Chapter 2 - Chapter 2 REACTIVITY AND REACTIONS IN THIS...

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Unformatted text preview: Chapter 2 REACTIVITY AND REACTIONS IN THIS CHAPTER: V Reaction Mechanisms V Types of Organic Reactions |/ Electrophiiic and Nucleophilic Reagents V Thermodynamics V Bond-Dissociation Energies V Rates of Reactions V Chemical Equilibrium V Transition State Theory and Energy Diagrams V Bronsted Acids and Bases V Lewis Acids and Bases V Solved Problems 10 CHAPTER 2: Reactivity and Reactions 1 1 Reaction Mechanisms The sequence of bond-making and bond—breaking processes in a reac— tion 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- 12 ORGANIC CHEMISTRY Types of Organic Reactions Most organic reactions fall into one of the following categories: I. Substitution. An atom or group of atoms in a molecule or ion is replaced by another atom or group. 2. Addition. Two molecules combine to yield a single molecule. Addition frequently occurs at a double or triple bond and some— times at three—membered rings. 3. 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 (B—elimination). a multiple bond is formed. Removal of two atoms or groups from the same atom (or- elimination) produces a carbene. 4. Rearrangement. Bonds in the molecule are scrambled, converting it to its isomer. 5. Oxidation-reduction (redox). These reactions involve transfer of electrons or change in oxidation number. An increase in the num- ber 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 5— end of a polar bond or (c) has C=C 1t electrons. Such elee tron—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 8+ end of a polar bond. These electron~deficient sites are electrophilic and the species possess- ing such sites are called electrophiles or electron-acceptors. Many reactions occur by covalent bond formation between a nucleophilic and an electrophilic site. CHAPTER 2: Reactivity and Reactions 13 Thermodynamics 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) AH, the change in enthalpy. the heat transferred to or from a sys— tem. AH of a chemical reaction is the difference in the enthalpies of the products and the reactants: AH = [( H of products)— (H of reactants)] If the bonds in the products are stronger than the bonds in the reactants, energy is released, and AH is negative. The reaction is exothermic. (2) AS 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, AS = [(S of products) — (S of reactants)] (3) AG is the change in free energy. At constant temperature. AG = AH — TAS (T = absolute temperature) For a reaction to be spontaneous, AG must be negative. Bond-Dissociation Energies The bond-dissociation energy is the energy needed for the endother- mic homolysis of a covalent bond A:B —> A- + ‘B; AH is positive for these reactions. Bond formation, the reverse of this reaction, is exother— mic and the AH values are negative. Stronger bonds require more ener- gy to break, so they have larger AH values. The AH of a reaction is the sum of all the (positive) AH values for bond cleavages PLUS the sum of all the (negative) AH values for bond formations. AH = AH AH (bonds broken) _ (bonds formed) 14 ORGANIC CHEMISTRY 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 = l‘;[A]"[B]y where k is the rate constant at the given temperature, T. and [A] and [B] are molar concentrations (moi/L). Chemical Equilibrium 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. For dA+ e8 —9 fC+gD, [Crib]g KC 2 q [AidtBr- The AG of a reaction is related to ch by the expression AG : —RT an, where R is the gas constant (R: 8.314 Jmol“K“') and T is the absolute temperature (in K). Transition State Theory and Energy Diagrams When reactants have collided with sufficient energy of activation (E:I or AG) and with the proper orientation, they pass through a transition state in which some bonds are breaking while others are forming. The CHAPTER 2: Reactivity and Reactions 15 transition state is the highest energy state between reactants and prod- ucts. 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 forma- tion of molecules of lower energy is favored. In this reaction, the prod- ucts (C+ D) are favored. The reaction rate is actually related to the free energy of activation, AG‘, where AGi 2 AH: — TASi. Bronsted Acids and Bases In the Bronsted 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 conju- gate bases, and the strongest bases have the weakest conjugate acids. The stronger an acid, the larger its ionization constant Ka and the small- er its pKZl value. For the acid HA, HA —> H+ + A‘, K“ = [WlIA‘l/[HAI pKa = —logl<u 16 ORGANIC CHEMISTRY 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 apro— tic acid (no available proton), such as B133. H F H F HINI B. F———>H. N.BZ F H F H F Lewis Base Lewis Acid You Need t6 Know ~ Nucleophiles 7, ' - Electrophiles fli — ‘ AG AH: AS _—, - Brensted Acids and Bases * - Lewis Acids and BaSes Solved Problems Problem 2.1 Consider the following sequence of steps: (1) A —* B (2) B + C —’ D + E (3)13 + A ——> 2F (a) Which species may be described as (i) reactant, (ii) product, and (iii) intermediate? (b) Write the net chemical equation. (c) Indicate the molecularity of each step. CHAPTER 2: Reactivity and Reactions 17 (d) If the second step is rate determining, write the rate expression. (a) (i) A, C; (II) D. F; (iii) 13‘ E. (b) 2A + C —» D + 2F (add steps I, 2. and 3) (c) (I) unimoleculan (2) bimoleculaix (3) bimoleeular. ((1) Rate = k[C][A], since A is needed to make the intermediate. B. Problem 2.2 Give the conjugate acid of (a) CH3NH2, (b) C1430“, (c) CH3OH, (d) :H', (e) :CH; (f) H2C=CHT (a) CH3NH3+, (b) CH3OH‘ (c) CHSOHZ”. (d) H2. (e) CH4,(f) H3CCH;. ...
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