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sodium salt of unknown aromatic carboxylic acid + soluble impurities + insoluble impurities total mass: ? g
dissolve in water, filter
insoluble impurities mass: ? g
aq. sol’n of unk. sodium salt + sol. impurities
decolorize, filter
colored dye(s) mass: ~ 20 mg
aq. sol’n of unk. sodium salt + sol. impurities
conc. HCl; filter
crude unk. carboxylic acid mass: ? g
water-soluble imp. recrystallize “pure” unk.
carboxylic acid mass: ? g
melting point (“mp”)
mp, MW, pKa, derivative mp
Identify unknown carboxylic acid
account for all masses; calculate %recovery CH361/361H
Week1LectureLaboratoryNotebookKeeping,MeltingPointDetermination,Recrystallization,&Yields
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ExperimentIOverview:Prepn andCharactn ofanUnknownCarboxylicAcid
preparationofthefreeacidfromitssodiumsalt
purificationoftheacidbyrecrystallization
measurementofmeltingpointofpureacid
determinationofmolecularweightbyend-pointtitration
determinationofpKa bypotentiometrictitration
preparationofanamidederivative
majoractivitiesinexperimentI
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•
•
•
•
•
O
O
R
+ insoluble impurities+ soluble impurities
Na OH
O
R
• impure sodium salt of an aromatic carboxylic acid
• pure ‘free’ aromatic carboxylic acid
NH
O
R
• amide derivative
R´
REPO
RT1
What is the identity of your randomly
assigned carboxylic acid?
•establishbymeasuring
properties,rationalization,
andcomparingtovalues
publishedinliterature
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•
•
•
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SafetyTips:UseofGlovesandAvoidingContamination
weargloveswhenhandlingchemicals
BUT,takethemoffwhenleavingthelabtoavoidtrackingcontamination- VIDEO
washhandsBEFOREusingrestroom(andafterasusual!)
avoidcontaminatingscreenofsmartphone,considerusingastylus
thebalanceroomcanbecomemessyveryquickly!cleanupafteryourself
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ProperMaintenanceofaLaboratoryNotebook
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•
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yourlabnotebookistheprimaryrecordofyourexperimentaldata,useittorecordnotonly
procedures/numericaldatabutanyobservationsthatarepotentiallyrelevant;youmay,and
should,cross-referenceothermaterials(e.g.,CH361labmanual);seeMohrigChapter3
useapennotapencil,donot eraseorobliteratemistakes,rulethroughneatly;makesure
originalpageisproperlyduplicated(justtothepageimmediatelybeneath)
entriesmustbedatedandsigned/counter-signedoffonattheendofday;turninduplicatepagesforgradingatendofFriday’slabeachweek
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ProperMaintenanceofaLaboratoryNotebook
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DevelopmentofaPurificationStrategy:ExploitPropertiesofNaSaltvs.FreeAcid
O
O
RNa OH
O
R
• ionic compound• high solubility in H2O
• polar non-ionic compound• sparing solubility in H2O
+ insoluble impurities+ soluble impurities
O
O
RNa
+ soluble impurities
insolubleimpurities
NaCl+
some soluble impurities
+ related impurities
(a)dissolveinH2O
(b)vacuumfiltration
(a)acidifyw.HCl
(b)vacuumfiltration
residue(solid) filtrate(liquid)
filtrate(liquid)
precipitate(solid)
this week!
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sodium salt of unknown aromatic carboxylic acid + soluble impurities + insoluble impurities total mass: ? g
dissolve in water, filter
insoluble impurities mass: ? g
aq. sol’n of unk. sodium salt + sol. impurities
decolorize, filter
colored dye(s) mass: ~ 20 mg
aq. sol’n of unk. sodium salt + sol. impurities
conc. HCl; filter
crude unk. carboxylic acid mass: ? g
water-soluble imp. recrystallize “pure” unk.
carboxylic acid mass: ? g
melting point (“mp”)
mp, MW, pKa, derivative mp
Identify unknown carboxylic acid
account for all masses; calculate %recovery
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Melting Point Determination
melting point:
melting range: range of temps. over which a solid sample becomes liquid
Alq3, tris(8-hydroxyquinolinato)aluminum
melting point determination serves two purposes: 1)identity 2)purity
temperature at which solid and liquid are in equilibrium
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Melting Point Determination
Figure from “Laboratory Techniques in Organic Chemistry, 4th Edition” by Mohrig, Alberg, Hofmeister, Schatz, Hammond
eutectic point: point on phase diagram that represents particular ratio of 2-component mixture with the lowest melting point
Two-component (A & B) phase diagram
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Melting Point Determination – practical considerations
Preparation of sample tube
Rate of temperature increase
Melting point standards
• 1 - 2 mm compound • tightly packed • never re-use sample tubes
• first time: fast increase • thereafter: 1 - 2 oC/minute • consult chart for voltage settings
• choose standard with mp +/- 10 oC your sample • heat standard sample alongside your sample
Pitfalls • decomposition • sublimation • rate of heating is too fast
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Melting Point Determination – practical considerations
Images downloaded from: http://www.csi.edu/ip/physci/faculty/rex/MPTips.htm
72 oC 73 oC 74 oC 75 oC
VIDEO CLIP
https://www.youtube.com/watch?v=D-AwTmJ7Nf4
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Recrystallization
Why?? need pure compound to conduct analytical tests very common lab technique
How? exploit temperature-dependence of solubility
solid, crude sample (unk. Ar-CO2H + imp.)
dissolve in minimum vol. of appropriate
solvent
minimal solubility at room temp.
heat to ~boiling point of solvent
all solids dissolve at high temp.
slowly cool
imp. stay dissolved; unk. Ar-CO2H
crystallizes
filter
pure crystals of unk. Ar-CO2H
impurities dissolved in filtrate (“mother liquor”)
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Recrystallization
What makes an “appropriate” solvent for recrystallization? • very low solubility at low temp; very high solubility at high temp • impurities are soluble at all temps • gives good recovery (>70%) and significant purification • does not co-crystallize with compound of interest • chemically inert
Figure from “Laboratory Techniques in Organic Chemistry, 4th Edition” by Mohrig, Alberg, Hofmeister, Schatz, Hammond
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Recrystallization – practical considerations
Progressive approach: 1)solvent selection on micro-scale (~20 mg) 2)medium batch (~100 – 200 mg) 3) large batch (~3 – 5 g) – also involves hot filtration
Solvent selection • sample must be completely dry! • careful, detailed observations are crucial! • begin with 20 mg in ~0.1 mL solvent; incrementally add more solvent • if > 1.0 mL solvent required, solvent is not suitable
Solvent choices • water • ethanol (CH3CH2OH) • hexane (CH3CH2CH2CH2CH2CH3) • toluene (methylbenzene)
water/ethanol
hexane/toluene
Mixed solvent systems
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Recrystallization – practical considerations
Common “pitfalls”:
1) crystals do not form seed crystal “scratching” too much solvent?
3) low recovery
filtrate (mother liquor) contains compound of interest too much solvent? insufficient cooling
2) crystals are of insufficient purity
was solution cooled slowly? inappropriate solvent sample “melting” at boiling point of recrystallization solvent
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Images downloaded from: http://orgchem.colorado.edu/Technique/Procedures/Crystallization/Crystallization.html
Recrystallization
Crystallization that occurs with slow cooling:
Crystallization that occurs with fast cooling:
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TasksforWeek1:DecolorizeNaSalt,PrecipitateFreeAcid(TrialRecrystallizations)
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•
•
decolorizefiltrateusingactivatedcharcoal,2-3timesmightbenecessary,try
differentbrandsofcharcoalin2nd (/3rd)roundformaximumeffect
acidifyNasaltwithconc.HCltoprecipitatethefreeacid;isolateacidbyvacuum
filtration,suckdryforca.10-15min,thenleavetoairdryinhooduntilnextperiod
ovendryprecipitatedfreeacid(afteratrialmpdetermination);TIMEALLOWING,
begintrialrecrystallizationswithsmallamountsofacid
sodium salt of unknown aromatic carboxylic acid + soluble impurities + insoluble impurities total mass: ? g
dissolve in water, filter
insoluble impurities mass: ? g
aq. sol’n of unk. sodium salt + sol. impurities
decolorize, filter
colored dye(s) mass: ~ 20 mg
aq. sol’n of unk. sodium salt + sol. impurities
conc. HCl; filter
crude unk. carboxylic acid mass: ? g
water-soluble imp. recrystallize “pure” unk.
carboxylic acid mass: ? g
melting point (“mp”)
mp, MW, pKa, derivative mp
Identify unknown carboxylic acid
account for all masses; calculate %recovery
precipitate(solid)
OH
O
R
• polar non-ionic compound• sparing solubility in H2O
+ related impurities
Wednesday
Friday
Wed.(/Fri.)