goc

Electronegativity

Element	Symbol	Electronegativity
Hydrogen	H	2.20
Carbon	C	2.55
Nitrogen	N	3.04
Oxygen	O	3.44
Fluorine	F	3.98
Chlorine	Cl	3.16
Bromine	Br	2.96
Iodine	I	2.66
Phosphorus	P	2.19
Sulfur	S	2.58
Silicon	Si	1.90
Boron	B	2.04
Lithium	Li	0.98
Sodium	Na	0.93
Magnesium	Mg	1.31
Potassium	K	0.82
Calcium	Ca	1.00
Aluminum	Al	1.61
Tin	Sn	1.96
Lead	Pb	2.33
Iron	Fe	1.83
Copper	Cu	1.90
Zinc	Zn	1.65

Inductive Effect

• Displacement of sigma electron towards more electronegative atom of sigma bond.
• During this effect partial charges develop.
• EWG(electron withdrawing group) (-I effect) - the group which pulls electron towards itself, they reduce negative charge and increase positive charge
• EDG(electron donating group) (+I effect) - the group which pushes electron away from itself, they reduce positive charge and increase negative charge
• This effect travels and decreases with distance, it travels upto 3 carbon and then it ceases.
• More the difference between electronegativity of sigma bond, more will be magnitude of inductive effect.
• More the distance between EN element and carbon, less will be magnitude of inductive effect.
• More the EN of element, more will be it’s desire to take electron
• Order of -I effect in various groups $\text{ce}{-}^{+}NF3>{-}^{+}NR3>{-}^{+}NH3>-NO2>-CN-CHO$ $\text{ce}>COR(ketone)>COOH>F>Cl>Br>I>-OR$ $\text{ce}>OH>NH2>Phenyl>-CH=CH2>H$
• Order of +I effect in various groups $\text{ce}CH{2}^{-}>N{H}^{-}>O^{-}>CO{O}^{-}>tertalkylgroup$ $\text{ce}secondaryalkylgroup>primaryalkylgroup>alkyl$

Acidic order

$\text{ce}HA<=>H++A^{-}$

• More the stability of -ve ion, stronger will be the acid
• Less the charge, more will be stability of acidic species
• More the magnitude of -I effect, more will be acidic nature, less will be basic nature
• More the magnitude of +I effect, less will be acidic nature, more will be basic nature

Resonance

- pi bonds are weak in nature so their electrons move around alot, this is what causes resonance and many other such phenomenons

• Shifting/Displacement/Movement of $\pi$ electrons / lone pairs within the compound is called resonance.
• It is an intramolecular and hypothetical phenomenon
• All the resonating structures are imaginary structures, they are not actually present inside the molecules
• Hybrid structure is a combination of all the resonating structures of a compound and it is the actual(real) structure of the compound and is actually present
• The condition for resonance is presence of atleast 1 of the following conjugate systems(in that order)
  • Pi bond, Sigma bond, +ve charge
  • Pi bond, Sigma bond, lone pair
  • Pi bond, Sigma bond, free radical
  • Pi bond, Sigma bond, Pi bond
  • lone pair, Sigma bond, +ve charge
• Total charge on all resonating structures must be same
• In resonance, only electrons move around, not the atoms [difference between resonance and tautomerism]

Stability of Resonating Structures

There are 5 rules to follow in order to determine the stability of resonating structures

- These rules should be used in this order

1. Check which structure has more covalent bonds, more the number of covalent bonts, more will be stability of resonating structure

   Energy is required when breaking bonds but energy is released when making bonds

2. Neutral species are generally more stable than charged species
3. When -ve charge is on more EN atom, it is more stable than when it is on any other atom.

   it’s vice versa is also true, when +ve charge is on more EN atom, it is less stable than when it is on any other aton

4. For following type of structure $\text{ce}A--A+-A-\text{ or }\text{ce}A+-A--A+$
   • If two like charges are seprated far or two unlike charges are close, stability increases
   • If two unlike charges are seprated far or two like charges are close, stability decreases
5. Inductive effect also plays a huge rule in determining stability of resonating structures

Mesomeric Effect

NOTE

Conjugation is needed for mesomeric effect as well.

• If any group or atom (which is not carbon) donates|withdraws electron(through resonance) then that effect is called mesomeric effect
• so, any resonance effect between carbon atoms|groups does not classify under mesomeric effect
• all mesomeric effect are resonance but not all resonance are mesomeric effect
• if electron is donated in mesomeric effect, then it is called +M effect
• if electron is withdrawed in mesomeric effect, then it is called -M effect
• Mesomeric effect do not depend on distance, only inductive effect depend on distace
• this effect changes electron density
  • +M effect increases electron density, it increases spatially at ortho and para position, it is not applicable for meta position
  • -M effect decreases electron density, it decreases spatially at ortho and para position
• Mesomeric effect dominates over inductive effect
  • the only exception to this rule is halogen in which inductive effect dominate over mesomeric effect
  • Magnitude of mesomeric effect is more than that of inductive effect because it both takes|gives more electron than inductive effect
• More the electron density on the compound, more will be it’s tendancy to show electrophillic substituition reaction
• Less the electron density on the compound, more will be it’s tendancy to show neucleophillic substituition reaction

  Flourobenzene has more electron density than chlorobenzene

• Order of -M effect $\text{ce}-CH2+>-NO2>-CN>-CHO>-COR>-COOH$
• Order of +M effect $\text{ce}-CH{2}^{-}>N{H}^{-}>O^{-}>-NH2>-NHR>-NR2>-OH>-OR>-NHCOCH3>-COCH3$

Mesomeric effect with Benzene

• If any group X is attatched directly with a benzene ring and if X has lone pair electron or -ve charge then that group will donate electron and +M effect will take place
• If any group X attatched directly with a benzene ring, is also attatched to an even more electronegative atom|group Y, then Y will take electron from X which will result in -M effect