Subha Arjunan

Mechanism of a reaction gives the information about the process, in which reactants are transferred into products. Mechanism provides the information about: o whether the reaction proceeds through single or multistep o the transition state(T.S) and/or intermediates and the structure of the T.S and/or intermediate o The bond formation and cleavage (if both occurs in a single step mechanism gives their order of occurrence) o The stereochemistry of the reaction o Any selectivity shown by the reaction

Single Step Reaction Also called Concerted Reaction Conversion of reactants into products involves only one step (i.e) A B A Reactant B Product The reactant may be more than one. (Ex.) A+P B Alkaline Hydrolysis of MeI Multistep Reaction Conversion of reactants into products involves more than one step (i.e) A B C D Hydrolysis of t-butyl bromide o Whether the reaction is reversible or not o If two or more products formed whether the reaction shows Kinetic or Thermodynamic Control

Mechanism: Single step reaction has no intermediate and has only one transition state Mechanism: This type of reaction has n-1 no. of intermediate and n no. of transition state. n number of steps in a reaction. For ex., 2 step reaction has 2 T.S and 1 intermediate

Hydrolysis of t-butyl bromide-Mechanism: (Sn1 type reaction)

Hydrolysis of MeI Mechanism: (Single step reaction) (Sn2 type reaction) Note: Sn1 reaction involves two steps (multi step) Sn2 reaction involves single step

Transition State T.S is highly unstable hence its lifetime is very short (zero lifetime). So T.S cannot be isolated from the reaction mixture. T.S occurs at energy maxima in the energy profile. In T.S, bond formation or cleavage occur only partially. (refer previous slide) Intermediate Intermediates are stable. Some are even stable for years. So it can be easily isolated. Life time sec. Intermediates occur at energy minima in the energy profile. Intermediates have complete bond formation and bond cleavage.

Energy Profile: The energy profile renders the information about energies of the reaction In an energy profile, horizontal axis depicts the progress of the reaction, knows as “REACTION COORDINATE”(R.C). R.C represents the changes in bond orders, bond distances and geometries when reactants are transformed into the products. vertical axis depicts the free energy of the reaction

Energy Profile: (Free energy changes of a single step reaction)

Free energy changes of a multistep reaction:

Types of Organic Reactions: Organic Reactions Substitution Reaction Addition Reaction Elimination Reaction Rearrangement Redox Reactions

Electrophiles Electron Deficient species Electron Loving Since e - deficient it can accept an e - pair from nucleophile to obtain a stable configuration Ex: Lewis acids(BF 3,AlCl 3,ZnCl 2 ), positively charges species Nucleophiles E lectron Rich species Nucleus loving It can donate an e - pair to Electrophiles Ex: neutral species having lone pair (H 2 O, NH 3 ), Anions ( - OH, Cl - )

Leaving GroupA functional group that is eliminated with electrons of the σ bond in a reaction SolventUsed to separate and solvate the ions involved in the reaction Activation energy or Energy barrier Minimum amount energy which molecules must have to react RearrangementMigration of an atom or a group of atoms from one atom to another within a molecule(Intra molecular rearrangement) or from one molecule to another(Intermolecular rearrangement) CatalystSubstance which used to change (may accelerate or decrease) the rate of the reaction perThis represents the O-O bond. Ex: Hydrogenperoxide(H- O-O-H)

References:  Organic Chemistry – L.G. Wade JR  Organic Chemistry – Clayden, Greeves, Warren and Wothers  Organic Chemistry (An Intermediate Text) – Robert V. Hoffman