Study of Chemical Reactions

Study of Chemical Reactions

Introduction to Mechanisms There are four different types of organic reactions: Additions Eliminations Substitutions Rearrangements 149

Addition Reactions Occur when 2 reactants add together to form a single product with no atoms left over A + B C 150

Elimination Reactions Are the opposite of addition, i.e. single reactant splits into 2 (or more) products A B + C 151

Substitution Reactions Occur when 2 reactants exchange parts to give 2 new products A B + C D A C + B D 152

Rearrangement Reactions Occur when a single reactant undergoes a reorganization of bonds and atoms, leading to an isomeric product A A' 153

Reaction Mechanism An overall description of how a reaction occurs. It is a representation of the electron flow that would accompany the breaking of bonds and the formation of new bonds. In this process, reactive intermediates are normally formed. 154

Pushing electrons is a good graphic way to describe each step. We use arrows to describe the electronic movement in a mechanism. Arrow Representations reactants going to products equilibrium resonance represents the movement of electrons 155

Bonds can break in 2 different ways: Homolytic cleavage: each partner takes one electron (Half-headed arrow = 1 electron moves) Heterolytic cleavage: one partner takes 2 electrons, the other partner takes none. (normal-headed arrow = 2 electrons move) 156

Radical reactions: process involving the symmetrical bond breaking and bond making. Polar Reactions: process involving the unsymmetrical bond breaking and bond making. 157

Radical Reactions normally require three kinds of steps: Initiation (formation of a radical) Propagation (radical cascade process) Termination (2 radicals join to end the reaction) We have already seen one example in the Chapter dealing with Alkanes: Monochlorination of Methane 158

Mechanism of monochlorination of methane homolytic cleavage Cl Cl h 2Cl initiation step Cl H CH 3 HCl + CH 3 first propagation step CH 3 Cl Cl CH 3 Cl + Cl second propagation step Cl Cl Cl 2 CH 3 CH 3 CH 3 CH 3 termination steps Cl CH 3 CH 3 Cl 159

Polar Reactions: in polar reactions, the electron rich site of one reactant react with the electron poor site of the other reactant. IMPORTANT: understand dipole moment 160

Electron Rich Site: the reactant with the electron rich site is often referred to as: Nucleophile Cl - OH - CH 3 NH 2 H 2 O nucleophiles since they have apairofelectronstoshare Electron Poor Site: the reactant with the electron poor site is often referred to as: Electrophile H + CH 3 CH 2 + BH 3 electrophiles since they can accept a pair of electrons 161

Example of polar reaction: Curved arrows are used to describe the reaction that took place? 162

Rule 1 Electrons move from a nucleophilic source (Nu:) to an electrophilic site. (Nu: must have an electron pair available, either as lone pair or a multiple bond) O E N E C - E E 163

The electrophilic site must be able to accept the electrons. Normally, this site has a positively charged atom or a positively polarized atom. Nu: Nu: Nu: Nu: C + C Halogen H O O 164

Rule 2 The nucleophile can either be neutral or charged Rule 3 The electrophile can either be neutral or charged Rule 4 The octet rule must be followed 165

In each step of our example, which species is the nucleophile and which is the electrophile? step1 H Br + H step2 Br - Br H 166

Let s look at the mechanism of this reaction with arrow representation. 167

Reactive Intermediates Are short-lived species that are never present in high concentrations because they react almost as quickly as they are formed. Types: species with trivalent carbon. They are classified according to their charge. C+ C C - C carbocation radical carbanion carbene 168

Carbocations and radicals: are stabilized by inductive effect. Inductive effect: is the sharing of electrons density through the σ bonds of molecules 169

Rule more highly substituted (with alkyl groups) carbocations (or radicals) are more stable. R R C + R more stable R R C + H R C + H H H C + H H 3 o 2 o 1 o methyl carbocation least stable 170

Same analysis is valid for radicals 171

Carbanions: are sp 3 hybridized. They are electron rich and nucleophilic Stability of carbanions is opposite to that of carbocations due to the inductive effect. 172

Like all other reactions, organic reactions require collision between species in order to take place Therefore, a certain amount of energy is required for these reactions to proceed 173

Activation Energy (E a ) The minimum kinetic energy the molecules must possess to overcome the repulsions between their electron clouds when they collide When enough energy is provided, the species involved in the reaction reach the transition state Figure 4-3 174

Transition state Highest energy state in a molecular collision that leads to reaction A B + C A B C A + B C reactants transition state products 175

Figure 4-4 When reactive intermediates are formed, there are usually more than one transition states. One example would be the first propagation step in the chlorination of methane. 176

Energy diagrams are also useful in understanding the stability of reactive intermediate. For example, the chlorination of propane should result in the formation of 2-chloropropane rather than 1-chloropropane due to the difference in stability of the radicals formed. Figure 4-8 177

Figure 4-12 Hammond Postulate Related species that are similar in energy are also similar in structure.the structure of a transition state resembles the structure of the closest stable species 178

Mechanisms Mechanism are very important to understand organic reactions. When you are drawing mechanisms, always remember: Are the structures respecting Lewis rules? Is there 2 electrons moving and is it from the Nu to E? Are there charges to the new species formed? Here are some general rules: 179

How to decide on charges? Neutral molecules as Nu When a lone pair/bonding pair is used as Nu to form a new bond, the resulting species is normally positively charged. In the original alkene, both carbon provided one electron each to form the double bond. But in the product, the red carbon lost one electron and is positively charged 180

Charged Nu When a charged nucleophile attacks a neutral molecule, the resulting species is normally negatively charged: O _ O NC _ CN 181

Neutral Nu and Neutral E When a neutral nucleophile and a neutral electrophile react, the resulting species normally has both negative and positive charges O Cl H2O + _ O O H Cl H 182

Questions: Complete the mechanism by adding the proper arrows and charges O NC O- H3C C CH3 -CN H CN NC OH + CN - 183

Ph3P CH3 Br + Ph 3P CH2 H B- + - Ph3P CH2 Ph3P=CH2 184

+ - Ph3P CH2 Ph3P=CH2 O C + O- PPh3 O PPh3 Ph3P=O + 185

O - + CH2S(CH3)2 O C O + CH2 S(CH3)2 C + (CH3)2S 186

HO O- N O H O- + HO N + + H O + H2O + NO2 187

4-36 Study Problems Draw a reaction-energy diagram for a one-step exothermic reaction. Label the parts that represent the reactants, products, transition state, activation energy and heat of reaction. 188

4-43 Write a mechanism for the light-initiated monochlorination of cyclohexane with chlorine producing chlorocyclohexane. Identify each of the steps. 189