerful base and a stronger nucleophile than an amine. Consequently a carbanion is enough basic to remove a proton from ammonia.

• Free Radicals

Structure and Geometry of Free Radicals: A free radical is a species which has one or more unpaired electrons. In the species where all electrons are paired the total magnetic moment is zero. In radicals, however, since there are one or more unpaired electrons, there is a net magnetic moment and the radicals as a result are paramagnetic. Free radicals are usually detected by electron spin resonance, which is also termed electron paramagnetic resonance. Simple alkyl radicals have a planar (trigonal) structure, i.e., these have sp² bonding with the odd electron in a p orbital. The pyramidal structure is another possibility when the bonding may be sp³ and the odd electron is in an sp³ orbital. The planar structure is in keeping with loss of activity when a free radical is generated at a chiral center. Thus, a planar radical will be attacked at either face after its formation with equal probability to give enantiomers. Unlike carbocations, the free radicals can be generated at bridgehead shows that pyramidal geometry for radicals is also possible and that free radicals need to be planar.

Stability of Free Radicals: As in the case of carbocation, the stability of free radicals is tertiary > secondary > primary and is explained on the basis of hyperconjugation. The stabilizing effects in allylic radicals and benzyl radicals is due to vinyl and phenyl groups in terms of resonance structures. Bond  dissociation energies shown that 19 kcal / mol  less energy is needed  to form the benzyl radical from toluene than the formation of methyl radical from methane. The triphenyl methyl type radicals are no doubt stabilized by resonance, however, the major cause of their stability is the steric  hindrance to dimerization. 

• Carbenes

Carbenes are neutral intermediates having bivalent carbon, in which a carbon atom is covalently bonded to two other groups and has two valency electrons distributed between two non bonding orbitals. When the two electrons are spin paired the carbene is a singlet, if the spins of the electrons are parallel it is a triplet.

Structure of Carbenes : A singlet carbene is thought to possess a bent sp2 hybrid structure in which the paired electrons occupy the vacant sp2 orbital. A triplet carbene can be either bent sp2 hybrid with an electron in each unoccupied orbital, or a linear sp hybrid with an electron in each of the unoccupied p-orbital. It has however, been shown that several carbenes are in a non-linear triplet ground state. However, the dihalogenocarbenes and carbenes with oxygen, nitrogen and sulphur atoms attached to the bivalent carbon, exist probably as singlets. The singlet and triplet state of a carbene display different chemical behaviour. Thus addition of singlet carbenes to olefinic double bond to form cyclopropane derivatives is much more stereoselective than addition of triplet carbenes.

Generation of Carbenes: Carbenes are obtained by thermal or photochemical decomposition of diazoalkanes. These can also be obtained by a-elimination of a hydrogen halide from a haloform with base, or of a halogen from a gem dihalide with a metal.

Reactions of Carbenes: These add to carbon double bonds and also to aromatic systems and in the later case the initial product rearranges to give ring enlargement products (a car-benoids –oranometallic or complexed intermediates which, while not free carbenes afford  products expected from carbenes are usually called carbenoids).

When a carbene is generated  in a three membered ring allenes are formed by rearrangement. However, a similar formation at a cyclopropylmethyl carbon gives ring expansion. Carbenes are also involved in Reimer —Tiemann reaction.

• Arenium Ions

A considerable amount of experimental evidence indicates that electrophiles attack the p system of benzene to form a delocalized non-aromatic carbocation known as arenium ion or sometimes a s complex CMR spectroscopic evidence is available in favour of  s complex.

• Benzynes

It is a reactive intermediate in some nucleophilic aromatic substitutions. It is a benzene with two hydrogen atoms removed. It is usually drawn with a highly strained triple bond in the six membered ring. Benzyne intermediate has been observed spectroscopically and trapped.

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