Reactivity and Reactions Help
The sequence of bond-making and bond-breaking processes in a reaction is called a mechanism. A reaction may occur in one step or, more often, by a sequence of several steps. For example, A+B→X+Y may proceed in two steps:
(1) A → I + X
(2) B + I → Y
Substances that are formed in early steps and consumed in later steps (such as I in the reaction above) are called intermediates. Sometimes the same reactants can give different products via different mechanisms. Intermediates often arise from one of two types of bond cleavage:
Heterolytic (polar) cleavage. Both electrons go with one group, e.g.,
A:B → A+ + :B– or (A: – + B+)
Homolytic (radical) cleavage. Each group takes one electron, e.g.,
A:B → A· + B·
Types of Organic Reactions
Most organic reactions fall into one of the following categories:
- Substitution. An atom or group of atoms in a molecule or ion is replaced by another atom or group.
- Addition. Two molecules combine to yield a single molecule. Addition frequently occurs at a double or triple bond and sometimes at three-membered rings.
- Elimination. This reaction is the reverse of addition. Two atoms or groups are removed from a molecule. If the atoms or groups are taken from adjacent atoms (β-elimination), a multiple bond is formed. Removal of two atoms or groups from the same atom (α-elimination) produces a carbene.
- Rearrangement. Bonds in the molecule are scrambled, converting it to its isomer.
- Oxidation-reduction (redox). These reactions involve transfer of electrons or change in oxidation number. An increase in the number of H atoms bonded to C and a decrease in the number of bonds to other atoms such as C, O, N, Cl, Br, F, and S signal reduction.
Electrophilic and Nucleophilic Reagents
Reactions generally occur at the reactive sites of molecules and ions. These sites fall mainly into two categories. One category has a high electron density because the site (a) has an unshared pair of electrons or (b) is the δ– end of a polar bond or (c) has C=C π electrons. Such electron- rich sites are nucleophilic and the species possessing such sites are called nucleophiles or electron-donors. The second category (a) is capable of acquiring more electrons or (b) is the δ+ end of a polar bond. These electron-deficient sites are electrophilic and the species possessing such sites are called electrophiles or electron-acceptors. Many reactions occur by covalent bond formation between a nucleophilic and an electrophilic site.
The thermodynamics and the rate of a reaction determine whether the reaction proceeds. The thermodynamics of a system is described in terms of several important functions:
(1) ΔH, the change in enthalpy, the heat transferred to or from a system. ΔH of a chemical reaction is the difference in the enthalpies of the products and the reactants:
ΔH = [(H of products) – (H of reactants)]
If the bonds in the products are stronger than the bonds in the reactants, energy is released, and ΔH is negative. The reaction is exothermic.
(2) ΔS is the change in entropy. Entropy is a measure of randomness. The more the randomness, the greater is S; the greater the order, the smaller is S. For a reaction,
ΔS = [(S of products) – (S of reactants)]
(3) ΔG is the change in free energy. At constant temperature,
ΔG = ΔH – TΔS (T = absolute temperature)
For a reaction to be spontaneous, ΔG must be negative.
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