UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry Introduction to Organic Reactions Prepared by Lydia Fe V. Landingin, RPh UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry REACTION MECHANISM • Definition → Overall description of a chemical reaction process • Reactions occur in defined steps that lead from reactant to product • Must account for all reactants and products • Reactant → ___________________________ • Product → ____________________________ • Chemical Reactions • Bond-breaking • Bond-making UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry REACTION MECHANISM • Parts of a chemical reaction A + B [C] D + E UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry SYMBOLS USED IN ORGANIC REACTIONS Symbol Meaning Forward Reaction (Reactants to Products) Reverse/ Backward Reaction (Product to Reactant) Forward and Backward Reaction ⇌ Equilibrium state (Forward and Backward reaction occurs at equal rates) Resonance Structures Arrowheads with a complete head → Movement of two electrons (asymmetrical process) Arrowheads with a half head (fish-hook) → Movement of one electron (symmetrical process) UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry SYMBOLS USED IN ORGANIC REACTIONS Symbol Meaning Alternative to (g), used to indicate gaseous product Reactants are heated 2 atm Pressure at which reaction is carried out, in this case is 2 atm pressure Pressure at which reaction is carried out exceed normal atm 0OC MnO2 hV Temperature at which reaction is caried out, in this case is 0OC Catalyst, in this case is Manganese (IV) oxide Represents light Represents radiation UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry ORGANIC REACTIONS 1. Addition Reaction (A + B → AB) • Two reactants combining to form a single product • Shortcut → loss of pi (double or triple) bond UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry ORGANIC REACTIONS 2. Elimination Reaction (AB → A + B) • Single reactant splitting into two products • Reverse of addition • Shortcut → addition of pi (double or triple) bond UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry ORGANIC REACTIONS 3. Substitution Reaction (AB + CD → AC + BD) • Two reactants exchanging parts to form two new products • Shortcut → No change in bond order UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry ORGANIC REACTIONS 4. Rearrangement Reaction (A → A [Isomer]) • Single reactant yielding an isomeric product through reorganization of its bonds and atoms UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry ORGANIC REACTIONS Practice Examples. Classify each of the following reactions as an addition, elimination, substitution, or rearrangement • a)CH3Br + KOH → CH3OH + KBr • b) CH3CH2Br → H2C═CH2 + HBr • c) H2C═CH2 + H2 → CH3CH3 UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry BOND FORMATION AND BREAKAGE 1. Symmetrical → Homolytic • Product: Radicals → _______________________________________ • Chemically reactive UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry BOND FORMATION AND BREAKAGE 2. Asymmetrical → Heterolytic • Product: Charged Species / particles → Nucleophile and Electrophile UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry RADICAL REACTIONS • Processes that involve symmetrical bond-breaking and bond-making • Radicals react to complete electron octet of valence shell UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry RADICAL REACTIONS • A radical can add to an alkene to give a new radical, causing an addition reaction UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry RADICAL REACTIONS • Steps of radical reactions 1. Initiation • 2 radicals in the product side 2. Propagation • 1 radical each in the reactant and product side 3. Termination • 2 radicals in the reactant side UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry RADICAL REACTIONS • Steps of radical reactions 1. Initiation • Breaking of bond to produced the radicals • Uses high energy → or hV or UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry RADICAL REACTIONS • Steps of radical reactions 2. Propagation • Reaction with molecule to generate radical • Common step in radical mechanism UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry RADICAL REACTIONS • Steps of radical reactions 3. Termination • Combination of 2 radicals to form a stable product • Radical destruction → Not a common step in radical mechanism UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry POLAR REACTIONS • Processes involving unsymmetrical bond-breaking and bond-making • Molecules can contain local unsymmetrical electron distributions • Cause bond polarity • Partial positive → atoms that pushes e• Partial negative → atoms that pulls e• Bonds in functional groups are polar UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry POLAR REACTIONS • Bonds in functional groups are polar • Polarizability → Tendency of atoms in a molecule to undergo polarization • Larger atoms, easily polarizable • Smaller atoms, less polarizable UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry POLAR REACTIONS UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry POLAR REACTIONS • Nucleophile (Nu-) • Electron-rich species, donate electron pairs • Lewis ___________ • Attacks positions with (+) charge or atoms with low electron density • Electrophile (E+) • Electron-poor species, accept electron pairs • Lewis ___________ • Attacks positions with (-) charge or atoms with high electron density UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry POLAR REACTIONS UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry POLAR REACTIONS UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry POLAR REACTIONS • Is the structure nucleophile or electrophile? UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry POLAR REACTIONS • Rules for using curved arrows 1. Electrons move from a nucleophilic source to an electrophilic sink 2. The nucleophilic site can be neutral or negatively charged UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry POLAR REACTIONS • Rules for using curved arrows 3. The electrophilic site can be neutral or positively charged 4. The octet rule should be followed UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry POLAR REACTIONS • Practice Example. Add curved arrows to the following polar reaction to indicate the flow of electrons UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry POLAR REACTIONS • Practice Example. Add curved arrows to the following polar reaction to indicate the flow of electrons UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry POLAR REACTIONS • Practice Example. What product would you expect from reaction of cyclohexene with HBr? UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry POLAR REACTIONS • Practice Example. Addition of HBr to Ethylene • Reaction Mechanism → Electrophilic Addition UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry POLAR REACTIONS • Practice Example. Addition of HBr to Ethylene • Reaction Mechanism → Electrophilic Addition 1 2 UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry POLAR REACTIONS • Practice Example. What product would you expect from reaction of cyclohexene with HBr? UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry POLAR REACTIONS • Practice Example. What product would you expect from reaction of cyclohexene with HBr? • Answer: UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry REACTION INTERMEDIATE • Definition → Species that are neither the reactant nor the final product UNIVERSITY of the IMMACULATE CONCEPTION College of Pharmacy and Chemistry REACTION INTERMEDIATE • Types of Carbon Intermediates
