Lecture 4 Chem 237 Feb. 10, 2015This Week Start Experiment 2: Preparation of Synthetic Banana Oil (Isoamyl Acetate) Purify product – simple distillation NMR and IR analysisPreparation of Synthetic Banana Oil – Isoamyl acetate Shift equilibrium by removal of waterDriving an Equilibrium by H2O Removal Dean Stark tube(apparatus)water collectshere (mark level)Fractional Distillation fractional distillationDoing Multiple Distillations with One Column 160 oC140 oC120 oC100 oCa:b 1:1a:b 3:1a:b 10:1a:b 20:1Refer to your lab manual for pictures – instructions: pp 53-55Fractional Distillation Columns eg if a column can achieve same as 5 simple distillations, the column is said to have 5 theoretical platesVigreuxWidmerGlassbeadsMost laboratory columns have 2-15 theor. plates Height Equivalent to Theoretical Plate (HETP) = length / # theoretical platesFractional Distillation Issues Several Problems:1) Low throughput2) Hold-up – loss of material on large surface area3) Non-ideal liquid-vapor equilibrium in column - floodingWhy not use packed columns to maximize theoretical plates?Type Description Throughput (mL/min) Hold-up (mL/plate) HETP (cm) Vigreux tube with indentations 10 1 10 Metal sponge packed tube filled with stainless steel sponge 5 1 5 Glass helices 1/8” glass helices 2-8 0.8 3-4 Widmer see picture 1-2 0.4 2Distillation Vapor pressure of pure A, PA0Dalton’s Law P = PA + PBWhen P = Pair, solution boilsSeparation of a mixture of liquids based on _____________________ of the components If we distill a mixture of A & B, what is the composition of the distillate? Raoult’s Law:PA =PA =XA = mole fraction of A in liquiddifferent vapor pressures liquid A liquids A & BXAPA0partial pressure of ARelative Volatility α What is the composition of the vapor in distillations? XAPAoXBPBoYB= =XAXBαYAXA = XB = 0.5 & A more volatile than Bafter 1 distillation: YA = 9 YBvapor is 90 % pure ASingle distillation is only useful if α > 20 (Δ b.p. > 80 oC)Suppose: 1:1 mixture of A & B in liquid and α = 9 How can we readily tell the composition of vapor? How do we purify liquids with α < 20? YA = mole fraction of A in vaporPhase Diagrams & Vapor Composition (Acetone/Benzene) Liquid line Vapor line If Acetone:Benzene = 1:1, phase transition (boiling) occurs at ______ vapor composition at _____ is _____ Benzene, or _____ Acetone. Can’t purify by simple distillation Liquid boils here Vapor has this composition 62 °C 0.31 62 °C 62 °C ~31% ~69%Phase Diagrams & Vapor Composition Large α VS Small αLarge αSmall αSimple vs. Multiple Distillations “Fractional Distillation” Liquid line Vapor line Plate 1Plate 2Plate 3Plate 4The more plates in a distillation, the more the condensate becomes enriched in the more volatile componentMinimum Boiling Azeotrope: Boiling point of the mixture is _____ than either pure compound Non-Ideal Mixtures may have unusual phase diagrams lower 1st thing to distill is the __________ Once the smaller component is depleted, 2nd Compound distills _____ Can only get ONE compound pure! Azeotrope Pure1st Compound 2nd Compound Can only get ONE compound pure! Non-Ideal Mixtures may have unusual phase diagrams Maximum Boiling Azeotrope: Boiling point of the mixture is _____ than either pure compound higher Pure AzeotropeIdentification of Organic Compounds Absorption spectroscopy Infrared spectroscopy Ultraviolet and visible spectroscopy NMR spectroscopy spectral data molecular structureAbsorption Spectroscopy Molecules absorb energy in the form of electromagnetic radiation when the energy difference between two states, ΔE, is perfectly matched to the energy of the radiation Excited state Ground state Ψ*Ψ0 ΔE photon of energy hν Ψ*Ψ0 Condition for absorption: ΔE = = –––– h = 9.537 x 10-14 kcal·s·mol-1 c = 3.0 x 108 m·s-1 ν = frequency λ = wavelength hν hc λstates involvednuclear spin statesvibrational stateselectronic statesenergy regionradio frequencyinfrared frequencyUV-visibleElectromagnetic Spectrum and Molecular Spectroscopy Type of spectroscopyNMRIRUV-visElectromagnetic Spectrum and Molecular Spectroscopy Infrared spectroscopy is diagnostic of the molecule’s _______________ functional groupsInfrared Spectroscopy Identification of Functional Groups Molecules are not static – they ____________________________. IR spectroscopy measures _______________________. translate, rotate, and vibrate molecular vibrationsMolecular vibrations can be though of as harmonic oscillations, like a spring. a b Hooke’s Law for harmonic oscillation: kf2πcma + mbmamb1/2ν =ν depends on kf - _______________ stronger bond à ν depends on __________________ heavier atoms à strength of bond masses of atoms higher νlower νBond C-H C-D C-C C=C C󲰨C ΔH0 (kcal/mol) 105 105 90 173 229 3,000 2,100 1,200 1,600 2,200 ν (cm-1) wavenumber ν = 1/λThese ν’s can be probed with infrared lightIntensity of absorption depends on the change in dipole moment (Δµ) R2C Oδ δµstretch contract + -+ -larger µ smaller µlarge Δµ, strong IR signal When there is little or no Δµ, little or no IR signal is observed H3CC CCH3CH3H3CO C O(symmetric stretch) These are termed _________________ vibrations IR inactive4000 3000 2000 1500 1000 6000100ν (cm-1)% TransmittanceA given functional group will absorb IR radiation at characteristic ν’s X-H X󲰨Y X=Y X-Y 4,000 – 1,600 cm-1 is where most diagnostic stretching occurs 1,600 – 600 cm-1 is called the ________________. It is a complex mixture of stretching and bending vibrations, and is ___________ to each compound. Fingerprint Region uniqueMolecular Vibrations can be Probed with IR light ΔE = hcνvibrational ground state vibrational excited state InfraredSourceIncident Beam (Io)TransmittedBeam (IT)sample'At certain!ν,!!IT!!!!I0!!because molecules have _________________ % Transmittance,!T!= !I0ν_x 100 %IT<!absorbed photonsPreparing samples – salt plates Nujol mull – solids ground into thin pasteCDCl3 solution – solids or oils KBr Pellet – good for solidsFilm – oil placed between platesAlkyl C-H stretch <3,000 cm-1 Ester C=O stretch at 1743 cm-1 Broad O-H stretch carboxylic acid Acid C=O stretch at 1712 cm-1 Two N-H stretches >3,000 cm-1 Alkenyl C-H stretch >3,000 cm-1 Alkenyl C=C at 1,661 cm-1 Terminal alkene C-H bend at 888 cm-1CH3Cl CD3Cl C-H stretch C-D stretch C-H bend C-D bend Effect of Increased Mass on IR SpectrumExample of