Molecular Orbital (MO) Theory

Transcription

1 Molecular rbital (M) Theory Molecular orbital mixing takes place at all times along reaction coordinate. (S N 2) + + C- σ As - approaches, and as - departs, orbitals mix. sp 3 C- σ

2 Molecular rbital (M) Theory Molecular orbital mixing takes place at all times along reaction coordinate. (S N 2) + + C- σ sp 3 C- σ As reaction proceeds, C- bonding mix is stretched apart; destabilizing

3 Molecular rbital (M) Theory Molecular orbital mixing takes place at all times along reaction coordinate. (S N 2) + + C- σ sp 3 C- σ owever, as reaction proceeds, lone pair is gradually redistributed over to : stabilizing

4 Molecular rbital (M) Theory Molecular orbital mixing takes place at all times along reaction coordinate. (S N 2) + + Point of most e - redistribution Point of least bonding overlap (transition state)

5 Molecular rbital (M) Theory Molecular orbital mixing takes place at all times along reaction coordinate. (S N 2) + + C- σ sp 3 C- σ

6 Molecular rbital (M) Theory Molecular orbital mixing takes place at all times along reaction coordinate. (S N 2) + + C- σ sp 3 C- σ

7 Thermodynamics n the Fly ΔG = Δ -TΔS = T(lnK) eminder of some good rules of thumb: At room temperature, ΔG = 1.4 kcal/mol is equivalent to an order of magnitude in equilibrium constant K. Creating a new molecule in the gas phase yields ΔS 50 cal/mol K ( entropy units, eu); 15 kcal/mol at room temperature New molecules in solution are worth less (< 35 eu) Example: N N 2 Δ = +14 kcal/mol; ΔG TP 0. So, K eq 1 M. (Standard energy means units refer to 1 M of all reactants and products.)

8 Thermodynamics n the Fly : eaction Energetics from Standard Bond Dissociation Energies (BDEs) Concept: Δ for reaction can be estimated from types of bonds that are created and broken in reaction. Don t need to know mechanism to do this; just look at bonds. Example: xidation of methane to methanol (to convert gas to liquid fuel). 3 C + 3 C + We don t know the mechanism of this reaction. What if we just pulled bonds apart and put them back together again? Enthalpy of reaction (state function) would be the same.

9 Thermodynamics n the Fly : eaction Energetics from Standard Bond Dissociation Energies (BDEs) 3 C + 3 C + Strategy: Use Bond Dissociation Energies (BDEs), enthalpy of individual homolytic bond cleavage, to calculate sum of bond breaking and bond making processes. For Δ r = BDE

10 Standard Bond Dissociation Energies (BDEs) Bond Type C C N N F F Si Si P P S S Br Br I I C N F BDE (kcal/mol) Bond Type Br I B S Si P B F B C N N C C C C S C F C C Br BDE (kcal/mol) Bond Type C I C B C Si C P N S Si F Si Si P P Br P C=C N=N = C=N From BDE (kcal/mol) Bond Type C= (C 2 ) C= (aldehyde) C= (ketone) C= (ester) C= (amide) C= (halide) C=S (CS 2 ) N= (N 2 ) P= (P 3 ) P=S (PS 3 ) S= (S 2 ) S= (DMS) P=P P P C C C N N C N BDE (kcal/mol)

11 Standard Bond Dissociation Energies (BDEs) From Anslyn & Dougherty, p. 72. Plenty more available on the Web.

12 Thermodynamics n the Fly : eaction Energetics from Standard Bond Dissociation Energies (BDEs) 3 C + 3 C +

13 Thermodynamics n the Fly : eaction Energetics from Standard Bond Dissociation Energies (BDEs) Concept: So, to determine Δ for entire reaction, sum up Δ values (BDEs) for all bonds broken and made. Don t need to know mechanism to do this; just look at bonds. Example: xidation of methane to methanol (to convert gas to liquid fuel). 3 C + 3 C + 3 C Cost: BDE( 3 C-) BDE(-) 105 kcal/mol 119 kcal/mol

14 Thermodynamics n the Fly : eaction Energetics from Standard Bond Dissociation Energies (BDEs) Concept: Δ for reaction can be estimated from types of bonds that are created and broken in reaction. Don t need to know mechanism to do this; just look at bonds. Example: xidation of methane to methanol (to convert gas to liquid fuel). 3 C + 3 C + 3 C Cost: BDE( 3 C-) BDE(-) 105 kcal/mol 119 kcal/mol BDE(-) 104 kcal/mol Gain

15 Thermodynamics n the Fly : eaction Energetics from Standard Bond Dissociation Energies (BDEs) Concept: Δ for reaction can be estimated from types of bonds that are created and broken in reaction. Don t need to know mechanism to do this; just look at bonds. Example: xidation of methane to methanol (to convert gas to liquid fuel). 3 C + 3 C + 3 C Cost: BDE( 3 C-) BDE(-) BDE( 3 C-) BDE(-) Gain 105 kcal/mol 119 kcal/mol 92 kcal/mol 104 kcal/mol

16 Thermodynamics n the Fly : eaction Energetics from Standard Bond Dissociation Energies (BDEs) Concept: Δ for reaction can be estimated from types of bonds that are created and broken in reaction. Don t need to know mechanism to do this; just look at bonds. Example: xidation of methane to methanol (to convert gas to liquid fuel). 3 C + 3 C + verall Δ = = +28 kcal/mol. (Endothermic.) Cost: BDE( 3 C-) BDE(-) BDE( 3 C-) BDE(-) Gain 105 kcal/mol 119 kcal/mol 92 kcal/mol 104 kcal/mol

17 Thermodynamics n the Fly : eaction Energetics from Standard Bond Dissociation Energies (BDEs) Concept: Δ for reaction can be estimated from types of bonds that are created and broken in reaction. Don t need to know mechanism to do this; just look at bonds. Warning: Using BDEs to calculate Δ r works okay, but is not very accurate. Good enough for thermo on the fly though.

18 Discussion Question + + aldehyde diol acetal For the reaction above, 1. What bonds are made and broken? 2. What do the BDEs of these bonds mean for the overall Δ for this reaction? 3. If you wanted the reaction to proceed to 99% completion ([acetal]/[aldehyde] = 100), what would the ratio [ 2 ]/[diol] have to be? ow might you pull/push this reaction to this degree of completion (if necessary)?

19 Discussion Question + + aldehyde diol acetal

20 Discussion Question + + aldehyde diol acetal 177 kcal/mol 2(111 kcal/mol) 2(85.5 kcal/mol) 2(111 kcal/mol) Δ r = +6 kcal/mol

21 Discussion Question + + aldehyde diol acetal Δ r = +6 kcal/mol. Assuming ΔS 0 (number of molecules is the same on both sides), ΔG r = +6 kcal/mol. ur rule of thumb says that this means K eq = [acetal][ 2 ] [aldehyde][diol] So, to reach [acetal] [aldehyde] = 100, [ 2 ] [diol] must