Organic Reactions & Their Mechanisms PDF

COLLEGE OF ENGINEERING Department of Mechanical and Industrial Engineering Organic Reactions & Their Mechanisms  Lesson Plan Factors influencing Chemical Reactions Inductive Effect Mesomeric Effect Electromeric Effect...

COLLEGE OF ENGINEERING Department of Mechanical and Industrial Engineering Organic Reactions & Their Mechanisms  Lesson Plan Factors influencing Chemical Reactions Inductive Effect Mesomeric Effect Electromeric Effect Hyperconjugation Reagents Electrophilic Nucleophilic Substitution Reactions SN1 SN2 ESR Free Radicals Resonance  Factors influencing Chemical Reactions A reaction may or may not occur depending upon the density of electrons at the sites of the reaction in the substrate. The density of electron is affected by following factors - Inductive Effect Mesomeric Effect Electromeric Effect Hyperconjugation  Inductive Effect Process of electron displacement along the chain of carbon atoms is called inductive effect (I effect) It is a permanent effect and appear in form of dipole moments in molecule. It does not depend upon the presence of a reagent. -I (Electron Withdrawing) Effect : When atom or group of atoms have greater electron attracting capacity than hydrogen. Like Cl2, NO2, COOH etc.  Inductive Effect (- I effect) When EW group is present, electrons are displaced towards them. They acquire small negative (δ-)charge, while carbon atom acquire small positive charge (δ+). This effect drops drastically as we move away from first carbon atom. NO2> CN>COOH>F>Cl> Br> I > OH> OCH3 > C6H5 > H  Inductive Effect (+ I effect) +I (Electron Repelling) Effect: When atoms or group of atoms having lesser electron attracting power than hydrogen, they repel electrons towards carbon atom. And they acquire small positive (δ+)charge, while carbon atom acquire small negative charge (δ-). Inductive effect affects reactivity of organic molecules by changing electron density around the site of attack.  Relative stabilities of carbocations Relative stabilities of carbanions  Mesomeric Effect (M effect) This effect refers to the polarity produced in a molecule as a result of interaction between two pi bonds or a pi bond and lone pair of electron. It is transmitted along the chain. It is very prominent in conjugated system. -C=C-C=C-C=C-C=C-C=C- In this system pi electrons get delocalized and give number of resonance structures of the molecule.  Mesomeric Effect (M effect) In carbonyl group (>C=O) oxygen is more electronegative so pi electrons displaced towards the oxygen atom. It is also permanent effect and does not depend on reagent. Inductive and mesomeric effects change the electron density in a molecule and hence decide the point of attack in molecule.  Electromeric Effect This effect is shown by those compounds containing double or triple bonds. When a double or triple bond is exposed to an attack by a reagent, pi electrons transferred completely from one atom to another. It is a temporary effect. This effect will remain till the attacking reagent is present. As soon as the reagent is removed, the polarized molecule will come back to the original state.  Electromeric Effect When the transfer of electrons takes place towards the attacking reagent, it is called +E effect. When the transfer of electrons takes place away from attacking reagent, it is called –E effect  Hyperconjugation Hyperconjugation is the interaction of sigma electrons (usually C–H or C–C) with an adjacent empty (or partially filled) p-orbital, It gives an extended molecular orbital that increases the stability of the system. It is important in stabilizing carbocations and substituted alkenes.  Reagents Reagents are substances or compounds that are added to a system in order to bring about a chemical reaction or are added to see if a reaction occurs. Some reagents are just a single element. However, most processes require reagents made of chemical compounds.  Reagents Electrophile: A reagent which can Nucleophile: A reagent which can accept electron pair in a reaction. donate an electron pair in a reaction. Electron loving. Electron hating. Electron deficient. Electron rich. It attacks on region of high electron It attacks on region of low electron density. density. H+, Cl+, I+, R3C+, AlCl3 Cl-, I-, CN-, OH-, NH2  Substitution Reactions Reactions which involves the replacement or substitution of one or more atoms or groups by other atoms or groups are known as substitution Reactions. CH3CH2Cl + OH- → CH3CH2OH + Cl- Depending on type of initiation it is termed as follows - Electrophilic substitution Reactions Nucleophilic substitution Reactions SN1 SN2 Free radical substitution Reactions Analyse these images and identify the difference  SN2 (Bimolecular Nucleophilic substitution) It is one step and second order reaction. Rate of reaction depends on concentration of both substrate and nucleophile. Rate α [substrate] [nucleophile] Rate determining step involves the participation of both substrate and nucleophile. Due to back side attack configuration of carbon is inverted like umbrella blown inside out. It’s called Walden inversion. Transition State  SN1 (Unimolecular Nucleophilic substitution) It is two step reaction. Single order reaction. Rate α [substrate] Step1: Formation of carbocation (rate determining step) Step 2: Attack of nucelophile on planar carbocation from either side to give product.  Video Resources SN1 & SN2 https://www.youtube.com/watch?v=TnY1S5IdVqI Hyperconjugation: https://www.youtube.com/watch?v=p0fHr_G-SOc  Electrophilic Substitution Reaction (ESR) In ESR, the functional group attached to a compound is replaced by an electrophile. The displaced functional group is typically a hydrogen atom. Electrophilic substitution reactions proceed via a three-step mechanism: The generation of an electrophile The formation of a carbocation (which is an intermediate) The removal of a proton from the intermediate  Free Radical Substitution Reaction Example Chlorination of methane CH4 + Cl2 → CH3Cl + HCl Three steps: Initiation Propagation and Termination Initiation: Cl2 molecule undergoes homolytic fission by light or temperature to give chloride free radical. Cl2 → Cl. + Cl.  Free Radical Substitution Reaction Propagation: Chlorine free radical attacks on methane molecule to give methyl free radical. Termination: Combination of various free radicals to give stable molecule.  Resonance Molecule is expressed by two or more structures. These various possible structures are called resonance structures Contributing structures differ only in the position of electrons, not in the position of nuclei. The actual structure is an approximate intermediate between the resonance forms and it is called a resonance hybrid. Overall effect of resonance is the delocalization of pi electrons over the full structure of molecule.  Resonance Delocalization of pi electrons in benzene due to resonance  Tutorial Question Evaluate key differences between Inductive & mesomeric effect Inductive & electromeric effect SN1 and SN2 Name three +M, +I groups