6: An Overview of Organic Reactions
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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.
- 6.1: 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
- 6.3: 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.
- 6.5: 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.
- 6.6: 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.
- 6.9: 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.