First Order and Second Order Kinetics

A first-order reaction can be defined as a chemical reaction in which the reaction rate is linearly dependent on the concentration of only one reactant.
A second kind of second-order reaction has a reaction rate that is proportional to the product of the concentrations of two reactants

1. First order and second order kinetics Md Altamash A hmad 1850951001 Pharm. D 1 st year

2. nucleophile Nucleophile is a chemical species that donates an electron pair to an electrophile to form a chemical bond in relation to a reaction. All molecules or ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons , they are by definition Lewis bases .

3. Leaving group A  leaving group , is an atom (or a group of atoms) that is displaced as stable species taking with it the bonding electrons . Typically the leaving group is an anion ( e.g.  Cl - ) or a neutral molecule ( e.g.  H 2 O ). Good leaving groups are weak bases . Weak bases have strong conjugate acids . The best leaving groups "want" electrons . They don't want to share them with other atoms .

4. First order reaction kinetics A  first order reaction  depends on the concentration of only one reactant. Like  Sn¹ reaction ( substitution nucleophilic unimolecular ) An Sn¹ reaction proceeds in two steps. The first step is the rate determining step & form a carbocation intermediate.  Its slow step reaction . In the second step  (fast step)  ,the intermediate carbocation is attacked by the nucleophile to give the final product .

5. SN 1 reaction

6. Second order reaction kinetics Rate depends on the product of two concentration terms When you have two components reacting with each other or one component reacting with itself E.g. – 2HI  H 2 + I 2 , here the reaction is not simply a matter of an HI molecule falling apart, but relies on the collision of two HI molecules. The rate of reaction from the law of mass action is given by- Rate = k[HI][HI] = k[HI] 2

7. SN 2 Reaction and mechanism The SN2 reaction (also known as bimolecular nucleophilic substitution ) is a type of nucleophilic substitution , where a lone pair from a nucleophile attacks an electron deficient electrophilic centre and bonds to it, expelling another group called a leaving group. SN 2 process proceeds in one step via a transition state . The rate of reaction is proportional to both the substrate and nucleophile concentration. Rate = k[RX][nu - ]

8. Mechanism of SN 2 Reaction In SN2 process there is attack by the nucleophile from the opposite side of the carbon atom of substrate bearing the leaving group, such that C-X bond of the substrate breaks only as the new C-Nu bond of the product is forming Therefore , in transition state, the nucleophile-carbon bond is partially formed and the C-X bond is partially broken. The approach of the nucleophile from the side of the molecule bearing the leaving group is unfavourable due to electrostatic repulsion and also due to steric hindrance.

9. stereochemistry The branch of chemistry that studies the properties of stereoisomers Stereoisomers : Isomers that have same formula and connectivity but differ in the position of the atoms in space I somers that differ only in the position of atoms in space, and that cannot be interconverted by rotation around a single bond. Stereocenter : A carbon atom bearing 4 different atoms or group of atoms.

10. Optically Active: the ability of some compounds to rotate plane polarized light . Chiral : any molecule that is non superimposable with its mirror image (i.e. A and B are chiral). Achiral : any molecule that is not chiral. Racemic mixture : a 1:1 (equimolar) mixture of two enantiomers

11. Enantiomers : stereoisomers that are non superimposable mirror images . Distereomers : stereoisomers that are not mirror images .

12. Meso compound : A compound with at least 2 Stereocenter that is achiral due to the presence of a plane of symmetry

13. Cis and trans R and S

14. E and Z

15. Phase transfer catalysis Phase transfer catalysis (PTC) uses catalytic amounts of phase transfer agents which facilitate interphase transfer of species, making reactions between reagents in two immiscible phases possible. PTC is used widely in the synthesis of various organic chemicals in both liquid-liquid and solid-liquid systems.

16. Role of solvents S olvents need to get everything into solution - both the substrate (that which is undergoing the reaction) and the nucleophile (that which is reacting with the substrate to form product ). It is best if the solvent is not reactive with either the substrate or nucleophile. SN 2 reactions uses Polar Aprotic Solvents like DMSO, DMF, tetrahydrofuran SN 1 reactions uses Polar Protic Solvents like Ethanol, water, acids