Reactivity and Reactions Help (page 2)
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.
The bond-dissociation energy is the energy needed for the endothermic homolysis of a covalent bond A:B → A · + · B; ΔH is positive for these reactions. Bond formation, the reverse of this reaction, is exothermic and the ΔH values are negative. Stronger bonds require more energy to break, so they have larger ΔH values. The ΔH of a reaction is the sum of all the (positive) ΔH values for bond cleavages PLUS the sum of all the (negative) ΔH values for bond formations.
ΔH = ΔH(bonds broken) + ΔH(bonds formed)
Rates of Reactions
The rate of a reaction is how quickly reactants disappear or products appear. For the general reaction dA + eB + fC + gD, the rate is given by a rate equation:
Rate = k[A]x[B]y
where k is the rate constant at the given temperature, T, and [A] and [B] are molar concentrations (mol/L).
Every chemical reaction can proceed in either direction, even if it goes in one direction only to a microscopic extent. A state of equilibrium is reached when the concentrations of reactants and products no longer change because the reverse and forward reactions are taking place at the same rate. The equilibrium constant, Keq, is defined in terms of molar concentrations as indicated by the square brackets.
The ΔG of a reaction is related to Keq by the expression ΔG = –RT lnK, where R is the gas constant (R= 8.314 Jmol–1K–1) and T is the absolute temperature (in K).
Transition State Theory and Energy Diagrams
When reactants have collided with sufficient energy of activation (Ea or ΔG‡) and with the proper orientation, they pass through a transition state in which some bonds are breaking while others are forming. The transition state is the highest energy state between reactants and products. The relationship of the transition state (TS) to the reactants (R) and products (P) is shown by the energy diagram below, which corresponds to a one-step exothermic reaction A+B →C+D. At equilibrium, formation of molecules of lower energy is favored. In this reaction, the products (C+ D) are favored. The reaction rate is actually related to the free energy of activation, ΔG‡, where ΔG‡ = ΔH‡ – TΔS‡.
Brønsted Acids and Bases
In the Brønsted definition, an acid donates a proton and a base accepts a proton. The strengths of acids and bases are measured by the extent to which they lose or gain protons, respectively. In these reactions, acids are converted to their conjugate bases and bases to their conjugate acids. Acid-base reactions go in the direction of forming the weaker acid and the weaker base. The strongest acids have the weakest conjugate bases, and the strongest bases have the weakest conjugate acids. The stronger an acid, the larger its ionization constant Ka and the smaller its pKa value.
For the acid HA, HA →H+ + A–,
Ka = [H+][A–]/[HA]
pKa = –logKa
Lewis Acids and Bases
A Lewis acid (electrophile) shares an electron pair furnished by a Lewis base (nucleophile) to form a covalent (coordinate) bond. The Lewis concept is especially useful in explaining the acidity of an aprotic acid (no available proton), such as BF3.
Practice problems for these concepts can be found at: Reactivity and Reactions Practice Problems
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