ChE 551 Lecture 28 Solvents As Catalysts 1

Literature does not usually consider solvents to be catalysts but I think of them as catalysts. Solvents can stabilize intermediates Solvents can raise rates by

Solvents as Catalysts Solvents can initiate reactions Solvents stabilize intermediates Solvents stabilize transition states and thereby modify the intrinsic barriers to reactions Solvents act as efficient means for energy transfer Solvents can donate or accept electrons Mass transfer limitations are more important when solvents are present. 3

Effect of Solvents Large for ionic reactions Small for radical reactions 4

Examples of the Role of Solvents 5 Effect small for non-ionic reactions.

Example of the Role of Solvents 6 Big effect for ionic reactions

More Examples 7 Intermediate effect when Ions form during reaction.

Ionic Strength Also Affects Rate 8 A.Co (NH 3 ) 5 Br 2+ + Hg 2+  B.S 2 O I -  C.[(Cr(urea) 6 ] 3+ + H 2 O  D.Co(NH 3 ) 5 Br 2+ + OH -  products E.Fe 2+ + Co (C 2 O 4 ) 3-  products

Why Do Solvents Change Rates? Solvents stabilize intermediates Solvents can initiate reactions Solvents stabilize transition states and thereby modify the intrinsic barriers to reactions Solvents act as efficient means for energy transfer Solvents can donate or accept electrons Mass transfer limitations are more important when solvents are present. 9

Example: Cl+CH 3 Br  ClCH 3 +Br - 10 Figure 13.3 A comparison of the free energy changes during the reaction Cl- + CH 3 Br  ClCH 3 + Br-. Data from p131 in Reichardt[1988]

SN 2 Reactions Prefer Polar Aprotic Solvents 11 Table 13.2 The rate of the S N 2 reaction NaCl + CH 3 I  CH 3 Cl + NaI at 350 K. Data from A. J. Parker, Chem Rev 69 (1969) 1. The gas phase rate in the table is estimated from the abinitio calculations of Glukhovsten et al SolventRate constant, lit/mole- sec SolventRate consta nt, lit/mol e-sec gas phase about water 3.5  methylcya nide 0.13 methano l 3.1  dimethylfo rmamide 2.5

SN 1 Reactions prefer Protic Solvents 12

Basis For Which Solvent Is Best Want the solvent to Stabilize intermediates Lower the energy of the transition state relative to the reactants 13

How Do Solvents Affect The Stability Of Transition States? Reactants and transition state can have different solvation energy. If solvation energy of transition state is greater than the solvation energy of the reactants, the transition state will be stabilized by the solvent Leads to a higher rate 14

Solvation Energy Energy to take a molecule out of a pure phase and put the molecule into a solution Three step process Remove molecule from pure solute phase Create a hole in solvent to hold solute Transfer the solute into the hole Gain energy due to solute/solvent attractions 15

Solvation Energy Solvation energy= (Energy of the solute- solvent bonds formed) – (Energy of the solvent-solvent bonds broken) – (Energy of solute-solute bonds broken) 16

Eaxmple: Solubility Of Salt In Water 17 Table. Solubility of some alkyl halides in water saltsolubility, moles/liter Lattice energy (  H solute ), kcal/mole LiF NaCl KBr CsI LiI CsF

Which Solvent to Use? Protic solvents: good for S N 1 reactions of anions water, ethanol, methanol, acetic acid, formic acid, ammonia, ethane thiol Polar Aprotic solvents: good for S N 2 reactions of anions acetone, dimethyl sulfoxide (DMSO) [(CH 3 ) 2 S=O], dichloromethane, ethers, Dimethylformamide (DMF) [(CH 3 ) 2 NCHO], cyclohexanone, acetaldehyde Non-polar Aprotic solvents: good for radical reactions ethylene, benzene 18

Hydrophobic Effects: SN2 - TST larger than reactants (want aprotic solvent) SN1 -TST smaller than reactants (want protic solvent) 19

Hughes-Ingold Rules If the transition state for a reaction has a larger charge than the reactants, then the rate of reaction will increase as the polarity of the solvent increases. If the transition state for a reaction has a smaller charge than the reactants, then the rate of reaction will decrease as the polarity of the solvent increases. If the net charge remains the same, but the charge is dispersed, then there will be a small decrease in rate as the polarity of the solvent increases. If the net charge remains the same, but the charge is localized, then there will be a small increase in rate as the polarity of the solvent increases. 20

Equations For Solvent Effect Single sphere model Double sphere model Regular solution theory 21 Assume solvent-solute interactions control rate Assume hydrophobic interactions control rate

Double Sphere Model 22 R C 3 + Y - Transition State r AB ‡ R C 3 + Y - Reactants r AB R

Single Sphere Model 23

Fit to Data Mixed 24

Regular Solution Theory Assume hydrophobic interactions dominate After pages of algebra 25

Predictions 26 S N 1 reaction: the solvolysis of p-methoxyneophyl-toluene-sulfonate

Summary Solvents are catalysts Solvents enhance initiation reactions, stabilize intermediates, transition states Key energy: solvation energy includes solvent-solvent, solvent-solute, solute-solute interactions Leads to large variations in rate for ionic reactions Much smaller effects for radical reactions 27

Query What did you learn new in this lecture? 28