Introduction to General Organic Chemistry

BOND CLEAVAGE

Organic reactions take place through the formation of reactive intermediates. These intermediates are formed due to cleavage of covalent bonds. These intermediates can be

(i) free radicals like $\stackrel{\bullet }{C}{H}_3$

(ii) carbocation like $\stackrel{\oplus }{C}{H}_3$

(iii) carbanion like $\stackrel{-}{C}{H}_3$

Homolytic (symmetrical) cleavage

In which the two electrons shared in a s bond become unpaired as the bond is broken.

The species formed are called free radicals

(i) They are electrically neutral.

(ii) They are extremely reactive.

Their stability is in the order of

Benzylic and allylic free radicals are resonance stabilized hence are more stable than alkyl free radicals.

Thus greater the stability easier will be formation of the species. $\stackrel{\bullet }{C}{H}_3$(Methyl radical) is

sp² – hydridized (bearing three $\sigma$ $C-H$ bonds and singly occupied p – orbital) with HCH angle 120⁰ and three $C-H$ bonds coplanar. Thus when a methyl radical is formed in the homolytic cleavage of CH₃­ - X bond, the carbon undergoes a geometric change from tetrahedral to planar and rehybridisation from sp³ to sp².

Heterolytic (unsymmetrical) cleavage

When a covalent bond joining two atoms A and B breaks in such a way that both the electrons of the covalent bond (i.e. shared pair) are taken away by one of the bonded atoms, the mode of bond cleavage is called heterolytic cleavage. Heterolytic cleavage is usually indicated by a curved arrow which denotes a two electron displacement. For example

As shown above heterolytic fission results in the formation of charged species, i.e. cations and anions. It usually occurs in polar covalent bonds and is favoured by polar solvents.

In the formation of carbocation, we also find that sp³ hybridised carbon (in CH₃ $-$X) changes to sp² hybridised carbon.

An organic ion with a pair of available electrons and a negative charge on the central carbon atom is called carbanion and stability is in order

Electron attracting group ($-$CN, > C = O) increases stability and electron – releasing group

($-$ CH₃ etc) decreases stability of carbanion.

Benzyl carbanion is again stabilized by resonance.

REACTION INTERMEDIATES

Most of organic reactions occurs through the involvement of certain chemical species. These are generally short lived (10^-6 seconds to a few seconds) and highly reactive and hence can not be isolated. These short lived highly reactive chemical species. Through which the majority of the organic reactions occur are called reactive intermediates. These intermediates are detected by spectroscopic methods or trapped chemically or their presence is confirmed by indirect evidence. On the other hand, synthetic intermediate are stable products which are prepared isolated and purified and subsequently used as starting materials in a synthetic sequence.

Carbocations (Earlier Called As Carbonium Ions)

Carbocations are the key intermediates in several reactions and particularly in nucleophilic substitution reactions and electrophilic addition reaction.

(a) Structure:

Generally in the carbocations the positively charged carbon atom is bonded to three others atoms and has no nonbonding electrons. It is sp² hybridized with a planer structure and bond angles are of about 120⁰. There is a vacant unhybridised p orbital which (e.g in the case of $\stackrel{+}{C}{H}_3)$ lies perpendicular to the plane of $C-H$bonds.

(b) Stability:

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 > $C{H}_3-CH$= 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 3⁰ > 2⁰ > 1⁰ > $\stackrel{+}{C}{H}_3$

Electron release: Disperses charge, stabilizasion.

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, due to resonance the positive charge is spread over two atoms instead of being concentrated only on one. This explains the stability associated with the allylic cations. The benzylic cations are stable, since one can draw canonical forms as for allylic.

The benzyl cation stability is affected by the presence of substituents on the ring. Electron donating p – methoxy and p – amino group stabilize. The carbocation by 14 and 26 kcal/mole, respectively. The electron withdrawing groups like e.g, p – nitro destabilize by 20 kcal/mol.

A heteroatom with an unshared pair of electrons when present adjacent to the cationic centre strongly stabilizes the carbocation. The methoxy methyl cation has been obtained as a stable solid $\text{C}{\text{H}}_{\text{3}}\text{O}\stackrel{\text{+}}{C}{\text{H}}_{\text{2}}\text{Sb}{\stackrel{\text{-}}{F}}_{\text{6}}$cylopropylmethyl 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. The carbocations are planar is shown by the fact these are difficult or impossible to form at bridgeheads, where they can not be planar.

The stability order of carbocation is explained by hyperconjugation. In vinyl cations$(C{H}_2=\stackrel{+}{C}H)$, resonance stability lacks completely and therefore are very much less stable.

Stability $\propto$ hyperconjugated structures $\propto$ number of $\alpha$ hydrogen.

Carbanions

Chemical species bearing a negative charge on carbon and possessing eight electrons in its valence shell are called carbonions. These are produced by heterolylic cleavage of covalent bonds in which the shared pair of electrons remain with the carbon atom.

(a) Structure:

A carbanion posses 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. Carbonions 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 bridgeheads carbon does not undergo reaction in which it is converted to a carbocation. However, the reactions which involve carbanions at such centre take place with ease, and stable bridgehead carbanion are known. In case this structure is correct and if all three R groups on a carbanion are different, the 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.

(b) Stability and Generation:

The Grignard regent 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 is 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 organometallic compounds can serve as a source form 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)

c) Properties:

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 is enough basic to remove a proton from ammonia.

Illustration 1.

Solution:

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 defected 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 centre. 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 bridge. This shows that pyramidal geometry for radicals is also possible and that free radicals need to be planar

Stability

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 shows that 19 kcal/mol less energy is needed to form the benzyl radicals from toluene than the formation of methyl radical from methane. The triphenyl methyl type radicals are no doubt stalbilized by resonance, however the major cause of their stability is the steric hindrance to dimerization.

$\Delta$H = +85 kcal

Ease of formation of alkyl free radicals, benzyl > 3⁰ > 2⁰ > 1⁰ > $\stackrel{+}{C}{H}_3$ > Vinyl

Illustration 2: Alkenes undergo electrophilic addition reaction and benzene undergoes electrophilic substitution whereas both proceeds through carbocation intermediate. Explain.

Solution: $\pi$ electrons are available in case of alkanes whereas they are delocalized in case of benzene. After attack of electrophile a stable delocalized carboncation is formed on benzene ring. Whereas a carbocation which can rearrange is formed by addition of electrophile on alkane.

(C) Both are equally stable

(D) None