Inductive Effect: It is well known fact that a covalentbond is formed by the equal sharing of electrons between the two atoms.In case of a covalent bond of the two similar atoms, the electron pairof the bond occupies a central position between the two nuclei of theconcerned atoms. Such a covalent bond is known as non polar.
Incase of a covalent bond between the two dissimilar atoms the electronpair forming the s ?bond is never shared absolutely equal between thetwo atoms but is attracted a little more towards the moreelectronegative atom. For example, in the compound C-X, X-is moreelectronegative than C, The electron pair forming the C-X is some whatmore attracted towards the atom X with the result of its attains apartial negative charge(denoted by –d) while the carbon atom attains apartial positive charge denoted by +d).
Inductiveeffect may be defined as the permanent displacement of electron forminga covalent bond towards the more electronegative element or group.
For example:The inductive effect is always transmitted along a chain of carbonatoms in which one terminal carbon atom is joined to a chlorine atom.
Now since chlorine has a greater electron-affinity than carbon, the electron pair between C1 and Cl will be displaced from the middle to the chlorine atom with the result of a small negative charge and C1 a small positive charge. Now further since C1 is positively charged it will attract towards itself the electron pair forming the covalent bond between C1 and C2 with the result the C2 will also acquire a small positive charge, but the charge on C2 will be smaller than on C1 since the chlorine atom has been transmitted through C1 to C2. Similarly, C3 will also acquire a small positive charge, but again due to the above reason the charge on C3 will be smaller than on C2.This effect can be relayed still further although it would decrease inintensity considerably as distance from the source increases. In factit will be negligible even on C3 for all practical purposes.
Theinductive effect is represented by the symbol (arrow), the arrowpointing towards the more electronegative element or group of elements.Thus in the case of n-butyl chloride inductive effect may berepresented as below.
Thedecrease in the effect is denoted by using a greater number of sign d.Inductive effect decreases with the increase in distance from theelectronegative atom.
Electron withdrawing Group or –I Effect Group:
Theeffect is said to be –I if a group pulls the electrons from the carbonchain. Such a group is called Electron withdrawing group or –I effectgroup.
Due to –I effect (electron withdrawing nature) electron density decreases, hence
-basic nature is decreased
-acidic nature is increased
-Analcohol is a weak acid compared with a carboxylic acid. There isdifference in electro negativity between carbonyl carbon and oxygenwhich results in the formation of partial positive charge on thecarbonyl carbon which in turn induces a polarization of electrons inthe O-H bond away from hydrogen. This electron withdrawing inductiveeffect of the carbonyl group weakens the O-H bond and favors ionizationof a carboxylic acid compared with an alcohol.
-Electron with drawing substituents when present near the carboxyl group increases acidity of carboxylic acids.
-Chloroacid is stronger acid than acetic acid since Cl show –I effect,electron-density is decreased and O-H bond is weakened causingionization of (-COOH) to a greater extent than CH3COOH. Also inductive effect of the chlorine destabilized the acid and stabilizes the conjugate base.
-Electronwith drawing substituents when present near the carboxyl groupincreases acidity of carboxylic acids, as it goes away from the sourceelectron withdrawing tendency decreases; hence, acidic nature alsodecreases
-Multiple halogen substitution increases the acidity further increases.
Electron Releasing Group or +I Effect Group:
The effect is said to be +I if a group pushes the electron towards the carbon chain. Such group is called
Electron releasing group or +I effect group.
-Dueto electron releasing groups electron density is increased, hence basicnature is also increased and naturally acidic nature is decreased. Thusalkyl group are acid weakening.
Applications of Inductive Effect: The phenomenon of inductive effect is very important in organic chemistry as it explains several facts are:
1. Effect on Bond Length:Since the inductive effect leads to ionic character in the bond, theincrease in –I effect usually decrease in bond length. For example, thebond length between and halogen in alky halide decreases with theincrease in the inductive effect of the halogen atom.
2. Dipole Moment:Since the inductive effect leads to a dipolar character in themolecule, it develops some dipole moment in the molecule. Furtherdipole moment increases with the increase in the inductive effect.
3. Reactivity of Alkyl Halides:Alkyl halides, in general are more reactive than the correspondingalkanes, the reason being the presence of C-X bonding alkyl halides dueto which they undergo inductive effect, e.g., methyl chloride
Furthermore tertiary butyl chloride is more reactive than the methyl chloridebecause the +I effect of the three methyl groups enhance the –I effectof chlorine atom by supplying electron towards the tertiary carbon atom.
Thus the chlorine atom in t-butyl chloride can be very easily replaced by other atom.
Electromeric effect:
In the presence of electrophiles, there is complete transfer of p-electronsfrom one atom to other to produce temporary polarity on atoms joined bymultiple bonds; It is called electrometric effect.
Positive Electromeric Effect: When a p-electrontransfer takes place C to C (as in alkane, alkenes and alkynes), it iscalled positive electrometric effect (denoted by +E)
Negative Electromeric Effect: When pelectron transfer takes place to more electronegative atom (O, N, S)joined by multiple bonds, it is called negative electrometric effect(dented by -E).
Mesomeric Effect: Mesomeric effect may be defined as the permanent effect in which the pelectrons are transferred from a multiple bond to an atom; or from amultiple bond to a single covalent bond or from an atom with lone pairof electrons to the adjacent single covalent bond. In short, the effectcaused by the phenomenon of resonance or mesomerism in a molecule iscalled the resonance effect or mesomeric effect.
For example,
Butene and 1,3-butadiene differ not only in the number of p bonds,
But has also s bond and pbonds at alternate positions. This type of the system is calledconjugate system. Following are some examples of the conjugate systems:
In such systems, p electron shifting takes place consecutively giving permanent polarity on the chain. This type of p electron shift in the conjugate systems is called mesomeric effect or conjugate effect.
-The p electron gets delocalized as a result of mesomeric effect giving a number of resonance structures.
-Dueto mesomeric effect terminal carbon is almost as positive as firstcarbon. This is quite different from inductive effect due to whichcharge decreases as one move away from the source.
Positive Mesomeric Effect:A group or atom is said to have +M effect when the direction ofelectro-displacement is away from it. Such groups have lone-pair ofelectrons, and release the pair for conjugation with an attachedunsaturated (conjugated) system.
Examples executing +M effect are:
-This effect extends the degree of delocalization and imparts stability to the molecule.
-The +M effect of halogen atom in vinyl halide and aryl halide explains their low reactivity.
-Lone-pair of N in aniline get delocalized and thus basic nature of aniline is less than tat of NH3.
Negative Mesomeric effect (-M):If electron-withdrawing groups are attached to the conjugate system, pielectron displacement takes place towards such groups. (but away fromconjugate system). This is said to have –M effect.
-M effect is shown by groups such as
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