REACTION INTERMEDIATES

Introduction

Majority of organic reactions take place via the formations of intermediates.

These intermediates are of transitory existence and take part in the reaction as

soon as they are formed. Being very reactive, these cannot be isolated under

normal reaction conditions. However, their structures are established by indirect

means either chemically or spectroscopically, or sometimes by isolating them at

very low temperature. For a clear understanding of the reaction mechanism, a

thorough knowledge and understanding of the reaction intermediates is essential.

CARBOCATIONS

It is a common knowledge that in the heterolytic fission of a C-X bond in an

organic molecule, if X is more electronegative than carbon atom, the takes away

the bonding electron pair and becomes negatively changed (:X-).In this case an ion

bearing a positive charge isalso produced.

This positively changed species is a carbocation. For many these species were

called ‘carbonium’ ion’. However, now they are referred to as carbocations. A

carbocation is also defined as a carbon atom which is trivalent, contains an even

number of electrons and carries a positive chang. The carbon atom in a

carbocation is sp2 hybridized and it uses all its three hybridized orbitals for

forming bonds with other atoms. The remaining pz, orbital is empty and is

perpendicular to the plane of other three bonds. Acarbocation may be described

as plant (trigonal coplanar) with bond angle of 120.Because carbocation assumes

a planar structure, its formation is inhibited in compounds which do not permit

attainment of a planar geometry as in bridge head compounds.

Stability of carbocations

If an electron –releasing group such as alkyl group is resent adjacent to the

carbon atom bearing positive change, then the stability of the carbocation

increases. This explains why a tertiary carbocation is more stable then a

secondary carbocation which in turn is more stable than a primary carbocation.

Thus, ethyl carbocation(CH3CH2+)is more stable than methyl carbocation (CH3

+).

Two factors are deemed to play a role in this case:(i) electron-donating inductive

effect of alkyl group, the more alkyl groups attached to the C, the greater is the

stabilizing effect and (ii) by hyperconjugation some change delocalization occurs

between the p orbital of positively charged carbon and o bond of the b C-H.

However, the presence of electron attacting group the carbocation less stable.

Thus, O2NCH2CH2+ is less stable than CH3CH2CH2

+

The stability of a carbocation can aiso be explained by resonance. By resonance

the positive change on the carbocation. The more the canonical structures for a

carbocation, the more stable it wil be. The explains why benzyl carbocation

(C6H5CH2+)and carbocations are more stable than propyl carbocation (CH3CH2CH2

+)

The basic requirement for a carbocation to be stable is it should be planer, as it is

only in this configuration that effective delocalization can occur. Also on basis of

quantum mechanical calculations for simple alkyl carbocations it has been found

that the planer (sp2) configuration is more stable than the pyramidal

(sp3)configuration by about 84 KJ (20kcal)mol-1. The difficulty in the formation of

carbocations increases as the attainment of planarity is inhibited. As an

illustration, 1-bromotritycene is very resistance to SN1 attack. This is attributed to

the assume planar configuration of carbocation. The planr configuration of simple

carbications has been confirmed by NMR analysis and ir spectra.

Some carbocations are very stable and so can be isolated and studied. The

stability of such carbications is explained in terms of resonance, i.e, the positive

change on the carbon is distributed uniformly throughout a number of structures.

In other words a number of canonical strures are possible. The most common

example is of triarylmethyl carbocarion (e.g., triphenylmethyl carbocation).

In certain cases, the carbocations are so stable that their solid salts have been

isolated. For example, triphenylmethyl perchlorate exists as crystalline solid and

tropylium bromide has been isolated as a yellow solid. The tropylium bromide

stabilized by aromatization. The tropylium cation is planar 6 electrons like

benzene.

The order of some carbocations is viven below:

Tropylium > (c6H5)3c+ < c6H5+cH2 > ch2=Ch-c+H2

Tertiary butyl > lsopropyl > Ethyl > methyl

Carbocation caebocation carbocation carbocation

The stability of carbocations is increased due to or more of following reasons.

(i) When the positrive carbon is in conjugation with a double bond

the stability of carbocation is grater due to increased

delocalization of positive change, which spreads over two atoms

instead of being concentrated on one. Conjugated dienes in

concentrated sulfuric give stable allyice type carbocations.

(ii)The carbocation stability is also increased due to the presence of

heteroatom having an unshared pair of electrons, e.g., oxygen nitrogen or

halogen, adjacent to the cationic centre. Such carbocations are stabilized by

resonance.

The methoxymethyl cation is obtained as stable solid MeOCH2 +SbF4