Mechanism of Organic Reactions

How the reactions occurs in organic compounds?Breaking of bonds in organic reactions(cleavage):

Heterolytic cleavage, Homolytic cleavageTypes of organic reactionsTerms in reaction: substrate, attacking reagent , Leaving

group & productAttacking reagents- Electrophile, Nucleophile & Free

radicalsAddition reactionSubstitution reaction- SN1 & SN2Elimination reaction- E1 & E2Rearrangement reaction- Hoffman rearrangement reaction

Fundamentals of Organic reactions: How the reaction occurs in organic compounds?

In an organic reaction some covalent bonds are broken and to form a new compound some covalent bonds are formed. Difference of energy for bond breaking and new bond formation defines the type or possibility of reactions.The chemical properties of attacking reagent and medium also plays a key role in a organic reaction. The different type of reaction medium or polarity differences determines the reaction type between substitution or elimination. We will see it in the later topics.Organic reactions and its products or reaction mechanism is determined by the functional groups present in the reactants. The reaction can be defined as a characteristic reaction in organic chemistry. As example, alcohols produce ester reacting with organic acid or organic acid derivatives.CH3 –CO- Z + C2H5-O-H CH3-CO-O-C2H5 + HZ

*Z=Cl/OH/-O-CO-R

Transition state:

In the transition state theory, the mechanism of interaction of reactants is not considered; the important criterion is that colliding molecules must have sufficient energy to overcome a potential energy barrier (the activation energy) to react.

For a bimolecular reaction, a transition state is formed when the two molecules’ old bonds are weakened and new bonds begin to form or the old bonds break first to form the transition state and then the new bonds form afterward. The theory suggests that as reactant molecules approach each other closely they are momentarily in a less stable state than either the reactants or the products. In the example below, the first scenario occurs to form the transition state:

It takes a lot of energy to achieve the transition state, so the state is a high-energy substance. The potential energy of the system increases at this point because: • The approaching reactant molecules must overcome the mutual repulsive forces between the outer shell electrons of their constituent atoms • Atoms must be separated from each other as bonds are broken

This increase in potential energy corresponds to an energy barrier over which the reactant molecules must pass if the reaction is to proceed. The transition state occurs at the maximum of this energy barrier. • The transition state is an unstable transitory combination of reactant molecules that occurs at a potential energy maximum

Breaking bonds in organic reactions

Homolysis: When a bond breaks and of the two electrons making up the covalent bond, one goes to each fragment; such bond-breaking is called homolysis.

Heterolysis: In heterolysis both bonding electrons go to the same fragment.

A:B A + B Homolysis A:B A + B Heterolysis

Terms in reaction

Substrate: a substrate is a molecule upon which an attacking reagent acts.

Attacking reagent: a molecule or groups of molecules or atom which acts on the substrate and sometime initiate a reaction is called attacking reagent.

Leaving groups: In a type of reaction the group which readily leaves from the reaction state and facilitate the specific type of reaction is said the leaving group.

Nu:- + R-L R-Nu + :L-

Leaving group

Substrate

Attacking reagent

Attacking reagents

Nucleophile:Nucleophile is a species with an unshared electron pair. Its

nucleus attracting group and able to donate electron in a reaction. Expressed as Nu:- or Nu:

2 types of nucleophiles : Negatively charged: :CH3

-, :Cl-, :Br-, :CN-, :OH-, :OR-

Neutral nuclephiles: NH3, H2O, R-OH Reaction: 1. With (-)ve nucleophiles: 2. With neutral Nu:

Attacking reagents

Electrophile:Electrophiles are electron attracting group. They are deficient of

a pair of electron at the outermost orbital or outermost bonding orbital and can accept a pair of electron at the deficient orbital. Expressed as E or E+

Electrophiles are of 2 types: 1. (+)vely charged: +CH3, Br+, H+, H3O+, NO2

+, +NO 2. Neutral Electrophile: AlCl3, BF3, FeCl3, SO3

Reaction: 1. With (-)ve electrophiles: 2. With neutral lectrophiles:

Free radicals:Free radicals are the molecules which are produced by

homolysis of a sigma bond and contains an unpair electron.They are highly reactive and chargeless as the unpair electron is

their own electron. Being highly reactive these free radicals exist a while and creates a stable compound by reacting instantly with another molecule or another free radical.

Free radical reaction or chain reaction: The halogenation reactions of alkanes take place by the free

radical chain reaction mechanism. The homolysis of Cl2 is necessary to produce the initiator free radical Cl . This light promoted reaction is highly efficient.

Step-1: Chain InitiationCl2 2 Cl

Step-2 & 3: Chain PropagationCH4 + Cl CH3 + H-Cl

CH3 + Cl2 CH3Cl + Cl

Step-4: Chain Termination

CH3 + Cl CH3Cl

CH3 + CH3 CH3-CH3

Cl + Cl Cl-Cl

Heatlight

Chain Reaction of Free radicals

Types of Organic reactions

Organic reactions are classified into 4 groups:1. Substitution reaction2. Addition reaction3. Elimination reaction & 4. Rearrangement or isomerization reaction

Substitution reaction: The reaction in which one group of molecules is replaced by

more reactive groups of molecules and forms a new compound is called substitution reaction.e.g.C2H5-Cl + OH- C2H5-OH + Cl-

Addition reaction:The reaction in which two different molecules are added to produce an

added compound is the addition reaction. e.g.H2C=C2H + Br-Br H2C-CH2

Unsaturation test: Addition reaction is used to identify the unsaturation in an organic

compound. In the previous reaction we can see the alkene contains a pie bond and a sigma bond. The bromine solution in CCl4 is red and when reacts with the alkene the pie bond is broken to give the addition product and red color of the solution disappears . This is also a 1,2-addition reaction. 1,2 addition reaction occurs in unsaturated organic comp.

Br Br

CCl4

1,2- dibromo alkane

Markinov’s Rule

Addition of hydrogen halides to alkenes:

The addition of HX to an asymmetric alkene could conceivably occur in 2 ways. In practice, however, one product usually predominate. The addition of HBr to propene, for example, could conceivably lead to either 1-bromopropane or 2-bromopropane. The main product is 2-bromopropane.

Markinov’s Rule

One way to state this rule is to say that in the addition of HX to alkene, the hydrogen atom adds to the carbon atom of the double bond that already has the greater number of hydrogen atoms.

Modern statement of Markonikov’s rule: In the ionic addition of an unsymmetrical reagent to a double bond, the positive protion of the adding reagent attaches itself to a carbon atom of the double bond so as to yield the more stable carbocation as an intermediate.

Markinov’s Rule

Markonikov’s rule can define the regioselevtivity of the addition of hydorgen halide or other reactants to the alkenes.

Regioselective reactions: When a reaction that can potentially yield two or more constitutional isomers actually produces only one (or a predominance of one), the reaction is said to be regioslelective.

Antimarkonikov’s rule

SUBSTITUTION REACTION

Nucleophilic substitution reactions: General types of nucleophilic substitution (SN)

Because of the substitution is initiated by a neucleophile, it is called a nucleophilic substitution or SN reaction.

The R-X bond can be broken by two ways-

Thus 2 types: SN1 & SN2

SN2 REACTION:

Bimolecular substitution Order of the reaction is 2, thus rate of the reaction depends

on the concentration of two reactants e.g both alkyl halide & base.

The nucleophile approaches the carbon bearing the leaving group from the back side, opposite to the leaving group.

MECHANISM OF SN2 REACTION

The negative OH- ion pushes a pair of electrons into the partially positive carbon from the back side. The chlorine begins to move away with the pair of electrons that may bonded it to the carbon

Transition state

A bond between oxygen and carbon in partially formed and the bond between carbon and chlorine is partially broken. The configuration of the carbon atom begins to change

Bond between the oxygen and carbon has formed and the chloride ion has departed.

Configuration of the carbon has inverted.

Walden Inversion

Configuration: The particular arrangement of groups around that atom in space.

SN2..

Characteristics of SN2 reaction:1. C-Nu bond provides energy to break the C-L bond2. Involves only one step 3. No intermediate compound4. Reaction proceeds through the formation of an unstable

arrangement of atoms that is transition state which exist for an extremely brief period of time

5. Always lead to inversion of configuration6. 3D factor is the rate determining factor

Stereochemistry of SN2Walden inversion- in acyclic and cyclic carbon skeleton

Unimolecular substitution reaction in which reaction rate depends on one reactants concentration.

SN1 REACTION:

MECHANISM OF SN1 REACTION

Multi-step reactionIntermediate products are produced in the steps but no

transition state

MECHANISM OF SN1 REACTION..