Fundamentals of Organic chemistry

Factors influencing electron availability: - (electron displacement effect or electronic effect)

Those effects, which are produced due to the movement of electron from one position to other, so that polarities are developed on the atoms, are known as electron displacement effects. The covalent bond, which has been considered as an ideal covalent bond, may attack by an attacking reagent, which may be negatively or positively charged. For reaction to take place on the covalent bond, it should posses oppositely charged centers. The reactant molecules although as a whole is electrically neutral, it must develop polarity on its carbon. Thus any factor that influence the relative availability of electrons in particular bonds or at particular atoms, in a compound might be expected to affect very considerably its reactivity towards a particular reagent. It is well known that a positive species always tends to attack the electron rich center and vice versa.

The electron displacement in molecule in turn may be due to certain effects some of which are permanent and other may be temporary (time variable effects). Such important effects are discussed in this chapter.

Permanent effect: -

Inductive effect: -

When the carbon atom is joined to a more electronegative atom (like Cl, O or N) by single covalent sigma bond than sigma electrons moves slightly towards more electronegative atom so that more electron rich acquire partial negative charge while carbon atom having electron deficiency acquires partial positive charge. Such type of electron displacement, which takes place due to the different in electro negativity of atoms, is called inductive effect.

Inductive effect may be defined as a type of electronic effect in which partial polarities are developed on the atoms joined by single covalent bond due to the slight displacement of sigma electron caused by different  in electronegativity of atoms.

 

 Characteristic of inductive effects: -

i)      It is represented by symbol I and indicated by arrow (®), pointing towards the more electronegative atom i.e. towards the direction of displacement of electron.

ii)   It is generally occurs in single covalent bond and it involves the displacement of electrons.

iii) Since it is always occurs in single covalent bond. It is permanent effect.

iv)  Due to inductive effect partial polarities are developed on the atoms due to slight displacement of sigma electrons.

v) The inductive effect can be transmitted along with carbon chain. Inductive effect decreases with increase in distance from the sources (i.e. more electronegative atom). The inductive effect may also be defined as type of electronic effects, which involves the successive polarization of one bond by another polar bond.

vi) For the measuring of inductive effect, hydrogen is taken as standard element and is supposed to be having zero effect i.e. neither repels nor withdraws the electrons.

vii) Atoms or groups having higher electron attracting power then hydrogen are called electron withdrawing or –I groups.

              E.g. –CN. –COOH, - CN, -NO₂, -C₆H₅ etc.

viii) Atoms or groups of having lower electron attracting power then hydrogen is called electron releasing groups or + I group for eg . –CH₃, -C₂H₅, -OR, etc.

Inductive effect may be negative or positive, It is –I when electronic displacement take place toward an atom or away from the carbon. It is +I when electronic displacement takes places away from the atom or towards the carbon atom. 

Effect of inductive effect:-

As it is permanent effect in organic compounds plays great role in its reactivity and other properties.

a)      Strength of acid and base:--I effect increases the strength of acid and vice versa.

e.g. Chloroacetic acid is stronger then acetic acid.

The chlorine atom is electron-withdrawing constituent. i.e. it has a –I effect. If it is substituted for one of the hydrogen atom of the CH₃— radical in Ethanoic acid, it brings about the electron displacements as shown

Similar displacements occur in the chloroethanoate anion. In this case the inductive effect of the chlorine atom increases the partial positive charge on the hydrogen atom of the —OH group. While it decreases the partial negative charges on the oxygen atoms of the chloroethanoate anion. The ionization of the acid is thus made easier and the recombination of ions more difficult. Hence Chloroacetic acid is a stronger acid than acetic acid.

 

Here H has fewer electrons deficient due to the present of +I group (-CH₃). The proton is difficult to abstract by base. So that lower tendency to donate proton thus weak acid.

+I effect increases the strength of base.

E.g. alkyl amine is stronger base than ammonia.

All amines are basic; they are stronger bases than ammonia. The basic character of amines is due to the availability of lone pair of electrons, on nitrogen atom, for protonation. Since the alkyl groups are electron releasing (+I effect) compared to hydrogen, It increases the availability of electrons for protonation. The greater the number of electrons releasing alkyl groups (+I effect) the one pair will became more available for protonation. The basicity of the amines wills therefore increases. The introduction of one methyl group into ammonia strengthens the base the second methyl group further strengthens the base. How ever the introduction of third methyl group decreases the strength of the base. E.g.

                (CH₃)₂NH > CH₃ NH₂> (CH₃)₃NH >NH₃

The introduction of third methyl group increases crowding (steric effect). This steric effect retards the protonation of nitrogen and weakens the base.

                                   

The alkyl group is electron-releasing groups. So alkyl group releases electron towards nitrogen. Thus nitrogen acquires higher tendencies to donate electron and exhibits more basic nature.

The hydrogen doesn't releases the electron towards nitrogen atom so that nitrogen became comparatively less electron rich. Thus it acquires lower tendencies to donate electrons pair, which is weak base.

b) Reactivity of alkyl halides: - More reactivity of alkyl halide than alkanes can be explained on the basis of inductive effect. More the number of alkyl group attached with the halide carbon more will be the polarity of C-Cl bond in alkyl halide thus more reactive.

Among three types of alkyl halides, tertiary alkyl halides consist of three alkyl groups (+I effect) attached with the halide carbon. Thus the carbon chlorine bond in tertiary alkyl halides is more polar than other alkyl halides.

The reactivity order of three types of alkyl halides is as follows.

	H3C—Cl < (H3C)2CHCl  < (H3C)3CCl

 

While the primary and secondary alkyl halides undergo hydrolysis by alkali through SN₂ mechanism, the hydrolysis of tertiary halides proceeds through SN₁ mechanism. The ionization of tertiary halides is facilitated due to the +I effect of the three-alkyl groups attached to the carbon carrying the halogen atom.

 

Mesomeric effect, conjugative effect or Resonance effect: -

The effect takes palace in unsaturated and conjugated system via their Pi- orbital. It is a permanent effect. Consider an example of carbonyl group whose all of the properties can neither satisfactorily be represented by the classical formula (I) not by polar structure (II).

 

The actual structure   is somewhat in between the two forms I and II, known as the resonance hybrid III.

                                                      

Hence the carbonyl group is present in compound with the conjugated system the polarization of electrons is further transmitted through pi- electrons. This type of displacement of electrons may also be caused by the presence of an atom having at least one lone pair of electron in the conjugation with a conjugated system.

This type of resonance may occur in between lone pair of electron and pi-bond.

     Or between positive charge and pi-bond.

 

The mesomeric effects like inductive effects are permanent polarizations and always operate in non-reacting molecules like inductive effect. It also affects the physical properties of the molecules.

Characteristic of mesomeric effects: -

1)                  It is represented by symbol M and indicated by (→) arrow pointing towards the direction of movement of electrons.

2)                  It is generally occurs in conjugated system

3)                  It is permanent effects.

4)                  In this the electron pairs is transferred completely with the result full positive and negative charges   are created.

5)                  It can also me transmitted from one end to the other of quite large molecules provided the conjugation

Mesomeric effect may be +ve M or –ve M. It is +M when the transference of electron pair is away from the atom and –M when transference of electron pair towards the atom.

                           

+M effect is the shown by the atoms or groups of atoms containing lone pair of electrons pair attached with conjugated system. e.g. Halogen, —OR, —OH, —NH₂, —NR₂, —SR and -M effect is shown by groups such as:- >C=O, —NO₂, —CN, —SO₃H etc. when attached to conjugated system.

There are several compounds, which cannot be represented by a single structure. Such compounds are represented by a set of hypothetical structures. The real structure of such compounds is represented by an intermediate of all such hypothetical structures. The true structure is known as resonance hybrid. The other hypothetical structures are known as resonating or canonical or contributing structures and the phenomenon is mesomerism or resonance. Thus resonance may be defined on the phenomenon in which two or more structures involving indicial position of atoms but different by the position of electrons and canonical structures of a system may be defined as any set of hypothetical structure which are sufficient to define all the possible electron distribution. Similarly resonance hybrid can be defined as the actual structure of all different possible structures that can be written for the molecules with out violation the rules of covalence maxima for the atom.

e.g. Benzene can not be represented by a single structures but is said to be a resonance hybrid of following resonating structures or canonical structure.  

 

The resonance defines the stability of molecules. It is found that greater the number of canonical or contributing or resonating structures greater will be its contribution in the formation of resonance hybrid, more will be the stability of resonance hybrid.