REACTION MECHANISM

Electronic Effects

The effect which appears due to electronic distribution is called electronic effect.

Inductive Effect

Inductive effect may be defined as a permanent shifting of s bond pair electrons due to difference in Electronegativity. (Polar bond).

- I Effect

The group which withdraws electron cloud is known as -I group and its effect is called -I effect. Various groups are listed in their decreasing -I strength as follows.

+I Effect

The group which release electron cloud is known as +I group and effect is +I effect.

Key Note :-

The hydrogen atom is reference for +I and -I series. The inductive effect of hydrogen is assumed to be zero.

Resonance & Mesomeric effect

Resonance is the phenomenon is which two or more structures involving identical position of atom, can be written for a particular species, all those possible structures are known as resonating structures or canonical structure.

The ultimate target of resonance is to find the real structure of molecule or ion.

Resonance Hybrid

It is the actual structure of the species without violating the rules of covalence maxima for the atoms.

Resonance Energy

The difference in energy between the hybrid and the most stable canonical structure is referred to as the resonance energy.

Relative stability of the canonical form

Nonpolar (uncharged) structure are most stable. Charge separation decreases stability. Separating opposite charges requires energy. Therefore, structures in which opposite charges are separated have greater energy (lower stability) than those that have no charge separation.

Structures in which all of the atoms have a complete valence shell of electrons (i.e., the noble gas electronic configuration) are especially stable and make large contributions to the hybrid.

Structures with more covalent bonds are more stable than other structures.

Structure that carries negative charge on a more electronegative atom and positive charge on less electronegative atom are comparatively more stable.

Key Notes :-

The structure with incomplete octet is least stable among all, and charge separation decreases stability.

Charge separation is endothermic process, which increases energy and decreases stability of R.S.

-M Effect

When the group withdraws electron from the conjugated system, it shows -m effect.

+M Effect

When the group donates electron to the conjugated system it shows +m effect.

Hyperconjugation

It is delocalisation of sigma electron with p-orbital. Also known as s-p conjugation or no bond resonance. It may take place in alkene, alkynes, carbocation, free radical, benzene nucleus.

Key Note :-

Presence of at least one a-hydrogen in Alkenes, Alkynes, carbocations, free-radical and benzene nucleus is necessary for operation of hyperconjugation.

Applications of hyperconjugation

(a)        Stability of Alkenes: “More alkylated alkenes are more stable”. Stability of alkenes µ no. of hyperconjugative structures.

(b)        Heat of hydrogenation: Stability of alkenes µ no. of hyperconjugative structure

(c)        Bond Length    (d)    Dipole moment    (e)      Stability of carbocation (f) Stability of free radical

Relative Stability Order

(A)       Stability of carbocation

(B)        Stability of free radical

(C)        Stability of Carbanion

Some important orders in organic chemistry

Basic strength of amine:

             In aqueous medium

             R Þ -CH₃                                   2⁰ > 1⁰ > 3⁰ > NH₃

             R Þ -CH₂CH₃                            2⁰ > 3⁰ > 1⁰ > NH₃

             In gaseous medium:

             R Þ -CH₃                                   3⁰ > 2⁰ > 1⁰ > NH₃

             R Þ -CH₂CH₃                            3⁰ > 2⁰ > 1⁰ > NH₃

Reactivity towards nucleophile (NAR)

(1) HCHO > CH₃CHO > (CH₃)₂CO

(2) CCl₃CHO > CHCl₂CHO > CH₂ClCHO

Reactivity order towards acyl nucleophilic substitution reaction

Acid chloride > anhydride > ester > amide

Order of electronic effect

Mesomeric > Hyperconjugation > Inductive effect

Stability of alkene µ no. of a hydrogen

R₂C = CH₂ >R₂C = CHR > R₂C = CH₂ >

RCH = CHR > RCH = CHR

Trans form     cis form

RCH = CH₂ > CH₂ = CH₂

Heat of hydrogenation *

Some important order of acidic strength

Ortho Effect

Acidic Strength

It is common observation that generally ortho substituted benzoic acids are highly acidic as compared to their isomers and benzoic acids itself.

Basic Strength:-

Ortho-substituted anilines are mostly weaker bases than aniline itself.

Ortho-substituent causes steric hinderance to solvation in the product (conjugate acid i.e. cation).

Carbenes

There is a group of intermediates in which carbon forms only two bonds. These neutral divalent carbon compounds are called carbenes.

Nitrenes

There is a group of intermediates in which nitrogen forms only one bond. These neutral Monovalent nitrogen compounds are called nitrenes.

Benzyne

The benzene ring has one extra C - C m bond in benzyne

Methods to stabilize Intermediate

Intermediates are stabilised by movement of electrons

Electronic movement is of two types

Permanent electron movement

Inductive Effect

Shifting of s electrons due to DEN

It is of two types

(1) +I Effect Þ

Eg. –NH- > -O- > -COO- > -³0R >

- 2⁰R > -1⁰R > -CH₃

(2) –I Effect Þ

Eg. NF₃⁺ > NR₃⁺ > NH₃⁺ > NO₂ > CH >

F > Cl > Br > I > OH > OR > H

Inductive effect is defined with respect to hydrogen.

Inductive effect of protium is zero while deuterium has +I effect.

Negatively charged groups have very high tendency to release electrons.

Hyper-Conjugation

Transfer of s electrons of a C-H bond of a sp³ hybridized carbon.

Also called “no bond resonance”, Baker Nathan effect.

Can be of two types

(1) isovalent (2) sacrificial

Stability of alkenes

Stability of carbocation & free radical

Activation of benzene ring by alkyl group, methyl is most activating while tertiary being least activating.

[Sacrificial Hyperconjugation]

Resonance

Delocalisation of p bond electrons

Also known as mesomeric effect.

Presence of conjugate system is must

It is of two types:

Effectiveness of various effects: Dancing resonance > Resonance > Hyper conjugation > Inductive effect

Organic Reaction

Addition Reaction

EAR-Electrophilic Addition Reaction

* bond electrons are mobile in nature and donated to electrophile.

In first step addition of electrophile takes place, that’s why known as EAR.

Compound with multiple bond between similar atoms.

E.g.: Alkene, Alkyne

NAR-Nucleophilic Addition Reaction

Multiple bonds are present in b/w different atoms having difference in E.N.

In first step addition of nucleophile takes place.

E.g.: Addition Reaction of aldehyde, ketone and Cyanides.

FRAR-Free Radical Add. Reaction

Metal adsorb hydrogen on its surface and converts molecular H₂ into atomic H. (H₂ ® H^· + H^·)

Eg. 1: Reduction by H₂ in the presence of metal (Ni, Pd, Pt)

Eg.2 : Alkene + HBr in presence of peroxide.

Follow anti-markonikov’s rule (HBr + Peroxide)

Substitution Reaction

ESR-Electrophilic Substitution Reaction

Characteristic Reaction of benzene and its derivatives to maintain aromaticity.

Substituted E⁺ is always H⁺.

Protonic acids like HCl, HF etc does not directly react with benzene.

Eg.: Friedel craft Alkylation and Friedel craft Acylation (Anhy. AlCl₃ Produces electrophile), and other reaction of benzne and its derivatives.

FRSR-Free Radical Substitution Reaction

Free radical substituted by other free radical.

Generally in alkanes.

3⁰H > 2⁰H > 1⁰H (Reactivity order)

Eg.: Halogenation of alkanes (X^· replace H^·)

NSR-Nucleophilic Substitution Reaction

 Nucleophile is substituted by another nucleophile.

S_N¹ Þ 3⁰R-X > 2⁰R-X >1⁰R-X

S_N² Þ 1⁰R-X > 2⁰R-X > 3⁰R-X

Ar SN Þ Ph-X

S_NAE Þ Acid derivatives.

Elimination Reaction

E¹-Elimination Reaction

I.M. is Carbocation and can undergo rearrangement.

1^st Order Reaction

R.O.R. µ [Substrate]

3⁰ > 2⁰ > 1⁰.

Polar protic solvent and high temperature is required.

Eg.: Dehydration of alcohols.

E²-Elimination Reaction

No carbocation is formed.

2^nd order reaction

R.O.R. µ [Substrate] [Nu^-]

1⁰ > 2⁰ > 3⁰.

In presence of strong Nu^-

Substrate should be sterically crowded.

Eg.: Dehydrohalogenation.

Elimination unimolecular conjugate base reaction E¹-CB

R.O.R. = [C.B]¹

Most stable carbocation is formed but not rearrangement occurs.

Oxidation Reaction

Oxidation Number Increases

Via Abstraction of Hydrogen

By weak/moderate oxidizing agent

PCC, PDC, CrO₂Cl₂, Cu/573 K Jones reagent – [CrO₃ + dil. H₂SO₄ + acetone]

Via Introduction of Nascent [O]

Strong oxidizing agent.

Eg.: KMnO₄, K₂Cr₂O₇, CrO₃, O₃.

Reduction Reaction

Oxidation Number Decreases

Via adding of Hydrogen [H⁺]

(Pd + CaCO₃)/H₂

Ni/Pt/Pd + H₂

Via introducing of Hydride [H^-]

Li A lH₄ (stronger reducing agent) NaBH₄.

Di-BAL-H

(diisobutylaluminium hydride)

[SnCl₂ + HCl] (H^- + Sn⁺⁴ + 3Cl^-)

Acid Base Reaction

*Most Important Reaction

*90% students get confused by this reaction

70% reactions of organic chemistry are acid base reactions.

Fastest Reactions.

Salt is produced in this reaction and anionic part of salt can act as Nucleophile.

Other Reaction

Peri-Cyclic Reaction

Polymerisation Reaction

Rearrangement Reaction

Aromatization Reaction

Condensation Reaction

Isomerisation Reaction

Combustion Reaction

Simplified Addition Reaction

Simplified Substitution Reaction

Selectivity in the Reaction

Chemo-Selective

When reaction occurs at a specific functional group.

Selectivity in the Reaction

Regio-Selective

Regio = Region

There Reaction are Region specific Reactions

Selectivity in the Reaction

Stereo-Selective

One of the stereo isomer is the main product.

Selectivity in the Reaction

Stereo-Specific

Product of this reaction depends on the stereo chemical of substrate.

Electrophilic Substitution Reaction

MechanismÞ

Nucleophilic Addition Reaction

Catalytic Hydrogenation

Homogeneous

Catalyst gets dissolved in reaction mixture.

Eg.: (1) Wilkinson’s catalyst, [Rh(Ph₃P)₃]Cl

(2) Metal + H₂ mixed in quinoline.

Heterogeneous

Eg.: (1) Raney Ni(Ni + Al)

More porous

Size difference in Al and Ni.

Large surface area.

Eg.: (2) Metal + H₃

Ni/Pt/Pd/Ni₂B

Metal with H+ donor

Birch Reduction (Anti addition)

Liq. NH₃ + Na

Bouveault-Blace Reduction

Na + C₂H₅OH

Stephen’s Reaction

(SnCl₂ + HCl)

Clemmensen’s Reaction

[Zn – Hg + HCl]

Step 1- e^- released by metal.

Step 2- Abstraction of H⁺ from H⁺ donar

Reduction (In Organic Chemistry)

Metal Hydride (H^- Releasing)

LiAlH₄

It reduces all except Alkene, Alkyne, Benzene

NaBH₄

Weak Reducing agent

Specific for carbonyl compound.

DiBAL-H

Specific for ester and cyanide

Used in synthesis of aldehyde

Other

Red P + HI

Oxygen containing functional group (Alkanes [Converts into])

Merrwin Ponndorf Verley reducing

Wolf Kishner’s Reducing

[NH₂-NH₂, NaOH]

Oxidation of Alkenes/Alkynes

Weak O.A.

Mild O.A

Strong O.A.

Oxidation of Alcohols

Hydrogen Abstractive O.A. (mild O.A.)/[Limited (O)]

Mild O.A. can reduce only C-H bond containing compounds. C-H bond has tendency to trap [O]

Eg. (1) P.C.C. (Pyridinium chlorochromate): [C₅H₅NH]+[CrO₃Cl]^-

(2) PC – Collin’s Reagent

(3) PDC – (Pyridinium di-chromate)

(4) Cu/500⁰C      

(5) CrO₃/H₂SO₄ (Jones’ Reagent)

Oxygen Releasing O.A. (Strong O.A.)

It Oxidises C-H bond as well as C-C bond.

Eg. (1) KMnO₄/H⁺

(2) K₂Cr₂O₇/H⁺

(3) HIO₄

HIO₄: (periodic cleavage) It is used for compounds having more than one OH group. Eg.

: (periodic cleavage) It is used for compounds having more than one OH group. Eg.

Oxidation of Alkane

Alkanes generally resist oxidation.

Only alkane which can be oxidized.

Oxidation (In Organic Chemistry)

Oxidation of Carbonyl Compounds

Strong Oxidizing agent

Eg. KMnO₄/H⁺, K₂Cr₂O₇/H⁺

Popoff’s rule- During the oxidation of ketones, keto group always stays with the smaller alkyl group.

Mild Oxidizing agent

Used in differentiation of aldehyde and ketones.

Ketones cannot be oxidized by mild O.A.

Eg. (1) Benedict’s Solution

Sodium Citrate + CuSO₄.5H₂O

Red ppt. of Cu₂O forms

(2) Schiff Reagent

(3) Fehling’s solution

(4) Tollen’s Reagent

It is also known as Silver Mirror Test

Ammonical Silver Nitrate – [AgNO₃ + NH₄OH]