CH361/361H Week 7 Lecture Gas Chromatography and Thermodynamics

Size: px

Start display at page:

Download "CH361/361H Week 7 Lecture Gas Chromatography and Thermodynamics"

April Paul
5 years ago
Views:

1 CH36/36H Week 7 Lecture Gas Chromatography and Thermodynamics

2 Experiment II Overview: Preparation & Equilibration of Isomeric 2,3-Dimethylbutenes H + cat K eq =? ΔG =? ΔH =? ΔS =? 2,3-dimethyl-2-butene 2-ene 2,3-dimethyl--butene -ene major activities in experiment II REPORT 2 synthesis of 2-ene (major) / -ene mixture by Grignard reaction followed by acid mediated dehydration (E elimination) synthesis of -ene by Wittig olefination purification and isomer separation by distillation / fractional distillation REPORT 3 equilibration of alkene isomers and analysis of alkene isomer distributions by gas chromatography (GC) determination of thermodynamic properties (DG, DH, DS ) by analyzing temperature dependence of equilibrium constant (K eq )

Gas Chromatography An analytical separation method; used on samples that can be volatilized without undergoing decomposition Chromatography: chroma = color; graphein = to write Stationary phase and

3 Gas Chromatography An analytical separation method; used on samples that can be volatilized without undergoing decomposition Chromatography: chroma = color; graphein = to write Stationary phase and mobile phase Compounds ( analytes ) partition between stationary and mobile phases In GC, partitioning depends on strength of attractive forces between stationary phase and analyte, and the analyte s vapor pressure Illustration of analyte partitioning between stationary and mobile phases in a gas chromatography experiment stationary phase mobile phase Thin layer chromatography (TLC) of plant pigments (Image Attribution: Wikipedia, CC BY-SA 2.5 Licensing) analyte partitioning event

4 Gas Chromatography: Instrument Schematic Metal column packed with firebrick, coated with polydimethylsiloxane (DC-200) Capillary columns also commonly used (Image Attribution: Wikipedia, CC BY-SA 2.5 Licensing)

5 Gas Chromatography: Instrument Schematic Thermal conductivity detector - Wheatstone bridge Other detectors commonly used: Flame ionization; electron capture; mass spec (Image Attribution: Wikipedia, CC BY-SA 2.5 Licensing)

6 Gas Chromatography: Instrument Schematic Trace output: Retention time (depends on several factors); must run standards to verify retention time Peak integration Response factors must be measured; responsiveness varies with heat capacity of analytes (Image Attribution: Wikipedia, CC BY-SA 2.5 Licensing)

7 Thermodynamics of Isomerization Reaction acid catalyzed isomerization -ene 2-ene H + = tosic acid (p-toluenesulfonic acid)

8 Thermodynamics of Isomerization Reaction K eq = [2-ene] [-ene] -ene 2-ene Polymerization reaction polymer

9 Thermodynamics of Isomerization Reaction Why do reactions occur? consider the reaction A B 3 possible values for DG: Gibbs energy, G DG > 0 DG < 0 (change is spontaneous) DG > 0 (change is non-spontaneous) DG = 0 (equilibrium) G = G + RTlnQ pure A c B, mole fraction B pure B

10 Thermodynamics of Isomerization Reaction G = G + RTlnQ where, DG o : standard change in Gibbs energy (standard conditions: atm, 298K) R: gas constant T: temperature Q: reaction quotient, [2-ene]/[-ene] If the system is at equilibrium, DG = 0 and Q = K therefore, and, G = G + RTlnK = 0 G = RTlnK

11 Thermodynamics of Isomerization Reaction G = H TS G = H T S where, DG o : standard change in Gibbs energy DH o : standard change in enthalpy DS o : standard change in entropy largely related to bond energy disorder ; related to number and type of bonds, free rotation, symmetry, and other factors

12 Thermodynamics of Isomerization Reaction G = RTlnK G = H T S Equilibrate at 60 o C, 80 o C, 00 o C, 20 o C, 40 o C RTlnK = G = H T S RTlnK = H T S RTlnK RT = H T S RT lnk = H R T + S R Plot lnk vs. /T slope = H R y int = S R

13 Thermodynamics of Isomerization Reaction Equilibrate at 60 o C, 80 o C, 00 o C, 20 o C, 40 o C lnk = H R T + R S G = H T S Plot lnk vs. /T slope = H R y int = R S Plot DG o vs. T slope = S y int = H

14 Experiment II: Tasks for Week H + cat K eq =? ΔG =? ΔH =? ΔS =? 2,3-dimethyl-2-butene 2-ene 2,3-dimethyl--butene -ene Grignard (2-ene) folks Wednesday complete dehydration / simple distillation of 2-ene; GC analysis of simple distillation sample of 2-ene (TIME ALLOWING, start to practice making ampules) Friday join your 2-ene group (~3 people) and conduct fractional distillation (in hood); TIME ALLOWING, perform GC analysis of your fract. distln. purified sample of 2-ene Wittig (-ene) folks Wednesday perform GC analysis of your groups sample(s) of -ene from fractional distillation; practice making ampules. Friday make stock solution (see p ) and analyze by GC.

Experiment II Overview: Preparation & Equilibration of Isomeric 2,3-Dimethylbutenes 2 3 4 H + cat. 2 3 4 K eq =? ΔG =? ΔH =? ΔS =?

15 Experiment II Overview: Preparation & Equilibration of Isomeric 2,3-Dimethylbutenes H + cat K eq =? ΔG =? ΔH =? ΔS =? 2,3-dimethyl-2-butene 2-ene 2,3-dimethyl--butene -ene major activities in experiment II REPORT 2 synthesis of 2-ene (major) / -ene mixture by Grignard reaction followed by acid mediated dehydration (E elimination) Report 2 due Wed. synthesis of -ene by Wittig olefination Nov. 2 by 2 noon purification and isomer separation by distillation / fractional distillation REPORT 3 equilibration of alkene isomers and analysis of alkene isomer distributions by gas chromatography (GC) determination of thermodynamic properties (DG, DH, DS ) by analyzing temperature dependence of equilibrium constant (K eq )

CH361/361H Week 6 Lecture Simple and Fractional Distillation

CH361/361H Week 6 Lecture Simple and Fractional Distillation Experiment II Overview: Preparation & Equilibration of Isomeric 2,3-Dimethylbutenes 1 2 3 4 H + cat. 1 2 3 4 K eq =? ΔG =? ΔH =? ΔS =? 2,3-dimethyl-2-butene