OBJECTIVES : Find the route of mechanism. We can able to predict the product. We can able to tell the property of the molecule. We can able to predict the stability. 2 TYPES : 1. Inductive effect(I) 2. Electromeric effect(E) 3. Resonance effect(R) 4. Hyperconjugation 3 Electronegativity plays a important role in inductive effect Loves electron and pull towards myself EN atoms N, O, X(halogens) 4 What is INDUCTIVE EFFECT? The process of electron shift along a chain of atoms due to the presence of electron withdrawing group(-I group) or electron donating group (+I group) is called inductive effect. (OR) The polarization of sigma bond due to electron withdrawing or electron donating effect of adjacents groups is called inductive effect. 5 The direction of displacement is shown by placing an Arrow head midway along the line presenting the sigma bond. This effect decrease as the distance from the electronegativity atom inreases. This is a permanent effect and is almost negligible beyond two carbon atom from the active group. 6 7 INDUCTIVE EFFECT PUSH PULL +I EFFECT -I EFFECT 8 -I effect : electron withdrawing groups -R3N+ > -NO2 > -SO2R > -CN > -COOH > -F > -Cl >- Br > -I > -OR > -COR > -OH > -C6H5 > -CH=CH2 > -H +I effect : electron donating groups (CH3)3C- > (CH3)2CH- > CH3CH2- > -CH3 > -D > -H 9 APPLICATION Stability of carbocation by +I effect of methyl group Acidity increased by presence of –I (group) effect 10 11 12 13 SUMMARY The inductive effect plays a vital role in deciding the acidity and basicity of a molecule Acidity increases with increasing the +I group Basicity increases with increasing the –I group > > 14 QUESTION 1 15 ELECTROMERIC EFFECT 16 ELECTROPHILE • ELECTRON LOVING SPECISES NULEOPHILE • NUTRON LOVING SPECISES 17 ELECTROMERIC EFFECT . Electrons of pi bond are loosely held (due to sideways overlap) and easily polarisable. therefore., when a compound having pi bond approached by a charged reagent (electrophile or nucleophile) the electrons of the bond are completely polaraised or displaced towards the one of the constituent atoms. 18 . This is a temporary effect operating in unsaturated compounds only. It involves complete transfer of pi electrons of multiple bond. 19 +E effect : When the transfer of pi electrons takes place towards the attaking reagent (electrophile).this effect is called +E effet. -E effect : When the transfer of pi electrons occurs away from the attacking reagent (nuleophile).this effect is called –E effect. 20 APPLICATION 1) Electrophilic addition reaction of unsaturation compounds the polarisation of the carbon – carbon double bond in the presence of attacking electrophile like H+ etc., electrophilic addition involving +E effect 2) Nucleophilic addition reation of carbonyl compounds the polarisation of the carbon – oxygen double bond in the presence of attacking nucleophile nucleophilic addition involving –E effect 3) Ring polarisation polarisation and the attack by electrophile may actually take plae concurrently in a concentrated mechanism. 21 INDUCTIVE EFFECT vs ELECTROMERIC EFFECT INDUCTIVE EFFECT ELECTROMERIC EFFECT It is a permanent effect involving only the It is a temporary effect involving a complete displacement of electrons transfer of pi electrons. This has no such requirements It is operate only under the influence of a suitable attacking reagent This requires only a polar covalent bond This effect is shown only by compounds containing one or more multiple bonds Electron transfer shown by arrow head midway Electron transfer shown by curved arrow 22 DELOCALIZATION : In conjugated system pi electrons are distributed over whole of the molecule embracing all the carbon atom and do not remain localized. this phenomenon is called delocalization. Deloalization of electrons does not only take place in alternate single bond and double bond in carbon atoms(1,3 buta diene and benzene) but in may also occur in a conjugated system involving carbon and other than carbon. Delocalization by pi – pi overlap Delocalization by pi – p orbital overlap 24 RESONANCE OR MESOMERIC EFFECT Resaonance effect describes the polarity produced in a molecule by the interation between the lone pair and a pi bond or the interaction of two pi bonds in adjustant atoms. The resonance(canonical) effect is a chemical phenomenon observed in compounds characteristic of double bonds of organic compounds. 25 It is usually found in molecules with conjugated double bonds or in molecules having at least one lone pair and one double bond. Understanding resonance is important in understanding stability of the compound and its energy state Aromatic compounds are especially stable due to this bond delocalization and resonance effect. 26 TYPES -M or -R effect : It is shown by substituents or groups that withdraw electrons by delocalization mechanism from rest of the molecule and are denoted by -M or -R. The electron density on rest of the molecular entity is decreased due to this effect. E.g. -NO2, Carbony group (C=O), -C≡N, -COOH, -SO3H etc. +M or +R effect : The groups show positive mesomeric effect when they release electrons to the rest of the molecule by delocalization. These groups are denoted by +M or +R. Due to this effect, the electron density on rest of the molecular entity is increased. E.g. -OH, -OR, -SH, -SR, -NH2, -NR2 etc. 27 +R effect -R effect 28 RESONANCE THEORY 1. Whenever a molecule can be represented by two or more structure that differ from one another only in the arrangement of electrons and not In atomi nuclei.the molecule is said to be a RESONANCE HYBRID of all these structure. 1. Each contributing structure must have the same number of unpaired electrons. 2. The greater the stability of a contributing structure, greater is its contribution to the hybrid. 3. Resonance is most important when the contributing structures are equivalent. 4. The resonance hybrid is more stable than any of the contributing structures. 5. The smaller the difference in the energy contents (or stabilities) of the contributing structures, the greater the resonance energy of that hybrid. 6. The polar contributing structure involving distinct charges are less stable than those which do not involve any charges. 8. The larger number of bonds in contributing structure, the greater the stability of that structure. 29 RESONANCE ENERGY Resonance hybrid is more stable than any single discrete structure.this can be explaind in terms of RESONANCE ENERGY, This can be quantised from the heat of hydrogenation released on addition of hydrogen to double bond. the difference between the observed and calculated value (energy released) is called resonance energy. Minimum energy content is more stable than high energy content. 30 APPLICATION 1.Aromatic compound and conjugated diene stability due to resonance effect 2.Triphenyl methyl cation : This is the only one of the most stable carbocations known.its stability is due to extensive deloalization of +ve charge by three benzene ring. 3.Active methylene compound: Malonic ester and acetoacetic ester have more acidic than alpha-hydrogens of carbonyl compounds Due to stability of forming carbanion. 31 QUESTN…? Why carboxylic acid do not show the reaction of the carbonyl group? ANS : The double bond character of the carbon –oxygen linkage in carboxylic acis is considerably dereased due to resonace. 32 HYPERCONJUGATION 33 The delocalization of σ-electrons or lone pair of electrons into adjacent π-orbital or p-orbital is called hyperconjugation. It occurs due to overlapping of σ-bonding orbital or the orbital containing a lone pair with adjacent π-orbital or p-orbital. It is also known as "no bond resonance" or "Baker-Nathan effect“ There must be an α-CH group or a lone pair on atom adjacent to sp2 hybrid (double bond)carbon or other atoms like nitrogen, oxygen etc. E.g., Alkenes, alkyl carbocations, nitro compounds with α- hydrogen 34 The displacement of σ-electrons towards the multiple bond occurs when there are hydrogens on the α-carbon (which is adjacent to the multiple bond). This results in the polarization of the multiple bond . E.g. In propene, the σ-electrons of C-H bond of methyl group can be delocalized into the π-orbital of doubly bonded carbon as represented below. In the same way, the other hydrogens on the methyl group also participate in the hyperconjugation. This is possible due to free rotation of C-C bond so that the other C-H bonds can also participate in the hyperconjugation. Thus the propene molecule can show following resonance structures, which confer stability to it. 35 In the contributing structures: (II), (III) & (IV) of propene, there is NO bond between an α-carbon and one of the hydrogen atom. Hence the hyperconjugation is also known as "no bond resonance". These equivalent contributing structures i.e., (II), (III) & (IV) are also polar in nature and hence are responsible for the dipole moment of propene (0.36 D). The C-C bond lengths in propene are equal to 1.48. Its value is in between 1.54 (of C- C) and 1.34 (of C=C). It is because the bond order of C-C bonds is approximately 1.5 due to hyperconjugation. This type of hyperconjugation is also referred to as sacrificial hyperconjugation since one bond is missing. 36 APPLICATION : 1) Stability of alkenes: A general rule is that, the stability of alkenes increases with increase in the number of alkyl groups (containing hydrogens) on the double bond. It is due to increase in the number of contributing no bond resonance structures. For example, 2-butene is more stable than 1-butane 2)Stability of carbocation : This is because, the σ-electrons of the α-C-H bond in ethyl group are delocalized into the empty p-orbital of the positive carbon . This also gives no bond resonance. 37 3) Reactivity & orientation of electrophilic substitution on benzene ring : In Toluene, the methyl group releases electrons towards the benzene ring partly due to inductive effect and mainly due to hyperconjugation.