Fundamental concepts in Organic Reaction in Mechanism

FUNDAMENTAL CONCEPTS IN ORGANIC REACTION MECHANISM

 

In an organic reaction, the organic molecule (also referred as a substrate) reacts with an appropriate attacking reagent and leads to the formation of one or more intermediate(s) and finally product(s)
The general reaction is depicted as follows:

 

A sequential account of each step, describing details of electron movement, energetics during bond cleavage and bond formation, and the rates of transformation of reactants into products (kinetics) is referred to as reaction mechanism.

Fission of a Covalent Bond

A covalent bond can get cleaved either by: (i) heterolytic cleavage, or by (ii) homolytic cleavage.

 

Homolytic cleavage

In this, one of the electrons of the shared pair in a covalent bond goes with each of the bonded atoms. the neutral chemical species thus formed, is called free radical. Generally, homolytic fission takes place in non-polar, covalent molecules in the presence of sunlight or high temperature.

 

Free radicals are highly reactive. Neutral and electron deficient species.

Heterolytic cleavage

In this, the bond breaks in such a fashion that the shared pair of electrons goes with one of the fragments.

Carbon bearing a positive charge is called carbocation and carbon bearing negative charge is called carbanion.

Heterolytic fission generally takes place in polar covalent molecules but in non-polar molecules, it takes place in the presence of catalyst like AiCI3 (anhy.), FeCl3 (anhy.) etc.

 

Nucleophiles and electrophiles

 

Nucleophiles

These are electron rich species i.e, behave as Lewis bases. These attack at electron deficient area. The following species behave as nucleophiles:

·         All anions e.g.,

·         Lewis bases e.g.,

 

·         Benzene, alkenes etc.

Nucleophilicity order is

 

In case of same nucleophilic site, nucleophilicity parallels basicity i.e., as the basicity increases, nucleophilicity also increases.

If nucleophilic sites (or attacking atoms) are different nucleophilicity varies inversely with electronegativity)

 

Electrophiles

These are electron deficient species i.e., behave as Lewis acids. The following species behave as electrophiles:

·         All non-metal cations and metal cations which have vacant d-orbitals.

 

·         Lewis acids (incomplete octet) e.g., BF3, FeCl3 (anhydrous), AlCl3 (anhydrous), etc.

·         Non-metal (acidic) oxides e.g., CO2, SO2 etc.

Electron movement in organic reaction

All organic reactions can be understood by following the electron movements, i.e. the electron redistribution during the reaction. The electron movement depends on the nature of the substrate, reagent and the prevailing conditions. The flow of electrons is represented by curved arrows which show how electrons move as shown in the figure. These electron movements result in breaking or formation of a bond (sigma or pi bond). The movement of single electron is indicated by a half -headed curved arrows.

There are three types of electron movement viz.,

·           lone pair becomes a bonding pair.

·           bonding pair becomes a lone pair

·           a bond breaks and becomes another bond.

Type 1: A lone pair to a bonding pair

 

Type 2: A bonding pair to a lone pair

 

Type 3: A bonding pair to an another bonding pair

 

Electron Displacement Effects in covalent compounds

Some of the properties of organic molecules such as stability, reactivity, basicity etc., are affected by the displacement of electrons that takes place in its covalent bonds. This movement can be influenced by either the atoms/groups present in close proximity to the bond or when a reagent approaches a molecule. The displacement effects can either be permanent or a temporary. In certain cases, the electron displacement due to an atom or a substituent group present in the molecule cause a permanent polarisation of the bond and it leads to fission of the bond under suitable conditions. The electron displacements are catagorised into inductive effect (I), resonance effect (R), electromeric effect (E) and hyper conjugation.

 

Inductive effect

Inductive effect is defined as the change in the polarisation of a covalent bond due to the presence of adjacent bonds, atoms or groups in the molecule. This is a permanent phenomenon.

Let us explain the inductive effect by considering ethane and ethylchloride as examples. The C-C bond in ethane is non polar while the C-C bond in ethyl chloride is polar. We know that chlorine is more electronegative than carbon, and hence it attracts the shared pair of electron between C-Cl in ethyl chloride towards itself. This develops a slight negative charge on chlorine and a slight positive charge on carbon to which chlorine is attached. To compensate it, the C1 draws the shared pair of electron between itself and C2. This polarisation effect is called inductive effect. This effect is greatest for the adjacent bonds, but they also be felt farther away. However, the magnitude of the charge separation decreases rapidly, as we move away from C1 and is observed maximum for 2 carbons and almost insignificant after 4 bonds from the active group.

 

It is important to note that the inductive effect does not transfer electrons from one atom to another but the displacement effect is permanent. The inductive effect represents the ability of a particular atom or a group to either withdraw or donate electron density to the attached carbon. Based on this ability the substituents are classified as +I groups and -I groups. Their ability to release or withdraw the electron through sigma covalent bond is called +I effect and -I effect respectively.

Highly electronegative atoms and atoms of groups which are carry a positive charge are electron withdrawing or -I group

Example: -F, -Cl, -COOH, -NO2, NH2

Higher the electronegativity of the substitutent, greater is the -I effect. The order of the –I effect of some groups are given below.

NH3+> NO2> CN > SO3H > CHO > CO > COOH > COCl > CONH2> F > Cl > Br > I > OH > OR > NH2> C6H5> H

Highly electropositive atoms and atoms are groups which carry a negative charge are electron donating or +I groups.

Example. Alkali metals, alkyl groups such as methyl, ethyl, negatively charged groups such as CH3, C2H5O, COO etc

Lesser the electronegativity of the elements, greater is the +I effect. The relative order of +I effect of some alkyl groups is given below

–C(CH3)3> –CH(CH3)2>–CH2CH3>–CH3

 

Resonance structure

 There are many organic molecules whose behaviour cannot be explained by a single Lewis structure. An example is that of 

 

 

Benzene

benzene. Its cyclic structure containing alternating C–C single and C=C double bonds shown is inadequate for explaining its characteristic properties.

 

As per the above representation, benzene should exhibit two different bond lengths, due to C–C single and C=C double bonds. However, as determined experimentally benzene has a uniform C–C bond distances of 139 pm, a value intermediate between the C–C single(154 pm) and C=C double (134 pm) bonds. Thus, the structure of benzene cannot be represented adequately by the above structure. Further, benzene can be represented equally well by the energetically identical structures I and II.

 

Therefore, according to the resonance theory the actual structure of benzene cannot be adequately represented by any of these structures, rather it is a hybrid of the two structures (I and II) called resonance structures. The resonance structures (canonical structures or contributing structures) are hypothetical and individually do not represent any real molecule. They contribute to the actual structure in proportion to their stability.

 Another example of resonance is provided by nitromethane (CH3NO2) which can be represented by two Lewis structures, (I and II). There are two types of N-O bonds in these structures.

 

However, it is known that the two N–O bonds of nitromethane are of the same length (intermediate between a N–O single bond and a N=O double bond). The actual structure of nitromethane is therefore a resonance hybrid of the two canonical forms I and II.

The energy of actual structure of the molecule (the resonance hybrid) is lower than that of any of the canonical structures. The difference in energy between the actual structure and the lowest energy resonance structure is called the resonance stabilisation energy or simply the resonance energy. The more the number of important contributing structures, the more is the resonance energy. Resonance is particularly important when the contributing structures are equivalent in energy.

The following rules are applied while writing resonance structures:

The resonance structures have (i) the same positions of nuclei and (ii) the same number of unpaired electrons. Among the resonance structures, the one which has more number of covalent bonds, all the atoms with octet of electrons (except hydrogen which has a duplet), less separation of opposite charges, (a negative charge if any on more electronegative atom, a positive charge if any on more electropositive atom) and more dispersal of charge, is more stable than others.

 

Resonance Effect

The resonance is a chemical phenomenon which is observed in certain organic compounds possessing double bonds at a suitable position. Certain organic compounds can be represented by more than one structure and they differ only in the position of bonding and lone pair of electrons. Such structures are called resonance structures (canonical structures) and this phenomenon is called resonance. This phenomenon is also called mesomerism or mesomeric effect.

For example, the structure of aromatic compounds such as benzene and conjugated systems like 1,3-butadiene cannot be represented by a single structure, and their observed properties can be explained on the basis of a resonance hybrid.

In 1,3 buta diene, it is expected that the bond between C1-C2 and C3 –C4 should be shorter than that of C2-C3, but the observed bond lengths are of same. This property cannot be explained by a simple structure in which two π bonds localised between C1-C2 and C3 –C4. Actually the π electrons are delocalised as shown below.



These resonating structures are called canonical forms and the actual structure lies between these three resonating structures, and is called a resonance hybrid. The resonance hybrid is represented as below.

 

Similar to the other electron displacement effect, mesomeric effect is also classified into positive mesomeric effect (+M or +R) and negative mesomeric effect (-M of -R) based on the nature of the functional group present adjacent to the multiple bond.

 

Positive Resonance Effect

Positive resonance effect occurs, when the electrons move away from substituent attached to the conjugated system. It occurs, if the electron releasing substituents are attached to the conjugated system. In such cases, the attached group has a tendency to release electrons through resonance. These electron releasing groups are usually denoted as +R or +M groups.

Examples : -OH, -SH, -OR,-SR, -NH2, -O-etc...

This effect in aniline is shown as :

 

Negative Resonance Effect

Negative resonance effect occurs, when the electrons move towards the substituent attached to the conjugated system. It occurs if the electron withdrawing substituents are attached to the conjugated system. 

In such cases, the attached group has a tendency to withdraw electrons through resonance. These electron withdrawing groups are usually denoted as -R or -M groups. Examples : NO2, >C=O, -COOH,-C≡N etc

For example in nitrobenzene this electron displacement can be depicted as :

 

Electromeric Effect

Electromeric is a temporary effect which operates in unsaturated compounds (containing >C=C<, >C=O, etc...) in the presence of an attacking reagent.

Let us consider two different compounds (i) compounds containing carbonyl group (>C=O) and (ii) unsaturated compounds such as alkenes (>C=C< ).

When a nucleophile approaches the carbonyl compound, the π electrons between C and O is instantaneously shifted to the more electronegative oxygen. This makes the carbon electron deficient and thus facilitating the formation of a new bond between the incoming nucleophile and the carbonyl carbon atom.

 

On the other hand when an electrophile such as H+ approaches an alkene molecule, the π electrons are instantaneously shifted to the electrophile and a new bond is formed between carbon and hydrogen. This makes the other carbon electron deficient and hence it acquires a positive charge.

 

The electromeric effect, is denoted as E effect. Like the inductive effect, the electromeric effect is also classified as +E and -E based on the direction in which the pair of electron is transfered to form a new bond with the attacking agent.

 

Positive Electromeric Effect

When the π electron is transferred towards the attacking reagent, it is called + E (positive electromeric) effect.

 

The addition of H+ to alkene as shown above is an example of +E effect.

 

Negative Electromeric Effect

When the π electron is transfered away from the attacking reagent, it is called, -E (negative electromeric) effect

 

The attack of CN- on a carbonyl carbon, as shown above, is an example of -E effect.

 

Hyperconjugation

The delocalisation of electrons of σ bond is called as hyper conjugation. It is a special stabilising effect that results due to the interaction of electrons of a σ-bond (usually C-H or C-C) with the adjacent, empty non-bonding p-orbital or an anti-bonding σ* or π*-orbitals resulting in an extended molecular orbital. Unlike electromeric effect, hyper conjugation is a permanent effect.

It requires an α-CH group or a lone pair on atom like N, O adjacent to a π bond (sp2 hybrid carbon). It occurs by the overlapping of the σ-bonding orbital or the orbital containing a lone pair with the adjacent π-orbital or p-orbital.

 

To understand hyperconjugation effect, let us take an example of (ethyl cation) in which the positively charged carbon atom has an empty p orbital. One of the C-H bonds of the methyl group can align in the plane of this empty p orbital and the electrons constituting the C-H bond in plane with this p orbital can then be delocalised into the empty p orbital as depicted in Fig.

 

This type of overlap stabilises the carbocation because electron density from the adjacent σ bond helps in dispersing the positive charge.

 

Example 1:

In propene, the σ-electrons of C-H bond of methyl group can be delocalised into the π-orbital of doubly bonded carbon as represented below.

 

In the above structure the sigma bond is involved in resonance and breaks in order to supply electrons for delocalisation giving rise to 3 new canonical forms. In the contributing canonical structures: (II), (III) & (IV) of propene, there is no bond between an α-carbon and one of the hydrogen atoms. Hence the hyperconjugation is also known as “no bond resonance” or “Baker-Nathan effect”. The structures (II), (III) & (IV) are polar in nature.

 Example 2:

Hyper conjugation effect is also observed when atoms / groups having lone pair of electrons are attached by a single bond, and in conjugation with a π bond. The lone pair of electrons enters into resonance and displaces π electrons resulting in more than one structure

 

Example 3:

When electronegative atoms or group of atoms are in conjugation with a π -bond,they pull π - electrons from the multiple bond.

 

In case of carbocations, greater the number of alkyl groups attached to the carbon bearing positive charge, greater is number of the hyper conjugate structure. thus the stability of various carbocations decreases in the order

3º Carbocation > 2 º Carbocation > 1 º Carbocation

 

Types of Organic Reactions and mechanism

Organic compounds undergo many number of reactions, however in actual sense we can fit all those reactions into the below mentioned six categories.

•     Substitution reactions

•     Addition reactions

•     Elimination reactions

•     Oxidation and reduction reactions

•     Rearrangement reactions

 

1. Substitution reaction (Displacement reaction)

 

In this reaction an atom or a group of atoms attached to a carbon atom is replaced by a new atom or a group of atoms. Based on the nature of the attacking reagent, this reactions can be classified as

i. Nucleophilic substitution

ii. Electophilic substitution

iii. Free radical substitution 

i. Nucelophilic substituion:

This reaction can be represented as

 

Here Y– is the incoming nucleophile or and attacking species and x– is the leaving group.

Example: Hydrolysis of alkyl halides

 

Aliphatic nucleophilic substitution reactions take places either by SN1 or SN2 mechanism.

Types of nucleophilic substitution reactions

Depending upon substrate, nucleophilic substitution reactions are classified into

1.     SN1 reactions

2.     SNreactions

3.     SNI reactions

4.     SNAr reaction

 

ii. Electrophilic Substitution

 

Example: Nitration of Benzene

 

Types of electrophilic substitution reactions

Depending upon the type of substrate, electrophilic substitution reactions are classified as:

1.     Aliphatic electrophilic substitution reactions

2.     Aromatic electrophilic substitution reactions

 

iii. Free radical substitution

In a free radical substitution reaction, a free radical attack a substrate and replace the leaving group. In such reactions, attacking species and leaving species both are free radicals. They are also named chain reactions. They proceed in the following three steps:

  • Initiation
  • Chain propagation
  • Chain termination

 

2. Addition reactions

 

It is a characteristic reaction of an unsaturated compound (compounds containing C-C localised double or triple bond). In this reaction two molecules combine to give a single product. Like substitution this reaction also can be classified as nucleophilic, electrophilic and freeradical addition reactions depending the type of reagent which initiates the reaction. During the addition reaction the hydridisation of the substrate changes (from sp2 → sp3 in the addition reaction of alkenes or sp → sp2 in the addition reaction of alkynes) as only one bond breaks and two new bonds are formed.

 

 

Types of addition reactions

Addition reactions are classified into four different classes

1.     Electrophilic addition reactions

2.     Nucleophilic addition reactions

3.     Free radical addition reactions

4.     Cyclic addition reactions  

Electrophilic Addition reaction

A general electrophilic addition reaction can be represented as below.

Brominatin of alkene to give bromo alkane is an example for this reaction.

Nucleophilic addition reaction

 

Example: addition of HCN to acetaldehyde

Free radical addition Reaction:

A General freeradical addition reaction can be represented as below.

 

 

In the above reaction, Benzoyl peroxide acts as a radical initiator. The mechanism involves free radicals.

Cyclic addition reactions

 

Cyclic addition reactions are the single steps addition reactions (simultaneous addition). In these reactions, the initial attack is not on the one-carbon of double or triple bond but both carbon atoms are simultaneously attacked. This leads to a cyclic transition state.

 

3. Elimination reactions:

In this reaction two substituents are eliminated from the molecule, and a new C-C double bond is formed between the carbon atoms to which the eliminated atoms/groups are previously attached. Elimination reaction is always accompanied with change in hybridisation.

Example: 

n-Propyl bromide on reaction with alcoholic KOH gives propene. In this reaction hydrogen and Br are eliminated.

 

Types of elimination reactions

    • 𝜶 Elimination reactions
    • 𝜷 Elimination reactions
    • 𝜸 Elimination reactions
    • Conjugate Elimination reactions
    • Pyrolytic Elimination reactions
    • Extrusion reactions

𝜶 Elimination reactions

In 𝜶 elimination reactions both the groups are removed from the same atom. This results in the formation of carbene in the case of carbon and nitrene in the case of nitrogen.

 

𝜷 Elimination reactions

In 𝜷 elimination reactions, two groups are removed from adjacent carbon atoms. One is from 𝜶 carbon and the second is from 𝞫 carbon. This is the most common type of elimination reaction.

𝜷 elimination reactions can proceed in a single step (E2) as well as in two steps (E1).

 


Types of 𝞫 Elimination reactions

𝞫 elimination reactions are subdivided into three types

  • E1 elimination reactions (unimolecular)
  • E2 elimination reactions (bimolecular)
  • E1cB elimination reactions

𝜸 Elimination reactions

𝜸 elimination reactions make a three-membered ring structure. Here one atom is removed from 𝜶 carbon and the other is removed from 𝜸 carbon. Three-member rings are very unstable due to angle strain.

4. Oxidation and reduction reactions:

Many oxidation and reduction reactions of organic compounds fall into one of the four types of reaction that we already discussed but others do not. Most of the oxidation reaction of organic compounds involves gain of oxygen or loss of hydrogen Reduction involves gain of hydrogen and loss of oxygen.

Examples:

 

 

5. Rearrangement reactions

The reactions in which an atom or group of atoms moves from one atom to another atom within a molecule are called rearrangement reactions. They result in the formation of structural isomers of original molecules. Connectivities of groups changes within the molecule but most of the migrations are from one atom to an adjacent atom and are called 1,2 shifts. However, there can be some migrations over a long distance

 

Types of rearrangement reactions

  • Nucleophilic rearrangement reactions
  • Electrophilic rearrangement reactions
  • Free radical rearrangement reactions

Nucleophilic rearrangement reactions

Nucleophilic rearrangement reactions are the most common rearrangement reactions and are also named as anionotropic rearrangements. In these rearrangements, the migrating group is a nucleophile. It will move with its electron pair.

Electrophilic rearrangement reactions

Nucleophilic rearrangement reactions are also named as cationotropic rearrangements or prototropic rearrangements. In these rearrangements, the migrating group is an electrophile. It will move without its electron pair.

Free radical rearrangement reactions

Free radical rearrangement reactions are not common. In these rearrangements, the migrating group is a free radical. It will move with just one electron.

 

Functional Group inter conversion

 

Organic synthesis involves functional group inter conversions. A particular functional group can be converted into other functional group by reacting it with suitable reagents. For example: The carboxylic acid group (–COOH) presents in organic acids can be transformed to a variety of other functional group such as – CH2–OH, – CONH2, – COCl by treating the acid with LiAlH4, NH3 and SOCl2 respectively.

Some of the important functional group interconversions of Organic compounds are summarised in the below mentioned Flow chart.

 

 

Stability of carbocation

Chemical species bearing a positive charge on carbon and carrying six electrons in its valence shell are called carbocation.

These are formed by heterolytic cleavage of the covalent bonds in which the leaving group takes away with it the shared pair of electrons.

 

Carbocation are also formed during dehydration of alcohols in presence of conc. H2SOat 430-440 K.

 

Classification

Carbocation are classified as primary (1°), secondary (2°) and tertiary (3°) according as the positive charge is present on a primary, secondary and tertiary carbon atom.

 

Stability of carbocation

The stability of carbocation follows the order : 3° > 2° > 1°> methyl

Reason:

Inductive effect:

An alkyl group has +I effect .When an alkyl group is attached to a positively charged carbon atom of a carbocation, it tends to release electrons towards that carbon.In doing so, it reduces the positive charge on the carbon.In other words, the positive charge gets dispersed and the alkyl group becomes somewhat positively charged.This dispersal of the positive charge stabilizes the carbocation.

More the number of alkyl groups on the carbon atom carrying the +ve charge, greater would be the dispersal of the charge and hence more stable would be the carbocation.Thus the stability of the carbocation decreases in the order :

3° > 2° > 1°

 

Stability decreases as +I effect of the alkyl group decreases.

Resonance Effect

Carbocations in which the positively charged carbon atom is attached to a double bond or a benzene ring are stabilized by resonance.

 

 

More the number of phenyl groups, greater is the stability

For Example: (C6H5)3C+ > (C6H5)2CH> (C6H5)3CH2+

The presence of electron-donating group such as -CH3, -OCH3, -OH in the benzene ring increases the stability while the presence of electron withdrawing group such as -NO2, -CN, -COOR, -Cl  decreases the stability of the carbocation.

Hyperconjugation Effect

tert-Butyl carbocation has 9 α-hydrogens and hence nine hyperconjugation structures can be written for it:

 

For isopropyl carbocation, six hyperconjugation structures can be written as :

 

Three hyperconjugation structures can be written for ethyl carbocation:

 

For CH3+ carbocation, no hyperconjugation occurs.

Reactivity

Carbocations are highly reactive chemical species since the carbon atom carrying the positive charge has only six electrons in its valence shell and thus has a strong tendency to complete its octet.

The order of reactivity of any chemical species is reverse that of its stability.

The order of reactivity of carbocations follows the sequence : 1° > 2°>3°

Orbital Structure

 

The carbocation are planar chemical species.

The carbon atom carrying the positive charge is sp2 hybridized.

The three sp2– hybridized orbitals of this carbon form three σ-bonds with monovalent atoms or groups which lie in a plane and are inclined to one another at an angle of 120°.

The unhybridized 2p-orbital which is perpendicular to the plane of the three σ-bonds is however empty.

 

Stability of carboanion

Carbanions

Chemical species bearing a negative charge on carbon and possessing eight electrons in its valence shell are called carbanions.

These are produced by heterolytic cleavage of covalent bonds in which the shared pair of electrons remains with the carbon atom.

 

 

Classification of Carbanion

Carbanions are also classified as primary (1°), secondary (2°) and tertiary (3°) according as the negative charge is present on a primary, secondary and tertiary carbon atom.

 

Stability of Carbanions

The stability of carbanions follow the order : CH3‾ >1°>2°>3°

Inductive effect : An alkyl group has +I effect. When an alkyl group is attached to a negatively charged carbon atom of the carbanions, it tends to release electrons towards the carbon .In doing so, it increases the intensity of the negative charge on the carbon and thus destabilizes the carbanion.

 

More the number of alkyl group on the carbon atom carrying the negative charge, more would be the intensity of the negative charge on the carbon atom and hence less stable is the carbanion.

Stability decreases in the order: CH3‾ >1°>2°>3°

 

Resonance Effect

Allyl and benzyl carbanions are stabilized by resonance.

 

 

More the number of phenyl group, greater is the stability

 

The presence of electron withdrawing group such as -NO2, -CN, -COOR,-Cl in the benzene ring tend to disperse the negative charge and hence increases the stability of the carbanion while the presence of electron donating group such as -CH3, -OCH3, -OH intend to intensify the negative charge and hence decreases the stability of carbanion.

s-character

Stability of the carbanion increases with the increase in s-character of the carbon carrying negative charge.

 

Reactivity of Carbanions

The reactivity of carbanions is reverse of the stability i.e. 3° > 2° >1° > CH3

Orbital structure of Carbanion

 

 

The structure of alkyl carbanion is usually pyramidal.

The carbon atom carrying the negative charge is sp3 hybridized.

Three of the four sp3 hybridized orbitals form three σ-bonds with monovalent atoms or groups while the fourth sp3 -orbital contain lone pair of electrons.

The carbanions which are stabilized by resonance are planar.In these carbanions, the carbon atom carrying the negative charge is sp2 hybridized.

Stability of free radicaL

 

Free Radical

free radical may be defined as an atom or a group having an odd or unpaired electron. These are generally produced by the homolytic cleavage of a covalent bond.

 

 

Classification of Free Radicals

Free radicals are also classified as primary (1°) , secondary (2°) and tertiary (3°) according as the carbon carrying the unpaired electron is primary, secondary and tertiary.

 

Stability of Free Radicals

The order of stability of free radicals is the same as that of carbocations i.e. 3° >2° >1°

This order of stability can be explained on the basis of hyperconjugation.

Greater the number of alkyl groups attached to the carbon atom carrying the odd electrons, greater is the delocalization of the odd electrons and hence more stable is the alkyl free radical.

 

Allyl and benzyl free radicals are stabilized by resonance.

 

Greater the number of phenyl groups more stable is the free radical.

Free radicals are also very short-lived highly reactive chemical species because of the strong tendency of the carbon atom carrying the odd electron to acquire one more electron to complete its octet.

Orbital structure of Free Radicals

 

Alkyl free radicals are planar chemical species. In free radicals, the unhybridized p-orbital contains the odd electron.

Like carbanions , free radicals can also assume pyramidal shape since the energy difference  between planar and pyramidal shape is not much.