Basis of Organic Reactions
Charged Intermediates Neutral Intermediates
Carbocation Free radical
Polar effect: Polar effect is the displacement of electron in a covalent bond. This displacement may be temporary or permanent.
INDUCTIVE or I EFFECT: Relatively permanent displacement of electron towards electronegative atom in a sigma bonded molecule. It induces permanent dipole moment in a molecule.
Reactivity may be decided by other factors but possibility of a reaction is mostly decided by I effect.
It is of two types:
+I effect: Electron releasing effect. Alkyl groups show this effect.
It plays an important role in determining the basic strength of a molecule. According to Lewis concept, a base is a species which can donate a pair of electrons. +I effect increases the electron density and enhances the basic strength. But Steric hindrance is a more important factor in determining the basic strength.
e.g. in case of amines, the order of the basic strength is
2o Amine > 3o Amine > 1o Amine > NH3
If only +I effect is taken into consideration then 3o amine should have the highest basic strength because it has three electron releasing alkyl groups but the three alkyl groups also hinder the electron donation.
Electron donating groups decrease acidity.
-I effect: Electron withdrawing effect. It is shown by a large no. of electronegative atoms or groups. E.g.-
-I effect depends upon
(1.) Oxidation state: Increases with increase in oxidation state.
e.g. NO2 will have greater –I effect than NH2.
O.N. of N=+5 O.N. of N= -2
(2.) s character : Increases with increase in s character. E.g.
e.g. –F > -Cl > -Br > -I
-OH will have greater inductive effect than -OR because +I effect of R (release of electrons) will lower the –I effect.
Acidic Strength & -I effect
e.g. Order of acid strength
Problem: Arrange the following in the order of decreasing acid strength.
Role & Importance of resonance effect
Acidic and basic character
Acidity of a compound is decided by the stability of its conjugate base. Delocalization of negative charge after the removal of H+ stabilizes a conjugate base. More stabilized a conjugate base, stronger is the acid.
In ethoxide, the negative charge is localized on one oxygen atom, whilst in acetate the charge is delocalized over two oxygens and in methane sulfonate, it is spread over three oxygens.
e.g. In case of Haloarenes
Problem: Arrange the following in order of decreasing acid strength.
Water, ethanol, phenol
Ans: Phenol > water > ethanol
Phenoxide ion is stabilized by resonance. RO- is destabilized because of electron releasing effect of alkyl group.
Note: In a reaction of alcohol and water, water acts as proton donor.
Sodium ethoxide is a stronger base than Sodium hydroxide due to the similar reason. (RO- is a stronger base than OH-.)
A carbocation is a trivalent positively charged sp2 hybrid planar carbon having vacant p orbital( perpendicular to the plane of the carbocation). It is diamagnetic and a Lewis acid.
Methods of formation
Factors of stability
A carbocation is a charged particle. So, it is more stabilised in a polar solvent.
As it is a charged intermediate. So, any factor that decreases positive charge or delocalises the charge will stabilise and these factors are
Methods of preparation
Factors of stability
As it is a charged particle, it is more stabilised in polar solvent than non-polar solvent. Order of stability in
Polar protic solvent > polar aprotic solvent > non-polar
Inductive effect: - I effect increases stability while + I effect decreases it.
S character: Greater the s character, greater is the stability.
Free Radical Intermediate
It is trivalent, planar (sp2) paramagnetic and free from any charge (neutral) formed by homolytic cleavage of a covalent bond.
Stability is just like stability of carbocations except the solvent factor.
CH3 < 1o < 2o < 3o
They are more stable in non-polar solvent. Unlike carbocation and carbanion, solvents do not affect the reaction of free radicals.
Free radical reactions take place in presence of peroxide or very high temperature or UV radiation.
Formation of free radical takes place by homolytic fission of the bond.
Halogenation of alkanes
This is an example of free radical substitution reaction. It takes place in the presence of diffused sunlight or UV light.
The first propagation step i.e, the attack of chlorine free radical on methane to generate methyl free radical is the rate determining step.
The overall reaction is the sum of above two propagation steps.
Divalent neutral carbon is called as carbene. With six electrons in valence shell (incomplete octet), they are electron deficient. They are of two types, singlet and triplet. Singlet is sp2 (angular), diamagnetic, electrophilic, less stable, less reactive and triplet is sp (linear), paramagnetic, biradical, more stable, more reactive.
Generation of carbene:
From Chloroform (CHCl3) Using a strong base: As in Reimer-Tiemann and Carbyl amine reactions.