Organic compounds mainly contain covalent bonds.
The displacements of the electron pairs in these covalent bonds may take place either on their own or under the influence of other species.
Three types of electron displacements are generally noticed in the mechanism of organic reactions.
Inductive Effect
Whenever the withdrawing electron group (such as halogen) is attached to the end of a carbon chain, a permanent effect called inductive effect takes place.
To understand this, consider a chain of carbon atoms having Cl atom at one end:
C-C-C-C-Cl
Since chlorine (Cl) is more electronegative than C, the electron pair shared between C, and Cl is displaced towards the Cl atom, due its larger electronegativity. A partial negative charge (-) is thus acquired by Cl and C acquires a partial positive charge (+).
This process of electron displacement of electrons along the chain of carbon atoms is called inductive effect (or I-effect). This is a permanent effect and results due to the presence of a polar covalent bond at one end of the chain.
An arrow as shown generally represents this effect:
It may be noted that this effect decreases sharply as we move away from the atoms involved in the initial polar bond.
From the fourth atom onwards, the effect becomes negligible.
For example, NO2 >CN >COOH >F >Cl >Br >I>OH>OCH3 >C6H5 >HI
Atoms or groups of atoms, having smaller electron attracting power than hydrogen are referred to as having +I (electron repelling) effect.
Inductive effect plays very significant role in understanding reactions of organic chemistry. Carbon of methyl chloride, for example, acquires partially positive charge due to shift of electrons towards chlorine and provides a site for attack by a negatively charged species (say OH).
Electromeric Effect
This is a temporary effect and takes place between two atoms joined by a multiple bond, i.e., a double or triple bond. It occurs at the demand of the attacking reagent, and involves instantaneous transfer of a shared pair of electrons of the multiple bond to one of the linked atoms.
It is temporary in nature because the molecule acquires its original electronic condition upon removal of the attacking reagent.
This phenomenon of movement of electrons from one atom to another at the demand of attacking reagent in multibonded atoms is called electromeric effect, denoted as E effect.
The electromeric shift of electrons takes place only at the moment of reaction. Like the inductive effect, the electromeric effect is also classified as +E and -E:
When the transfer of electrons takes place towards the attacking reagent, it is called + E (positive electromeric) effect. For example,
When the transfer of electrons takes place away from the attacking reagent, it is called, -E (negative electromeric) effect. For example,
Resonance or Mesomeric Effect If two or more structures can be assigned to a molecule, none of which is capable of describing all the known properties of the compound, then the actual structure lies in between these structures and is known as resonance hybrid or an intermediate of these structures. The various structures written are called resonating structures and this phenomenon is called resonance. The phenomenon of resonance can be explained with the help of CO2 molecule. It has been represented by the structural formula: The formula satisfies the conventional valency requirements, but it fails to explain some experimental facts. The actual carbon oxygen bond in CO has been found to be 1.15 Å whereas the normal carbon-oxygen double bond is 1.22 Å and that for triple bond is 1.10 Å.
This means that carbon-oxygen bond in carbon dioxide is intermediate between a double and triple bond. To account for this, two other structures were proposed: and The actual molecule is said to be a resonance hybrid of these three structures: Benzene molecule is also regarded as a resonance hybrid of the following two structures: The resonating structures of carboxylic acid may be written as:
Types of Bond Fission
Organic reactions usually involve making and breaking of covalent bonds. Fission of bonds can take place in two ways:
Homolytic Fission
When the cleavage of covalent bond between two atoms takes place in a manner, which enables each atom to retain one electron of the shared pair, it is known as homolytic fission. This fission is symmetrical and leads to the formation of atoms or groups of atoms having unpaired electrons, called free radicals. The free radicals are denoted by putting dot over the symbol of atom or group of atoms. For example,
Heterolytic Fission
Heterolytic fission is unsymmetrical wherein one of the fragments takes both the electrons of the shared pair, leaving none on the other. This results into two charged particles as:
Reaction Intermediates
The species produced during cleavage of bonds are called reaction intermediates. The important reaction intermediates are:
Free Radical
A free radical may be defined as an atom or group of atoms having an unpaired electron. Free radicals are produced during the homolytic fission of a covalent bond.
Free radicals are very reactive as they have strong tendency to pair up their unpaired electron with another electron from wherever available. These pairs are very short lived and occur only as reaction intermediates during reactions.
For example, dissociation of chlorine gas in the presence of ultra-violet light produces chlorine free radicals:
The alkyl free radical may be obtained when free radical chlorine attacks methane.
Free radicals may be classified as primary, secondary or tertiary depending upon whether one, two or three carbon atoms are attached to the carbon atom carrying the odd electron:
Stability of free radicals
The order of stability of alkyl free radicals is: CH3 < 1o < 2o < 3o
This order of stability can easily be explained on the basis of hyperconjugation. Larger the number of alkyl groups, attached to the carbon atom carrying the odd electron, greater is the delocalisation of the odd electron and hence more stable is the free radical. Accordingly, the tertiary free radical with three alkyl groups attached to the carbon atom carrying the odd electron is more stable than the secondary free radical containing two alkyl groups and so on.
Structure of alkyl free radical
The carbon atom in alkyl free radicals involves sp2 hybridization. Therefore, it has a planar structure. Three hybrid orbitals are used in the formation of three -bonds with three H atoms or alkyl group. The unpaired electron is present in unhybridized p orbital.
Orbital structure of free radicals
Carbocation or Carbonium Ion
It is defined as a group of atoms, which contain positively charged carbon having only six electrons. A carbonium ion is obtained by heterolytic fission of a covalent bond involving carbon atom.
The carbocations are also classified as primary (1o), secondary (2o) or tertiary (3°) depending upon whether one, two or three carbon atoms are attached to carbon bearing the positive charge as:
Relative Stability of Carbocation
We know that methyl group is electron releasing i.e. has +I inductive effect. The alkyl group attached to positively charged carbon atom tends to release electrons towards carbon. This decreases the positive charge on the carbon atom but the carbon itself is somewhat positive. As a result, the positive charge on the carbon atom gets dispersed, giving it stability. Therefore, more the number of alkyl groups, the greater will be the dispersal of charge and therefore, more stable will be the carbocation.
Thus, tertiary carbocation in which there are three alkyl groups attached to positive carbon is more stable than a secondary carbocation with two alkyl groups.
Similarly, a secondary carbocation is more stable than a primary carbocation, which in turn is more stable than methyl carbocation:
Thus, the order of stability is: CH3+ < 1o < 2o < 3o
Other carbocations are benzyl carbocation, which is primary in nature but is highly stable because of resonance stabilization.
Triphenyl carbocation, Ph3C+ is the most stable because of resonance stabilization among three phenyl groups.
Structure of Carbocation The carbon atom in carbocation is sp2 hybridized. Since it involves sp2 hybridization, its structure is planar with a bond angle of 120o. The three sp hybrid orbitals form three -bonds with hydrogen or other carbon atoms of alkyl groups.
The unhybridized p orbital of carbon remains vacant.
Carbanion
A species containing a carbon atom carrying a negative charge may be defined as carbanion.
These are generated by the heterolytic fission of covalent bond involving carbon atom, where the atom linked to carbon goes away without the bonding electrons. This causes the carbon to acquire a negative charge.
For example, removal of hydrogen of methyl part of acetaldehyde molecule as H+ ion leaving both the electrons on carbon:
Carbanions are also very reactive species
Like carbocations, carbanions are also classified as primary (1°), secondary (2°) and tertiary (3°) depending upon whether one, two or three carbon atoms are attached to the carbon bearing negative charge as:
The order of stability of carbanions is reverse of that of carbocations and free radicals. CH3- > 1o > 2o > 3o
This order of stability is explained on the basis of the +I, inductive effect of the alkyl group. Alkyl groups have electron releasing tendency and cause an increase in the electron density on the negatively charged carbon atom. This creates unstabllity. Larger the number of alkyl groups attached to the negatively charged carbon atom, greater will be the electron density on the carbon atom and lower will be its stability.
Structure of carbanion The negatively charged carbon atom in carbanion is sp3 hybridized. Therefore, it has a tetrahedral structure. Three of the four sp3 hybridized orbitals form 3-bonds with hydrogen or carbon atom of the alkyl group. The fourth sp3 hybrid orbital contains the lone pair of electrons.
Tetrahedral structure of carbanion
Carbenes
Carbenes are reactive neutral species in which the carbon atom has six electrons in the valence shell, two of which are shared.
The simplest carbene is methylene (CH2). It is formed when diazomethane is decomposed by the action of light. It is very reactive.
These electrons may have anitparallel spins (singlet state) or parallel spins (triplet state)
Types of Attacking Reagents The attacking reagents are classified into three types: Free radicals These are very reactive species having unpaired electrons. Electrophiles Positively charged or neutral species, which are deficient of electrons and can accept a pair of electrons are called electrophiles. These are also called electron loving (philic) species. For example, H+, H3O+, Cl+, CH3+, NO2+ (Positively charged) AlCl3, BF3, SO3 (Neutral) Both Al and B act as electrophiles as they have total of six electrons i.e. two less than the octet, and so they try to complete their octets. These are also called as Lewis acids. Nucleophiles A nucleophile is a reagent containing an atom having unshared or lone pair of electrons. As a nucleophile is electron rich it seeks electron deficient sites i.e., nucleus (nucleus loving). According to Lewis concept of acids and bases, nucleophiles behave as Lewis bases. For example, NH3, H2O, ROH, ROR (neutral)