10 Chemical reactions in aqueous solutions

Transcription

1 The Physics and Chemistry of Water 10 Chemical reactions in aqueous solutions Effects of water in reactions Hydration of reactants cause steric barriers Increases attraction between nonpolar reactants Strong cohesive energy promotes volume-reducing reactions Surface-adsorbed water affects bulk/surface composition ratios May swell polymers Necessary for acid-base reactions Water as a medium for organic reactions Increasingly used for environmental reasons. Efficient in many situations where the transition state or other intermediates are polar. Sometimes strong reaction promotors at low concentrations.

2 Elementary reaction dynamics I (After Atkins) The activated complex is an energetic cluster of reactants, near the region of the reaction rearrangement, where some vibrational mode is replaced by a translation along the reaction coordinate. The transition state is the configuration at the peak (saddle point) in the potential energy curve (surface). The reaction rate is the frequency of activated complexes passing over the energy barrier.

3 Elementary reaction dynamics II Consider a bimolecular reaction: A + B [AB] products The equilibrium constant for formation of the activated complex is K = [AB] [A][B] The rate of the reaction is d[a] = [AB] (rate of passage over barrier) dt The rate of passage equals the frequency ν with which the complex separates as products, i.e. when one of its vibrations becomes a translation. This is a thoroughly excited vibration at temperature T, with classical energy ɛ = kt, and ν = ɛ/h, so that ν = kt /h, and the reaction rate is therefore d[a] = k 2 [A][B] = K [A][B] kt dt h where the rate constant is (the Eyring equation): k 2 = K kt h

4 Elementary reaction dynamics III Some thermodynamic quantities: The activation free energy is G = RT ln K Since G = H T S, the Eyring equation becomes k 2 = kt h e G/RT = kt h e S/R e H/RT Now H = E + (P V ), but in liquids (and solids) the P V term is negligible, so that d ln k 2 dt = E a RT = H + RT 2 RT 2, and E a = H+RT r G is the standard reaction free energy.

5 Diels-Alder reactions I A nonpolar 4+2 cycloaddition, forming a 6- membered ring from from two unsaturated fragments, via an ordered transition state, where the reactant s π-electrons form a common cloud. Outcome generally predictable even for complicated cases. Usually does not require catalyst or initiator. The aromatization of the transition state decreases its energy, so the activation barrier is low. This is a reaction that can be considerably enhanced by hydrophobic effects!

6 Diels-Alder reactions II (Rideout & Breslow, J. Am. Chem. Soc. 102, 7817 (1980)) Cyclopentadiene + Butenone Rate constants at 20 C Solvent Additive k 2 (10 5 M 1 s 1 ) Isooctane 5.94 MeOH 75.5 H 2 O 4400 H 2 O LiCl (4.86 M) H 2 O C(NH 2 ) + 3 Cl (4.86 M) 4300 The rate is higher in MeOH than isooctane, where the former is slightly more polar. LiCl is expected to increases the hydrophobic interaction between nonpolar molecules, due to the strong tendency of Li + to induce order in water. (In old style terminology it is a structure maker, and salts out nonpolar material dissolved in water). Guanidinium chloride (C(NH 2 ) + 3 Cl ) disrupts the organized water strcture (a structure breaker ), and is expected to reduce hydrophobic association, though the effect on the rate is only small.

7 Diels-Alder reactions III (Rideout & Breslow, J. Am. Chem. Soc. 102, 7817 (1980)) Anthracene-9-carbinol + N-Ethylmaleimide Rate constants at 30 C Solvent k 2 (10 5 M 1 s 1 ) Isooctane butanol 666 MeOH 344 CH 3 CN 107 H 2 O The reaction is fastest in the least polar of the organic solvents. The dramatic rate increase in water remains. Hydrophobic interaction, driving the reactants together at a rate higher than in organic solvents, causes the rate increase, rather than just the polarity of the solvent!

8 Carboxymethylation of starch in isopropanol-water media I (Kooijman et al., Starch/Stärke 55, 495 (2003)) Native starch (above) is insoluble in cold water, but carboxymethylated starches are, provided that the degree of substitution is higher than 0.2. The reaction is carried out with sodium monochloroacetate (SMCA) in the presence of a strong base, to enhance nucleophilicity of the hydroxyl groups, and to swell the starch particles. R OH + NaOH R ONa + H 2 O R ONa + ClCH 2 COONa R O-CH 2 COONa + NaCl NaOH also participates in undesired side-reactions, reducing the selectivituy of SMCA: NaOH + ClCH 2 COONa H O-CH 2 COONa + NaCl The reaction medium is typically a 9:1 mixture of alcohol:water, where alcohol is required for solubility, and water for efficiency.

9 Water has a clear influence on the degree of substitution (DS) and the selectivity of SMCA for the reaction with starch (rather than NaOH). DS for arrowroot (solid line) and potato (dashed line) starch. Conversion (solid line) and SMCA selectivity (dashed line). Water swells the starch particles to make them more accessible for the reagents. Water also hydrates the starch, thus affecting the distribution of the various components between the bulk liquid and the starch particles.

10 Water in radical reactions I (Yorimitsu et al., J. Am. Chem. Soc. 122, (2000)) Radical cyclization of allyl iodoacetate in various solvents solvent yield dielectric E T cohesive energy (%) constant (kcal/mol) density (cal/mol) H 2 O DMSO formamide DMF acetonitrile methanol ethanol THF < benzene < hexane < Radical reactions are generally rather insensitive to the medium, so why does water improve the yield?

11 Water in radical reactions II (Yorimitsu et al., J. Am. Chem. Soc. 122, (2000)) The large dielectric constant of water reduces the activation energy for the rotation 1 3, and the cyclization 3 5, relative to other solvents. The high cohesive energy density assists in the rotation 1 3, in that the size of the cavity required for the reactant is reduced. The dipole moment has a minor effect, but from 1 to 4, the dipole moment increases, thus being promoted by highly polar solvents.

12 Solvation effects on reactions (Bergsma et al., J. Chem. Phys. 86, 1356 (1967)) MD simulation of a nucleophilic displacement reaction in water Cl + CH 3 Cl ClCH 3 + Cl Contrast between reactions in gas and water free energy profile along the reaction coordinate: The polar solvent will stabilize the higher charge density reactants and products more than the lower charge density transition state complex. The energy wells in the gas phase are characteristic of metastable charge dipole complex forming close to the transition barrier.

13 Solvation effects on reactions (Bergsma et al., J. Chem. Phys. 86, 1356 (1967)) MD simulation of a nucleophilic displacement reaction in water Cl + CH 3 Cl ClCH 3 + Cl Contrast between reactions in gas and water free energy profile along the reaction coordinate: The polar solvent will stabilize the higher charge density reactants and products more than the lower charge density transition state complex. The energy wells in the gas phase are characteristic of metastable charge dipole complex forming close to the transition barrier.

14 Solvation effects on reactions (Bergsma et al., J. Chem. Phys. 86, 1356 (1967)) MD simulation of a nucleophilic displacement reaction in water The results of the reaction simulation differs significantly from a description with equilibrium solvation, where the free energy of the solvent is equilibrated to the reaction system along the reaction coordinate. The outcome of the reaction is highly dependent on (or determined by) the local configuration of the solvent at the transition state. The reaction in the transition barrier is characterized by a frozen solvent configuration, i.e. nonadiabatic solvation. A significant fraction of the transtion states reappear as reactants again. There are two reasons for this: 1. The time scale of the reaction is much shorter than the characteristic time scale of solvent reorientation. 2. There is rapid variation of charge distribution along the reaction coordinate near the transition barrier, leading to a pronounced coupling between the reacting system and the solvent, in addition to the ion-dipole-forces present for fixed charges.

15 H-bond effects (Yin et al., Organometallics 20, 1216 (2001)) Transition-metal-catalyzed hydrogenation of CO 2 to formic acid is accelerated by small amounts of added water in organic solvents. Free energy profiles (kcal/mol) for the reactions of TpRu-(PH 3 )(CH 3 CN)(H) with CO 2, without (top) and with (bottom) water. H-bonding of H 2 O to CO 2 enhances the electrophilicity at the carbon, resulting in significant reduction of the reaction barrier.

16 General references G. Wilse Robinson et al., Water in biology, chemistry and physics, Singapore: World Scientific W. A. Smit et al., Organic synthesis, Cambridge: Royal Society of Chemistry 1998.