Chapter 6: An Overview of Organic Reactions
Most organic reactions are additions, eliminations, substitutions, or rearrangements
The great majority of the reactions encountered in an introductory course can be sorted into four classes, and the classification is made not by the reagents but by the net structural change. An addition reaction unites two reactants into a single product; a pi bond is broken and two new sigma bonds form in its place, so a double bond becomes a saturated linkage. The addition of hydrogen bromide to but-2-ene to give 2-bromobutane illustrates the pattern. An elimination is the formal reverse of an addition. A single molecule loses two atoms or groups from adjacent carbons, a new multiple bond…
Bonds break evenly to give radicals or unevenly to give ions
Reactivity begins with bond cleavage, and a covalent bond can come apart in two fundamentally different ways. In homolytic cleavage the two bonding electrons separate evenly, one going to each fragment, so two neutral species each bearing an unpaired electron, called radicals, result. Weak and nonpolar bonds break this way under light or heat; molecular chlorine, for instance, absorbs light and splits into two chlorine atoms. Radical reactions are chains of such single-electron steps and are drawn with single-barbed fishhook arrows. In heterolytic cleavage the electrons stay together and both…
Curved arrows track the movement of electron pairs
Curved arrows are the grammar of polar mechanisms, and using them correctly is the single most transferable skill in the subject. The notation itself was set up in Chapter 2: each arrow moves one electron pair from a source — a lone pair or an existing bond — to a destination, and the two moves named there, association and displacement, cover every polar step to come. What is new here is the arrowhead. A full head with two barbs denotes a moving pair; radical steps use single-barbed fishhook arrows for one electron at a time. The two rules from Chapter 2 still prevent the most common errors:…
The equilibrium constant and free energy measure how favorable a reaction is
Two independent questions can be asked of any reaction: how far does it go, and how fast. The first is thermodynamic and is answered by the equilibrium constant, the ratio of product to reactant activities once the system has settled. When the constant exceeds one, products are favored at equilibrium; when it is less than one, reactants are favored. The equilibrium constant is not arbitrary; it is fixed by the standard free-energy change of the reaction. That free-energy change has two contributions. The enthalpy term measures the net heat of the reaction, essentially the difference between…
Activation energy and the transition state control reaction rate
Kinetics addresses the second question about a reaction: not whether it is favorable, but how quickly it occurs. The answer lies in the energy profile that connects reactants to products. Along that path the energy first rises to a maximum and then descends. The maximum is the transition state, and the energy needed to climb from the reactants to it is the activation energy. This barrier, not the overall free-energy change, sets the rate. The size of the activation energy has a direct molecular meaning. Molecules react only when they collide with enough energy, and in the correct orientation,…
Energy diagrams reveal transition states and reactive intermediates
The energy diagram is the chapter's central tool for reasoning about a mechanism, because it displays in one picture both the thermodynamics and the kinetics of a transformation. The horizontal axis is the reaction coordinate, a measure of progress from reactants to products; the vertical axis is energy. Reading the diagram is a matter of interpreting its peaks and valleys. A one-step reaction produces the simplest diagram: a single maximum, the transition state, between the reactant valley on the left and the product valley on the right. The height of the maximum above the reactants is the…
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