Learning Objectives After you have completed Chapter 6, you should be able to fulfill the detailed objectives listed under each individual section. identify the polarity pattern in the common functional groups, and explain the importance of being able to do so. describe the essential differences between polar and radical reactions, and assign a given reaction to one of these two categories. discuss how kinetic and thermodynamic factors determine the rate and extent of a chemical reaction. use bond dissociation energies to calculate the ΔH° of simple reactions, and vice versa . draw and interpret reaction energy diagrams. define, and use in context, the new key terms. This chapter is designed to provide a gentle introduction to the subject of reaction mechanisms. Two types of reactions are introduced—polar reactions and radical reactions. The chapter briefly reviews a number of topics you should be familiar with, including rates and equilibria, elementary thermodynamics and bond dissociation energies. You must have a working knowledge of these topics to obtain a thorough understanding of organic reaction mechanisms. Reaction energy diagrams are used to illustrate the energy changes that take place during chemical reactions, and to emphasize the difference between a reaction intermediate and a transition state. 6.0: Why This Chapter? To understand both organic and biological chemistry, it’s necessary to know not just what occurs but also why and how chemical reactions take place. In this chapter, we’ll start with an overview of the fundamental kinds of organic reactions, we’ll see why reactions occur, and we’ll see how reactions can be described. Once this background is out of the way, we’ll then be ready to begin studying the details of organic chemistry in future chapters. 6.1: Kinds of Organic Reactions If you scan any organic textbook you will encounter what appears to be a very large, often intimidating, number of reactions. These are the "tools" of a chemist, and to use these tools effectively, we must organize them in a sensible manner and look for patterns of reactivity that permit us make plausible predictions. Most of these reactions occur at special sites of reactivity known as functional groups, and these constitute one organizational scheme that helps us catalog and remember reactions 6.2: How Organic Reactions Occur - Mechanisms Having looked at the kinds of reactions that take place, let’s now see how they occur. An overall description of how a reaction occurs is called a reaction mechanism. A mechanism describes in detail exactly what takes place at each stage of a chemical transformation—which bonds are broken and in what order, which bonds are formed and in what order, and what the relative rates are for each step. A complete mechanism must also account for all reactants used and all products formed. 6.3: Polar Reactions Polar reactions occur because of the electrical attraction between positively polarized and negatively polarized centers on functional groups in molecules. Most organic compounds are electrically neutral; they have no net charge, either positive or negative. However, certain bonds within a molecule, particularly the bonds in functional groups, are polar, which is a consequence of an unsymmetrical electron distribution in a bond due to the difference in electronegativity of the bonded atoms. 6.4: An Example of a Polar Reaction - Addition of HBr to Ethylene This page looks at the reaction of the carbon-carbon double bond in alkenes such as ethene with hydrogen halides such as hydrogen chloride and hydrogen bromide. Symmetrical alkenes (like ethene or but-2-ene) are dealt with first. These are alkenes where identical groups are attached to each end of the carbon-carbon double bond. 6.5: Using Curved Arrows in Polar Reaction Mechanisms Understanding the location of electrons and being able to draw the curved arrows that depict the mechanisms by which the reactions occur is one of the most critical tools for learning organic chemistry since they allow you to understand what controls reactions, and how reactions proceed. 6.6: Radical Reactions Because of their high reactivity, free radicals have the potential to be both extremely powerful chemical tools and extremely harmful contaminants. Much of the power of free radical species stems from the natural tendency of radical processes to occur in a chain reaction fashion. Radical chain reactions have three distinct phases: initiation, propagation, and termination. 6.7: Describing a Reaction - Equilibria, Rates, and Energy Changes Every chemical reaction can go in either a forward or reverse direction. Reactants can go forward to products, and products can revert to reactants. As you may remember from your general chemistry course, the position of the resulting chemical equilibrium is expressed by an equation in which Keq, the equilibrium constant, is equal to the product concentrations multiplied together, divided by the reactant concentrations multiplied together, with each concentration raised to the power of its coeff 6.8: Describing a Reaction - Bond Dissociation Energies e’ve just seen that heat is released (negative ΔH) when a bond is formed because the products are more stable and have stronger bonds than the reactants. Conversely, heat is absorbed (positive ΔH) when a bond is broken because the products are less stable and have weaker bonds than the reactants. The amount of energy needed to break a given bond to produce two radical fragments when the molecule is in the gas phase at 25 °C is a quantity called the bond strength, or bond dissociation energy. 6.9: Describing a Reaction - Energy Diagrams and Transition States When we talk about the thermodynamics of a reaction, we are concerned with the difference in energy between reactants and products, and whether a reaction is ‘downhill’ (exergonic, energy releasing) or ‘uphill (endergonic, energy absorbing). When we talk about kinetics, on the other hand, we are concerned with the rate of the reaction, regardless of whether it is uphill or downhill thermodynamically. 6.10: Describing a Reaction- Intermediates Many important organic reactions do not occur in a single step; rather, they are the sum of two or more discreet bond-forming / bond-breaking steps, and involve transient intermediate species that go on to react very quickly. 6.11: A Comparison between Biological Reactions and Laboratory Reactions Some reaction that are important in laboratory chemistry yet don’t occur in nature and others that have counterparts in biological pathways. In comparing laboratory reactions with biological reactions, several differences are apparent. For one, laboratory reactions are usually carried out in an organic solvent such as diethyl ether or dichloromethane to dissolve the reactants and bring them into contact, whereas biological reactions occur in the aqueous medium within cells. 6.12: Chemistry Matters—Where Do Drugs Come From? It has been estimated that major pharmaceutical companies in the United States spent some $200 billion on drug research and development in 2020, while government agencies and private foundations spent another $28 billion. What does this money buy? From 1983 to 2022, the money resulted in a total of 1237 new molecular entities (NMEs)—new biologically active chemical substances approved for sale as drugs by the U.S. Food and Drug Administration (FDA). 6.13: Key Terms 6.14: Summary This summary highlights key organic reaction types, including addition, elimination, substitution, and rearrangement reactions. It emphasizes the importance of understanding reaction mechanisms, the role of functional groups, and how molecular structure influences reactivity. The chapter also introduces concepts such as reaction energy profiles and the significance of catalysts in enhancing reaction rates. Overall, it provides a foundational overview for studying organic chemistry. 6.15: Additional Problems