Basis of Organic Reactions

• An Intermediate is a short-lived species formed during the course of a reaction.

• It is formed in case of multi-step reaction.

• It may be charged or neutral.

• If it is charged, it gets stabilised in polar solvent.

• If it is neutral then it gets stabilised in non-polar solvent.

Charged Intermediates Neutral Intermediates

Carbocation Free radical

Carbanion Carbene

Nitrene

Benzyne

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:

1. + Inductive (+I) effect: Electron releasing effect

2. - Inductive (-I) effect: Electron withdrawing effect

+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.

3. Electronegativity : Increases with increase in electronegativity.

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

• -I effect increases the acidic strength as the groups which withdraw electrons can stabilise the negative charge on the conjugate base. (Smaller the pKa value, stronger is the acid/ larger the Ka value, stronger is the acid).

e.g. Order of acid strength

• Both the inductive effects are distance dependent effect i.e, they decrease with increasing distance.

Problem: Arrange the following in the order of decreasing acid strength.

MESOMERIC/RESONANCE EFFECT

• Displacement of π− electrons in π− bonded compound.

• This effect operates through π− bond, + charge, - charge, lone pair of electrons and odd electrons present at alternate position.

• Extension of resonance via π- bond, + charge, - charge, lone pair of electrons etc. Is called conjugation.

Role & Importance of resonance effect

• It reduces the bond length of single bond. For a molecule with alternate single and double bond, all the bond lengths are found to be equal.

Aromaticity

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.

• Comparing the acidities of HClO, HClO2, HClO3, and HClO4. They have pKa values 7.5, 2, –1, and about –10, respectively. In each case the acidic proton is on an oxygen attached to chlorine, that is, we are removing a proton from the same environment in each case. Then, why is the difference? Once the proton is removed, we end up with a negative charge on oxygen. For hypochlorous acid, this is localized on the one oxygen. With each successive oxygen, the charge can be more delocalized, and this makes the anion more stable. For example, with perchloric acid, the negative charge can be delocalized over all four oxygen atoms.

• Acidity of ethanol (pKa = 15.9), acetic acid (pKa = 4.8) and methane sulphonic acid (pKa = -1.9).

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.

• pKa  of cyclohexanol is 16 while that of penol is 10.

• Similarly, Aniline is a weaker base than Cyclohexylamine. In case of cyclohexylamine, lone pair is localised (easily available for donation) and in aniline, the lone pair is in partial delocalisation with the ring (partial because the plane of the lone pair is 40o away from the plane of the ring).

• Amides are weaker bases than amines. No protonation can occur on nitrogen of amides. In case of amides, Nitrogen is sp2 hybridised with lone pair in the p orbital having good overlap with the carbonyl group. This delocalisation binds the ‘lone pair’.

• Note: Mesomeric effect is always dominating than inductive effect and it is always the deciding factor if both effects are present.

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-.)

Hyperconjugation

• Delocalisation of σ electrons.

• Separated proton always remains closer to the molecule. It is also called ‘No bond resonance’ because at least in one canonical form, it is separated from the molecule.

• It is also called Second order resonance due to migration of σ electrons.

• No. Of hyperconjugated forms = n, where n is the no. Of σ hydrogens

Effects

• To decide the stability of unconjugated alkene.

• More the no. Of hyperconjugated structures, more is the stability of the molecule.

• If the no. Of hyperconjugated structure is same then more symmetrical structure will be more stable.

• Hyperconjugation decides the stability of alkyl carbocation in the same way, more the no. Of hyperconjugated structures, more is the stability of the carbocation.

• Hyperconjugation effect is a weak effect than mesomeric effect because of breaking of C-H σ bond but stronger than inductive effect because it is the actual delocalisation of electrons.

Carbocations

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

1. By breaking of C-X bond as in first step of SN1 reaction.

2. Dehydroxylation of alcohols

3. Protonation of alkenes

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

1. I effect: + I effect increase the stability while – I effect decreases it

2. Hyperconjugation: More the no. Of hyperconjugated structures, more is the stability of the carbocation.

3. Mesomerism

4. Aromaticity: Whenever during the formation of a carbocation, an aromatic structure is achieved then this carbocation becomes most stable carbocation.

5. S character: Higher the s character of the carbon atom on which the
   + charge is present, lower will be the stability of carbocation.

                                   CARBANION

• Negatively charged trivalent carbon.

• Either planar (sp2) or pyramidal (sp).

• Carbanions with bulky alkyl groups are sp2 hybridised.

• They are important intermediate in several types of condensation reactions like Aldol condensation. Carbonyl chemistry is governed mainly by carbanion formation. It acts as a nucleophile. There are so many examples of nucleophilic addition reaction.

Methods of preparation

• Using a strong base to abstract H+ .

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.

Aromaticity

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.

Mechanism:

1. Initiation: Breaking of Cl-Cl bond to generate chlorine free radical

2. Propagation: Chlorine free radical attacks on methane to generate methyl free radical and then methyl again attacks on chlorine to generate chlorine free radical. The process gets repeated.

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.

3. Termination occurs via following reactions.

Carbene

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.

Reimer-Tiemann reaction