Understanding Organic Reactions

When two sequential reactions are carried out without drawing any intermediate compound, the steps are usually numbered above the reaction arrow. This convention signifies that the first step occurs before the second step, and the reagents are added in sequence, not at the same time.

Kinds of Organic Reactions

A substitution is a reaction in which an atom or a group of atoms is replaced by another atom or group of atoms. In a general substitution, Y replaces Z on a carbon atom. Substitution reactions involve s bonds: one s bond breaks and another forms at the same carbon atom. The most common examples of substitution occur when Z is a hydrogen or a heteroatom that is more electronegative than carbon.

Elimination is a reaction in which elements of the starting material are “lost” and a p bond is formed. In an elimination reaction, two groups X and Y are removed from a starting material. Two s bonds are broken, and a p bond is formed between adjacent atoms. The most common examples of elimination occur when X = H and Y is a heteroatom more electronegative than carbon.

Addition is a reaction in which elements are added to the starting material. In an addition reaction, new groups X and Y are added to the starting material. A p bond is broken and two s bonds are formed. Addition and elimination reactions are exactly opposite. A p bond is formed in elimination reactions, whereas a p bond is broken in addition reactions.

Bond Making and Bond Breaking

A reaction mechanism is a detailed description of how bonds are broken and formed as starting material is converted into product. A reaction can occur either in one step or a series of steps.

Homolysis generates two uncharged species with unpaired electrons. A reactive intermediate with a single unpaired electron is called a radical. Radicals are highly unstable because they contain an atom that does not have an octet of electrons.

Heterolysis generates a carbocation or a carbanion. Both carbocations and carbanions are unstable intermediates. A carbocation contains a carbon surrounded by only six electrons, and a carbanion has a negative charge on carbon, which is not a very electronegative atom.

Radicals and carbocations are electrophiles because they contain an electron
deficient carbon. Carbanions are nucleophiles because they contain a carbon with a lone pair Bond formation occurs in two different ways. Two radicals can each donate one electron to form a two-electron bond. Alternatively, two ions with unlike charges can come together, with the negatively charged ion donating both electrons to form the resulting two-electron bond. Bond formation always releases energy.

Bond Dissociation Energy

The energy absorbed or released in any reaction, symbolized by DH0, is called the enthalpy change or heat of reaction. Bond dissociation energy is the DH0 for a specific kind of reaction—the homolysis of a covalent bond to form two radicals. Because bond breaking requires energy, bond dissociation energies are always positive numbers, and homolysis is always endothermic.

Comparing bond dissociation energies is equivalent to comparing bond strength.The
stronger the bond, the higher its bond dissociation energy. Bond dissociation energies decrease down a column of the periodic table. Generally, shorter bonds are stronger bonds. Bond dissociation energies are used to calculate the enthalpy change (DH0) in a reaction in which several bonds are broken and formed.

Thermodynamics

For a reaction to be practical, the equilibrium must favor products and the reaction rate must be fast enough to form them in a reasonable time. These two conditions depend on thermodynamics and kinetics respectively. Thermodynamics describes how the energies of reactants and products compare, and what the relative amounts of reactants and products are at equilibrium. Kinetics describes reaction rates. The equilibrium constant, Keq, is a mathematical expression that relates the amount of starting material and product at equilibrium.

The size of Keq expresses whether the starting materials or products predominate once equilibrium is reached. When Keq > 1, equilibrium favors the products (C and D) and the equilibrium lies to the right as the equation is written. When Keq < 1, equilibrium favors the starting materials (A and B) and the equilibrium lies to the left as the equation is written. For a reaction to be useful, the equilibrium must favor the products, and Keq > 1. The position of the equilibrium is determined by the relative energies of the reactants and products. DG0 is the overall energy difference between reactants and products.


Enthalpy and Entropy

DG0 depends on DH0 and the entropy change, DS0. Entropy change, DS0, is a measure of the change in the randomness of a system. The more disorder present, the higher the entropy. Gas molecules move more freely than liquid molecules and are higher in entropy. Cyclic molecules have more restricted bond rotation than similar acyclic molecules and are lower in entropy.

DS0 is (+) when the products are more disordered than the reactants. DS0 is (-) when the products are less disordered than the reactants. Reactions resulting in increased entropy are favored. DG0 is related to DH0 and DS0 by the following equation:

Energy Diagrams

An energy diagram is a schematic representation of the energy changes that take place as a reactants are converted to products. An energy diagram plots the energy on the y axis versus the progress of reaction, often labeled as the reaction coordinate, on the x axis.

The energy difference between reactants and products is DH0. If the products are lower in energy than the reactants, the reaction is exothermic and energy is released. If the products are higher in energy than the reactants, the reaction is endothermic and energy is consumed.

The unstable energy maximum as a chemical reaction proceeds from reactants to products is called the transition state. The transition state species can never be isolated. The energy difference between the transition state and the starting material is called the energy of activation, Ea.

Kinetics

Kinetics is the study of reaction rates. Recall that Ea is the energy barrier that must be exceeded for reactants to be converted to products. The higher the concentration, the faster the rate. The higher the temperature, the faster the rate. DG0, DH0, and Keq do not determine the rate of a reaction. These quantities indicate the direction of the equilibrium and the relative energy of reactants and products. A rate law or rate equation shows the relationship between the reaction rate and the concentration of the reactants. It is experimentally determined.


Referenssi
Janice Gorzynski Smith. Organic Chemistry, First Edition Chapter 6 Lecture Outline
. University of Hawaii