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Melting Points Melting Points

Melting Point&Recrys

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Page 1: Melting Point&Recrys

Melting PointsMelting Points

Page 2: Melting Point&Recrys

Background InformationBackground Information

Melting point:Melting point: temperature at which solid is in equilibrium with its temperature at which solid is in equilibrium with its liquidliquid

Melting point range:Melting point range: span of temperaturespan of temperaturess from the point at which from the point at which the crystals first begin to liquefy to the point at which the enthe crystals first begin to liquefy to the point at which the entire tire sample is liquid sample is liquid

Pure compounds usually melt over a narrow temperature range of 1Pure compounds usually melt over a narrow temperature range of 1--22°°CC

Presence of soluble impurity almost always decreases and Presence of soluble impurity almost always decreases and broadens the melting point rangebroadens the melting point range

Page 3: Melting Point&Recrys

Melting Point of Mixture (X and Y)Melting Point of Mixture (X and Y)

Eutectic temperatureEutectic temperature:: lowest possible melting point for a mixture of lowest possible melting point for a mixture of 2 compounds 2 compounds Eutectic composition:Eutectic composition: composition at which both solid X and solid composition at which both solid X and solid Y are in equilibrium with the liquid mixture (at eutectic tempeY are in equilibrium with the liquid mixture (at eutectic temperature)rature)

Eutectic pointEutectic point

Page 4: Melting Point&Recrys

Taking a Melting Point: PackingTaking a Melting Point: Packing

Pack the capillary tube by pressing the Pack the capillary tube by pressing the open end gently into a sample of the open end gently into a sample of the compound to be analyzed.compound to be analyzed.

(Capillary tubes break easily!)(Capillary tubes break easily!)

The solid should fill the tube to a The solid should fill the tube to a depth of depth of ~~2 mm2 mm. Tap the bottom of . Tap the bottom of the capillary on a hard surface so the capillary on a hard surface so that the crystals pack down into the that the crystals pack down into the bottom of the tube.bottom of the tube.

Page 5: Melting Point&Recrys

Melting Point Apparatus: MelMelting Point Apparatus: Mel--TempTemp

Pack the crystals at the bottom of the Pack the crystals at the bottom of the tube and place the tube in the slot tube and place the tube in the slot behind the eyebehind the eye--piece on the Melpiece on the Mel--Temp. Temp. Turn on the MelTurn on the Mel--Temp and turn the Temp and turn the knob to adjust heating rate (not too fast knob to adjust heating rate (not too fast or too slow). or too slow).

Watch through eyepiece and record Watch through eyepiece and record temperature at which the crystals temperature at which the crystals first begin to liquefy and record first begin to liquefy and record temperature at which the entire temperature at which the entire sample is liquid (sample is liquid (mp rangemp range).).

Page 6: Melting Point&Recrys

Experiment: Part I Experiment: Part I -- Practice Melting RangePractice Melting Range

Use only Use only benzoic acidbenzoic acid (mp = 122(mp = 122--123123°°C).C).

Use proper size of sample (not more than 2 mm in capillary) Use proper size of sample (not more than 2 mm in capillary) just just

enough to see!!!enough to see!!!

Do not move the MelDo not move the Mel--TempTemp to your bench or remove to your bench or remove the probethe probe..

Increase Temperature by 10Increase Temperature by 10°°C/min until 10C/min until 10--1515°°C below the C below the

known mp range, then reduce rate to 1known mp range, then reduce rate to 1--22°°C/min.C/min.

Allow mp apparatus to cool down naturally after experiment (Allow mp apparatus to cool down naturally after experiment (no no

wet paper towels or ice wet paper towels or ice –– may use airmay use air))

Page 7: Melting Point&Recrys

Experiment: Part III Experiment: Part III -- Identifying Your UnknownIdentifying Your Unknown

Record the code Record the code of your of your unknownunknown..

Take 2 melting points (2 different samples):Take 2 melting points (2 different samples):

11stst: : taken quickly to approximate range (~10taken quickly to approximate range (~10°°C/min)C/min)

22ndnd: : taken slower (1taken slower (1--22°°C/min) when 10C/min) when 10°°C below mp range approximated C below mp range approximated from 1from 1stst runrun

Choose the Choose the knownknown compound that could match the mp of your compound that could match the mp of your unknownunknown(table 1 (table 1 -- p. 39).p. 39).

Mix Mix knownknown vsvs unknownunknown on a watch glass (1:1 mixture) on a watch glass (1:1 mixture) thoroughly by thoroughly by grinding with a spatula.grinding with a spatula.

Take mp of the mixture:Take mp of the mixture:

>If mp is the same as mp of unknown, unknown is identified.>If mp is the same as mp of unknown, unknown is identified.

>If mp of mixture is different from mp of unknown, repeat proces>If mp of mixture is different from mp of unknown, repeat process with s with another compound.another compound.

Page 8: Melting Point&Recrys

Unknown mp = 133Unknown mp = 133--135135°°CC

⇒⇒ Benzoin (mp = Benzoin (mp = 135135--137137°°C) or C) or transtrans--CinnamicCinnamic Acid Acid (mp = (mp = 133133--134134°°C)C)

mp =mp = 135135--137137°°CC

⇒⇒ Unknown is BenzoinUnknown is Benzoinmp mp <<<<133133°°CC

⇒⇒ Unknown is not BenzoinUnknown is not Benzoin

mp mp <<<<133133°°CC

⇒⇒ Unknown is not Unknown is not transtrans--Cinnamic AcidCinnamic Acid

mp mp = 133= 133--134134°°CC

⇒⇒ Unknown is Unknown is transtrans--Cinnamic Cinnamic AcidAcid

Mix Unknown and BenzoinMix Unknown and Benzoin

Mix Unknown and Mix Unknown and transtrans--CinnamicCinnamic AcidAcid

Part IIIPart III -- Identifying Your UnknownIdentifying Your Unknown

Page 9: Melting Point&Recrys

Waste HandlingWaste Handling

DonDon’’t discard capillaries in the trash. Put them in the collection t discard capillaries in the trash. Put them in the collection

beaker labeled beaker labeled ““3221 Melting Capillaries3221 Melting Capillaries””. . No glass in the trash!!No glass in the trash!!

Any unused unknown is to be returned to the TA in the original Any unused unknown is to be returned to the TA in the original

sample vial (properly capped).sample vial (properly capped).

Page 10: Melting Point&Recrys

Purifying Acetanilide by Purifying Acetanilide by RecrystallizationRecrystallization

Page 11: Melting Point&Recrys

DefinitionDefinition

Recrystallization:Recrystallization: purification process used to remove purification process used to remove impurities from organic compounds which are solid at room impurities from organic compounds which are solid at room temperature.temperature.

Principle:Principle:Solubility of a compound in a solvent increases with temperatureSolubility of a compound in a solvent increases with temperature..As the solution cools, crystals form and grow.As the solution cools, crystals form and grow.Molecules in a crystal have a greater affinity for molecules of Molecules in a crystal have a greater affinity for molecules of the the same kind than for impurities.same kind than for impurities.

Page 12: Melting Point&Recrys

The compound being purified must be The compound being purified must be insoluble in solvent at insoluble in solvent at room temproom temp butbut soluble in boiling solventsoluble in boiling solvent..

SolventSolvent’’s boiling point must be s boiling point must be lower thanlower than the the compoundcompound’’s s melting pointmelting point..

An An abundant quantity of crystalsabundant quantity of crystals must be recoverable from the must be recoverable from the

cool solvent.cool solvent.

Choosing a Choosing a RecrystallizingRecrystallizing SolventSolvent

Not Not SolubleSoluble

Not Not SolubleSolubleSolubleSolubleCold SolventCold Solvent

0.63 g0.63 g0.85 g0.85 gNANAMass Mass RecoveredRecovered

SolubleSolubleSolubleSolubleSolubleSolubleSolventSolvent’’s s Boiling PointBoiling Point

Not Not SolubleSoluble

Not Not SolubleSolubleSolubleSolubleRoom Room

TemperatureTemperature

CCBBAA Note:Note:

1. Be careful that your 1. Be careful that your compound does not melt before compound does not melt before the solvent boils. Why? the solvent boils. Why? ––Oiling OutOiling Out

2. Make sure that you record 2. Make sure that you record your observations in a chart your observations in a chart like the one shown here.like the one shown here.

Page 13: Melting Point&Recrys

Step 1 Step 1 –– Choosing the Right Solvent for AcetanilideChoosing the Right Solvent for Acetanilide

WaterWater

(polar)(polar)

EthanolEthanol

(polar)(polar)

AcetoneAcetone

(polar)(polar)

Petroleum Petroleum EtherEther

Record whether the Record whether the acetanilide is soluble or not at acetanilide is soluble or not at room temperatureroom temperature..

Heat the Heat the solution(ssolution(s) to boiling (in a hot water bath). Record whether the ) to boiling (in a hot water bath). Record whether the acetanilide is soluble or not in each acetanilide is soluble or not in each hot solventhot solvent..

Let them cool at room temperature first, and then in an ice wateLet them cool at room temperature first, and then in an ice water bath. r bath. Record your Record your observations observations –– Does crystal form?Does crystal form?..

Choose the appropriate solvent by Choose the appropriate solvent by

consulting with your TAconsulting with your TA and go to the next step.and go to the next step.

Page 14: Melting Point&Recrys

Step 2 Step 2 –– Dissolving the CompoundDissolving the Compound

Dissolve the compound in a Dissolve the compound in a MINIMUMMINIMUM volume of boiling solvent.volume of boiling solvent.After all of the compound dissolves, add 5% excess to prevent After all of the compound dissolves, add 5% excess to prevent premature crystallizationpremature crystallization. (For example: if 10 mL of boiling solvent is . (For example: if 10 mL of boiling solvent is required to just dissolve a compound, 0.5 mL will be added)required to just dissolve a compound, 0.5 mL will be added)

11-- Heat some solvent to Heat some solvent to boiling. Place the solid boiling. Place the solid to be recrystallized in an to be recrystallized in an Erlenmeyer flask.Erlenmeyer flask.

22-- Add small amount (1 Add small amount (1 mL) of the hot solvent mL) of the hot solvent into the flask.into the flask.

(Use a Pasteur pipet)(Use a Pasteur pipet)

33-- Swirl the flaskSwirl the flask

44-- Place the flask on the Place the flask on the hot plate to keep the hot plate to keep the solution warmsolution warm

55-- Repeat steps 2 to 4 until you have dissolved Repeat steps 2 to 4 until you have dissolved the compound, and then add the 5% excess.the compound, and then add the 5% excess.

Page 15: Melting Point&Recrys

Step 3 Step 3 -- DecolorizingDecolorizing

Decolorizing the solutionDecolorizing the solutionAdd a spatula tip of activated carbon Add a spatula tip of activated carbon after the solution after the solution is removed from the heat and has stopped boiling !!! is removed from the heat and has stopped boiling !!! Stir in the carbon and then allow it to sit briefly.

Hot Filtration!!!Hot Filtration!!!The The hot solutionhot solution is filtered (in this step) by is filtered (in this step) by gravity gravity filtrationfiltration through a through a stemlessstemless funnel containing a funnel containing a fluted fluted filter paperfilter paper. Keep the paper and funnel warm to avoid . Keep the paper and funnel warm to avoid crystals forming during filtration.crystals forming during filtration.

Be quickBe quick!!! at the hot filtration step to avoid !!! at the hot filtration step to avoid premature premature crystallizationcrystallization in the filter.in the filter.

Hot & Ready!Hot & Ready!

Page 16: Melting Point&Recrys

Step 4 Step 4 –– Forming CrystalsForming Crystals

Recrystallizing Pure CompoundRecrystallizing Pure CompoundSlowly coolSlowly cool the solution at room temperature before placing in an icethe solution at room temperature before placing in an ice--water bath to maximize crystal formation.water bath to maximize crystal formation.

TIP: TIP: If crystals do not form, induce crystallization yourself:If crystals do not form, induce crystallization yourself:

-- scratchingscratching the inside of the flask with a glass stirring rodthe inside of the flask with a glass stirring rod

-- or or concentratingconcentrating by evaporation (last resort!).by evaporation (last resort!).

Prepare ice cold solvent for washing step!

Page 17: Melting Point&Recrys

Vacuum Vacuum filtrationfiltration: : PrePre--weigh your filter paper and record mass!weigh your filter paper and record mass!

Step 5 Step 5 –– Collecting, Washing, and Drying the CrystalsCollecting, Washing, and Drying the Crystals

Page 18: Melting Point&Recrys

Sandwich your crystals between two filter Sandwich your crystals between two filter papers. Let your crystals dry for papers. Let your crystals dry for 1 week 1 week in your lab drawer.in your lab drawer.

Next Week:Next Week:

Measure and record the Measure and record the massmass and and melting pointmelting point..

Submit your crystals to the TA in a Submit your crystals to the TA in a labeled vial (your name, compoundlabeled vial (your name, compound’’s s name, mass of compound, and MP).name, mass of compound, and MP).

Step 6 Step 6 –– Recovery Analysis Recovery Analysis (Next Week)(Next Week)

mass of recrystallized compound, gmass of recrystallized compound, g% Recovery = % Recovery = —————————————————————————————————— x 100x 100

mass of crude compound, gmass of crude compound, g