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Chem51LA

Winter‘11

Lecture1:MelngPoint

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OrganicChemistryTechniques

•  InChem51LAwewillbuildatoolboxoftechniquesintwomaincategories:

 – Purificaon:separangmixturesofcompoundsbasedontheirchemicalorphysicalproperes

 – Idenficaon:determiningtheidentyofacompoundthroughphysicalproperesor

structureelucidaon

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Experiment1:MelngPoint

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MelngPoint(Idenficaon)

•  Themelngpointofacompoundisaphysicalproperty

 –  Canbeusedtodetermineidentyandpurity.

•  Melng:transionfromsolidphasetoliquid –  Requiresenergytobreakupcrystallace –  Solidheatedtopointwherevaporpressureofsolidequalsvaporpressureofliquid.

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LoweringtheMelngPoint

•  MelngpointofanimpuresubstancewillALWAYSbelowerandbroaderthanthepure

substance!Why?

 – Ittakeslessenergytobreakupalacewithcontaminants.

 – Vaporpressue!

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Freezing(Melng)PointDepression

•  Solidandliquidarealwaysinequilibrium.•  Formelngtooccur,thevaporpressureofthesolidmustequalthevaporpressureoftheliquid.

•  Vaporpressuresofsolidsdon’tchangewithcomposion(norealmixing)butdochangewithtemperature.

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Freezing(Melng)PointDepression(II)

•  Assomesolidstartstomelt,thecontaminantsubstance(B)dissolves,loweringthevaporpressureoftheliquid(soluon,A).

•  Remember,solidonlyneedstobeheatedenoughtoreachvaporpressureequaltoliquid’svaporpressure.

•  SO...Asmorecontaminantdissolvesinliquid,vaporpressureofliquiddecreases,andamountofheatrequiredtomeltsoliddecreases.

LiquidA+DissolvedB

SolidB

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MelngPointComposionDiagram

h[p://www.chem.arizona.edu/~salzmanr/480a/480ants/2comppd/2comppd.html

Melngpointsarenotloweredindefinitely.

Theeuteccpointrepresentsthelowestpossiblemelngtemperatureforamixtureof

twosubstances.

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HowCanWeUseMelngPoint?

•  Idenficaon:Melngpointcanhelpustoidenfyacompound

 – DoALLcompoundshavecompletelydifferentmelngpoints?

•  Ex:o-anisicacid(98-100ºC)vs4-tert -butylphenol(98-101ºC)

•  Howcanwetellthemapart(usingonlymelngpoint)?

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MixedMelngPointandIdenficaon

•  Scenario:Yourunknowncompoundmeltsat97-100°C.

 –  Couldbeo-anisicacid(98-100ºC)(A) – Orcouldbe4-tert-butylphenol(98-101ºC)(B)

• Tryamixedmelngpointexperiment! – Mixunknown+o-anisicacid:melngpointislowered – Mixunknown+4-tert-butylphenol:nochange

•  Yourcompoundis4-tert -butylphenol!

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WhatYou’reDoinginLab

1.  Calibraoncurve(knowncompounds)2.  Determinemelngpointofunknowns3.  Tryamixedmelngpoint(known

compounds)4.  Usemixedmelngpointtoconfirmidenty

ofunknowns

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What’saCalibraonCurve?

•  Arethermocouplesalwaysaccurate?•  Determinemelngpointoftwoknowncompounds.

•  Makegraphcomparingtheorecalmelngpointtoobservedmelngpoint.

•  Usethisgraphtoadjustalltemperaturereadings!

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HowtoDetermineMelngPointofan

Unknown

1.  Doa“quickanddirty”run.•  Quicklymeltasampletodetermineapproximatemelng

point.

2.  Doamorecarefulrun.•  Heatquicklyunl~20°Cbelowapproximatemelng

point.

•  Slowheangto1-2°CperminuteandWATCHCLOSELY!

3.  Doitagain.• 

Youneedatleast2determinaonswheremelngtemperaturesagree.

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TipsandTricks

•  Grindyoursolidwithaspatulaonawatchglassbeforeloadingintocapillarytube.•  Usethe“drop”methodtopacksample(TAwilldemo).

•  Mel-Tempshave3slots,sodomorethanonesampleatame!

•  Startwithlower-melngsamples.•  Useafreshsampleeachme.•  Don’tforgettoletyourmel-tempcoolbetweenruns.

•  USEYOURCALIBRATIONCURVE!